for Journals by Title or ISSN
for Articles by Keywords
help
Followed Journals
Journal you Follow: 0
 
Sign Up to follow journals, search in your chosen journals and, optionally, receive Email Alerts when new issues of your Followed Journals are published.
Already have an account? Sign In to see the journals you follow.
Journal Cover Combustion and Flame
  [SJR: 3.12]   [H-I: 124]   [119 followers]  Follow
    
   Full-text available via subscription Subscription journal
   ISSN (Print) 0010-2180
   Published by Elsevier Homepage  [3089 journals]
  • Near-field flame dynamics of liquid oxygen/kerosene bi-swirl injectors at
           supercritical conditions
    • Abstract: Publication date: April 2018
      Source:Combustion and Flame, Volume 190
      Author(s): Xingjian Wang, Yixing Li, Yanxing Wang, Vigor Yang
      The flame dynamics of liquid bi-swirl injectors are numerically investigated using the large eddy simulation technique. Liquid oxygen (LOX) and kerosene at subcritical temperatures are injected into a supercritical pressure environment. The theoretical framework is based on the full conservation laws and accommodates real-fluid thermodynamics and transport theories over the entire range of fluid states. Turbulence/chemistry interaction is modeled with a laminar flamelet library approach, the validity of which is demonstrated in the present work. The near-field flow and flame characteristics are carefully studied. The flame is anchored in the wake of the inner injector post by two counter-rotating vortices, and further stabilized by center and corner recirculation zones in the downstream region. Differences in the flow patterns between the cold-flow and combustion cases are recognized. Various geometric parameters, including recess region, post thickness, and kerosene annulus width, are examined in depth to explore their influence on flame characteristics. A recess region is found to be necessary to achieve efficient mixing and combustion. The absence of a recess region increases the penetration depth of the kerosene stream in the downstream region and reduces the thermal protection provided to the injector faceplate. On the other hand, a thicker LOX post or a wider kerosene annulus protects the faceplate more efficiently, and introduces larger recirculation zones near the LOX post surface and thus higher flow residence time to better anchor the flame. However, the flame attachment for thicker post and wider annulus induces a stronger heat flux to the post surface, and thus increases the risk of thermal failure of the injector device. The dynamic characteristics of the flame field are also discussed. The flow oscillations within the injector are found to be dominated by a quarter acoustic wave, while the oscillatory field near the injector exit is characterized by vortex shedding. The characteristic frequency of the vortex shedding is similar for different LOX post thicknesses and annulus widths, and is determined by the exit velocity profiles.

      PubDate: 2017-12-12T17:40:59Z
       
  • Framework for submodel improvement in wildfire modeling
    • Abstract: Publication date: April 2018
      Source:Combustion and Flame, Volume 190
      Author(s): Mohamad El Houssami, Aymeric Lamorlette, Dominique Morvan, Rory M. Hadden, Albert Simeoni
      An experimental and numerical study was carried out to assess the performance of the different submodels and parameters used to describe the burning dynamics of wildfires. A multiphase formulation was used and compared to static fires of dried pitch pine needles of different bulk densities. The samples were exposed to an external heat flux of 50 kW/m2 in the FM Global Fire Propagation Apparatus and subjected to different airflows, providing a controlled environment and repeatable conditions. Submodels for convective heat transfer, drag forces, and char combustion were investigated to provide mass loss rate, flaming duration, and gas emissions. Good agreement of predicted mass loss rates and heat release rates was achieved, where all these submodels were selected to suit the tested conditions. Simulated flaming times for different flow conditions and different fuel bulk densities compared favorably against experimental measurements. The calculation of the drag forces and the heat transfer coefficient was demonstrated to influence greatly the heating/cooling rate, the degradation rate, and the flaming time. The simulated CO and CO2 values compared well with experimental data, especially for reproducing the transition between flaming and smoldering. This study complements a previous study made with no flow to propose a systematic approach that can be used to assess the performance of the submodels and to better understand how specific physical phenomena contribute to the wildfire dynamics. Furthermore, this study underlined the importance of selecting relevant submodels and the necessity of introducing relevant subgrid-scale modelling for larger scale simulations.

      PubDate: 2017-12-12T17:40:59Z
       
  • An investigation of pyrolysis and ignition of moist leaf-like fuel subject
           to convective heating
    • Abstract: Publication date: April 2018
      Source:Combustion and Flame, Volume 190
      Author(s): Babak Shotorban, Banglore L. Yashwanth, Shankar Mahalingam, Dakota J. Haring
      The burning of a thin rectangular-shape moist fuel element, representing a living leaf subject to convective heating, was investigated computationally. The setup resembled a previous bench-scale experimental setup (Pickett et al., Int. J. Wildland Fire 19, 2010, 153-162), where a freshly harvested horizontally oriented manzanita (Arctostaphylos glandulosa) leaf was held over a flat flame burner and burned by its convective heating. Computations were performed by FDS coupled with an improved version of Gpyro3D. This improvement was concerned with the calculation of the mean porosities in the computational cells to account for the net volume reduction that the condense phase experiences within the computational cells during moisture evaporation and pyrolysis. The dry mass was assumed to consist of cellulose, hemicellulose and lignin undergoing the pyrolysis reactions proposed by Miller and Bellan (Combust. Sci. Technol. 126, 1997, 97-137) for biomass. The reaction scheme was initially validated against published experimental and computational TGA results. Then, the burning of leaf-like fuels with three initial fuel moisture contents (40%, 76%, 120%), selected as per the range of experimentally measured values, was modeled. The time evolutions of the normalized mass were good for the modeled fuels with 76% and 120% FMCs and fair for the one with a 40% FMC, as compared to the experimental burning results of four manzanita leaves with unspecified FMCs. The computed ignition time was also in good agreement with the measurement. The computed burnout time was somewhat shorter than the measurement. Modeling revealed the formation of unsteady flow structures, including vortices and regions with high strain rates, near the fuel that acted as a bluff body against the stream of the burner exit. These structures played a significant role in the spatial distribution of gas phase temperature and species around the fuel, which in turn, had an impact on the ignition location. Fuel moisture content primarily affected the temperature response of the fuel and solid phase decomposition.

      PubDate: 2017-12-12T17:40:59Z
       
  • Probing the low-temperature chemistry of ethanol via the addition of
           dimethyl ether
    • Abstract: Publication date: April 2018
      Source:Combustion and Flame, Volume 190
      Author(s): Yingjia Zhang, Hilal El-Merhubi, Benoîte Lefort, Luis Le Moyne, Henry J. Curran, Alan Kéromnès
      Considering the importance of ethanol (EtOH) as an engine fuel and a key component of surrogate fuels, the further understanding of its auto-ignition and oxidation characteristics at engine-relevant conditions (high pressures and low temperatures) is still necessary. However, it remains difficult to measure ignition delay times for ethanol at temperatures below 850 K with currently available facilities including shock tube and rapid compression machine due to its low reactivity. Considering the success of our recent study of toluene oxidation under similar conditions [38], dimethyl ether (DME) has been selected as a radical initiator to explore the low-temperature reactivity of ethanol. In this study, ignition delay times of ethanol/DME/‘air’ mixtures with blending ratios of 100% EtOH, 70%/30% EtOH/DME and 50%/50% EtOH/DME mixtures were measured in a rapid compression machine and in two high-pressure shock tubes at conditions relevant to internal combustion engines (20–40 atm, 650–1250 K and equivalences ratios of 0.5–2.0). The influence of these conditions on the auto-ignition behavior of the mixture blends was systematically investigated. Our results indicate that, in the low temperature range (650–950 K), increasing the amount of DME in the fuel mixture significantly increases the reactivity of ethanol. At higher temperatures, however, there is almost no visible impact of the fuel mixture composition, whereas DME shows a lower reactivity. Furthermore, with the addition of DME, different kinetic regimes were observed experimentally: the reactivity is controlled by ethanol when the addition of DME is less than 30% while it is dominated by DME when the proportion of DME is over 50%. Literature mechanisms show reasonable agreement with the new experimental data for the 100% EtOH and the 70%/30% EtOH/DME mixtures but under-predict the reactivity of the 50%/50% EtOH/DME mixtures at temperatures below 850 K, suggesting that further refinement of the low-temperature chemistry of ethanol/DME is warranted. An updated binary fuel mechanism is therefore proposed by incorporating the latest experimental and/or theoretical work in the literature, as well as adding new reaction pathways. Results indicate that the proposed model is in satisfactory agreement with all of the mixtures investigated.

      PubDate: 2017-12-12T17:40:59Z
       
  • Modelling turbulent premixed flame–wall interactions including flame
           
    • Abstract: Publication date: April 2018
      Source:Combustion and Flame, Volume 190
      Author(s): Dominik Suckart, Dirk Linse
      A level-set flamelet model for turbulent premixed combustion that accounts for the main effects of flame–wall interaction, quenching and near-wall turbulence, is proposed based on the G-equation. The structure of laminar unquenched and quenched flames is analysed and a consistent G-equation valid for both flame types is derived. For the modelling of the turbulent quenching process, it is argued that the state of individual flamelets can be described as either quenched or unquenched. This binary mechanism leads to a kinematic description of turbulent flame–wall interactions in which the fraction of unquenched flames is described by the unquenched factor Q. It is shown that Q allows for a general and appropriate scaling of the turbulent burning velocity due to the fact that only unquenched flamelets contribute to the overall propagation speed. For the modelling of turbulent premixed combustion, a unified G-equation valid for unquenched and quenched flames is derived. Modelling closures accounting for near-wall turbulence as well as unsteady flame development effects are introduced. The modelling approach is analysed a priori as well as a posteriori using a turbulent channel flow. With regard to the turbulent burning velocity, it is found that the effect of quenching is dominant compared to the effect of wall-bounded turbulence. The latter becomes important for a proper estimation of the turbulent flame length. Moreover, the model is compared against available DNS data of flame–wall interaction in a turbulent channel flow. It is shown that the turbulent burning velocity, the turbulent flame thickness as well as the reactive flame surface density near the wall are correctly reproduced. The present modelling approach thus allows for a consistent modelling of flame–wall interactions, which can also be transferred to other combustion models that are based on turbulent flame speed correlations.

