Journal Cover Combustion and Flame
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  • On the particle evolution in iron pentacarbonyl loaded counterflow
           methane–air flame
    • Authors: Abhishek Raj; Zhongchao Tan; Dong Zhu; Eric Croiset; John Z. Wen
      Pages: 1 - 14
      Abstract: Publication date: August 2018
      Source:Combustion and Flame, Volume 194
      Author(s): Abhishek Raj, Zhongchao Tan, Dong Zhu, Eric Croiset, John Z. Wen
      The present work analyzes these particle evolution processes along the plane axis in an iron precursor (iron pentacarbonyl) loaded methane/air counterflow diffusion flame. The addition of iron pentacarbonyl into methane led to the formation of iron-based nanoparticles in the flame, together with the pre-existing soot particles. These two types of nanoparticles were found to be distinct in their shapes, chemical compositions, geometric mean particle diameters, total particle number concentrations, and particle size distributions. Nanoparticles produced in the flame were sampled from various axial and radial locations by means of a vacuum pump and their particle size distributions were characterized using a scanning mobility particle sizer (SMPS). The representative nanoparticle samples were also collected on the probe and examined using scanning electron microscopy (SEM) and energy dispersive spectroscopy (EDAX). SMPS, SEM and EDAX analysis revealed the nucleation, growth, agglomeration and interaction of these nanoparticles formed in the flame, evidenced by the changes of particle morphology, averaged particle size and elemental composition. Adding iron precursors was found to promote particle inception, leading to a greater total particle number concentration but a smaller mean particle diameter. Near the flame location, combustion of soot particles was accelerated due to the catalytic role of iron-based nanoparticles, which agrees the observation of primarily smaller iron-based nanoparticles dominating in the particle population. These findings shed light in studying the engine performance when the fuel borne catalysts are injected for abating particulate matters (PM) during regeneration of Diesel Particulate Filters (DPFs).

      PubDate: 2018-05-01T08:24:06Z
      DOI: 10.1016/j.combustflame.2018.04.003
      Issue No: Vol. 194 (2018)
       
  • Construction of a skeletal oxidation mechanism of n-pentanol by
           integrating decoupling methodology, genetic algorithm, and uncertainty
           quantification
    • Authors: Yachao Chang; Ming Jia; Bo Niu; Zhen Xu; Zihe Liu; Yaopeng Li; Maozhao Xie
      Pages: 15 - 27
      Abstract: Publication date: August 2018
      Source:Combustion and Flame, Volume 194
      Author(s): Yachao Chang, Ming Jia, Bo Niu, Zhen Xu, Zihe Liu, Yaopeng Li, Maozhao Xie
      Pentanol has attracted increasing attentions in recent years due to its ability to reduce the pollution emissions of engines and the dependence on fossil fuels. A skeletal oxidation mechanism composed of 47 species and 177 reactions is first developed for n-pentanol based on the decoupling methodology in this study. Then, the rate constants of the reactions in the fuel-related sub-mechanism are automatically optimized by using the genetic algorithm to reproduce the ignition delay times in shock tubes and rapid compression machines, and the major species concentrations in jet-stirred reactors. The final mechanism is determined based on the method of uncertainty minimization using polynomial chaos expansions by comparing the predicted uncertainty of the optimized mechanisms with available experimental data in shock tubes, rapid compression machines, and jet-stirred reactors. The final n-pentanol mechanism is validated against measurements in shock tubes, rapid compression machines, jet-stirred reactors, and premixed laminar flames over low-to-high temperatures. Good agreements between the measured and predicted results are obtained for various reactors. Due to the compact size and the reliable performance, the final mechanism is capable of well reproducing the combustion and emission behaviors of n-pentanol in a homogeneous charge compression ignition engine in coupling with a three-dimensional Computational Fluid Dynamics (CFD) model.

      PubDate: 2018-05-01T08:24:06Z
      DOI: 10.1016/j.combustflame.2018.04.012
      Issue No: Vol. 194 (2018)
       
  • Fast approximations of exponential and logarithm functions combined with
           efficient storage/retrieval for combustion kinetics calculations
    • Authors: Federico Perini; Rolf D. Reitz
      Pages: 37 - 51
      Abstract: Publication date: August 2018
      Source:Combustion and Flame, Volume 194
      Author(s): Federico Perini, Rolf D. Reitz
      We developed two approaches to speed up combustion chemistry simulations by reducing the amount of time spent computing exponentials, logarithms, and complex temperature-dependent kinetics functions that heavily rely on them. The evaluation of these functions is very accurate in 64-bit arithmetic, but also slow. Since these functions span several orders of magnitude in temperature space, some of this accuracy can be traded with greater solution speed, provided that the governing ordinary differential equation (ODE) solver still grants user-defined solution convergence properties. The first approach tackled the exp() and log() functions, and replaced them with fast approximations which perform bit and integer operations on the exponential-based IEEE-754 floating point number machine representation. The second approach addresses complex temperature-dependent kinetics functions via storage/retrieval. We developed a function-independent piecewise polynomial approximation method with the following features: it minimizes table storage requirements, it is not subject to ill-conditioning over the whole variable range, it is of arbitrarily high order n > 0, and is fully vectorized. Formulations for both approaches are presented; and their performance assessed against zero-dimensional reactor simulations of hydrocarbon fuel ignition delay, with reaction mechanisms ranging from 10 to 104 species. The results show that, when used concurrently, both methods allow global speed-ups of about one order of magnitude even with an already highly-optimized sparse analytical Jacobian solver. The methods also demonstrate that global error is within the integrator’s requested accuracy, and that the solver’s performance is slightly positively affected, i.e., a slight reduction in the number of timesteps per integration is seen.

      PubDate: 2018-05-01T08:24:06Z
      DOI: 10.1016/j.combustflame.2018.04.013
      Issue No: Vol. 194 (2018)
       
  • Optimizing in-situ char gasification kinetics in reduction zone of
           pulverized coal air-staged combustion
    • Authors: Denggao Chen; Zhi Zhang; Zhenshan Li; Zian Lv; Ningsheng Cai
      Pages: 52 - 71
      Abstract: Publication date: August 2018
      Source:Combustion and Flame, Volume 194
      Author(s): Denggao Chen, Zhi Zhang, Zhenshan Li, Zian Lv, Ningsheng Cai
      Reliable kinetics of char gasification are of great importance for accurate prediction of the reductive atmosphere in air-staged combustion of pulverized coal, which is critical in controlling nitrogen and sulfur species for the optimized design and operation of boilers and burners. In this study, a new method of obtaining char gasification kinetics from realistic air-staged combustion experiments is proposed. Firstly, a real combustion atmosphere, including the temperature and concentration fields of air-staged combustion in an actual boiler, is simulated in an electric-heated down-firing furnace by controlling the stoichiometric ratio of air to coal. In-situ char gasification is observed by obtaining gas and solid reaction data from staged combustions with stoichiometric ratios in the range of 0.6–0.9 at temperatures of 1200–1400 °C. Secondly, a detailed char combustion/gasification model with emphasis on coupling between the discrete phase and gas phase is developed and verified. The single film model based on nth order Arrhenius-type equations is used in the char combustion/gasification model, with consideration of particle boundary layer diffusion. The effect of reduced reactivity of the char at high degrees of conversion is included in the kinetic model. Thirdly, the in-situ char gasification kinetics are determined via a CFD-aided rigorous mathematical optimization process. A direct search algorithm is used to accelerate the optimization process. Finally, the determined kinetics are verified at wide ranges of temperatures, residence times, and stoichiometric ratios. Compared with char gasification kinetics from ex-situ char gasification experiment using the same coal, it is demonstrated that the kinetics of in-situ char gasification are very different from kinetics derived from ex-situ char gasification experiment. Therefore, reliable kinetics of in-situ gasification are necessary when predicting the fuel conversion and gas phase species in modern air-staged pulverized coal combustion boilers.

      PubDate: 2018-05-01T08:24:06Z
      DOI: 10.1016/j.combustflame.2018.04.015
      Issue No: Vol. 194 (2018)
       
  • On the particle sizing of torrefied biomass for co-firing with pulverized
           coal
    • Authors: Aidin Panahi; Mahmut Tarakcioglu; Martin Schiemann; Michael Delichatsios; Yiannis A. Levendis
      Pages: 72 - 84
      Abstract: Publication date: August 2018
      Source:Combustion and Flame, Volume 194
      Author(s): Aidin Panahi, Mahmut Tarakcioglu, Martin Schiemann, Michael Delichatsios, Yiannis A. Levendis
      In biomass harvesting and fuel preparation processes, grinding causes a prominent energy consumption penalty, which results in an analogous cost impact. This is due to the fibrous and tenacious nature of biomass. Torrefaction of biomass makes it brittle, as it diminishes its fibrous nature and, hence, it enhances its grindability. Nevertheless, grinding costs are still important and increase with decreasing targeted particle size. Therefore, this study introduces a methodology for assessing the torrefied biomass grind size that is suitable for firing or co-firing with coal in existing pulverized fuel boilers. It examines combustion of biomass of different origins, herbaceous, woody, or crop-related. Biomass was torrefied for 30 min at 275 °C in nitrogen. It was subsequently ground and sieved to various size cuts, which reflect the mean widths rather than the lengths of these typically elongated particles. Subsequently, the particles were burned, one at a time, in a drop tube furnace (DTF) under high temperature and high heating rate conditions. Luminous burnout times were observed pyrometrically and cinematographically for a number of single particles from various size cuts. Such burnout times were then contrasted with those of individual coal particles in the size range of 75–90 µm, i.e., at the upper end of particle sizes burned in coal-fired boilers. Based on this comparison, the nominal sieve size of the examined torrefied biomass particles whose overall observed burnout times matched those of the 75–90 µm coal particles was determined to be 212–300 µm. Hence, to minimize the grinding cost of co-firing such torrefied biomass with coal in existing boilers, its finer pulverization may not be necessary.

      PubDate: 2018-05-01T08:24:06Z
      DOI: 10.1016/j.combustflame.2018.04.014
      Issue No: Vol. 194 (2018)
       
  • Reduction of a detailed chemical mechanism for a kerosene surrogate via
           RCCE-CSP
    • Authors: Panos Koniavitis; Stelios Rigopoulos; W.P. Jones
      Pages: 85 - 106
      Abstract: Publication date: August 2018
      Source:Combustion and Flame, Volume 194
      Author(s): Panos Koniavitis, Stelios Rigopoulos, W.P. Jones
      Detailed mechanisms for kerosene surrogate fuels contain hundreds of species and thousands of reactions, indicating a necessity for reduced mechanisms. In this work, we employ a framework that combines Rate-Controlled Constrained Equilibrium (RCCE) with Computational Singular Perturbation (CSP) for systematic reduction based on timescale analysis, to reduce a detailed mechanism for a jet fuel surrogate with n-dodecane, methylcyclohexane and m-xylene. Laminar non-premixed flamelets are utilised for the CSP analysis for different strain rates and therefore different scalar dissipation rate, covering the flammable region of strain rates for the surrogate fuel. Two RCCE-reduced mechanisms are developed via an RCCE-CSP methodology, one with 17 and one with 42 species, and their accuracy is assessed in a range of cases that test the performance of the reduced mechanism under both non-premixed and premixed conditions and its dynamic response. These include non-premixed flamelets with varying strain rate, laminar premixed flames for a range of equivalence ratios and pressures, flamelets ignited by an artificial pilot or by hot air, and unsteady flamelets with time-dependent strain rate. The profiles of both major and minor species, as well as important combustion characteristics such as the ignition strain rate and the laminar flame speed, are investigated. The structure of non-premixed flamelets is very well predicted, while the premixed flames are overall well predicted apart from a few deviations in certain species and an underprediction in the laminar flame speed. Apart from the large reduction in dimensionality, the reduction in computational time is also considerable (up to 19 times). As the detailed mechanism comprises 367 species and 1892 reactions, this paper presents the first application of RCCE to a mechanism of this size, as well as a comprehensive validation in a set of cases that include non-premixed and premixed laminar flames, atmospheric and elevated pressures and steady-state and dynamic response.

