Journal Cover Combustion and Flame
  [SJR: 3.12]   [H-I: 124]   [126 followers]  Follow
    
   Full-text available via subscription Subscription journal
   ISSN (Print) 0010-2180
   Published by Elsevier Homepage  [3120 journals]
  • A reduced thermal diffusion model for H and H2
    • Abstract: Publication date: May 2018
      Source:Combustion and Flame, Volume 191
      Author(s): Jason Schlup, Guillaume Blanquart
      This work details the development of a new, reduced thermal diffusion model. The proposed model derives from the thermal diffusion model of Chapman and Cowling (1970). In its derivation, a set of mixture-averaged like approximations are made, which results in the number of operations being reduced from O ( n 2 ) to sub-linear, where n is the number of species in the chemical model. With these approximations, the new, reduced model thermal diffusion coefficients can be calculated independently for each species. The model is validated against multicomponent thermal diffusion cases using multiple fuel and diluent mixtures at various pressures, temperatures, and equivalence ratios. The resulting reduced model thermal diffusion fluxes agree well with the multicomponent values, with a multiplicative scaling factor identified using a least squares regression. Unstretched laminar flame speeds are compared using the multicomponent and newly developed models. Finally, an a posteriori comparison in a turbulent configuration shows excellent agreement of both the mean and fluctuations of the thermal diffusion coefficients.

      PubDate: 2018-02-05T09:13:59Z
       
  • Chemical-looping combustion of plastic wastes for in situ inhibition of
           dioxins
    • Abstract: Publication date: May 2018
      Source:Combustion and Flame, Volume 191
      Author(s): Haibo Zhao, Jinxing Wang
      Polychlorinated dibenzo-p-dioxins (PCDDs) and dibenzofurans (PCDFs) as toxic by-products are inevitably emitted during conventional air incineration processes of chlorine containing wastes (typically, plastic wastes). The presence of O2 in the conventional waste incineration not only participates in carbon gasification and rearrangement as an oxygen source during the de novo synthesis process of PCDD/Fs, but also promotes chlorination through generating more active Cl2 via Deacon reaction. Chemical looping combustion (CLC), which creates an O2-free atmosphere in the fuel reactor, was proposed to dispose plastic wastes. Comparative experiments [conventional PW incineration vs. in situ gasification-chemical looping combustion (iG-CLC) using CaO-decorated Fe2O3/Al2O3 as oxygen carrier] were conducted to measure the distribution properties of 17 toxic PCDD/Fs congeners. The total amount and toxic equivalency quantity of PCDD/Fs have been reduced by 94 % and 89 %, respectively, in iG-CLC. The absence of O2 in fuel reactor and lower Cl2 yield (due to the restriction of Deacon reaction and effective dechlorination by CaO) lead to the significant inhibition of the PCDD/Fs formation. Chlorine substitution modelling also demonstrated that the chlorine substitution probabilities for the formation of 7 toxic congeners of PCDDs and 10 toxic congeners of PCDFs are significantly reduced in iG-CLC.

      PubDate: 2018-02-05T09:13:59Z
       
  • Propagation of a reaction front in a narrow sample of energetic material
           with heat losses: Chaotic regimes, extinction and intermittency
    • Abstract: Publication date: May 2018
      Source:Combustion and Flame, Volume 191
      Author(s): Vadim N. Kurdyumov, Vladimir V. Gubernov
      The influence of heat-losses on the flame dynamics in narrow samples of energetic material is investigated numerically. The model is reduced to a one-dimensional form with the flame-sheet approximation applied for the reaction rate. Both the steady-state solutions and its linear stability analysis are treated analytically. A typical C-shaped response curve is found for the dependence of the flame-propagation velocity on the heat-loss parameter, with solutions along the lower branch of slower flames being always unstable. It is found that a part of the upper branch of the C-shaped response curve is also unstable and the Poincaré–Andronov–Hopf bifurcation takes place at a certain value of heat-loss intensity even if the steady state solution is stable under the corresponding adiabatic conditions. The numerical simulations show that an increase in heat-losses induces, for sufficiently high Zel’dovich numbers, the Feigenbaum’s cascade of period doubling bifurcations after which a chaotic dynamics is setting in. The chaotic dynamics precedes the flame extinction occurring for the further increase of the heat-loss parameter which, nevertheless, remains significantly lower than the steady extinction limit dictated by the C-shaped response curve. Apparently, the parametric dependence of the extinction time in these cases is also irregular with appreciable disparities in magnitude. Finally, the intermittency effect is detected slightly below the extinction limit with irregular dynamics alternating by apparently periodic stages. These results may be important for the flammability limits theory and practical fire safety applications.