      PubDate: 2017-12-12T17:40:59Z
       
  • Laminar burning velocity and structure of coal dust flames using a unity
           Lewis number CFD model
    • Abstract: Publication date: April 2018
      Source:Combustion and Flame, Volume 190
      Author(s): Chris T. Cloney, Robert C. Ripley, Michael J. Pegg, Paul R. Amyotte
      Despite decades of research, predictive methods remain unavailable to estimate flame propagation in dust clouds under industrial scenarios. The complexity of scaling the fundamental processes occurring in multiphase flames to industrial geometries, and a lack of tools to explore and extend knowledge in this area, may be key factors missing in the research literature. The main objective of this work is to verify the ability of a CFD model based on a unity Lewis number assumption to explore laminar burning velocity in coal dust clouds. A second objective is to perform parametric analysis including the role of surface reactions, particle diameter, and initial system temperature. The third and final objective is to explore the impact of discrete particle combustion on flame structure and burning velocity. Despite a simplified treatment of gas phase transport properties, single-step devolatilization, and single-step surface reaction, the current model correctly captures the effects of particle diameter and initial temperature on burning velocity and demonstrates good agreement with previous investigations once preheating in the experimental results is accounted for. Furthermore, the reduced model complexity may allow future investigation by the current authors and other research groups into different combustible dusts, more detailed system geometry, and turbulent flow conditions. Lastly, the results of the current study provide a baseline that more comprehensive modeling methods may be compared to, which is currently missing in the literature.

      PubDate: 2017-12-12T17:40:59Z
       
  • Effect of n-dodecane decomposition on its fundamental flame properties
    • Abstract: Publication date: April 2018
      Source:Combustion and Flame, Volume 190
      Author(s): Jennifer Smolke, Francesco Carbone, Fokion N. Egolfopoulos, Hai Wang
      The effect of fuel decomposition on fundamental flame properties was investigated computationally for atmospheric-pressure n-dodecane/air mixtures. The fuel decomposition was modeled under isobaric and adiabatic conditions for initial temperatures of 1100, 1200 and 1300 K, and equivalence ratios of 0.7, 1.0, and 1.4. For various extents of n-dodecane oxidative thermal decomposition, the combustion characteristic of mixtures of the resulting products with air were investigated by keeping the total enthalpy constant and equal to that of the n-dodecane/air mixture. The endothermic n-dodecane decomposition was found, to a large extent, to be decoupled from the subsequent oxidation of the attendant products that include largely hydrogen, ethylene, methane, and other small alkenes. The mass burning rates in freely propagating flames were found to increase with an increase in the extent of n-dodecane decomposition, but the change is limited to 15%, which occurs in the highest extent of decomposition. On the other hand, the extinction strain rate of decomposed, lean to stoichiometric mixtures increases notably compared to the corresponding un-decomposed fuel–air mixtures. Sensitivity analyses of mass burning rates and extinction strain rates to kinetics and binary diffusion coefficients reveal that the laminar flame speed is primarily sensitive to key heat release and radical branching reactions, and as such fuel decomposition has a small effect on the mass burning rate. On the other hand, the extinction strain rate of the fuel-lean mixtures is sensitive to the diffusivity of the fuel, and for this reason, fuel decomposition removes the difficulties associated with the transport of the large fuel molecules into the flame zone.

      PubDate: 2017-12-12T17:40:59Z
       
  • An LES-PBE-PDF approach for predicting the soot particle size distribution
           in turbulent flames
    • Abstract: Publication date: March 2018
      Source:Combustion and Flame, Volume 189
      Author(s): Fabian Sewerin, Stelios Rigopoulos
      In this article, we combine the large eddy simulation (LES) concept with the population balance equation (PBE) for predicting, in a Eulerian fashion, the evolution of the soot particle size distribution in a turbulent non-premixed hydrocarbon flame. In order to resolve the interaction between turbulence and chemical reactions/soot formation, the transport equations for the gas phase scalars and the PBE are combined into a joint evolution equation for the filtered pdf associated with a single realization of the gas phase composition and the soot number density distribution. With view towards an efficient numerical solution procedure, we formulate Eulerian stochastic field equations that are statistically equivalent to the joint scalar-number density pdf. By discretizing the stochastic field equation for the particle number density using an explicit adaptive grid technique, we are able to accurately resolve sharp features of evolving particle size distributions, while keeping the number of grid points in particle size space small. Compared to existing models, the main advantage of our approach is that the LES-filtered particle size distribution is predicted at each location in the flow domain and every instant in time and that arbitrary chemical reaction mechanisms and soot formation kinetics can be accommodated without approximation. The combined LES-PBE-PDF model is applied to investigate soot formation in the turbulent non-premixed Delft III flame. Here, the soot kinetics encompass acetylene-based rate expressions for nucleation and growth that were previously employed in the context of laminar diffusion flames. In addition, both species consumption by soot formation and radiation based on the assumption of optical thinness are accounted for. While the agreement of our model predictions with experimental measurements is not perfect, we indicate the benefits of the LES-PBE-PDF model and demonstrate its computational viability.

      PubDate: 2017-12-12T17:40:59Z
       
  • Modelling and analysis of the combustion behaviour of granulated fuel
           particles in iron ore sintering
    • Abstract: Publication date: March 2018
      Source:Combustion and Flame, Volume 189
      Author(s): Jiapei Zhao, Chin E. Loo, Hao Zhou, Jinliang Yuan, Xinbao Li, Yingying Zhu, Guohua Yang
      The combustion behaviour of solid fuels – for example, coke and biomass char – is an important consideration in iron ore sintering as it determines heat availability for the melt formation process. This behaviour is influenced by the presence of an adhering layer of fine material around the fuel particles. In this study, analytical results for the combustion of single granulated fuel particles – applicable to all Thiele modulus (ϕh ) values – are presented. For the conversion of an isothermal carbon particle, the conversion parameter α is found to depend on ϕh and the effectiveness factor ηh . For ϕh  < 9, α can be approximated by 0.4ηh , while for ϕh  > 9, α approaches ηh /3. The relationship between α and ηh is not altered by the presence of an adhering layer. However, at high temperatures and for reactive fuels, an adhering layer influences the combustion rate significantly. The fuel combustion process in iron ore sintering can be viewed as occurring in three regimes depending on factors such as fuel size, reactivity and temperature. To investigate the effect of fuel properties on sintering performance, the developed combustion model is integrated into a 2D iron ore sintering model. Good comparisons are obtained between model results and experimental data from laboratory sintering tests. In the study of fuel types, model results indicate that when biomass char replaced coke there was significant lowering of flame front temperature and combustion efficiency, while the speed of the flame front down the bed accelerated. These changes can be explained by the higher reactivity of the biomass char and its physical properties which influence the granulation process – resulting in changes in the thickness of the adhering layer and combustion behaviour. The flow-on effect of this on sintering performance is consistent with reported experimental results by other researchers.

      PubDate: 2017-12-12T17:40:59Z
       
  • Uncertainty quantification and sensitivity analysis of thermoacoustic
           stability with non-intrusive polynomial chaos expansion
    • Abstract: Publication date: March 2018
      Source:Combustion and Flame, Volume 189
      Author(s): Alexander Avdonin, Stefan Jaensch, Camilo F. Silva, Matic Češnovar, Wolfgang Polifke
      In this paper, non-intrusive polynomial chaos expansion (NIPCE) is used for forward uncertainty quantification and sensitivity analysis of thermoacoustic stability of two premixed flame configurations. The first configuration is a turbulent swirl combustor, modeled by the Helmholtz equation with an n − τ flame model. Uncertain input parameters are the gain and the time delay of the flame, as well as the magnitude and the phase of the outlet reflection coefficient. NIPCE is successfully validated against Monte Carlo simulation. It is observed that the first order expansion suffices to yield accurate results. The second configuration under investigation is a low order network model of a laminar slit burner, with the flame transfer function identified from weakly compressible CFD simulations of laminar reacting flow. Firstly the uncertainty and sensitivity of the growth rate due to three uncertain input parameters of the CFD model – i.e., flow velocity, burner plate temperature and equivalence ratio – are analyzed. A Monte Carlo simulation is no longer possible due to the computational cost of the CFD simulations. Secondly, two additional uncertain parameters are taken into account, i.e., the respective magnitudes of inlet and outlet reflection coefficients. This extension of the analysis does not entail a considerable increase in computational cost, since the additional parameters are included only in the low order network model. In both cases, the second order expansion is sufficient to model the uncertainties in growth rate.

      PubDate: 2017-12-12T17:40:59Z
       
  • Kinetic and thermodynamic measurements of the reactions of the positive
           ions, Mn+ and MnOH+, formed by adding manganese to fuel-rich flames of
           either H2 + O2 or C2H2 + O2
    • Abstract: Publication date: March 2018
      Source:Combustion and Flame, Volume 189
      Author(s): Nigel A. Burdett, Allan N. Hayhurst
      Mass spectrometric sampling of flat, fuel-rich flames of H2 or C2H2, burning at 1 atm. and at 1800–2650 K, has revealed that the ions MnOH+ and Mn+ are present in such flames, when seeded with amounts of manganese less than 1 p.p.m. Free electrons were the only negatively charged species. The concentrations of these two positive ions were shown to be linked by the reaction MnOH+ + H = Mn+ + H2O being close to equilibrium downstream of the reaction zone of a flame, provided it was hotter than 2100 K. The equilibrium constant of this reaction was measured in flames differing in temperature; the values showed that the reaction's forward step is exothermic by 233 ± 20 kJ mol−1. This magnitude indicates that the ionisation energy of MnOH is 763 ± 35 kJ mol−1. In addition, the rate constant of the forward step was measured and found to have an activation energy of 112 ± 25 kJ mol−1. In a simple flame of H2 + O2 + N2, ions were produced from the major species containing manganese, i.e. MnO and atomic Mn, by chemi-ionisation in: MnO + H → MnOH+ + e− and Mn + OH → MnOH+ + e−. Free atoms of Mn were roughly three times as abundant as molecules of MnO. The kinetics of these chemi-ionisation reactions were measured; the reactions resulted in elevated levels of ionisation because of a flame having concentrations of the free radicals H and OH in excess of those for equilibrium. In flames of C2H2, their significant natural ionisation was transferred to the metallic additive by: H3O+ + MnO → MnOH+ + H2O and also H3O+ + Mn → Mn+•H2O + H, followed by Mn+•H2O → Mn+ + H2O. These processes led to the level of ionisation early in a hydrocarbon flame being well above that for equilibrium, which was approached by ions recombining in MnOH+ + e− → MnO + H or Mn + OH, as well as in the three-body reaction: Mn+ + e− + M → Mn + M. Recombination coefficients were measured for these processes, as well as for MnOH+ + Cl− → Mn + OH + Cl and Mn+ + Cl− + M → Mn + Cl + M.