      PubDate: 2018-05-01T08:24:06Z
      DOI: 10.1016/j.combustflame.2018.04.004
      Issue No: Vol. 194 (2018)
       
  • A representative linear eddy model for simulating spray combustion in
           engines (RILEM)
    • Authors: Tim Lackmann; Alan R. Kerstein; Michael Oevermann
      Pages: 1 - 15
      Abstract: Publication date: July 2018
      Source:Combustion and Flame, Volume 193
      Author(s): Tim Lackmann, Alan R. Kerstein, Michael Oevermann
      The design of new combustion concepts for low emission, high efficiency internal combustion engines often leads to combustion under low temperature conditions. Under those conditions, the assumption of fast chemistry, which has been the cornerstone of many turbulent combustion models, is not strictly valid anymore and the validity and applicability of classical combustion models such as flamelet models might be limited. In this paper we present an updated version of a recently developed regime independent modeling approach for turbulent non-premixed combustion with an emphasis on applications to internal combustion engines. The model utilizes the mode- and regime-independent linear eddy model (LEM) as a combustion and micro-mixing model in a representative way. This is achieved by time advancing only one LEM realization representing the combustion process in the whole engine domain and coupling it to a RANS simulation with a presumed β-function PDF approach for the mixture fraction. The use of LEM rather than flamelet combustion closure has several benefits, an important one being regime independence. Additionally, LEM incorporates a physically based representation of the stochastic variability of turbulent eddy motions, implying an intrinsic representation of scalar dissipation rate fluctuations. In order to capture key features of engine spray-combustion environments, the LEM methodology is extended by introducing a conical LEM domain to approximate spray spatial development, fuel vapor input based on CFD-prescribed spray evaporation, and a representation of large scale turbulent motions distinct from the inertial-range turbulence that develops at smaller scales. The representative character of LEM states is evaluated by comparing mixture fraction statistics and scalar dissipation rates generated by LEM and the CFD. The performance and predictive capability of the model for typical engine applications is evaluated by simulating a standard test case – Spray B of the Engine Combustion Network (ECN) – and comparing the results with experimental data. The results demonstrate the capability of the model to represent the spray combustion process with reasonable accuracy but also reveal some limitations. The limitations and shortcomings of the model are discussed and an outlook for further development of the approach into a regime- and mode-independent combustion model for internal engine applications is given.

      PubDate: 2018-04-15T17:29:15Z
      DOI: 10.1016/j.combustflame.2018.02.008
      Issue No: Vol. 193 (2018)
       
  • Experimental study of gasoline vapor deflagration in a duct with an open
           end
    • Authors: Sheng Qi; Yang Du; Peili Zhang; Guoqing Li; Shimao Wang; Yangchao Li; Tong Dong
      Pages: 16 - 24
      Abstract: Publication date: July 2018
      Source:Combustion and Flame, Volume 193
      Author(s): Sheng Qi, Yang Du, Peili Zhang, Guoqing Li, Shimao Wang, Yangchao Li, Tong Dong
      An experimental investigation was conducted on gasoline vapor deflagration in a duct (100 × 100 mm, L/D = 4, 6, 10) with the ignition end closed and the other end open. The tests were focused on the flame behavior and the pressure generation inside and outside the duct. High-speed flame images, schlieren images, and the instantaneous pressure at different test points were recorded. The experimental results verified Bychkov's model [Combust. Flame 150 (2007) 263] in predicting the flame propagation at the early stages. Moreover, a new model was proposed to predict the axial flame moving distance when the flame front moved out of the tube. This model predicts the experimental data precisely when L/D = 4–10, and the deviation increases with larger L/D ratio. Effusive gas mixture gave rise to a vortex ring at the edge of the opening and leads to the formation of a mushroom-shaped gas cloud. A distortion of the flame skirt was observed several milliseconds after the flame ejection at about 10–40 mm away from the opening. The entire external flame was enveloped by the flammable gas cloud. Three peak pressures were obtained for each test point inside the vessel. The first peak was observed when the flame shirt touched the side walls. The second peak appeared when the flame front reached the opening. The third peak pressure was a result of the external explosion. For external test points, the pressure-time curves reached their peaks when the combustion reactions ended, and then dropped below zero due to the dissipation of energy and the low density of the burned gas.

      PubDate: 2018-04-15T17:29:15Z
      DOI: 10.1016/j.combustflame.2018.02.022
      Issue No: Vol. 193 (2018)
       
  • Flame propagation through three-phase methane-hydrate particles
    • Authors: Yuval Dagan; Tali Bar-Kohany
      Pages: 25 - 35
      Abstract: Publication date: July 2018
      Source:Combustion and Flame, Volume 193
      Author(s): Yuval Dagan, Tali Bar-Kohany
      A new mathematical solution for the laminar combustion of a spray containing three-phase particles is derived. The three-phase particles are represented by methane hydrate (MH) particles. These particles melt and evaporate following a spherical symmetric model that demonstrates the unique nature of that process. The methane-hydrate particles are uniformly distributed, yet the gaseous methane is not initially premixed, until they reach the particle depletion front. A one-dimensional, laminar flame then propagates into a homogeneous mixture of oxidizer, inert gases and small methane-hydrate particles. Characteristics of the laminar methane-hydrate spray combustion are examined at different methane-to-water mass ratio values within the particles. The methane-hydrate evaporation model serves as a building block for the evaluation of these characteristics. Previous constraints on the evaporation front are relaxed. Instead, the unique profiles of the evaporation process of the methane-hydrate particles are integrated over the entire evaporation period. Thus, an energy balance is employed to evaluate the flame location, velocity and temperature at which all liquid is evaporated. Using this approach it is shown that modeling the initial transient evaporation stage is crucial in MH and leads to non-linear evaporation characteristics. This in turn alters the dynamics of the supported flame temperature, location and velocity. Particle loading is shown to have significant impact on the evaporation and flame as well.

      PubDate: 2018-04-15T17:29:15Z
      DOI: 10.1016/j.combustflame.2018.02.026
      Issue No: Vol. 193 (2018)
       
  • Chemical interaction of dual-fuel mixtures in low-temperature oxidation,
           comparing n-pentane/dimethyl ether and n-pentane/ethanol
    • Authors: Hanfeng Jin; Julia Pieper; Christian Hemken; Eike Bräuer; Lena Ruwe; Katharina Kohse-Höinghaus
      Pages: 36 - 53
      Abstract: Publication date: July 2018
      Source:Combustion and Flame, Volume 193
      Author(s): Hanfeng Jin, Julia Pieper, Christian Hemken, Eike Bräuer, Lena Ruwe, Katharina Kohse-Höinghaus
      With the aim to study potential cooperative effects in the low-temperature oxidation of dual-fuel combinations, we have investigated prototypical hydrocarbon (C5H12) / oxygenated (C2H6O) fuel mixtures by doping n-pentane with either dimethyl ether (DME) or ethanol (EtOH). Species measurements were performed in a flow reactor at an equivalence ratio of ϕ = 0.7, at a pressure of p = 970 mbar, and in the temperature range of 450–930 K using electron ionization molecular-beam mass spectrometry (EI-MBMS). Series of different blending ratios were studied including the three pure fuels and mixtures of n-pentane containing 25% and 50% of C2H6O. Mole fractions and signals of a significant number of species with elemental composition CnH2n+xOy (n = 1–5, x = 0–(n + 2), y = 0–3) were analyzed to characterize the behavior of the mixtures in comparison to that of the individual components. Not unexpectedly, the overall reactivity of n-pentane is decreased when doping with ethanol, while it is promoted by the addition of DME. Interestingly, the present experiments reveal synergistic interactions between n-pentane and DME, showing a stronger effect on the negative temperature coefficient (NTC) for the mixture than for each of the individual components. Reasons for this behavior were investigated and show several oxygenated intermediates to be involved in enhanced OH radical production. Conversely, ethanol is activated by the addition of n-pentane, again involving key OH radical reactions. Although the main focus here is on the experimental results, we have attempted, in a first approximation, to complement the experimental observations by simulations with recent kinetic models. Interesting differences were observed in this comparison for both, fuel consumption and intermediate species production. The inhibition effect of ethanol is not predicted fully, and the synergistic effect of DME is not captured satisfactorily. The exploratory analysis of the experimental results with current models suggests that deeper knowledge of the reaction chemistry in the low-temperature regime would be useful and might contribute to improved prediction of the low-temperature oxidation behavior for such fuel mixtures.

      PubDate: 2018-04-15T17:29:15Z
      DOI: 10.1016/j.combustflame.2018.03.003
      Issue No: Vol. 193 (2018)
       
  • Effect of mixing methane, ethane, propane and ethylene on the soot
           particle size distribution in a premixed propene flame
    • Authors: Baiyang Lin; Hao Gu; Hong Ni; Bin Guan; Zhongzhao Li; Dong Han; Chen Gu; Can Shao; Zhen Huang; He Lin
      Pages: 54 - 60
      Abstract: Publication date: July 2018
      Source:Combustion and Flame, Volume 193
      Author(s): Baiyang Lin, Hao Gu, Hong Ni, Bin Guan, Zhongzhao Li, Dong Han, Chen Gu, Can Shao, Zhen Huang, He Lin
      The fuel mixing effect on soot formation was investigated based on a premixed propene flame, in which the C/O mole ratio was 0.6 and the maximum flame temperature was 1829 K. Different proportions of methane, ethane, propane and ethylene were mixed into the propene flame, with the constant C/O mole ratio and similar maximum flame temperatures and temperature–time histories. The particle size distribution function (PSDF) was measured in the burner stabilized stagnation (BSS) flame configuration, using the micro-orifice probe sampling technique and the scanning mobility particle sizer (SMPS). It was observed that the PSDF of the propene flame was not sensitive to methane, ethane or propane addition, while the number density of small nucleated particles was strengthened with ethylene addition. The particle growth in flame with a small amount of ethylene addition was stronger than that of either the pure propene or ethylene flame, indicating the synergistic effect between propene and ethylene on soot formation.

      PubDate: 2018-04-15T17:29:15Z
      DOI: 10.1016/j.combustflame.2018.03.002
      Issue No: Vol. 193 (2018)
       
  • A new emission reduction approach in MILD combustion through asymmetric
           fuel injection
    • Authors: Saurabh Sharma; Hrishikesh Pingulkar; Arindrajit Chowdhury; Sudarshan Kumar
      Pages: 61 - 75
      Abstract: Publication date: July 2018
      Source:Combustion and Flame, Volume 193
      Author(s): Saurabh Sharma, Hrishikesh Pingulkar, Arindrajit Chowdhury, Sudarshan Kumar
      This paper presents investigations with asymmetric liquid fuel injection (kerosene and biodiesel) into a combustor operating in MILD (moderate or intense low oxygen dilution) combustion regime with thermal inputs varying from 25 kW (6.34 MW/m3)–53 kW (13.3 MW/m3). Effect of air-preheat temperature on temperature distribution and pollutant emissions is investigated by varying the incoming air temperature from 300 to 800 K. The position of asymmetric fuel injection is optimized based on numerical studies to maximize internal recirculation rate, Rdil . Maximum Rdil values of 4.54 and 3.52 are obtained for asymmetric and symmetric fuel injection cases respectively. Different fuel injection pressures of 14, 30, and 48 bar with the same nozzle are used to achieve different mass flow rates of 2.5/2.45, 3.12/3.10, and 4.46/4.3 kg/h for kerosene/biodiesel fuels respectively. Shadowgraphy studies show that measured Sauter Mean Diameters (SMD) vary from 34 to 19 µm and 108 to 37 µm for kerosene and biodiesel respectively, with fuel injection pressure varying from 14 to 48 bar. The combustor showed increased flame stability up to a global equivalence ratio of ϕ = 0.2 for asymmetric fuel injection compared to ϕ = 0.6 in symmetric fuel injection case, due to higher temperatures measured in the central zone of combustor.