      PubDate: 2018-02-05T09:13:59Z
       
  • New roles of metal–organic frameworks: Fuels for aluminum-free energetic
           thermites with low ignition temperatures, high peak pressures and high
           activity
    • Abstract: Publication date: May 2018
      Source:Combustion and Flame, Volume 191
      Author(s): Hui Su, Jichuan Zhang, Yao Du, Pengcheng Zhang, Shenghua Li, Tao Fang, Siping Pang
      Aluminum-based thermites are widely used in gas generators, propulsions, explosives, and welding because these materials can release a large amount of stored energy on combustion. However, one of the biggest problems in the traditional aluminum-based thermite systems is excessive oxidation of the aluminum particle before combustion, resulting in a decrease of the active aluminum content. Here, we report a new-concept aluminum-free thermites based on an energetic metal–organic framework [Cu(atrz)3(NO3)2] n (MOF(Cu), atrz = 4,4′-azo-1,2,4-triazole) as a fuel. Compared with the traditional aluminum-based thermites, these new thermites exhibit superior performances such as low electrostatic discharge sensitivities, low ignition temperatures, high heats of reaction, high peak pressures, high activity, and production of very few solid residues. It is anticipated that this work will open a new field for the application of MOFs, while laying the groundwork for the development of new energetic materials.

      PubDate: 2018-02-05T09:13:59Z
       
  • Leading edge dynamics of lean premixed flames stabilized on a bluff body
    • Abstract: Publication date: May 2018
      Source:Combustion and Flame, Volume 191
      Author(s): Dan Michaels, Ahmed F. Ghoniem
      This paper examines the dynamics of the flame leading edge in a laminar premixed CH4/air flame stabilized on a bluff body in a channel. Harmonic fluctuations and step velocity change are used to simulate the flame response to acoustic oscillations, which are of primary importance in the study of thermo-acoustic instabilities. We use a fully resolved unsteady two-dimensional code with detailed chemistry and species transport, with coupled heat transfer to the bluff body. Calculations were conducted with different equivalence ratios, body materials, and steady state inlet velocity with step or harmonic perturbations. Results reveal that the flame leading edge dynamics displays a peak response around St = 0.5 suggesting that the leading edge motion is mainly due to the advection of appropriate ignition conditions as a result of the excitement of the wake recirculating flow. There is considerable augmentation of the flame wrinkles generated by the flame leading edge motion as result of the flow–flame interaction. Additionally, we show that a flame that anchors on average further upstream leads to stronger damping of the shear layer vortices and thus weaker vortex-flame interaction and heat release fluctuations. Hence, we identify two different mechanisms by which the flame leading edge location and oscillation amplitude impact heat release fluctuations. The study suggests a stronger dependence of the overall flame wrinkling and heat release fluctuations on the flame leading edge dynamics than recognized previously and the potential role it plays in combustion dynamics.

      PubDate: 2018-02-05T09:13:59Z
       
  • A wide-range experimental and modeling study of oxidation and combustion
           of n-propylbenzene
    • Abstract: Publication date: May 2018
      Source:Combustion and Flame, Volume 191
      Author(s): Yue-Xi Liu, Bing-Yin Wang, Jun-Jie Weng, Dan Yu, Sandra Richter, Thomas Kick, Clemens Naumann, Marina Braun-Unkhoff, Zhen-Yu Tian
      The oxidation of n-propylbenzene (NPB) was studied in a jet-stirred reactor (JSR) equipped with online GC and GC–MS for temperatures ranging between 700 and 1100 K, at φ = 0.4–2.0. In addition, laminar flame speeds were measured at p = 1, 3 and 6 bar at a preheat temperature of T = 473 K, and ignition delay times in a shock tube device behind reflected shock waves, for stoichiometric mixtures at around p = 16 bar. Mole fraction profiles of 25 intermediates including six species, namely 1-propenylbenzene, 2-propenylbenzene, α-methylstyrene, naphthalene, indene, and benzofuran were observed additionally. With φ increasing, NPB consumption shifts to higher temperatures, and the reaction temperature zone becomes broader. Based on the experimental measurements and on new calculations of the rate constants for the H-abstractions from NPB with OH, an updated kinetic model involving 292 species and 1919 reactions was developed with a reasonable agreement with the measured species profiles, flame speed values, and ignition delay times. Rate of production analysis reveals that NPB consumption is generally governed by CH bond cleavage to form three A1C3H6 radicals, which mostly transform to styrene under rich condition and to benzaldehyde under lean condition. Compared to the aromatics formed in the oxidation of two other aromatic C9 fuels, 1,3,5-trimethylbenzene and 1,2,4-trimethylbenzene, NPB exhibits to be the most reactive fuel with the least aldehyde intermediates. Moreover, the present model gives a reasonable agreement with the literature-reported ignition delay times and JSR data. These results can improve the understanding of the oxidation and combustion of NPB as a surrogate fuel constituent for kerosene and diesel.