      PubDate: 2017-12-12T17:40:59Z
       
  • MMC-LES of a syngas mixing layer using an anisotropic mixing time scale
           model
    • Abstract: Publication date: March 2018
      Source:Combustion and Flame, Volume 189
      Author(s): Son Vo, Andreas Kronenburg, Oliver T. Stein, Matthew J. Cleary
      This brief communication presents results from a sparse Lagrangian particle method coupled with large eddy simulations (LES) called MMC-LES. It is considered an addendum of an earlier DNS-based study of a syngas shear layer configuration [10] and verifies the enhanced predictive capabilities of a new MMC time scale model. The MMC-LES further demonstrate the correct scaling of the mixing time, the time scale’s independence of the filter scale of the underlying LES field and a desirable low sensitivity of the predictions to variations in the primary modelling parameter fm .

      PubDate: 2017-12-12T17:40:59Z
       
  • Experimental and modeling studies of a biofuel surrogate compound: laminar
           burning velocities and jet-stirred reactor measurements of anisole
    • Abstract: Publication date: March 2018
      Source:Combustion and Flame, Volume 189
      Author(s): Scott W. Wagnon, Sébastien Thion, Elna J.K. Nilsson, Marco Mehl, Zeynep Serinyel, Kuiwen Zhang, Philippe Dagaut, Alexander A. Konnov, Guillaume Dayma, William J. Pitz
      Lignocellulosic biomass is a promising alternative fuel source which can promote energy security, reduce greenhouse gas emissions, and minimize fuel consumption when paired with advanced combustion strategies. Pyrolysis is used to convert lignocellulosic biomass into a complex mixture of phenolic-rich species that can be used in a transportation fuel. Anisole (or methoxybenzene) can be used as a surrogate to represent these phenolic-rich species. Anisole also has attractive properties as a fuel component for use in advanced spark-ignition engines because of its high blending research octane number of 120. Presented in the current work are new measurements of laminar burning velocities, jet-stirred reactor (JSR) speciation of anisole/O2/N2 mixtures, and the development and validation of a detailed chemical kinetic mechanism for anisole. Homogeneous, steady state, fixed gas temperature, perfectly stirred reactor CHEMKIN simulations were used to validate the mechanism against the current JSR measurements and published JSR experiments from CNRS-Nancy. Pyrolysis and oxidation simulations were based on the experimental reactant compositions and thermodynamic state conditions including P = 1 bar and T = 675–1275 K. The oxidation compositions studied in this work span fuel-lean (ϕ = 0.5), stoichiometric, and fuel rich (ϕ = 2.0) equivalence ratios. Laminar burning velocities were measured on a heat flux stabilized burner at an unburnt T = 358 K, P = 1 bar and simulated using the CHEMKIN premixed laminar flame speed module. Ignition delay times of anisole were then simulated at conditions relevant to advanced combustion strategies. Current laminar burning velocity measurements and predicted ignition delay times were compared to gasoline components (e.g., n-heptane, iso-octane, and toluene) and gasoline surrogates to highlight differences and similarities in behavior. Reaction path analysis and sensitivity analysis were used to explain the pathways relevant to the current studies. Under pyrolysis and oxidative conditions, unimolecular decomposition of anisole to phenoxy radicals and methyl radicals was found to be important due to the relatively low bond strength between the oxygen and methyl group, ∼65 kcal/mol. Reactions of these abundant phenoxy radicals with O2 were found to be critical to accurately reproduce anisole's reactivity.

      PubDate: 2017-12-12T17:40:59Z
       
  • Autoignition of straight-run naphtha: A promising fuel for advanced
           compression ignition engines
    • Abstract: Publication date: March 2018
      Source:Combustion and Flame, Volume 189
      Author(s): Mohammed Alabbad, Gani Issayev, Jihad Badra, Alexander K. Voice, Binod Raj Giri, Khalil Djebbi, Ahfaz Ahmed, S. Mani Sarathy, Aamir Farooq
      Naphtha, a low-octane distillate fuel, has been proposed as a promising low-cost fuel for advanced compression ignition engine technologies. Experimental and modelling studies have been conducted in this work to assess autoignition characteristics of naphtha for use in advanced engines. Ignition delay times of a certified straight-run naphtha fuel, supplied by Haltermann Solutions, were measured in a shock tube and a rapid comparison machine over wide ranges of experimental conditions (20 and 60 bar, 620–1223 K, ϕ = 0.5, 1 and 2). The Haltermann straight-run naphtha (HSRN) has research octane number (RON) of 60 and motor octane number (MON) of 58.3, with carbon range spanning C3–C9. Reactivity of HSRN was compared, via experiments and simulations, with three suitably formulated surrogates: a two-component PRF (n-heptane/iso-octane) surrogate, a three-component TPRF (toluene/n-heptane/iso-octane) surrogate, and a six-component surrogate. All surrogates reasonably captured the ignition delays of HSRN at high and intermediate temperatures. However, at low temperatures (T < 750 K), the six-component surrogate performed the best in emulating the reactivity of naphtha fuel. Temperature sensitivity and rate of production analyses revealed that the presence of cyclo-alkanes in naphtha inhibits the overall fuel reactivity. Zero-dimensional engine simulations showed that PRF is a good autoignition surrogate for naphtha at high engine loads, however, the six-component surrogate is needed to match the combustion phasing of naphtha at low engine loads.

      PubDate: 2017-12-12T17:40:59Z
       
  • Fully resolved DNS of droplet array combustion in turbulent convective
           flows and modelling for mixing fields in inter-droplet space
    • Abstract: Publication date: March 2018
      Source:Combustion and Flame, Volume 189
      Author(s): Bosen Wang, Andreas Kronenburg, Giovanni L. Tufano, Oliver T. Stein
      The combustion of droplets in turbulent convective flows is simulated using direct numerical simulations (DNS). The liquid–vapour interface between kerosene droplets and the surrounding air is fully resolved with realistic boundary conditions for mass conservation, heat conduction and species diffusion. The study focuses on the characterisation of the mixing process between the evaporating fuel and the surrounding gas in regions that are dominated by small turbulent scales. For inertial droplets, these regions are characterised by Kolmogorov time scales and the mean relative velocity between droplets and the surrounding gas phase. Scaling laws for quantities of interest that require sub-grid modelling for LES, such as mixture fraction, its conditional scalar dissipation and probability density functions (PDF), are presented and assessed by comparisons with the DNS data. The scaling laws provide satisfactory estimates and are validated for different inflow Reynolds numbers, turbulence intensities and integral length scales, droplet diameters, inter-droplet distances, droplet combustion regimes and various instants of the transient evaporation process. Suitable modelling parameters are extracted from the DNS and functional dependencies of the parameters are suggested. This study demonstrates that the scaling laws are suitable to serve as sub-grid scale models for mixture fraction based approaches such as flamelet, conditional moment closure (CMC) or multiple mapping conditioning (MMC) methods.

      PubDate: 2017-12-12T17:40:59Z
       
  • Autoignition study of binary blends of n-dodecane/1-methylnaphthalene and
           iso-cetane/1-methylnaphthalene
    • Abstract: Publication date: March 2018
      Source:Combustion and Flame, Volume 189
      Author(s): Goutham Kukkadapu, Chih-Jen Sung
      An experimental study on autoignition of two binary blends, n-dodecane/1-methylnaphthalene and iso-cetane/1-methylnaphthalene, has been conducted using a rapid compression machine. Specifically, the ignition delays of the stoichiometric blend+air mixtures were measured at elevated pressures of PC  = 15 bar and 30 bar, compressed temperatures of TC = 626–944 K, and varying blending ratios of the constituents. For a given set of PC and TC , a nonlinear response of the blend reactivity with respect to the relative amount of the constituents was observed. Since a comprehensive chemical kinetic model for the blends investigated here is under development, the current ignition delay datasets serve as the needed targets for model validation. For selected conditions, ignition delay simulations were conducted to highlight and discuss the deficiencies of the literature models and the potential areas for model improvements, especially at low temperatures. Further chemical kinetic analyses were conducted to gain understanding of the blending behavior predicted by the available model.

      PubDate: 2017-12-12T17:40:59Z
       
  • Length scale effect on Mach reflection of cellular detonations
    • Abstract: Publication date: March 2018
      Source:Combustion and Flame, Volume 189
      Author(s): Jian Li, Huilan Ren, Xiahu Wang, Jianguo Ning
      An experimental study of the Mach reflection of cellular detonations over the wedge is reported in this paper. Throughout the experiments, high-quality smoked foil is obtained to record cell pattern variation. The initial pressures are varied to yield cellular detonations with varied transverse wave spacing (or cell size). This paper focuses on length scales controlling the deviation and recovery of self-similarity in an unsteady Mach reflection process. The results show that the frozen limit and equilibrium limit both exist for all the mixture compositions. The Mach reflection undergoes a frozen condition in the near field, and then goes through a transition process before asymptotically approaching an equilibrium state in the far field. The cell size variation in the Mach stem region also confirms the transient process. Although the final triple-point trajectory angle in the equilibrium state disagrees with classical three-shock theories, it is in agreement with the reactive three-shock theory when the wedge angle is less than 30°. The triple-point trajectory angle asymptotically approaches zero as the wedge angle increases to approximately 50°, indicating the critical wedge angle from Mach to regular reflection. The transition length associated with the equilibrium limit is found to be dependent on the mixture composition and has the same order of a hydrodynamic thickness, which is approximately a few cell lengths. This means that the hydrodynamic thickness is the characteristic length scale that most significantly dominates the Mach reflection process of cellular detonations.