      PubDate: 2018-04-15T17:29:15Z
      DOI: 10.1016/j.combustflame.2018.03.008
      Issue No: Vol. 193 (2018)
       
  • Flame spread between two droplets of different diameter in microgravity
    • Authors: Masato Mikami; Naoya Motomatsu; Kentaro Nagata; Yasuko Yoshida; Takehiko Seo
      Pages: 76 - 82
      Abstract: Publication date: July 2018
      Source:Combustion and Flame, Volume 193
      Author(s): Masato Mikami, Naoya Motomatsu, Kentaro Nagata, Yasuko Yoshida, Takehiko Seo
      This research investigates the flame-spread characteristics between two droplets, Droplets A and L, of different diameter. n-Decane droplets are placed at intersections of 14 µ SiC fibers. The flame spread from Droplet A to Droplet L was observed in microgravity. The results show that the flame-spread rate decreases with an increase in the droplet spacing or the initial diameter of Droplet L for a constant initial diameter of Droplet A. The flame-spread time is approximated as the summation of the thermal conduction time from the flame around Droplet A to Droplet L and the heating time of Droplet L, which is the time required to activate the vaporization of Droplet L. Both the thermal conduction time and the heating time of Droplet L increase with the droplet spacing. The latter also linearly increases with the squared initial droplet diameter of Droplet L. The results suggest that the ratio of the heating time of Droplet L to the thermal conduction time depends roughly on the droplet diameter of Droplet L alone for a constant initial diameter of Droplet A. The flame-spread-limit droplet spacing gradually decreases with an increase in the initial droplet diameter of Droplet L and increases sharply with the initial droplet diameter of Droplet A. The flame-spread time is limited by the burning lifetime of Droplet A and about 80% of the burning lifetime of Droplet A under the near-flame-spread-limit condition. The flame-spread limit is discussed considering the burning lifetime of Droplet A, the thermal conduction time, and the heating time of Droplet L.

      PubDate: 2018-04-15T17:29:15Z
      DOI: 10.1016/j.combustflame.2018.03.004
      Issue No: Vol. 193 (2018)
       
  • Influence of methane addition on soot formation in pyrolysis of acetylene
    • Authors: Alexander Eremin; Ekaterina Mikheyeva; Ivan Selyakov
      Pages: 83 - 91
      Abstract: Publication date: July 2018
      Source:Combustion and Flame, Volume 193
      Author(s): Alexander Eremin, Ekaterina Mikheyeva, Ivan Selyakov
      Time-resolved laser-induced incandescence for particle sizing and laser light extinction for soot volume fraction was applied simultaneously to study the influence of methane addition on soot formation in acetylene pyrolysis. Three series of the experiments with initial mixtures of 2% C2H2 + Ar, 1% CH4 + Ar and 2% C2H2 + 0.5/1/2% CH4 + Ar in the temperature range of 1600–2300 K and the pressure range of 4–5 bar behind reflected shock waves were carried out. The kinetic characteristic of the soot formation process—the induction time of soot particle inception as well as the temperature dependences of final values of soot volume fraction and particle sizes have been determined and analyzed. An essential increase of soot volume fraction, particle sizes and a decrease of induction time of soot inception at methane addition to acetylene were observed. The gas phase kinetic modeling of the investigated processes up to the soot nuclei precursors formation has been performed. Analysis of gas kinetic stages of acetylene decomposition with methane addition has demonstrated the significant increase of the rates of pyrene formation followed by PAH growth due to effective propargyl C3H3 formation.

      PubDate: 2018-04-15T17:29:15Z
      DOI: 10.1016/j.combustflame.2018.03.007
      Issue No: Vol. 193 (2018)
       
  • Velocity and stretch characteristics at the leading edge of an
           aerodynamically stabilized flame
    • Authors: Ianko Chterev; Ben Emerson; Tim Lieuwen
      Pages: 92 - 111
      Abstract: Publication date: July 2018
      Source:Combustion and Flame, Volume 193
      Author(s): Ianko Chterev, Ben Emerson, Tim Lieuwen
      High swirl flows with vortex breakdown are frequently used to create recirculating flow regions for flame stabilization. This paper experimentally characterizes flame stabilization in such flows, motivated by the fact that conditions under which they enable (or do not enable) aerodynamically stabilized flames are not well understood. Such flames are substantially more difficult to experimentally characterize than shear layer stabilized flames, as the flame leading edge moves around significantly and may or may not coincide with a diagnostic laser sheet, in contrast to the near-fixed leading edge of shear layer stabilized flames. This paper characterizes the leading edge conditions, based upon simultaneous stereo Particle Image Velocimetry (sPIV), OH Planar Laser-Induced Fluorescence (PLIF) and OH* chemiluminescence data. Frames are conditionally analyzed when the flame's most upstream point was captured in the laser sheet. These conditioned frames were used to determine the coordinates, local flow velocity and full, three-dimensional, hydrodynamic strain component of flame stretch at the dynamically evolving flame leading edge. Results are taken at two conditions, 70 m/s and 45 m/s axial velocity, referred to as case 1 and 2, respectively. These data show that the leading edge of the flame precesses off-axis, with additional uncorrelated radial and axial motions. The conditioned mean flame stretch rate is positive in both cases, due to the bulk flow deceleration from the high velocity nozzle flow into the larger diameter combustor. Mean strain or velocity values conditioned on the leading flame edge are substantially different from mean values calculated at the same location. For example, the case 1 mean conditioned axial velocity at the flame leading edge is 9 m/s, while the mean axial velocity at the same location is 4 times higher, 36 m/s. In addition, the case 1 mean hydrodynamic strain rate is 3.6 times higher than the corresponding laminar flame extinction stretch rate, κ ext, calculated from detailed kinetics for the high velocity case, while less than half of κ ext in case 2. The mean strain rate predicted from bulk velocity scaling changes by a factor of 70/45 between the two cases, a value confirmed by measurements. However, the conditioned mean stretch values differ by a factor of 14. This result illustrates that additional work is needed to understand stretch scalings in such flames. Furthermore, they show that current correlations for flame stability do not capture much of the intermittency that is present in swirl flows with precessing flow features, and illustrates the need for continued work in very basic physics of how and when flames are stabilized aerodynamically.

      PubDate: 2018-04-15T17:29:15Z
      DOI: 10.1016/j.combustflame.2018.02.024
      Issue No: Vol. 193 (2018)
       
  • A multipurpose reduced mechanism for ethanol combustion
    • Authors: Alejandro Millán-Merino; Eduardo Fernández-Tarrazo; Mario Sánchez-Sanz; Forman A. Williams
      Pages: 112 - 122
      Abstract: Publication date: July 2018
      Source:Combustion and Flame, Volume 193
      Author(s): Alejandro Millán-Merino, Eduardo Fernández-Tarrazo, Mario Sánchez-Sanz, Forman A. Williams
      New multipurpose skeletal and reduced chemical-kinetic mechanisms for ethanol combustion are developed, along the same philosophical lines followed in our previous work on methanol. The resulting skeletal mechanism contains 66 reactions, only 19 of which are reversible, among 31 species, and the associated reduced mechanism contains 14 overall reactions among 16 species, obtained from the skeletal mechanism by placing CH3CHOH, CH2CH2OH, CH3CO, CH2CHO, CH2CO, C2H3, C2H5, C2H6, S − CH 2 , T − CH 2 , CH4, CH2OH, CH3O, HCO, and O in steady state. For the reduced mechanism, the steady-state relations and rate expressions are arranged so that computations can be made sequentially without iteration. Comparison with experimental results for autoignition, laminar burning velocities, and counterflow flame structure and extinction, including comparisons with the 268-step, 54-species detailed San Diego Mechanism and five other mechanisms in the literature, support the utility of the skeletal and reduced mechanisms, showing, for example, that, in comparison with the San Diego mechanism, they reduced the computational time by a factor of 4 (71 % faster) and 12 (93 % faster), respectively. Measures of computation times and of extents of departures from experimental values are defined and employed in evaluating results. Besides contributing to improvements in understanding of the mechanisms, the derived simplifications may prove useful in a variety of computational studies.

      PubDate: 2018-04-15T17:29:15Z
      DOI: 10.1016/j.combustflame.2018.03.005
      Issue No: Vol. 193 (2018)
       
  • Effects of particle size and morphology of NQ on thermal and combustion
           properties of triple-base propellants
    • Authors: Binbin Wang; Xin Liao; Zeshan Wang; Luigi T. DeLuca; Zhitao Liu; Weidong He
      Pages: 123 - 132
      Abstract: Publication date: July 2018
      Source:Combustion and Flame, Volume 193
      Author(s): Binbin Wang, Xin Liao, Zeshan Wang, Luigi T. DeLuca, Zhitao Liu, Weidong He
      Particle size and morphology of nitroguanidine (NQ) are two of the dominant factors that influence the thermal and combustion properties of triple-base propellants. However, the investigations of these effects and the corresponding mechanisms are insufficient. In this work, we examined the thermal properties and nonisothermal decomposition kinetics using differential scanning calorimetry (DSC) and thermogravimetry (TG). Combustion properties i.e. flame structure, melting layer and burning rate were determined on the bases of windowed strand burner (3 MPa), high-speed video camera, interrupted closed vessel, scanning electron microscope (SEM) and closed vessel, respectively. It was found that the decomposition process of triple-base propellant could be divided into two steps. The first step was mainly related to the decomposition process of nitroglycerin (NG) with higher activation energy and lower peak temperature, which was enhanced due to reactions between the added NQ and the decomposition product of NG (NO2), while the second step mainly concerned the decomposition process of nitrocellulose (NC) with lower activation energy and higher peak temperature. The flame structure of triple-base propellants was heterogeneous and closely attached to the condensed phase without dark zone. When the NQ particle size was decreased, the active site number of NQ was increased, enhancing the intensity of the reaction between NO2 and NQ; the decomposition peak temperature was decreased; the flame structure became more heterogeneous and brighter, resulting in more heat feedback from gas phase to the condensed phase. Thus, the burning rate was increased. On the other hand, the apparent activation energy and thickness of the melting layer of triple-base propellants were also increased with the decrease of NQ particle size, which meant that the sensitivities of burning rate to the conditions of gas phase were reduced. Therefore, the burning rate pressure exponent was decreased.