      PubDate: 2017-12-12T17:40:59Z
       
  • Numerical investigation of soot formation from microgravity droplet
           combustion using heterogeneous chemistry
    • Abstract: Publication date: March 2018
      Source:Combustion and Flame, Volume 189
      Author(s): Alessandro Stagni, Alberto Cuoci, Alessio Frassoldati, Eliseo Ranzi, Tiziano Faravelli
      The use of isolated droplets as idealized systems is an established practice to get an insight on the physics of combustion, and an optimal test field to verify physical submodels. In this context, this work examines the dynamics of soot formation from the combustion of hydrocarbon liquid fuels in such conditions. A detailed, heterogeneous kinetic mechanism, describing aerosol and particle behavior through a discrete sectional approach is incorporated. The developed 1-dimensional model accounts for (i) non-luminous and luminous radiative heat losses, and (ii) incomplete thermal accommodation in the calculation of the thermophoretic flux. The combustion of droplets of n-heptane, i.e., the simplest representative species of real fuels, was investigated as test case; an upstream skeletal reduction of the kinetic mechanism was carried out to limit calculation times. After checking the performance of the reduced mechanism against gas-phase experimental data, the transient evolution of the system was analyzed through a comprehensive study, including fiber-suspended (D0 < 1 mm) as well as free (D0 > 1 mm) droplets. The different steps of soot evolution were quantified, and localized in the region between the flame front and the soot shell, where particle velocity is directed inwards because of thermophoresis, and residence times are much higher than what usually found in diffusion flames. As a result, growth, coalescence, and aggregation steps are significantly enhanced, and soot accumulates in the inner shell, with an evident modification of the particle size distribution, if compared to what observed in conventional combustion conditions. The model exhibits a satisfactory agreement with experimental data on flame temperature and position around the droplet, while for larger droplets an increasing sensitivity to the radiation model was observed. It is found that the latter has a significant impact on the production of soot, while scarcely affecting the location of the soot shell. On the other side, the inclusion of incomplete thermal accommodation in the thermophoretic law brought about more accurate predictions of both volume fractions and shell location, and highlighted the primary role of thermophoresis in these conditions, as already found in literature through more simplified approaches.

      PubDate: 2017-12-12T17:40:59Z
       
  • Multi-point LIBS measurement and kinetics modeling of sodium release from
           a burning Zhundong coal particle
    • Abstract: Publication date: March 2018
      Source:Combustion and Flame, Volume 189
      Author(s): Yingzu Liu, Yong He, Zhihua Wang, Kaidi Wan, Jun Xia, Jianzhong Liu, Kefa Cen
      A multi-point Laser-Induced Breakdown Spectroscopy (LIBS) method for quantitative measurement of sodium concentrations in the gas phase, the surface temperature and the particle diameter during the combustion of a Zhundong coal particle is presented. To obtain multi-point LIBS data, the laser focusing and signal collection optics are mounted on a translational platform which is able to traverse cyclically. With this setup multi-point LIBS measurements above a burning particle can be performed and the time-resolved sodium release process can be obtained. The results show that 42.2% of the total sodium mass is released during the burning of the Zhundong coal sample. For a 4 mm particle, in the char burnout stage sodium is released most strongly, i.e., 87% of the total released sodium mass, while in the de-volatilization and ash reaction stages the percentages are 5% and 8%, respectively. The atomic sodium and NaOH are the most favored species at chemical equilibrium in the plume according to CHEMKIN. The sodium release is found to be closely related to the particle burning stages by analyzing the sodium release, particle surface temperature and its diameter. A linear relationship is found between the residual sodium mass in the particle and the volume of the particle. The volatile sodium release rate obeys a two-step Arrhenius expression. Predictions by the developed two-step kinetics model agree well with the measured sodium release profiles in all the three coal-burning stages.

      PubDate: 2017-11-16T09:21:36Z
       
  • Coherent anti-Stokes Raman spectroscopy of a premixed ethylene–air flame
           in a dual-mode scramjet
    • Abstract: Publication date: March 2018
      Source:Combustion and Flame, Volume 189
      Author(s): Andrew D. Cutler, Emanuela C.A. Gallo, Luca M.L. Cantu, Robert D. Rockwell, Christopher P. Goyne
      As part of a broader effort to provide detailed measurements of turbulent flames in dual-mode scramjets, promote a deeper understanding of the relevant combustion physics, and aid appropriate computational model development, a high-subsonic cavity-stabilized premixed ethylene–air flame (typical of ramjet operation) is studied using coherent anti-Stokes Raman spectroscopy. This technique provides simultaneous measurements of temperature, separate O2 temperature, and mole fraction of N2, O2, CO, CO2, and C2H4. The experiments reveal a highly unsteady turbulent flame, approximately two-dimensional in the mean, which propagates downstream from the cavity and towards the observation wall. Measurements in the flame region reveal a flow that is largely divided into reactants (freestream fluid) and (nearly) equilibrium products, separated by flames that cannot be resolved by the CARS measurement volume, which is about 1 mm long. Mean and standard deviation of the resolved fluctuations of the temperature and mole fractions of species are quantified. The peak standard deviation in each profile across the flame occurs where the mean gradient is steepest, and is about 37% the difference between reactants (freestream) and products conditions. Several cases were investigated including limiting combustion cases near the lean fuel and low air temperature blowouts; in all cases the flame propagation angles are the same and distributions of suitably normalized mean and fluctuation parameters are similar at all locations and for all cases.

      PubDate: 2017-11-16T09:21:36Z
       
  • Nanothermite of Al nanoparticles and three-dimensionally ordered
           macroporous CuO: Mechanistic insight into oxidation during thermite
           reaction
    • Abstract: Publication date: March 2018
      Source:Combustion and Flame, Volume 189
      Author(s): Do Joong Shin, Whi Dong Kim, Seokwon Lee, Doh C. Lee
      We report the synthesis and characterization of thermite composites consisting of Al nanoparticles and three dimensionally-ordered macroporous (3DOM) CuO. The pores of 3DOM CuO have size ranging between 190 nm and 320 nm and size dispersion lower than 10%, while 70 nm Al particles we used in this study are dispersed uniformly over the entire composite structures. Both the size uniformity and homogeneous mixing enable quantitative correlation between structures and thermite reaction characteristics. Ignition of the thermite composites in a closed chamber initiates thermite reactions, and the combustion kinetics is recorded in terms of the transient pressure changes. Contrary to a premise that small CuO pores would result in mixing with Al nanoparticles at a smaller length scale and hence higher pressurization rate, 3DOM CuO with pore size smaller than 240 nm exhibits gradually lower pressurization rate as pore size decreases. It turns out that pressurization rate has the highest value when the pore size of CuO is about 240 nm. The size dependence indicates that two different pathways, solid-state and gaseous diffusion, account for oxygen transfer from CuO to Al in the thermite composites. With the pore size of CuO larger than 240 nm, gas-phase diffusion predominates and pressurization rate increases as the size of the pores decreases. On the other hand, at small length scale, i.e., with CuO pore size smaller than 240 nm, condensed-phase diffusion is becoming a visibly more influential factor, reversing the size dependence. The size-dependence of the pressurization rate from thermite composites of Al nanoparticles and geometry-controlled 3DOM CuO reveals that the thermite reaction has the highest combustion rate at the smallest length scale where the gaseous diffusion still surpasses condensed-phase diffusion.
      Graphical abstract image

      PubDate: 2017-11-16T09:21:36Z
       
  • Analysis of pulverized coal flame stabilized in a 3D laminar counterflow
    • Abstract: Publication date: March 2018
      Source:Combustion and Flame, Volume 189
      Author(s): Xu Wen, Kun Luo, Haiou Wang, Yujuan Luo, Jianren Fan
      In this paper, pulverized coal flames stabilized in a three-dimensional laminar counterflow configuration are simulated with detailed chemistry and the flame behaviors are analyzed in detail. Effects of radiation, coal particle mass flow rate and strain rate on the pulverized coal flame structure are investigated. The results show that the coal particles transported by the air stream tend to be ignited in a premixed combustion mode, which is followed by a non-premixed flame reaction zone, forming a typical double-flame type structure. The contribution of premixed combustion to the total heat release rate is sensitive to the studied operating conditions. Both volatile combustion and char off-gases combustion contribute to premixed combustion and their relative importance is influenced by the operating conditions. The pulverized coal combustion is significantly affected by radiative heat transfer. Without radiation, the reaction zone becomes thinner and the ignition is delayed. As the coal particle mass flow rate increases, the coal particles are ignited earlier and the combustion of char off-gases expands over a larger region. As the strain rate increases, both the premixed combustion share and the contribution of char off-gases combustion to the total heat release rate are decreased. For the extremely high strain rate case, the oxidizer can diffuse into the coal cloud from the oxidizer side to ignite the gas fuels at the fuel side (i.e. effect of oxidizer “leakage”). In order to properly consider the interphase heat transfers in gaseous flamelet models, a new tabulation method (compared to the conventional ones, e.g., Wen et al., (2017)) is proposed. The a priori analysis of the new tabulation method on different configurations shows that, compared to the conventional tabulation method, it better accounts for the heat transfers between the coal particle phase and gas phase, and can be applied to combustion systems with different oxidizer streams without introducing additional manifolds.

      PubDate: 2017-11-16T09:21:36Z
       
  • Regime identification from Raman/Rayleigh line measurements in partially
           premixed flames
    • Abstract: Publication date: March 2018
      Source:Combustion and Flame, Volume 189
      Author(s): Sandra Hartl, Dirk Geyer, Andreas Dreizler, Gaetano Magnotti, Robert S. Barlow, Christian Hasse
      Current methods for combustion regime characterization, such as the flame index, rely on 3D gradient information that is not accessible with available experimental techniques. Here, a method is proposed for reaction zone detection and characterization, which can be applied to instantaneous 1D Raman/Rayleigh line measurements of major species and temperature as well as to results of laminar and turbulent flame simulations, without the need for 3D gradient information. Several derived flame markers, namely the mixture fraction, the heat release rate, and the chemical explosive mode, are combined to detect and characterize premixed versus non-premixed reaction zones. The methodology is developed and evaluated using fully resolved simulation data from laminar flames. The fully resolved 1D simulation data are spatially filtered to account for the difference in spatial resolution between experiment and simulation. Then, starting from just temperature and major species, a constrained homogeneous constant pressure, constant temperature reactor calculation gives an approximation of the full thermochemical state at each sample location along the line. Finally, the chemical explosive mode and the heat release rate are calculated from this approximated state and compared to those calculated directly from the simulation data. As a further test, experimental uncertainty is superimposed onto the filtered numerical data to produce a Raman/Rayleigh equivalent state before running the constrained homogeneous reactor, and results are again compared. After successful tests using the numerical data, the approach is applied to Raman/Rayleigh line measurements from laminar counterflow flames and a mildly turbulent lifted flame. The results confirm that the reaction zones can be reliably detected and characterized using experimental data. Furthermore, the relative importance of premixed and non-premixed reaction zones within the same flame can be qualitatively assessed as demonstrated in the results.

      PubDate: 2017-11-16T09:21:36Z
       
  • Pressure wave evolution during two hotspots autoignition within end-gas
           region under internal combustion engine-relevant conditions
    • Abstract: Publication date: March 2018
      Source:Combustion and Flame, Volume 189
      Author(s): Haiqiao Wei, Ceyuan Chen, Gequn Shu, Xingyu Liang, Lei Zhou
      To further understand the intense pressure wave formation mechanism induced by multiple spontaneous ignition kernels under internal combustion engine-relevant conditions, a one dimensional code to solve the Navier–Stokes equations for reactive flows is adopted in this parametric study. The multiple autoignition kernels occurring in the end-gas region are simplified into two neighboring hotspots. Stoichiometric mixtures of PRF40/air (without an obvious Negative Temperature Coefficient (NTC) behavior, and showing a certain degree of anti-knock property) and PRF0/air (exhibiting a strong NTC behavior and weak anti-knock property) are both used to explore the effect of fuel characteristic. The initial pressure is 30.0 atm. Unburnt gas temperatures of 800.0 K and 890.0 K are below and within the NTC regime of stoichiometric PRF0/air under the initial pressure. The hotspots are modeled as sine waves in the initial temperature field. Different sine wave amplitudes are adopted to examine the effect of temperature inhomogeneity. Within a limited computational domain, a detonation wave tends to form under the initial conditions with a small interval between hotspots, a low initial temperature of the unburnt mixture and a large temperature inhomogeneity. During the formation of detonation wave, a process similar to the “explosion in the explosion” phenomenon that are found in previous experiments has been detected. Moreover, at the low initial temperature, a large interval between hotspots reduces the maximum intensity of pressure wave. In contrast, a wide interval increases the maximum pressure intensity at a higher initial temperature. The strongest pressure intensity induced by PRF0/air mixture autoignition is generally higher than that during the autoignition of PRF40/air mixture under the same initial condition, but a common intense pressure wave generation mechanism is shared. Furthermore, compared with the autoignition of the PRF40/air mixture, the distance for detonation formation within PRF0/air mixture becomes shorter.

      PubDate: 2017-11-16T09:21:36Z
       
  • Uncertainty reduction in laminar flame speed extrapolation for expanding
           spherical flames
    • Abstract: Publication date: March 2018
      Source:Combustion and Flame, Volume 189
      Author(s): Jialong Huo, Sheng Yang, Zhuyin Ren, Delin Zhu, Chung K. Law
      The quantification and reduction of the uncertainties in the extrapolation process in laminar flame speed measurements were studied using expanding spherical flames under positive Markstein length (Lb   > 0) conditions. The experimental and computational results were first compared showing their differences. The performance of three extrapolation formulas was then examined under a wide range of experimental conditions using various extrapolation ranges and pressures. It is found that the extrapolation uncertainty contains two sources of error, namely model error and random error. The individual effects of the upper and lower bounds of the extrapolation range under various Lb conditions were studied and the ratio of Lb and the flame radius range is found to be the controlling parameter of the model error. Small value of Lb/Rf allows the neglect of the model error by increasing the upper or the lower bound of the flame radius range. A new empirical parameter, Lb/Rf,new , was defined according to the experimental results, while it is recognized that the random error is mainly affected by the number of points for extrapolation and that at least 30 points should be used to remove the random error.

      PubDate: 2017-11-16T09:21:36Z
       
  • DNS of MILD combustion with mixture fraction variations
    • Abstract: Publication date: March 2018
      Source:Combustion and Flame, Volume 189
      Author(s): Nguyen Anh Khoa Doan, Nedunchezhian Swaminathan, Yuki Minamoto
      Direct numerical simulations of Moderate or Intense Low-oxygen Dilution combustion inside a cubical domain are performed. The computational domain is specified with inflow and outflow boundary conditions in one direction and periodic conditions in the other two directions. The inflowing mixture is constructed carefully in a preprocessing step and has spatially varying mixture fraction and reaction progress variable fields. Thus, this mixture includes a range of thermo-chemical states for a given mixture fraction value. The combustion kinetics is modelled using a 58-step skeletal mechanism including a chemiluminescent species, OH*, for methane–air combustion. The study of reaction zone structures in the physical and mixture fraction spaces shows the presence of ignition fronts, lean and rich premixed flames and non-premixed combustion. These three modes of combustion are observed without the typical triple-flame structure and this results from the spatio-temporally varying mixture fraction field undergoing turbulent mixing and reaction. The flame index and its pdf are analysed to estimate the fractional contributions from these combustion modes to the total heat release rate. The lean premixed mode is observed to be quite dominant and contribution of non-premixed mode increased from about 11% to 20% when the mean oxygen mole fraction in the inflowing mixture is reduced from about 2.7% to 1.6%. Also, the non-premixed contribution increases if one decreases the integral length scale of the mixture fraction field. All of these results and observations are explained on physical basis.

      PubDate: 2017-11-16T09:21:36Z
       
  • Experimental and numerical study of cap-like lean limit flames in
           H2-CH4-air mixtures
    • Abstract: Publication date: March 2018
      Source:Combustion and Flame, Volume 189
      Author(s): Zhen Zhou, Yuriy Shoshin, Francisco E. Hernández-Pérez, Jeroen A. van Oijen, Laurentius P.H. de Goey
      Lean limit flames of H2-CH4-air mixtures stabilized inside a tube with an inner diameter of 30 mm in a downward flow are studied experimentally and numerically. A transition from bubble-like flames, with a long decaying skirt, to cap-like flames with a sharp visible flame edge at the bottom is observed as the lean flammability limit is approached. This transition is accompanied by formation of a secondary weak flame front inside the cap-like flame. The CH* chemiluminescence distribution of the studied flames is recorded and the velocity field of the lean limit flames is measured using Particle Image Velocimetry (PIV). The flame temperature field is measured utilizing the Rayleigh scattering method. Numerical prediction with a mixture-averaged transport model and skeletal mechanism for CH4 qualitatively reproduces the above experimentally observed phenomena. The presence of negative flame displacement speed for the entire leading edge of the cap-like flames is numerically predicted and experimentally demonstrated. The secondary weak flame front is located in a region with reverse upward flow of the recirculation zone, which is found to support the propagation of the leading edge with a negative flame displacement speed. Furthermore, radiative heat loss has a significant influence on the lean flammability limit of the cap-like flames.

      PubDate: 2017-11-16T09:21:36Z
       
  • A simplified approach to simultaneous multi-scalar imaging in turbulent
           flames
    • Abstract: Publication date: March 2018
      Source:Combustion and Flame, Volume 189
      Author(s): Aaron W. Skiba, Campbell D. Carter, Stephen D. Hammack, Tonghun Lee
      This communication describes and demonstrates an approach to making simultaneous multi-scalar measurements with reduced equipment requirements. Specifically, this letter describes and demonstrates the ability to simultaneously acquire high-quality planar laser-induced fluorescence (PLIF) images of CH2O and either CH, OH, or a combination of CH and OH using a single Nd:YAG-pumped dye-laser system and two intensified cameras. Acquiring these images with common diagnostic equipment was facilitated by exciting strong transitions in the C–X (0,0) band of CH (near 314 nm) and in the A–X (0,0) and (1,1) bands of OH, which are spectrally adjacent. Additionally, Rayleigh scattering images were acquired simultaneously with these PLIF images using a second Nd:YAG laser and an un-intensified camera. However, it would be possible to conduct these measurements with a single, sufficiently energetic Nd:YAG laser.