      PubDate: 2018-04-15T17:29:15Z
      DOI: 10.1016/j.combustflame.2018.03.009
      Issue No: Vol. 193 (2018)
       
  • Influence of gap height and flow field on global stoichiometry and heat
           losses during opposed flow flame spread over thin fuels in simulated
           microgravity
    • Authors: Sarzina Hossain; Indrek S. Wichman; George W. Sidebotham; Sandra L. Olson; Fletcher J. Miller
      Pages: 133 - 144
      Abstract: Publication date: July 2018
      Source:Combustion and Flame, Volume 193
      Author(s): Sarzina Hossain, Indrek S. Wichman, George W. Sidebotham, Sandra L. Olson, Fletcher J. Miller
      This study characterizes thin fuel opposed flow flame spread in simulated microgravity for a range of gap heights and airflow velocities in a Narrow Channel Apparatus (NCA). One objective was to estimate gap heights that suppress buoyancy without promoting excessive heat losses to the channel walls. A corollary of this objective was to assess the dependence of heat losses on the channel height. A second objective was to determine the influence of global combustion stoichiometry on simulated microgravity flame spread in the NCA. Whatman 44 filter paper was used for NCA gap heights ranging from 6–20 mm (half-gap below and above sample) and average opposed flow velocities 1–40 cm/s. Flames at low flows were fuel rich when the forced flows were of the same magnitude as the diffusive flow. For thin fuels, a full gap of 10 mm (5 mm half-gap) provided a compromise between buoyancy suppression and heat loss. Calculations were made of flame stoichiometry and of the influence of the velocity profile on flame spread rates (comparing it with previous theory). This part of the analysis provided support for the velocity gradient theory of flame spread. The information provided in this work on the theoretical nature of opposed flow flame spread over thin fuels is based on experimental measurements in simulated microgravity conditions.

      PubDate: 2018-04-15T17:29:15Z
      DOI: 10.1016/j.combustflame.2018.02.023
      Issue No: Vol. 193 (2018)
       
  • Experimental study of CO2 diluted, piloted, turbulent CH4/air premixed
           flames using high-repetition-rate OH PLIF
    • Authors: Dong Han; Aman Satija; Jay P. Gore; Robert P. Lucht
      Pages: 145 - 156
      Abstract: Publication date: July 2018
      Source:Combustion and Flame, Volume 193
      Author(s): Dong Han, Aman Satija, Jay P. Gore, Robert P. Lucht
      High-speed planar laser-induced fluorescence (PLIF) was applied to turbulent premixed flames with CO2 addition. Instantaneous PLIF images capturing emissions from the OH radical via the A2Σ-X2Π (1,0) band were collected. Three flames with varying levels of CO2 addition were established at the same adiabatic flame temperature, Reynolds number, and Lewis number, thereby minimizing the effects of differences in flame temperature and transport. The chemical effects of CO2 addition were investigated through critical combustion parameters revealing turbulent flamelet structure and burning velocity. The flamelet structure analysis suggests that the development of the mean flame brush thickness follows the turbulent diffusion law with a secondary effect introduced by CO2 addition. The flame surface density of these flames is affected by CO2 addition mainly through modification of mean progress variable distributions. The flame length is extended by CO2 addition with an enhancement of unburned pocket formation in the downstream portion of the flame. The apparent size and consumption speed of the fine scale unburned reactant pockets are similar among flames with varying CO2 addition. The global consumption speed of flames with CO2 addition is reduced predominantly by a reduction in the laminar flame speed. The global combustion intensity shows a constant value within uncertainty limits for flames with CO2 addition. The local combustion intensity before x/D = 1.5 is observed to be lower for the flames with CO2 addition due to the attenuation of pocket formation in the flame brush development region. Its effect on global combustion intensity is counterbalanced by the contribution of pocket formation process in the downstream portion of the flame.

      PubDate: 2018-05-01T08:24:06Z
      DOI: 10.1016/j.combustflame.2018.03.012
      Issue No: Vol. 193 (2018)
       
  • The combustion kinetics of the lignocellulosic biofuel, ethyl levulinate
    • Authors: Manik Kumer Ghosh; Mícheál Séamus Howard; Yingjia Zhang; Khalil Djebbi; Gianluca Capriolo; Aamir Farooq; Henry J. Curran; Stephen Dooley
      Pages: 157 - 169
      Abstract: Publication date: July 2018
      Source:Combustion and Flame, Volume 193
      Author(s): Manik Kumer Ghosh, Mícheál Séamus Howard, Yingjia Zhang, Khalil Djebbi, Gianluca Capriolo, Aamir Farooq, Henry J. Curran, Stephen Dooley
      Ethyl levulinate (Ethyl 4-oxopentanoate) is a liquid molecule at ambient temperature, comprising of ketone and ethyl ester functionalities and is one of the prominent liquid fuel candidates that may be easily obtained from lignocellulosic biomass. The combustion kinetics of ethyl levulinate have been investigated. Shock tube and rapid compression machine apparatuses are utilised to acquire gas phase ignition delay measurements of 0.5% ethyl levulinate/O2 mixtures at ϕ = 1.0 and ϕ = 0.5 at ∼ 10 atm over the temperature range 1000–1400 K. Ethyl levulinate is observed not to ignite at temperatures lower than ∼1040 K in the rapid compression machine. The shock tube and rapid compression machine data are closely consistent and show ethyl levulinate ignition delay to exhibit an Arrhenius dependence to temperature. These measurements are explained by the construction and analysis of a detailed chemical kinetic model. The kinetic model is completed by establishing thermochemical-kinetic analogies to 2-butanone, for the ethyl levulinate ketone functionality, and to ethyl propanoate for the ethyl ester functionality. The so constructed model is observed to describe the shock tube data very accurately, but computes the rapid compression machine data set to a lesser but still applicable fidelity. Analysis of the model suggests the autooxidation mechanism of ethyl levulinate to be entirely dominated by the propensity for the ethyl ester functionality to unimolecularly decompose to form levulinic acid and ethylene. The subsequent reaction kinetics of these species is shown to dictate the overall rate of the global combustion reaction. This model is then use to estimate the Research and Motored Octane Numbers of ethyl levulinate to be ≥97.7 and ≥ 93, respectively. With this analysis ethyl levulinate would be best suited as a gasoline fuel component, rather than as a diesel fuel as suggested in the literature. Indeed it may be considered to be useful as an octane booster. The ethyl levulinate kinetic model is constructed within a state-of-the-art gasoline surrogate combustion kinetic model and is thus available as a tool with which to investigate the use of ethyl levulinate as a gasoline additive.

      PubDate: 2018-04-15T17:29:15Z
      DOI: 10.1016/j.combustflame.2018.02.028
      Issue No: Vol. 193 (2018)
       
  • An optimization method for formulating model-based jet fuel surrogate by
           emulating physical, gas phase chemical properties and threshold sooting
           index (TSI) of real jet fuel under engine relevant conditions
    • Authors: Wenbin Yu; Wenming Yang; Kunlin Tay; Feiyang Zhao
      Pages: 192 - 217
      Abstract: Publication date: July 2018
      Source:Combustion and Flame, Volume 193
      Author(s): Wenbin Yu, Wenming Yang, Kunlin Tay, Feiyang Zhao
      Two jet fuel surrogates (S1 and S2) were proposed in this work, aimed at improving the quantitative accuracy of fuel properties that affect both premixed and spray-guided combustion modes under engine relevant conditions by emulating real jet fuel properties including physical, gas phase chemical properties and threshold sooting index (TSI). An intelligent optimization approach was employed to calculate the composition that inherently satisfies both the physical and gas phase chemical characteristics as well as sooting tendency. The jet fuel surrogate S1 was composed of five components including decalin, n-dodecane, iso-cetane, iso-octane and toluene (0.005/0.4011/0.1249/0.098/0.371 by mole fraction), while surrogate S2 is a mixture of the same components but different in proportions (0.1449/0.3706/0.2059/0.0195/0.2591 by mole fraction). Based on the newly proposed surrogates, a skeletal jet fuel surrogate chemical reaction mechanism was developed by describing the chemistries for the oxidation of large molecules C4 Cn and smaller H2/CO/C1 molecules respectively. The skeletal jet fuel surrogate mechanism was significantly compacted into 74 species and 189 reactions, making it practical to be used in 3-dimensional (3-D) engine combustion simulations. This newly developed mechanism was verified against the experimental results of ignition delay times, species concentrations and laminar flame speed under a wide range of conditions, while 3-D validations were conducted for spray liquid and vapor penetrations in a constant volume chamber. As a result, the proposed fuel surrogate is capable of predicting the spray and combustion characteristics and main species profiles under engine relevant circumstance. Due to the stringent target properties used in this work, the surrogate S1 performed best in assessing ignition characteristics attributed to its elaborate chemical properties, making it the most suitable candidate for chemical dominated combustion like premixed engine combustion. Alternatively, S2 displayed outstanding spray-guided combustion behavior because of the stringent chemical and physical target properties assigned. It is worthwhile to note that this new method of formulating surrogates for different applications was efficient and time-saving. Finally, we aim to perform experimental validation tests for CN, density, viscosity, surface tension, TSI, sooting tendency and particle size distribution to further support the validity of the current proposed jet fuel surrogates (S1 and S2) in the future.

      PubDate: 2018-05-01T08:24:06Z
      DOI: 10.1016/j.combustflame.2018.03.024
      Issue No: Vol. 193 (2018)
       
  • Characterization of stratified fuel distribution and charge mixing in a
           DISI engine using Rayleigh scattering
    • Authors: Stina Hemdal
      Pages: 218 - 228
      Abstract: Publication date: July 2018
      Source:Combustion and Flame, Volume 193
      Author(s): Stina Hemdal
      The stratified fuel distribution and early flame development in a firing spray-guided direct-injection spark-ignition (DISI) engine were characterized applying optical diagnostics. The objectives were to compare effects of single and double injections on the stratified air–fuel mixing and early flame development. Vaporized in-cylinder fuel distributions resulting from both single and double injections before, during and after ignition were selectively visualized applying Rayleigh scattering. The optical investigation of the in-cylinder fuel distributions and early flame propagation corroborated the better mixing, showing that double injections were associated with more evenly distributed fuel, fewer local areas with high fuel concentrations, faster initial flame spread and more even flame propagation (more circular flame spreading). The results support the hypothesis that delivering fuel in closely coupled double injections results in better mixing than corresponding single injections. The improved mixing is believed to stem from the longer time available for mixing of the air and fuel in double injection events.

      PubDate: 2018-05-01T08:24:06Z
      DOI: 10.1016/j.combustflame.2018.03.020
      Issue No: Vol. 193 (2018)
       
  • Direct numerical simulation of a high Ka CH4/air stratified premixed jet
           flame
    • Authors: Haiou Wang; Evatt R. Hawkes; Bruno Savard; Jacqueline H. Chen
      Pages: 229 - 245
      Abstract: Publication date: July 2018
      Source:Combustion and Flame, Volume 193
      Author(s): Haiou Wang, Evatt R. Hawkes, Bruno Savard, Jacqueline H. Chen
      Direct numerical simulation (DNS) of a high Karlovitz number (Ka) CH4/air stratified premixed jet flame was performed and used to provide insights into fundamentals of turbulent stratified premixed flames and their modelling implications. The flame exhibits significant stratification where the central jet has an equivalence ratio of 0.4, which is surrounded by a pilot flame with an equivalence ratio of 0.9. A reduced chemical mechanism for CH4/air combustion based on GRI-Mech3.0 was used, including 268 elementary reactions and 28 transported species. Over five billion grid points were employed to adequately resolve the turbulence and flame scales. The maximum Ka of the flame in the domain approaches 1400, while the jet Damköhler number (Dajet) is as low as 0.0027. The flame shows early stages of CH4/air combustion in the near field and later stages in the far field; the separation of combustion stages can be largely attributed to the small jet flow timescale and the low Dajet. The gradient of equivalence ratio in the flame normal direction, dϕ/dn, is predominantly negative, and small-scale stratification was found to play an important role in determining the local flame structure. Notably, the flame is thinner, the burning is more intense, and the levels of the radical pools, including OH, O and H, are higher in regions with stronger mixture stratification. The local flame structure is more strained and less curved in these regions. The mean flame structure is considerably influenced by the strong shear, which can be reasonably predicted by unity Lewis number stratified premixed flamelets when the thermochemical conditions of the reactant and product are taken locally from the DNS and the strain rates close to those induced by the mean flow are used in the flamelet calculation. An enhanced secondary reaction zone behind the primary reaction zone was observed in the downstream region, where the temperature is high and the fuel concentration is negligible, consistent with the observed separation of combustion stages. The main reactions involved in the secondary reaction zone, including CO + OH⇔CO2 + H (R94), H + O2 + M⇔HO2 + M (R31), HO2 + OH⇔H2O + O2 (R82) and H2 + OH⇔H2O + H (R79), are related to accumulated intermediate species including CO, H2, H, and OH. The detailed mechanism of intermediate species accumulation was explored and its effect on chemical pathways and heat release rate was highlighted.