      PubDate: 2017-11-16T09:21:36Z
       
  • Impact of non-ideal behavior on ignition delay and chemical kinetics in
           high-pressure shock tube reactors
    • Abstract: Publication date: March 2018
      Source:Combustion and Flame, Volume 189
      Author(s): Gandhali Kogekar, Canan Karakaya, Gary J. Liskovich, Matthew A. Oehlschlaeger, Steven C. DeCaluwe, Robert J. Kee
      Here, we study real gas effects on high-pressure combustion by comparing simulated and experimentally-measured shock tube ignition delay measurements for n-dodecane/O2/N2 mixtures. Experiments and simulations occur at conditions relevant to diesel engines: 40–80 atm, 774–1163 K, equivalence ratios of 1.0 and 2.0, and O2 concentrations of 13–21%. At these conditions the fuel, oxidizer and intermediate species may exist in a supercritical state during combustion, requiring a real gas equation of state to incorporate non-ideal effects on thermodynamics, chemical kinetics, and the resulting ignition characteristics. A constant-volume, adiabatic reactor model is developed to simulate the reflected shock tube experiments, and simulations compare results for real and ideal gas equations of state, with different expressions for reacting species’ activity concentrations. This paper focuses particularly on the cubic Redlich–Kwong equation of state and thermodynamically consistent chemical kinetic rate calculations based on it. Results demonstrate that the equation of state can have considerable influence on ignition delay times with increasing pressure, particularly in the negative temperature coefficient region. Additionally, the results establish important practices for incorporating real gas effects, namely that (i) the compressibilities of key species (i.e. those participating in rate-limiting reactions) are the appropriate way to screen for real gas effects, rather than the average mixture compressibility; and (ii) incorporating a real gas equation of state without also incorporating thermodynamically consistent chemical kinetics significantly under-predicts the magnitude of real gas effects.

      PubDate: 2017-11-08T21:15:04Z
       
  • Moving boundary modeling for solid propellant combustion
    • Abstract: Publication date: March 2018
      Source:Combustion and Flame, Volume 189
      Author(s): Nguyen Dat Vo, Min Young Jung, Dong Hoon Oh, Jung Soo Park, Il Moon, Min Oh
      Moving boundary problem frequently occurs in chemical engineering to describe various problems when the boundaries to describe domain keep changing. Combustion of solid propellant is a notorious example, which involves three phases (solid, condensed, and gas phase) and the positions of the interface between phases change in relation to the state of phases. In this study, moving boundary modeling approach was suggested to develop a rigorous mathematical model of solid propellant combustion. The mathematical model of a solid propellant was divided into three sub-models for each phase, which include conservation equations (mass, energy, and momentum conservation) and constitutive equations within the framework of moving interface. Coordinate transformation was carried out to achieve a fixed interface formulation from the moving interface problem, which leads to a fixed domain of each phase ranging from 0 to 1. In order to validate the feasibility of this approach, the mathematical model for the combustion of ammonium perchlorate was developed and dynamic simulation was performed with various operating conditions. The simulation results, including burning rate, temperature, mole fraction, and phase thickness, were compared with various reference data. Based on the comparison, it was concluded that the suggested moving boundary modeling approach can be used for the combustion of solid propellant and can accurately predict dynamic behaviors of the combustion.

      PubDate: 2017-11-08T21:15:04Z
       
  • Propagation of sub-atmospheric methyl formate flames
    • Abstract: Publication date: March 2018
      Source:Combustion and Flame, Volume 189
      Author(s): Dong J. Lee, Robert Roe Burrell, Fokion N. Egolfopoulos
      Laminar flame speeds of methyl formate/air mixtures were measured at sub-atmospheric pressures for which limited data exist. The experiments were carried out in the counterflow configuration at an unburned mixture temperature of 333 K. The flow velocities were measured using particle image velocimetry. Particle phase slip correction was applied to low-pressure data sets for which the density disparity between the flow tracers and the gaseous phase is notable. The data were modeled using two recently developed kinetic models of methyl formate oxidation, and significant disagreements were realized at all pressures especially under fuel-rich conditions. Additionally, the computed species profiles of CO and CO2 in the burner-stabilized flame configuration using the two models were found to differ significantly. Reaction path analysis revealed that the kinetics of CH2OCHO that is produced directly from the fuel affects the overall reactivity, and the attendant rate constants differ between the two models. The variation of laminar flame speed with pressure revealed also a different behavior between experiments and simulations. Further insight into the sources causing the observed discrepancies were investigated and it was determined that reactions involving formyl radical, methanol, and formaldehyde could also be responsible for the reduction in reactivity specifically under fuel-rich conditions.

      PubDate: 2017-11-08T21:15:04Z
       
  • Fuel vaporization: Effect of droplet size and turbulence at elevated
           temperature and pressure
    • Abstract: Publication date: March 2018
      Source:Combustion and Flame, Volume 189
      Author(s): Cameron Verwey, Madjid Birouk
      This paper presents an extensive experimental study regarding the effect of turbulence and droplet size on the evaporation rate of suspended monocomponent alkane droplets at elevated temperature and pressure conditions of up to 100 °C and 10 bar, respectively. Individual droplets of n-heptane and n-decane were suspended at the intersection of two crossed micro-fibers in the center of a fan-stirred spherical vessel. The droplet size was varied in the range between 110 and 730 µm. Eight axial fans generated a controlled turbulent flow field with quasi-zero mean velocity and turbulence intensity up to 1.5 m/s. The results reveal a linear relationship between the initial droplet size and its turbulent steady-state evaporation rate, where larger droplets evaporate at a faster rate than their smaller counterparts at all elevated temperature and pressure conditions. The normalized turbulent evaporation rate increases with pressure, whereas elevated temperature produces the opposite effect. The ratio of the Kolmogorov length scale to initial droplet diameter is shown to be of paramount importance for interpreting the effect of turbulence, as the normalized evaporation rate increases dramatically at lower values of this ratio under all test conditions. However, the ability of turbulence to enhance the vaporization rate vanishes when this ratio approaches unity, suggesting that only droplets which are initially larger than the smallest turbulent eddies experience enhanced evaporation. In addition, the widespread belief that turbulence enhances less volatile fuels more than their high volatility counterparts also depends on the ambient pressure and initial droplet size. The ability of turbulence to generate small-scale structures and, subsequently, the interaction of these eddies with the available fuel concentration gradient at the surface of the droplet governs the relationship between fuel type, initial droplet size, and ambient temperature and pressure. A turbulent Reynolds number or a vaporization Damköhler number is used to correlate turbulent droplet evaporation rates at all explored test conditions.

      PubDate: 2017-11-08T21:15:04Z
       
  • Theory of first-stage ignition delay in hydrocarbon NTC chemistry
    • Abstract: Publication date: February 2018
      Source:Combustion and Flame, Volume 188
      Author(s): Wenkai Liang, Chung K. Law
      The first-stage ignition delay of n-heptane/air mixtures is computationally studied using detailed mechanism and theoretically studied using eigenvalue analysis of simplified systems. Results show that the delay has a turnover behavior as temperature increases, being dominated by the competition of low-temperature branching and termination channels as well as the competition of forward and reverse reaction channels. As temperature increases to the intermediate range, the termination and reverse pathways result in a minimum in the delay, the state of which is theoretically derived. Simple analytical solutions for the delay as well as the species evolutions are presented to identify the rate constants that control the first-stage ignition and quantify the influence of the mixture composition, initial temperature and system pressure. It is further demonstrated that the above results also hold for n-octane/air and iso-octane/air mixtures.

      PubDate: 2017-11-08T21:15:04Z
       
  • Combustion characteristics of well-dispersed boron submicroparticles and
           plasma effect
    • Abstract: Publication date: February 2018
      Source:Combustion and Flame, Volume 188
      Author(s): Dan Yu, Chengdong Kong, Jiankun Zhuo, Qiang Yao, Shuiqing Li, Mengze Wang, Zhen-Yu Tian
      Boron is an attractive high-energy fuel additive. But it could not burn efficiently in practical systems due to its high ignition temperature and slow burning velocity. Finding methods to enhance the combustion of boron is desired. This work focused on the combustion characteristics of boron submicroparticles with and without plasma discharges in a hot environment supported by CH4/N2/O2 flat flame based on the optical diagnostics. The boron submicroparticles were dispersed by the nebulization method to control the agglomeration. The well-dispersed boron flame exhibited two different burning modes, depending on the ambient temperature. As the ambient temperature was above 1520 K, the boron flame showed definitely two-stage characteristics where the upstream of particle flow was yellow, corresponding to the first-stage flame, while the downstream was green and diffusive, corresponding to the second-stage flame. The first-stage and second-stage burn times were respectively in the range of 0.46–1.08 ms and 0.92–1.87 ms, as the ambient temperature decreased from 1752 K to 1520 K. The chemical kinetics-controlled mechanism was confirmed by the nearly linear size dependence of the burn time (d1 law). Nevertheless, as the ambient temperature was below 1520 K, the boron submicroparticles were partially burned or oxidized, exhibiting a mildly orange stream. This mild boron flame could be enhanced using a plasma discharge. The ignition delay time was shortened from 3.06 ms to 0.77 ms when the discharge was introduced at the ignition delay stage. The two-stage combustion characteristics occurred when the discharge was introduced at the combustion stage.

      PubDate: 2017-10-18T13:36:07Z
       
  • Laser ignition of CL-20 (hexanitrohexaazaisowurtzitane) cocrystals
    • Abstract: Publication date: February 2018
      Source:Combustion and Flame, Volume 188
      Author(s): Andrew McBain, Vasant Vuppuluri, Ibrahim E. Gunduz, Lori J. Groven, Steven F. Son
      Energetic cocrystals are a new class of materials that consist of two or more energetic molecules in a crystal structure. The multicomponent cocrystals can possess significantly different properties than either component, and therefore it is possible that their ignition behavior could be different than a physical mixture. In this paper, we report the time to ignition and reaction dynamics of various energetic materials, including select cocrystal materials under CO2 laser heating. An effort was made to minimize the amount of material used in comparison to previous laser ignition studies due to the limited availability of materials. The 1:1 molar Trinitrotoluene (TNT): Hexanitrohexaazaisowurtzitane (CL-20) produced by slurry or precipitation methods and 1:2 molar cyclotetramethylene-tetranitramine (HMX):CL-20 produced by slurry or resonant mixing methods were investigated along with the individual constituents and equivalent molar physical mixes. Pressed cylindrical pellets with diameters of 3.2 mm and heights between 1 and 2 mm were ignited at irradiances ranging from 310 to 1446 W/cm2. Visual imaging was performed with a high speed camera and ultraviolet (UV) spectral data were collected with a spectrometer coupled to a streak camera. Additionally, high speed schlieren imaging was performed to investigate the ignition dynamics prior to light generation. Specific species identified with the spectrometer include OH and CN, which coincide with the observation of the secondary flame as indicated from schlieren and first-light in the visual video record. All the cocrystallized materials ignited in a similar manner to CL-20 with comparable times to secondary flame formation, but the cocrystallized materials were found to have a shorter onset to gasification than the constituents or physical mixtures.

      PubDate: 2017-10-18T13:36:07Z
       
  • Autoignition of methyl propanoate and its comparisons with methyl
           ethanoate and methyl butanoate
    • Abstract: Publication date: February 2018
      Source:Combustion and Flame, Volume 188
      Author(s): Kamal Kumar, Chih-Jen Sung, Bryan W. Weber, Justin A. Bunnell
      This work reports an experimental and computational study on the autoignition characteristics of methyl propanoate under high pressure and low-to-intermediate temperature conditions. Comparisons to its next higher and lower methyl esters are also presented. The methyl propanoate experiments have been conducted using a rapid compression machine over compressed pressure and temperature ranges of 15‒45 bar and 899‒1103 K, respectively, as well as covering both stoichiometric and fuel lean conditions. In addition, the performance of four chemical kinetic models reported in the literature is assessed by comparing the experimental ignition delay times to numerical simulations. Sensitivity analysis is also performed to identify important reactions influencing the model predictions for ignition delay times. Further, we provide a comparison between the experimental ignition delay times of methyl propanoate and its next higher and lower homologs, namely methyl butanoate and methyl ethanoate, under selected conditions. An unusual trend in the autoignition response with respect to the carbon number is observed among methyl ethanoate and methyl propanoate. Methyl ethanoate is found to be more reactive than methyl propanoate in the low-temperature regime of 850‒950 K despite having a lower carbon number, with a crossover in reactivity above 950 K. Methyl butanoate is the most reactive among the three esters investigated which is consistent with the notion of increase in reactivity with increasing carbon number. This experimental and computational investigation provides insights into the homogenous autoignition chemistry associated with small unsaturated methyl ester compounds under engine relevant conditions.

      PubDate: 2017-10-18T13:36:07Z
       
  • Laser diagnostics and chemical kinetic analysis of PAHs and soot in
           co-flow partially premixed flames using diesel surrogate and oxygenated
           additives of n-butanol and DMF
    • Abstract: Publication date: February 2018
      Source:Combustion and Flame, Volume 188
      Author(s): Haifeng Liu, Peng Zhang, Xinlei Liu, Beiling Chen, Chao Geng, Bo Li, Hu Wang, Zhongshan Li, Mingfa Yao
      Effects of oxygenated fuels on soot reduction strongly depend on the base fuel. Interesting candidates from oxygenated fuels in this respect include both n-butanol and 2,5-dimethylfuran (DMF), because they have already been used in diesel engines recently. However, information is rather limited on n-butanol and DMF added into a diesel fuel surrogate in fundamental flames to investigate the mechanism of soot reduction. In the current work, both n-butanol and DMF was successively added into diesel surrogate (80% n-heptane and 20% toluene in volume, named as T20) in co-flow partially premixed flames. The effects of different oxygenated structures on polycyclic aromatic hydrocarbons (PAHs) and soot were investigated at the same oxygen weight fractions of 4% and the same volume fractions of 20%. The diagnostics on PAHs, soot volume fractions and soot sizes were conducted by using both laser-induced fluorescence (LIF) and two-color laser-induced incandescence (2C-LII). A combined detailed kinetic model (n-heptane/toluene/butanols/DMF/PAHs) has been obtained in order to clarify the chemical effects of the different oxygenated fuels on PAHs formation. Results show that the reduced toluene content due to the addition of oxygenated fuels is the dominant factor for the reduction of soot, as compared with the base fuel of T20. The oxygenated structure of n-butanol has a higher ability to reduce PAHs and soot as compared with the addition of DMF. This is due to the fact that the consumption of DMF leads to much formation of C5H5 which enhances the formation of PAHs and subsequent soot. However, the formation of PAHs can be inhibited remarkably as blending n-butanol because only small hydrocarbons like C2H2 and C3H3 etc. are formed. The formation rate of A4 is more similar to that of soot in comparison with the smaller ring aromatics. For the size of soot particles, the distribution range is shrunk from 19–70 nm for T20 to 20–40 nm for the addition of oxygenated fuels. As compared to the effects of oxygenated structures, DMF20 presents a little wider distribution on soot sizes than that of B16.8. Some larger soot particles are detected in DMF20 flame but cannot be found in B20 flame.

      PubDate: 2017-10-18T13:36:07Z
       
  • Effect of equivalence ratio on the thermal autoignition of aqueous ammonia
           ammonium nitrate monofuel
    • Abstract: Publication date: February 2018
      Source:Combustion and Flame, Volume 188
      Author(s): Bar Mosevitzky, Gennady E. Shter, Gideon S. Grader
      The combustion of a carbon-free nitrogen-based monofuel consisting of an aqueous ammonium hydroxide/nitrate (AAN) solution was studied at different equivalence ratios ranging between 0.6 and 12. The main oxidizing and reducing agents in AAN are nitric acid and ammonia, respectively. A combined differential thermal/barometric analysis (DTA/DBA) study was used to investigate the effect of the equivalence ratio on the autoignition temperature (AIT) of this monofuel. Increasing the equivalence ratio was found to increase the AIT value and reduce the energy generated during the ignition. Kinetic gas-phase simulations were used to calculate the theoretical AIT values. Good agreement was found at equivalence ratios close to unity. Rate of production and sensitivity analyses were performed to explore the reaction kinetics leading to the thermal auto-ignition. These indicated that reactions reducing nitrogen dioxide to nitrogen oxide and molecular oxygen inhibit the ignition, while reactions between ammonia and high oxidation state NO x producing amidogen promote it. The results of this study shed light on the influence of non-stoichiometric conditions on the thermal auto-ignition of AAN.

      PubDate: 2017-10-18T13:36:07Z
       
  • A rapid compression machine study of autoignition, spark-ignition and
           flame propagation characteristics of H2/CH4/CO/air mixtures
    • Abstract: Publication date: February 2018
      Source:Combustion and Flame, Volume 188
      Author(s): Changpeng Liu, Heping Song, Peng Zhang, Zhi Wang, Margaret S Wooldridge, Xin He, Guotao Suo
      Recent years have seen increased interest in dedicated exhaust gas recirculation (D-EGR) systems integrated with reciprocating engines. By dedicating one cylinder to operate at fuel rich conditions, hydrogen is generated for use in the remaining cylinders to optimize combustion and mitigate pollutant emissions. In this study, experiments were performed using a rapid compression machine (RCM) to investigate the effects of hydrogen and carbon monoxide on methane ignition and flame propagation at fuel rich and stoichiometric conditions relevant to D-EGR engines. The experiments were conducted over a range of temperatures (T = 860–1080 K) and equivalence ratios (ϕ = 1.0–1.5) and at a pressure of P = 20 atm. The results showed hydrogen addition had little effect on ignition delay time at lower temperatures (T < 950 K), but hydrogen significantly promoted ignition at higher temperatures (T > 950 K). Carbon monoxide had little effect on ignition delay times at all conditions studied. Combustion products were acquired using a fast sampling system and analyzed using gas chromatography. The results showed at ϕ = 1.4 the hydrogen mole fraction was a maximum of 8.0% of the products of rich combustion which was consistent with predictions based on chemical equilibrium calculations and model simulation results. High speed images were taken to quantify the impact of equivalence ratio on flame speed for spark-ignited mixtures in RCM experiments using mixtures similar to those expected in the D-EGR and stoichiometric cylinders. The results indicated the H2 in the D-EGR promoted the combustion process in the stoichiometric cylinder, increasing flame speed and decreasing combustion duration. Specifically, when the equivalence ratio in the D-EGR cylinder was ϕ = 1.3, the flame speed increased by about 40% compared with ϕ = 1.0. However, higher equivalence ratios reduced the flame speed and thus adversely affected combustion in the D-EGR cylinder. The results indicate stability and effectiveness of the combustion in the D-EGR cylinder could be a major concern for D-EGR natural gas engines, and optimizing the equivalence ratio of the D-EGR cylinder is critical for D-EGR to enhance combustion performance in the engine overall.

      PubDate: 2017-10-18T13:36:07Z
       
  • Influence of flame-holder temperature on the acoustic flame transfer
           functions of a laminar flame
    • Abstract: Publication date: February 2018
      Source:Combustion and Flame, Volume 188
      Author(s): Daniel Mejia, Maxence Miguel-Brebion, Abdula Ghani, Thomas Kaiser, Florent Duchaine, Laurent Selle, Thierry Poinsot
      The occurrence of combustion instabilities in high-performance engines such as gas turbines is often affected by the thermal state of the engine. For example, strong bursts of pressure fluctuations may occur at cold start for operating conditions that are stable once the engine reaches thermal equilibrium. This observation raises the question of the influence of material temperature on the response of flames to acoustic perturbations. In this study, we assess the influence of the temperature of the flame holder for a laminar flame. Both experiments and numerical simulations show that the Flame Transfer Function (FTF) is strongly affected by the flame-holder temperature. The key factors driving the evolution of the FTF are the flame-root location as well as the modification of the flow, which affects its stability. In the case of the cooled flame-holder, the formation of a recirculation zone is identified as the main impact on the FTF.