      PubDate: 2018-05-01T08:24:06Z
      DOI: 10.1016/j.combustflame.2018.03.025
      Issue No: Vol. 193 (2018)
       
  • Effects of inflow Mach number on oblique detonation initiation with a
           two-step induction-reaction kinetic model
    • Authors: Pengfei Yang; Honghui Teng; Zonglin Jiang; Hoi Dick Ng
      Pages: 246 - 256
      Abstract: Publication date: July 2018
      Source:Combustion and Flame, Volume 193
      Author(s): Pengfei Yang, Honghui Teng, Zonglin Jiang, Hoi Dick Ng
      Oblique detonations induced by two-dimensional, semi-infinite wedges are simulated by solving numerically the reactive Euler equations with a two-step induction-reaction kinetic model. Previous results obtained with other models have demonstrated that for the low inflow Mach number M 0 regime past a critical value, the wave in the shocked gas changes from an oblique reactive wave front into a secondary oblique detonation wave (ODW). The present numerical results not only confirm the existence of such critical phenomenon, but also indicate that the structural shift is induced by the variation of the main ODW front which becomes sensitive to M 0 near a critical value. Below the critical M 0 ,cr , oscillations of the initiation structure are observed and become severe with further decrease of M 0. For low M 0 cases, the non-decaying oscillation of the initiation structure exists after a sufficiently long-time computation, suggesting the quasi-steady balance of initiation wave systems. By varying the heat release rate controlled by kR , the pre-exponential factor of the second reaction step, the morphology of initiation structures does not vary for M 0 = 10 cases but varies for M 0 = 9 cases, demonstrating that the effects of heat release rate become more prominent when M 0 decreases. The instability parameter χ is introduced to quantify the numerical results. Although χ cannot reveal the detailed mechanism of the structural shift, a linear relation between χ and kR exists at the critical condition, providing an empirical criterion to predict the structural variation of the initiation structure.

      PubDate: 2018-05-01T08:24:06Z
      DOI: 10.1016/j.combustflame.2018.03.026
      Issue No: Vol. 193 (2018)
       
  • On the consistency of state vectors and Jacobian matrices
    • Authors: Michael A. Hansen; James C. Sutherland
      Pages: 257 - 271
      Abstract: Publication date: July 2018
      Source:Combustion and Flame, Volume 193
      Author(s): Michael A. Hansen, James C. Sutherland
      The formulation of reactive flow problems can be both quite challenging and important to the efficiency and robustness of solution algorithms. In this article, we focus on the choice of the thermochemical state vector as it relates to recently-developed computational techniques for complex combustion chemistry problems. We identify over-specification of the state vector as a source of both ambiguity and error in the partial derivatives used in forming analytical forms of the chemical source Jacobian matrix. We review and compare several approaches taken to increase sparsity of the Jacobian matrix, as it relates to the use of Newton–Krylov methods for implicit time integration, and identify proper techniques for achieving sparsity that do not rely on ad-hoc choice of state variables with inconsistent Jacobians. Chemical explosive mode analysis and linearly-implicit methods, such as Rosenbrock methods, are identified as areas where Jacobian accuracy may be critical. The distinction between how Jacobian exactness impacts Rosenbrock and Newton–Krylov methods is demonstrated with a simple example. We demonstrate the errors in conservation obtained from over-specification of the state vector with auto-ignition calculations for hydrogen, ethylene, and n-heptane chemistry with a high-order implicit Runge–Kutta method.

      PubDate: 2018-05-01T08:24:06Z
      DOI: 10.1016/j.combustflame.2018.03.017
      Issue No: Vol. 193 (2018)
       
  • Effect of pressure on the transfer functions of premixed methane and
           propane swirl flames
    • Authors: Francesco Di Sabatino; Thibault F. Guiberti; Wesley R. Boyette; William L. Roberts; Jonas P. Moeck; Deanna A. Lacoste
      Pages: 272 - 282
      Abstract: Publication date: July 2018
      Source:Combustion and Flame, Volume 193
      Author(s): Francesco Di Sabatino, Thibault F. Guiberti, Wesley R. Boyette, William L. Roberts, Jonas P. Moeck, Deanna A. Lacoste
      This paper reports on the effect of pressure on the response of methane–air and propane–air swirl flames to acoustic excitation of the flow. These effects are analyzed on the basis of the flame transfer function (FTF) formalism, experimentally determined from velocity and global OH* chemiluminescence measurements at pressures up to 5 bar. In parallel, phase-locked images of OH* chemiluminescence are collected and analyzed in order to determine the associated flame dynamics. Flame transfer functions and visual flame dynamics at atmospheric pressure are found to be similar to previous studies with comparable experimental conditions. Regardless of pressure, propane flames exhibit a much larger FTF gain than methane flames. For both fuels, the effect of pressure primarily is to modify the gain response at the local maximum of the FTF, at a Strouhal number around 0.5 (176 Hz). For methane flames, this gain maximum increases monotonically with pressure, while for propane flames it increases from 1 to 3 bar and decreases from 3 to 5 bar. At this frequency and regardless of pressure, the flame motion is driven by flame vortex roll-up, suggesting that pressure affects the FTF by modifying the interaction of the flame with the vortex detached from the injector rim during a forcing period. The complex heat transfer, fluid dynamics, and combustion coupling in this configuration does not allow keeping the vortex properties constant when pressure is increased. However, the different trends of the FTF gain observed for methane and propane fuels with increasing pressure imply that intrinsic flame properties and fuel chemistry, and their variation with pressure, play an important role in controlling the response of these flames to acoustic forcing.

      PubDate: 2018-05-01T08:24:06Z
      DOI: 10.1016/j.combustflame.2018.03.011
      Issue No: Vol. 193 (2018)
       
  • Prediction of product distributions in coal devolatilization by an
           artificial neural network model
    • Authors: Kun Luo; Jiangkuan Xing; Yun Bai; Jianren Fan
      Pages: 283 - 294
      Abstract: Publication date: July 2018
      Source:Combustion and Flame, Volume 193
      Author(s): Kun Luo, Jiangkuan Xing, Yun Bai, Jianren Fan
      Currently most coal combustion simulations treat the devolatilization products as a mixture of light gases with a given proportion or a postulate substance, which is obviously different from the reality. To obtain a more accurate treatment on the product distribution from coal devolatilization, an artificial neural network (ANN) model is innovatively developed based on a training database constructed from diverse experimental data for a wide range of coal types under a wide range of heating conditions. The accuracy and applicability of the developed ANN model are validated and compared with that of the chemical percolation devolatilization coupled with the functional group (FG-CPD) model for the validation database, and the relative impact of each input parameter on the evolution of each devolatilization product is evaluated. The results show that the detailed product distributions of coal devolatilization predicted by the proposed ANN model are in good agreement with the experimental data for both the training and validation database, and the ANN model can give a more accurate prediction on both the yield of each component and its evolution compared with the FG-CPD model. The coal composition accounts for the most impact (above 60%) on the product distribution, and the relative impact of Cdaf, Hdaf, Odaf , coal particle diameter, instantaneous heating rates, particle residence time and particle temperature decrease successively. This ANN model has great potential to be coupled into coal combustion simulations to improve efficiency and accuracy, which will be studied in the future.

      PubDate: 2018-05-01T08:24:06Z
      DOI: 10.1016/j.combustflame.2018.03.016
      Issue No: Vol. 193 (2018)
       
  • Dynamic response of a vaporizing spray to pressure oscillations:
           Approximate analytical solutions
    • Authors: Kwassi Anani; Roger Prud'homme; Mahouton Norbert Hounkonnou
      Pages: 295 - 305
      Abstract: Publication date: July 2018
      Source:Combustion and Flame, Volume 193
      Author(s): Kwassi Anani, Roger Prud'homme, Mahouton Norbert Hounkonnou
      In this work, we study thermal conduction and convection combined effects on frequency response to pressure oscillations of a spray of repetitively injected drops in a combustion chamber. The theoretical model is based on Heidmann analogy of the so called “mean droplet” which is a single spherical vaporizing droplet with constant average radius, given that this droplet is continually fed at a stationary flow rate. The feeding comes from a source point placed at the mean spherical droplet center in such a way that the injection process can be assumed to be isothermal (isothermal feeding regime) or adiabatic (adiabatic feeding regime). Drawing upon the linear decomposition of the energy conservation equation, approximate analytical solutions for the perturbed temperature field inside the droplet are obtained from some derived double confluent Heun equations. Frequency response factor of the evaporating mass is then evaluated on the basis of the Rayleigh criterion by means of the linearized equations of the gas phase. Compared to the results obtained for the previous pure conduction model of the same “mean droplet”, frequency response factor curves seem to be similar with reference to each feeding regime. Moreover, due to the radial thermal convection effect introduced in the present work, a frequency response factor curve with the same characteristic times ratio may exhibit a relatively larger frequency range for the instability domain. Data are found to be correlated in terms of period of pressure oscillations, of vaporization characteristics times and of fuel thermodynamic coefficients. In the isothermal feeding regime in particular, due to some possible values that can be taken by a certain thermodynamic coefficient, high and non-linear frequency responses may appear in the system.

      PubDate: 2018-05-01T08:24:06Z
      DOI: 10.1016/j.combustflame.2018.03.015
      Issue No: Vol. 193 (2018)
       
  • Effect of ethanol addition on soot formation in laminar methane diffusion
           flames at pressures above atmospheric
    • Authors: Elizabeth A. Griffin; Moah Christensen; Ömer L. Gülder
      Pages: 306 - 312
      Abstract: Publication date: July 2018
      Source:Combustion and Flame, Volume 193
      Author(s): Elizabeth A. Griffin, Moah Christensen, Ömer L. Gülder
      Influence of ethanol addition on soot production in laminar diffusion flames of methane under elevated pressures was investigated experimentally. A high pressure vessel, equipped with a co-flow laminar diffusion flame burner having a 3 mm fuel nozzle diameter, was used for the soot experiments. The amount of ethanol in methane was 10% based on the total carbon of the fuel stream. Pressure range was from atmospheric to 6 bar. To have measurements to be comparable for the purpose of assessing the pressure dependence, the carbon mass flow rate of the ethanol and methane mixture was kept constant at 0.941 mg/s. Luminescent heights of the flames studied did not vary much with pressure excluding the heights of those at atmospheric pressure. Line-of-sight measurements of soot spectral emission were inverted by an Abel type algorithm, assuming axisymmetric flames, to evaluate variations of radial profiles of temperatures, soot concentrations, and soot yields of neat methane and ethanol-doped methane with pressure. Ethanol-doped methane flames displayed higher soot concentrations than those of neat methane flames at all pressures considered in the study; however, pressure dependence of maximum soot volume fraction is almost the same for both neat methane and ethanol-doped methane. The results showed that the maximum soot volume fractions scale with pressure as Pn , where n decreases from about 2.5 to 1.8, from atmospheric to 6 bar. These exponents are similar to measurements reported previously in the literature for gaseous paraffinic hydrocarbons. Maximum soot yields, on the other hand, are almost the same for both flames at 2 bar, but at 4 and 6 bar pressures ethanol-doped methane flames give higher maximum soot yields than neat methane flames.