      PubDate: 2017-10-11T09:13:47Z
       
  • Evolution of turbulence through a broadened preheat zone in a premixed
           piloted Bunsen flame from conditionally-averaged velocity measurements
    • Abstract: Publication date: February 2018
      Source:Combustion and Flame, Volume 188
      Author(s): Timothy M. Wabel, Aaron W. Skiba, James F. Driscoll
      This work assesses two hypotheses that predict how turbulence properties vary within premixed turbulent flames that lie in the regime of Broadened Preheat-Thin Reaction layers. There have been few prior measurements describing flames in this regime. The authors previously found that very broad preheat layers were achieved for turbulence levels (u′/SL ) up to 243. Surprisingly, the reaction layer thickness did not increase, despite having Kolmogorov scales smaller than the laminar reaction layer thickness. A first hypothesis is that the turbulence decays in the preheat layer (as the temperature rises and viscous forces increase), so that the reaction layer sees only a small fraction of the initial turbulence. It follows that this turbulence decay might be responsible for the observed non-linear bending of the curve of turbulent burning velocity versus turbulence level. A second hypothesis is that the total turbulent kinetic energy does not decrease significantly in the preheat zone; instead, the small eddies decay and cause the integral scale to increase. Conditional averages are required to assess these two hypotheses. Fluorescence imaging identified the reaction zone boundary and particle image velocimetry diagnostics were applied simultaneously. The velocity measurements were conditioned on η, the distance to the upstream boundary of the reaction zone in each image. Conditioned measurements of turbulent kinetic energy, average eddy rotational velocity, strain rate, enstrophy, and integral length scale were computed through the flame. Results indicate that the turbulence level does not decrease within the broad preheat layers, and therefore the first hypothesis is not valid. In fact, the turbulence level within the entire burner core does not vary appreciably. However, the second hypothesis was supported by the measurements, since the integral scale increased by 50% across the preheat layer. The total turbulent kinetic energy did not decrease significantly. One explanation for this result is that small eddies are dissipated in the preheat zone.

      PubDate: 2017-10-11T09:13:47Z
       
  • Scalar dissipation rates in a turbulent partially-premixed dimethyl
           ether/air jet flame
    • Abstract: Publication date: February 2018
      Source:Combustion and Flame, Volume 188
      Author(s): Frederik Fuest, Robert S. Barlow, Gaetano Magnotti, Jeffrey A. Sutton
      This paper presents the gradient structure of a turbulent partially premixed dimethyl ether (DME)/air jet flame operating at a jet Reynolds number of 29,300. Temperature and mixture fraction profiles from Raman/Rayleigh/CO-LIF line measurements are used to determine one-dimensional scalar dissipation rates at six axial locations. A major focus of the current work is to assess the effects of experimental artifacts, including spatial resolution, noise, and dimensionality, on the accuracy of the derived scalar dissipation rate. Two-dimensional probability density functions (PDFs) of the mixture fraction gradients are used to determine possible clipping effects due to insufficient spatial resolution. This resolution limit is compared to values determined from one-dimensional dissipation spectra and scaling laws. Spatial resolution also is investigated using laminar flame calculations in conjunction with optical-blur filters representing the experimental setup. The impact of noise is treated by error propagation methods. Monte Carlo simulations and experimental data from laminar flames are used to verify and validate the models used to predict noise propagation for the measurements of the absolute gradients, squared gradients, and scalar dissipation rates. Gradient and scalar dissipation rate detection limits and contribution from apparent dissipation (due to noise effects) are presented as functions of measurement signal-to-noise ratios. A noised lognormal function is introduced to investigate the impact of noise on derived PDFs and corresponding statistical moments of the measured scalar gradients and the scalar dissipation rate within the turbulent flame. Results from the turbulent flame measurements are presented in the form of scatter plots and conditional statistics to examine turbulence-chemistry interaction and develop a database for model assessment. Specifically, the results are compared to laminar flame calculations over a broad range of strain rates with multi-component and unity Lewis number transport assumptions. This comparison is used to assess the relevance of differential diffusion effects on scalar dissipation rates in the turbulent flame.

      PubDate: 2017-10-11T09:13:47Z
       
  • Influence of flash boiling spray on the combustion characteristics of a
           spark-ignition direct-injection optical engine under cold start
    • Abstract: Publication date: February 2018
      Source:Combustion and Flame, Volume 188
      Author(s): Jie Yang, Xue Dong, Qiang Wu, Min Xu
      Flash boiling occurs when liquid fuel is injected into an ambient environment below its saturation pressure. Compared to non-flash-boiling (liquid) spray, flash-boiling spray features a two-phase flow that constantly generates vapor bubbles inside the liquid spray thus results in much smaller drop size and faster evaporation, which are favorable for direct-injection gasoline engine combustion. In this study, the combustion characteristics of flash boiling spray was investigated under cold start condition in a spark-ignition direct-injection (SIDI) optical gasoline engine. Three spray conditions, including liquid, transitional flash boiling, and flare flash boiling spray were studied for comparison. Optical access into the combustion chamber was realized by a quartz insert on the piston. The crank angle resolved color flame images as well as in-cylinder pressure of 150 consecutive cycles were recorded simultaneously. From the color images, the blue flame generated by excited molecules and the yellow flame resulted from soot radiation was identified and analyzed separately alongside with the cylinder pressure. Results show an improvement of indicated mean effective pressure-gross (IMEPg) and a reduction of soot formation with the introduction of flash boiling spray under cold start condition. The emission measurement shows that the formation of soot is positively related to particulate number (PN) emissions. Further study on the transient development of in-cylinder flames shows that flash boiling spray leads to higher propagation rate of the blue flame, and a subsequent statistical analysis shows a positive correlation between IMEP and the propagation rate of the blue flame.

      PubDate: 2017-10-11T09:13:47Z
       
  • Effect of Lewis number on ball-like lean limit flames
    • Abstract: Publication date: February 2018
      Source:Combustion and Flame, Volume 188
      Author(s): Zhen Zhou, Yuriy Shoshin, Francisco E. Hernández-Pérez, Jeroen A. van Oijen, Laurentius P.H. de Goey
      The lean limit flames for three different fuel compositions premixed with air, representing three different mixture Lewis numbers, stabilized inside a tube in a downward flow are examined by experiments and numerical simulations. The CH* chemiluminescence distribution in CH4–air and CH4–H2–air flames and the OH* chemiluminescence distribution in H2–air flames are recorded in the experiments. Cell-like flames are observed for the CH4–air mixture for all tested equivalence ratios. However, for CH4–H2–air and H2–air flames, ball-like lean limit flames are observed. Flame temperature fields are measured using Rayleigh scattering. The experimentally observed lean limit flames are predicted qualitatively by numerical simulation with the mixture-averaged transport model and skeletal mechanism of CH4. The results of the simulations show that the entire lean limit flames of CH4–H2–air and H2–air mixtures are located inside a recirculation zone. However, for the lean limit CH4–air flame, only the leading edge is located inside the recirculation zone. A flame structure with negative flame displacement speed is observed for the leading edges of the predicted lean limit flames with all three different fuel compositions. As compared with 1D planar flames, the fuel transport caused by convection is less significant in the present 2D lean limit flames for the three different fuel compositions. For the trailing edges of the three predicted lean limit flames, a diffusion dominated flame structure is observed.

      PubDate: 2017-10-11T09:13:47Z
       
  • Experimental observation of pulsating instability under acoustic field in
           downward-propagating flames at large Lewis number
    • Abstract: Publication date: February 2018
      Source:Combustion and Flame, Volume 188
      Author(s): Sung Hwan Yoon, Longhua Hu, Osamu Fujita
      According to previous theory, pulsating propagation in a premixed flame only appears when the reduced Lewis number, β(Le-1), is larger than a critical value (Sivashinsky criterion: 4(1 + 3 ) ≈ 11), where β represents the Zel'dovich number (for general premixed flames, β ≈ 10), which requires Lewis number Le > 2.1. However, few experimental observation have been reported because the critical reduced Lewis number for the onset of pulsating instability is beyond what can be reached in experiments. Furthermore, the coupling with the unavoidable hydrodynamic instability limits the observation of pure pulsating instabilities in flames. Here, we describe a novel method to observe the pulsating instability. We utilize a thermoacoustic field caused by interaction between heat release and acoustic pressure fluctuations of the downward-propagating premixed flames in a tube to enhance conductive heat loss at the tube wall and radiative heat loss at the open end of the tube due to extended flame residence time by diminished flame surface area, i.e., flat flame. The thermoacoustic field allowed pure observation of the pulsating motion since the primary acoustic force suppressed the intrinsic hydrodynamic instability resulting from thermal expansion. By employing this method, we have provided new experimental observations of the pulsating instability for premixed flames. The Lewis number (i.e., Le ≈ 1.86) was less than the critical value suggested previously.

      PubDate: 2017-10-11T09:13:47Z
       
  • Transition condition and control mechanism of subatmospheric flame spread
           rate over horizontal thin paper sample
    • Abstract: Publication date: February 2018
      Source:Combustion and Flame, Volume 188
      Author(s): Jun Fang, Xuan-ze He, Kai-yuan Li, Jing-wu Wang, Yong-ming Zhang
      The horizontal flame spread over paper samples was investigated using a subatmospheric cabin with varied O2 concentration. The 25 kPa is found to be a clear turning point for the flame illumination and structure, radiative heat flux and flame spread rate (FSR), which leads to the transition boundary between the extinction limits and power law regions. In the extinction limits (non-linear) region below 25 kPa, the oxygen partial pressure is low with a small Da number. Consequently, the flame spread is gas phase kinetics controlled, resulting in low burning rate, low radiative heat loss and weak buoyancy, and thus the FSR is more sensitive to the oxygen concentration while less sensitive to the ambient pressure. In the power law (linear) region above 25 kPa, in contrast, the oxygen partial pressure is high and the Da number is large, and the flame spread is heat transfer controlled, which weakens the dependence of FSR on oxygen concentration and enhances the dependence on air pressure.

      PubDate: 2017-10-11T09:13:47Z
       
 
 
JournalTOCs
School of Mathematical and Computer Sciences
Heriot-Watt University
Edinburgh, EH14 4AS, UK
Email: journaltocs@hw.ac.uk
Tel: +00 44 (0)131 4513762
Fax: +00 44 (0)131 4513327
 
Home (Search)
Subjects A-Z
Publishers A-Z
Customise
APIs
Your IP address: 54.91.38.173
 
About JournalTOCs
API
Help
News (blog, publications)
JournalTOCs on Twitter   JournalTOCs on Facebook

JournalTOCs © 2009-2016