      PubDate: 2018-05-01T08:24:06Z
      DOI: 10.1016/j.combustflame.2018.04.001
      Issue No: Vol. 193 (2018)
       
  • Theoretical analysis and simulation of methane/air flame inhibition by
           sodium bicarbonate particles
    • Authors: Omar Dounia; Olivier Vermorel; Thierry Poinsot
      Pages: 313 - 326
      Abstract: Publication date: July 2018
      Source:Combustion and Flame, Volume 193
      Author(s): Omar Dounia, Olivier Vermorel, Thierry Poinsot
      The capacity of sodium bicarbonate (NaHCO3)s powder to chemically reduce flame speeds and mitigate the effects of accidental explosions is well established. The inhibition of premixed hydrocarbon/air flames by monodisperse (NaHCO3)s solid particles is investigated, here, using theory and numerical simulations. First, an analytical solution for the temperature history of a solid (NaHCO3)s particle crossing a flame shows that the size of the largest (NaHCO3)s particle which can decompose inside the flame front, and act on chemical reactions efficiently, strongly depends on the flame speed. For various fuels and a wide range of equivalence ratios, particles with a strong potential for flame inhibition are identified: hence a criterion, on the maximum particle size, for efficient inhibition is proposed. Thereafter, a one-dimensional methane/air flame traveling in a premixed gas loaded with sodium bicarbonate is simulated using a chemical mechanism based on GRI-Mech, extended to include inhibition chemistry and reduced to 20 species with a DRGEP method (Pepiot-Desjardins and Pitsch, 2008). Inhibitor particle size and mass loading are varied to study the flame response to inhibition by (NaHCO3)s powders. Finally, two-dimensional simulations of a planar flame traveling in a flow with a non-uniform inhibitor mass loading distribution are analyzed. In the case of strong particle stratification, an acceleration of the flame is observed, instead of a mitigation. This fundamental mechanism may limit the actual potential of inhibition powders in real configurations.

      PubDate: 2018-05-01T08:24:06Z
      DOI: 10.1016/j.combustflame.2018.03.033
      Issue No: Vol. 193 (2018)
       
  • Experimental study of the burning behaviors of thin-layer pool fires
    • Authors: Jinlong Zhao; Hong Huang; Grunde Jomaas; Maohua Zhong; Rui Yang
      Pages: 327 - 334
      Abstract: Publication date: July 2018
      Source:Combustion and Flame, Volume 193
      Author(s): Jinlong Zhao, Hong Huang, Grunde Jomaas, Maohua Zhong, Rui Yang
      The thin-layer burning behaviors of gasoline, including the heat flux feedback to the burning surface, the penetrating thermal radiation, the temperature profile of liquid layer, and the burning rate were studied in experiments of thin-layer pool fires in square, fireproof glass trays. Experiments with four different tray sizes (side lengths of 30 cm, 40 cm, 50 cm and 60 cm) and four different initial liquid thicknesses of 6 mm, 9 mm, 12 mm and 15 mm were conducted. The results indicate that the heat flux feedback from the flame remained approximately constant, except during the ignition and extinguishment periods, and was also independent of the initial fuel thickness. The penetrating thermal radiation, on the other hand, increased with decreasing liquid layer thickness, gradually assuming rapid exponential growth. Furthermore, a boiling layer was formed during the initial burning period and its maximum depth was close to 3.0 mm. Four typical burning phases including pre-heating burning, steady burning, thin-layer burning and extinguishment were identified. The penetrating thermal radiation was the main cause of the burning rate decrease for thin-layer burning. These findings can provide a basis on which to build a real-time burning rate model for thin-layer burning.

      PubDate: 2018-05-01T08:24:06Z
      DOI: 10.1016/j.combustflame.2018.03.018
      Issue No: Vol. 193 (2018)
       
  • Energetic and ecological effect of small amount of metalline powders used
           for doping waste-derived fuels
    • Authors: Roman I. Egorov; Timur R. Valiullin; Pavel A. Strizhak
      Pages: 335 - 343
      Abstract: Publication date: July 2018
      Source:Combustion and Flame, Volume 193
      Author(s): Roman I. Egorov, Timur R. Valiullin, Pavel A. Strizhak
      The synthesis of an effective composite fuel from industrial wastes almost always requires mixing several components with different properties. Features of certain components can effectively compensate for the limitations of others, eventually improving the fuel as whole. We propose the optimization of the waste-derived coal-water slurry (CWS) by doping it with a small amount (2–5 wt%) of non-hydrocarbon dopants (metal-bearing powders with aluminium, iron, copper and chalk). It allows the stabilization of the combustion temperature (or even its increase), while keeping the ignition delays at the reasonable level ( ∼ 2 s at 1000 K). Doping the CWS by 2–5 wt% of chalk powder allows a radical decrease in the production of harmful exhausts (up to an order of magnitude regarding the SO2 and 2–5 times regarding NO x ). A strong ecological effect was observed when the fuel is doped by iron powder, too. Therefore, the usage of non-organic dopants is a very promising way to prepare environmentally friendly fuel compositions with high power efficiency.

      PubDate: 2018-05-01T08:24:06Z
      DOI: 10.1016/j.combustflame.2018.03.040
      Issue No: Vol. 193 (2018)
       
  • Joint probability density function models for multiscalar turbulent mixing
    • Authors: Bruce A. Perry; Michael E. Mueller
      Pages: 344 - 362
      Abstract: Publication date: July 2018
      Source:Combustion and Flame, Volume 193
      Author(s): Bruce A. Perry, Michael E. Mueller
      Modeling multicomponent turbulent mixing is essential for simulations of turbulent combustion, which is controlled by mixing of fuel, oxidizer, combustion products, and intermediate species. One challenge is to find functions that can reproduce the joint probability density function (PDF) of scalar mixing states using only a small number of parameters. Even for mixing with only two independent scalars, several statistical distributions, including the Dirichlet, Connor–Mosimann (CM), five-parameter bivariate beta (BVB5), and statistically-most-likely distributions, have previously been proposed for this purpose, with minimal physical justification. This work uses the concept of statistical neutrality to relate these distributions to each other, relate the distributions to physical mixing configurations, and develop a systematic approach to model selection. This approach is validated by comparing the ability of these distributions to reproduce the evolution of the scalar PDF from Direct Numerical Simulations of three-component passive scalar mixing in isotropic turbulence with 11 different initial conditions that are representative of a wide range of mixing conditions of interest. The approach correctly identifies whether the Dirichlet, CM, and BVB5 distributions, which are increasingly complex bivariate generalizations of the beta distribution, can accurately model the joint PDFs, but knowledge of the mixing configuration is required to select the appropriate distribution. The statistically-most-likely distribution is generally less accurate than the appropriate bivariate beta distribution but still gives reasonable predictions and does not require knowledge of the mixing configuration, so it is a suitable model when no single mixing configuration can be identified.

      PubDate: 2018-05-01T08:24:06Z
      DOI: 10.1016/j.combustflame.2018.03.039
      Issue No: Vol. 193 (2018)
       
  • Modelling of diesel spray flames under engine-like conditions using an
           accelerated Eulerian Stochastic Field method
    • Authors: Kar Mun Pang; Mehdi Jangi; Xue-Song Bai; Jesper Schramm; Jens Honore Walther
      Pages: 363 - 383
      Abstract: Publication date: July 2018
      Source:Combustion and Flame, Volume 193
      Author(s): Kar Mun Pang, Mehdi Jangi, Xue-Song Bai, Jesper Schramm, Jens Honore Walther
      This paper aims to simulate diesel spray flames across a wide range of engine-like conditions using the Eulerian Stochastic Field probability density function (ESF-PDF) model. The ESF model is coupled with the Chemistry Coordinate Mapping approach to expedite the calculation. A convergence study is carried out for a number of stochastic fields at five different conditions, covering both conventional diesel combustion and low-temperature combustion regimes. Ignition delay time, flame lift-off length as well as distributions of temperature and various combustion products are used to evaluate the performance of the model. The peak values of these properties generated using thirty-two stochastic fields are found to converge, with a maximum relative difference of 27% as compared to those from a greater number of stochastic fields. The ESF-PDF model with thirty-two stochastic fields performs reasonably well in reproducing the experimental flame development, ignition delay times and lift-off lengths. The ESF-PDF model also predicts a broader hydroxyl radical distribution which resembles the experimental observation, indicating that the turbulence–chemistry interaction is captured by the ESF-PDF model. The validated model is subsequently used to investigate the flame structures under different conditions. Analyses based on flame index and formaldehyde distribution suggest that a triple flame, which consists of a rich premixed flame, a diffusion flame and a lean premixed flame, is established in the earlier stage of the combustion. As the combustion progresses, the lean premixed flame weakens and diminishes with time. Eventually, only a double-flame structure, made up of the diffusion flame and the rich premixed flame, is observed. The analyses for various ambient temperatures show that the triple-flame structure remains for a longer period of time in cases with lower ambient temperatures. The present study shows that the ESF-PDF method is a valuable alternative to Lagrangian particle PDF methods.

      PubDate: 2018-05-01T08:24:06Z
      DOI: 10.1016/j.combustflame.2018.03.030
      Issue No: Vol. 193 (2018)
       
  • Self-sustained, high-frequency detonation wave generation in a
           semi-bounded channel
    • Authors: Kyle Schwinn; Rohan Gejji; Brandon Kan; Swanand Sardeshmukh; Stephen Heister; Carson D. Slabaugh
      Pages: 384 - 396
      Abstract: Publication date: July 2018
      Source:Combustion and Flame, Volume 193
      Author(s): Kyle Schwinn, Rohan Gejji, Brandon Kan, Swanand Sardeshmukh, Stephen Heister, Carson D. Slabaugh
      Combustion dynamics in a linear, semi-bounded channel were studied to characterize injection, mixing, and ignition processes in a two dimensional analogue of a rotating detonation wave combustor. The linear channel was developed to operate using natural gas and oxygen and to provide large optical accessibility in support of high-fidelity diagnostics at thermal power density levels representative of rotating detonation wave combustors over a broad range of flow conditions. Measurements of pressure and heat release were performed using an array of high-frequency dynamic pressure probes and OH*-chemiluminescence, respectively. Self-sustained, periodic generation of detonation waves was observed. These traveling combustion waves were initiated from low amplitude deflagrative fronts and steepened as they traveled through the combustion channel. The sensitivity of this instability to changes in acoustic boundary conditions was explored to determine the origin of the high-amplitude dynamics. The change in acoustic boundary at the end-wall affected the transverse acoustic mode frequency as expected, but had no effect on the detonation wave initiation. The detonation wave dynamics were found to be correlated with resonant frequencies in the propellant manifolds. It was determined that the frequency of the self-sustained chamber dynamics depends on which manifold has the strongest injection pressure ratio. OH*-chemiluminescence and pressure measurements indicate that the observed transition to detonation is linked to the auto-ignition of the reactants and that the auto-ignition event is coupled with the fluctuation in reactant mass flow associated with the manifold dynamics.

      PubDate: 2018-05-01T08:24:06Z
      DOI: 10.1016/j.combustflame.2018.03.022
      Issue No: Vol. 193 (2018)
       
  • Including real fuel chemistry in LES of turbulent spray combustion
    • Authors: Anne Felden; Lucas Esclapez; Eleonore Riber; Bénédicte Cuenot; Hai Wang
      Pages: 397 - 416
      Abstract: Publication date: July 2018
      Source:Combustion and Flame, Volume 193
      Author(s): Anne Felden, Lucas Esclapez, Eleonore Riber, Bénédicte Cuenot, Hai Wang
      Large Eddy Simulation (LES) is progressively becoming a crucial design tool for the next generation of aeronautical combustion chambers. However, further improvements of the predictive capability of LES is required especially for predictions of pollutant formation. In general, the exact description of real fuel combustion requires to take into account thousands of unique chemical species involved in complex and highly non-linear chemical reaction mechanisms, and the direct integration of such chemistry in LES is not a viable path because of excessive computational demands and numerical stiffness. Modeling of real aeronautical transportation fuel is further complicated by the fact that kerosenes are complex blends of a large number of hydrocarbon compounds and their exact composition is very difficult to determine. In this work, we propose a new framework relying upon the Hybrid Chemistry (HyChem) approach and Analytically Reduced Chemistry (ARC) to allow a direct integration of real fuel chemistry in the compressible LES solver AVBP. The HyChem-ARC model is coupled with the Dynamically Thickened Flame LES model (DTFLES) and a Lagrangian description of the spray to investigate the turbulent two-phase flow flame in a lean direct injection combustor, fueled with Jet-A. The LES results are compared to experimental data in terms of gas velocity, temperature and species (CO2, H2O, CO, NO) mass fractions. It is found that the proposed methodology leads to very satisfying predictions of both the flow dynamics and the NO x levels. Additionally, the refined level of chemistry description enables to gain valuable insights into flame/spray interactions as well as on the NO x formation mechanism in such complex flame configurations. To improve further the results, a more detailed experimental characterization of the liquid fuel injection should be provided.

      PubDate: 2018-05-01T08:24:06Z
      DOI: 10.1016/j.combustflame.2018.03.027
      Issue No: Vol. 193 (2018)
       
  • Thermoanalytical studies on the thermal and catalytic decomposition of
           aqueous hydroxylammonium nitrate solution
    • Authors: Alan A. Esparza; Robert E. Ferguson; Ahsan Choudhuri; Norman D. Love; Evgeny Shafirovich
      Pages: 417 - 423
      Abstract: Publication date: July 2018
      Source:Combustion and Flame, Volume 193
      Author(s): Alan A. Esparza, Robert E. Ferguson, Ahsan Choudhuri, Norman D. Love, Evgeny Shafirovich
      Green monopropellants based on hydroxylammonium nitrate (HAN) are a promising alternative to hydrazine in space propulsion systems. In the present paper, thermal and catalytic decomposition of aqueous HAN solution was studied using thermogravimetric analysis (TGA), differential scanning calorimetry (DSC), and mass spectrometry. The tests were conducted at heating rates of 1, 2.5, 5, and 10 K/min. The values of the apparent activation energy obtained for thermal decomposition using TGA and DSC are 62.2 ± 3.7 kJ/mol and 57.5 ± 3.5 kJ/mol, respectively, and they are in agreement with the literature data for solid HAN and solutions with high concentrations of HAN. The obtained values of the pre-exponential factor, 2.24 × 104 s−1 for TGA and 3.55 × 103 s−1 for DSC, are lower by six to seven orders of magnitude than those reported in the literature for aqueous HAN solutions, apparently because of full vaporization of water from the HAN solution at the beginning of the TGA and DSC tests. The use of an iridium/rhodium foam catalyst decreased the temperature of full decomposition by over 60 °C. The value of the apparent activation energy obtained for the catalytic decomposition using TGA is 63.9 ± 2.5 kJ/mol, while the obtained value of the pre-exponential factor is 3.31 × 105 s−1.

      PubDate: 2018-05-01T08:24:06Z
      DOI: 10.1016/j.combustflame.2018.04.007
      Issue No: Vol. 193 (2018)
       
  • A two-phase MMC–LES model for turbulent spray flames
    • Authors: Nazmul Khan; Matthew J. Cleary; Oliver T. Stein; Andreas Kronenburg
      Pages: 424 - 439
      Abstract: Publication date: July 2018
      Source:Combustion and Flame, Volume 193
      Author(s): Nazmul Khan, Matthew J. Cleary, Oliver T. Stein, Andreas Kronenburg
      A multiple mapping conditioning/large eddy simulation model (MMC–LES) is formulated for turbulent spray flames. Heat and mass transfer between the gaseous and liquid phases is modelled using a conserved scalar form of the Spalding transfer number approach along with an evaporative mixing model that is local in mixture fraction space. The model and the numerical implementation are tested in a non-reacting and two reacting experimental acetone spray cases. Model predictions are generally in good agreement with the velocity, liquid volume flow rate and mean temperature data and are shown to have a low sensitivity to the sparse stochastic particle resolution. The consistency of the heat and mass transfer coupling between the hybrid Eulerian–Lagrangian–Lagrangian schemes is demonstrated by comparison of mixture fraction fields solved on both the LES and the stochastic particles. A comprehensive appendix contains a derivation of the phase-weighted form of the MMC–LES governing equations along with a derivation and single-droplet test of the conserved scalar heat and mass transfer formulation.

      PubDate: 2018-05-01T08:24:06Z
      DOI: 10.1016/j.combustflame.2018.03.023
      Issue No: Vol. 193 (2018)
       
  • Local extinction mechanisms analysis of spray jet flame using high speed
           diagnostics
    • Authors: Antoine Verdier; Javier Marrero Santiago; Alexis Vandel; Gilles Godard; Gilles Cabot; Bruno Renou
      Pages: 440 - 452
      Abstract: Publication date: July 2018
      Source:Combustion and Flame, Volume 193
      Author(s): Antoine Verdier, Javier Marrero Santiago, Alexis Vandel, Gilles Godard, Gilles Cabot, Bruno Renou
      This paper reports an experimental study where flame structure, flow topology and local extinction mechanisms of n-heptane spray flames are investigated. The burner consists of an annular non-swirling co-flow of air that surrounds a central hollow-cone spray injector, leading to a lifted spray flame. The experiments include measurements of droplet size and velocity by Phase Doppler Anemometry (PDA), flame structure by High-Speed Planar Laser Induced Fluorescence of OH radical (HS-OH-PLIF) simultaneously recorded with the velocity fields of the reactive flow obtained by High-Speed Particle Image Velocimetry (HS-PIV). The poly-disperse spray distribution yields small droplets along the centerline axis while the majority of the mass is located as large droplets along the spray borders. These large droplets associated with high velocities have ballistic trajectories and strongly interact with the inner wrinkled partially premixed flame front and the outer diffusion flame front. Simultaneous HS-OH-PLIF and HS-PIV images characterize the dynamics of extinction events in the spray jet flame. In the inner reaction zone, local flame extinctions are mainly controlled by the shear layer induced by the co-flow and the fuel–air heterogeneities due to the evaporation of small droplets in the vicinity of the flame front. The large scales of turbulence in the shear layer play a significant role in the dynamics of these extinctions. It is also found that the large inertial droplets penetrate the lower part of the inner front reaching the burned gases, where they evaporate rapidly. They also disturb the outer reaction zone due to the low droplets temperature and the rich mixture in the wake of droplet. These new results on local extinction of spray flames and droplet–flame interactions will also strengthen the CORIA Rouen Spray Burner (CRSB) database for the improvement of evaporation and combustion models for reacting sprays.

      PubDate: 2018-05-01T08:24:06Z
      DOI: 10.1016/j.combustflame.2018.03.032
      Issue No: Vol. 193 (2018)
       
  • An experimental chemical kinetic study of the oxidation of diethyl ether
           in a jet-stirred reactor and comprehensive modeling
    • Authors: Zeynep Serinyel; Maxence Lailliau; Sébastien Thion; Guillaume Dayma; Philippe Dagaut
      Pages: 453 - 462
      Abstract: Publication date: July 2018
      Source:Combustion and Flame, Volume 193
      Author(s): Zeynep Serinyel, Maxence Lailliau, Sébastien Thion, Guillaume Dayma, Philippe Dagaut
      The oxidation of diethyl ether was studied experimentally in a jet-stirred reactor. Fuel-lean, stoichiometric and fuel-rich mixtures were oxidized at a constant fuel mole fraction (1000 ppm), at temperatures ranging from 450 to 1250 K, pressures of 1 and 10 atm, and constant residence time (70 and 700 ms, respectively). In total, six mixtures were tested at both pressures. Mole fraction profiles were obtained using gas chromatography and Fourier transform infrared spectrometry. The fuel mole fraction profiles, as well as some reaction intermediate and product profiles indicated strong low-temperature chemistry at high pressure. On the other hand, at atmospheric pressure this behavior was observed to a very small extent and only with the lean and stoichiometric mixtures. These data were compared to modeling results using literature mechanisms for diethyl ether oxidation. None of these predicted low-temperature reactivity under present conditions. Therefore, a kinetic mechanism is proposed in this study, based on recently computed kinetic parameters from literature. It shows good performances for representing the present experimental data as well as experimental data found in literature consisting of ignition delay times, laminar flame speeds and flame structure.

      PubDate: 2018-05-01T08:24:06Z
      DOI: 10.1016/j.combustflame.2018.04.002
      Issue No: Vol. 193 (2018)
       
  • Role of low-temperature chemistry in detonation of
           n-heptane/oxygen/diluent mixtures
    • Authors: Wenkai Liang; Rémy Mével; Chung K. Law
      Pages: 463 - 470
      Abstract: Publication date: July 2018
      Source:Combustion and Flame, Volume 193
      Author(s): Wenkai Liang, Rémy Mével, Chung K. Law
      The ZND structure of mixtures of n-heptane, oxygen and diluent has been investigated using a reduced reaction scheme that includes the low-temperature chemistry (LTC) pathways. It is shown that for high CO2 contents (XCO2 > 0.82) such that the reaction temperature is relatively low, the structure is affected by LTC and exhibits two distinct stages of energy release caused by low- and high-temperature chemistry, respectively. Based on the ZND structures, the dynamic parameters such as cell size, direct initiation energy and critical tube diameter of detonation within the LTC affected regime have been evaluated using various semi-empirical models. Such detonation structures exhibiting two distinct length scales lead to two distinct values for each of these dynamic parameters. For the evolution of the induction length and cell size, although the total length scales do not show negative response with increasing temperature, the length scale of the first-stage ignition demonstrates negative response when the post-shock temperature decreases within the LTC controlled regime. For the evolution of direct initiation energy, the models based on critical curvature and critical decay rate show negative temperature response, but the model based on cell size predicts continuous increase of critical initiation energy with increasing dilution. For the critical tube diameter, the model based on the critical decay rate approach also exhibits the negative temperature response.

      PubDate: 2018-05-01T08:24:06Z
      DOI: 10.1016/j.combustflame.2018.03.035
      Issue No: Vol. 193 (2018)
       
  • Reduction of flame development time in nanosecond pulsed high frequency
           discharge ignition of flowing mixtures
    • Authors: Joseph K. Lefkowitz; Timothy Ombrello
      Pages: 471 - 480
      Abstract: Publication date: July 2018
      Source:Combustion and Flame, Volume 193
      Author(s): Joseph K. Lefkowitz, Timothy Ombrello
      The effects of discharge and flow parameters on ignition kernel development time are explored in flowing methane–air mixtures. A nanosecond pulsed high frequency discharge in a pin-to-pin configuration is used as the ignition source, providing 2.9 ± 0.23 mJ/pulse. The effects of pulse repetition frequency (PRF) in the range of 10–300 kHz, number of pulses in the range of 1–50 (≈2.9–145 mJ), equivalence ratio in the range of 0.55–0.65, gap distance in the range of 0.5–2.5 mm, and flow velocity in the range of 2.5–10 m/s are explored. For all conditions, the ignition events are in the “fully-coupled” regime, in which high ignition probability is achieved and locally extinguished ignition kernels are avoided. It is found that reducing the PRF reduces the kernel development time for fixed total energy deposition due to an increased volume of unburned mixture exposed to the discharge. Increasing the number of pulses at a given PRF also decreases the kernel development time, again by increasing the volume of gas exposed to the discharge. The equivalence ratio only has an effect on the kernel growth rate after the discharge, with identical kernel areas measured at all equivalence ratios while the discharge is active. Increasing gap distance decreases the kernel development time by providing a larger initial kernel volume as well as reducing heat and radical quenching at the electrode surfaces. Finally, the flow velocity has an effect on the kernel growth rate at velocities greater than 5 m/s, with larger flow velocities resulting in shorter kernel development time. This is due to the competing rates of self-propagating flame expansion and kernel growth due to convection past the discharge region. The combined effects of all of the above parameters on the kernel area after the discharge are summarized in a correlation equation, which predicts the trends in kernel growth rate based on an estimated plasma area defined by the discharge duration, flow velocity, and gap distance.

      PubDate: 2018-05-01T08:24:06Z
      DOI: 10.1016/j.combustflame.2018.04.009
      Issue No: Vol. 193 (2018)
       
  • Characterization of thermodiffusive and hydrodynamic mechanisms on the
           cellular instability of syngas fuel blended with CH4 or CO2
    • Authors: Denis Lapalme; Fabien Halter; Christine Mounaïm-Rousselle; Patrice Seers
      Pages: 481 - 490
      Abstract: Publication date: July 2018
      Source:Combustion and Flame, Volume 193
      Author(s): Denis Lapalme, Fabien Halter, Christine Mounaïm-Rousselle, Patrice Seers
      Thermodiffusive and hydrodynamic instabilities cause a departure from laminar combustion and, as such, are responsible for flame self-acceleration behavior. Due to the presence of hydrogen, syngas is a fuel prone to instabilities and is frequently diluted with CO2 or co-fired with CH4. This paper, thus presents an experimental study on how syngas mixed with CO2 and/or CH4 influence the onset of both kinds of instability. First, the results show that the laminar flame thickness is the controlling parameter in determining the critical radius at which cellularity appeared when CH4 is added to syngas. Moreover, higher levels of CO2 dilution translated into a constant critical radius, illustrating that the thermodiffusive mechanism is counterbalanced by the hydrodynamic one. To help differentiate between both kinds of instability, it is suggested herein to use the coefficient of self-acceleration or the ratio of the flame speed at the critical flame radius on the laminar flame speed. Finally, a correlation predicting the onset of cellularity is proposed based on the equation format proposed by Jomaas et al. (2007), derived from the stability analysis of a spherically expending flame. The correlation expresses the critical Peclet as a function of hydrodynamic and thermodiffusive instabilities and was successfully validated against experimental data from this study and the literature.

      PubDate: 2018-05-01T08:24:06Z
      DOI: 10.1016/j.combustflame.2018.03.028
      Issue No: Vol. 193 (2018)
       
  • Effects of fuel properties and free stream turbulence on characteristics
           of bluff-body stabilized flames
    • Abstract: Publication date: August 2018
      Source:Combustion and Flame, Volume 194
      Author(s): Bikram Roy Chowdhury, Baki M. Cetegen
      An experimental investigation of the effect of fuel properties and different levels of free stream turbulence intensities on the structure of bluff body stabilized lean, premixed flames is reported. The diagnostic techniques involving simultaneous imaging of hydroxyl (OH) and formaldehyde (CH2O) by planar laser induced fluorescence and particle image velocimetry (PIV) were used to study the interaction between the flame and the flow field. CH2O fluorescence and the pixel-by-pixel multiplication of OH and CH2O fluorescence signals were utilized to mark preheat and heat release regions, respectively. As the turbulence intensity increased from 4% to 14%, pronounced formation of cusps and unburnt mixture fingers were observed along the flame front. For the intense turbulence conditions, different characteristics of the flame front were observed which strongly depended on the properties of fuel/air mixture. For lean methane/- and ethylene/air (ϕ = 0.85), localized extinctions along the flame sheet and flamelet merging were observed which created isolated pockets of reactants in the flame envelope with heat release regions along their boundary. In addition to these features, propane/- and ethylene/air (ϕ = 0.655) flames exhibited the occurrence of flame fragmentation events which created multiple islands of OH filled regions separated by thick layers of CH2O. The overall flame shape for these conditions was observed to change intermittently from symmetric to asymmetric mode with increasing turbulence intensity. Several properties were measured to characterize the effects of turbulence–flame interaction which includes the preheat and reaction zone thicknesses, 2-D strain rate, burning fraction, flame brush thickness, flame surface density and turbulent to laminar flame area ratio.

      PubDate: 2018-05-18T15:11:47Z
       
  • Radiative extinction of large n-alkane droplets in oxygen-inert mixtures
           in microgravity
    • Abstract: Publication date: August 2018
      Source:Combustion and Flame, Volume 194
      Author(s): Vedha Nayagam, Daniel L. Dietrich, Michael C. Hicks, Forman A. Williams
      Experimental observations are presented concerning radiative extinction of large n-alkane droplets in diluent-substituted environments at moderately varied pressures in microgravity onboard the International Space Station. The fuels considered are n-heptane, n-octane, and n-decane with carbon dioxide, helium, and xenon used as inerts, replacing nitrogen as diluents at varying amounts. It is shown that a simple scaling analysis, based on the assumptions that radiative extinction occurs when the flame temperature drops to a critical value and that the radiative heat loss rate is a fraction of the heat-release rate at the flame, is able to correlate the measured droplet diameter at extinction as a function of its initial diameter and of the ambient gas-mixture properties.

      PubDate: 2018-05-17T15:11:29Z
       
  • A note on radiation preheating of some hydrocarbons by combustion products
    • Abstract: Publication date: August 2018
      Source:Combustion and Flame, Volume 194
      Author(s): Quentin Binauld, Philippe Rivière, Anouar Soufiani
      The aim of this paper is to provide upper bounds of the radiation emitted by hot combustion products, mainly CO2 and H2O, and absorbed by some gaseous cold hydrocarbons, or by cold CO2 and H2O. Line by line calculations with accurate spectroscopic data bases are used to compute the absorbed power under two geometrical configurations: a cold optically thin medium surrounded by spherical hot combustion products, and a thin cold slab in front of a hot combustion parallel slab. The absorbed power and the corresponding characteristic time scale are adjusted by simple and physically based formulas to provide quick estimations for various values of combustion product temperature, pressure, and the radius of the hot sphere or the hot slab thickness. The obtained time scales can be compared in a given application to hydrodynamic or diffusive time scales to decide whether it is necessary to take this preheating phenomenon into account or not.

      PubDate: 2018-05-17T15:11:29Z
       
  • Influence of aliphatic side chains of aromatic hydrocarbons on soot
           formation: Experimental and numerical investigation
    • Abstract: Publication date: August 2018
      Source:Combustion and Flame, Volume 194
      Author(s): Yoichiro Araki, Kaname Takahashi, Keiichi Kaga, Yasuhiro Saito, Yohsuke Matsushita, Hideyuki Aoki, Koki Era, Takayuki Aoki, Togo Yamaguchi
      The effect of aliphatic side chains in the feedstock on soot formation was studied with experimental and numerical simulation. Feedstocks of ethylbenzene, xylene, toluene, and benzene were pyrolyzed at different residence times and temperatures, and the particle size distribution (PSD) of the formed soot was measured. The initial nucleation rates of soot during ethylbenzene or toluene pyrolysis was high. However, after a long residence time, the amount of soot produced from benzene pyrolysis became higher than for these feedstocks. This result suggests that at long residence times, the growth rate of soot from benzene is higher than those of ethylbenzene and toluene. Numerical simulation for the pyrolysis of ethylbenzene, toluene, and benzene was conducted with the KAUST PAH Mechanism 2 (KM2) model, and the prediction accuracy was improved by adding some new reactions. The simulations were performed at two conditions similar to the experimental ones: The production rate of polycyclic aromatic hydrocarbons (PAHs) was very high at the beginning of reactions of ethylbenzene and toluene, indicating high rates of soot nuclei generation. In benzene pyrolysis, modification of the KM2 model led to a higher predicted PAH concentration than that with the original KM2. Most of the reactions we added to the KM2 model are those of PAH growth by phenyl addition (PA). Therefore, the PA mechanism would be an important reaction pathway for PAH growth in the pyrolysis of benzene.

      PubDate: 2018-05-17T15:11:29Z
       
  • Autoignition of trans-decalin, a diesel surrogate compound: Rapid
           compression machine experiments and chemical kinetic modeling
    • Abstract: Publication date: August 2018
      Source:Combustion and Flame, Volume 194
      Author(s): Mengyuan Wang, Kuiwen Zhang, Goutham Kukkadapu, Scott W. Wagnon, Marco Mehl, William J. Pitz, Chih-Jen Sung
      Decahydronaphthalene (decalin), with both cis and trans isomers, is a bicyclic alkane that is found in aviation fuels, diesel fuels, and alternative fuels from tar sands and oil shales. Between the two decalin isomers, trans-decalin has a lower cetane number, is energetically more stable, and has a lower boiling point. Moreover, trans-decalin has often been chosen as a surrogate component to represent two-ring naphthenes in transportation fuels. Recognizing the importance of understanding the chemical kinetics of trans-decalin in the development of surrogate models, an experimental and modeling study has been conducted. Experimentally, the autoignition characteristics of trans-decalin were investigated using a rapid compression machine (RCM) by using trans-decalin/O2/N2 mixtures at compressed pressures of PC   = 10–25 bar, low-to-intermediate compressed temperatures of TC   = 620–895 K, and varying equivalence ratios of ϕ = 0.5, 1.0, and 2.0. These new experimental data demonstrate the effects of pressure, fuel loading, and oxygen concentration on autoignition of trans-decalin. The current RCM data of trans-decalin at lower temperatures were also found to complement well with the literature shock tube data of decalin (mixture of cis + trans) at higher temperatures. Furthermore, a chemical kinetic model for the oxidation of trans-decalin has been developed with new reaction rates and pathways, including, for the first time, a fully-detailed representation of low-temperature chemical kinetics for trans-decalin. This model shows good agreement with the overall ignition delay results of the current RCM experiments and the literature shock tube studies. Chemical kinetic analyses of the developed model were further conducted to help identify the fuel decomposition pathways and the reactions controlling the autoignition at varying conditions.

      PubDate: 2018-05-17T15:11:29Z
       
 
 
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