      PubDate: 2018-02-05T09:13:59Z
       
  • Near-field flow stability of buoyant methane/air inverse diffusion flames
    • Abstract: Publication date: May 2018
      Source:Combustion and Flame, Volume 191
      Author(s): Xuren Zhu, Xi Xia, Peng Zhang
      Experiment and simulation were performed to investigate buoyant methane/air inverse diffusion flames, with emphasis on the near-field flow dynamics under non-reacting and reacting conditions. In the non-reacting flow, the initial shear flow and the buoyancy effect induce opposite-direction vortices, which interact with each other and cause flow instability similar to the mechanism forming the von Karman vortex street. The instability is greatly intensified at around unity Richardson number, when the two vortices are comparably strong. In the reacting flows, the density gradient is reversed due to chemical heat release and so is the buoyancy-induced vortex that has the same direction with the vortex of the initial shear flow. As a result, the buoyancy-induced vorticity generation would facilitate the growth of the initial shear layer, thus the near-field flow remains stable. However, the growing shear flow would eventually lead to the development of the Kelvin–Helmholtz instability in the far field.

      PubDate: 2018-02-05T09:13:59Z
       
  • Analysis of high-temperature oxidation of wood combustion particles using
           tandem-DMA technique
    • Abstract: Publication date: May 2018
      Source:Combustion and Flame, Volume 191
      Author(s): Heikki Lamberg, Olli Sippula, Jorma Joutsensaari, Mika Ihalainen, Jarkko Tissari, Anna Lähde, Jorma Jokiniemi
      Soot particles from combustion sources are known to have significant environmental effects. One of the major sources of soot particles is small-scale wood combustion, and there is an urgent need to develop methods to abate soot emissions from these appliances. The oxidation of soot particles in the combustion chamber is essential for the control of harmful emissions. Thus, the oxidation characteristics of wood combustion particles were studied in a high-temperature tandem differential mobility analyzer using various types of wood combustion particles. These studied particles were generated with a pellet boiler, operated under normal and deteriorated combustion conditions, and with a wood stove. Electron microscopy and chemical analyses, combined with thermodynamic equilibrium calculations, were carried out for interpreting the effect of ash species on the particle shrinking observed at various temperatures. Finally, kinetic parameters for assessing wood combustion soot oxidation under conditions representing the post-combustion zone of small appliances were derived. Pellet combustion soot particles were fully oxidized at 710 °C while wood stove particles were not fully oxidized at 900 °C. This difference is a result of the high alkali metal content in pellet combustion particles, which presumably leads to catalytically enhanced oxidation of soot particles. However, the wood stove particles with low alkali metal content also had lower oxidation temperatures compared to previously studied diesel combustion particles. According to the fitting parameters, about 70% particle reduction could be achieved with one second residence time at 800 °C. These results can be utilized in the development of strategies and technologies to abate soot emissions from small-scale wood-fired combustion appliances.
      Graphical abstract image

      PubDate: 2018-02-05T09:13:59Z
       
  • Stabilization of ultra-lean hydrogen enriched inverted flames behind a
           bluff–body and the phenomenon of anomalous blow–off
    • Abstract: Publication date: May 2018
      Source:Combustion and Flame, Volume 191
      Author(s): Carmen Jiménez, Dan Michaels, Ahmed F. Ghoniem
      This paper presents a fundamental study of ultra-lean flames stabilized behind a thin, highly conducting metallic rectangular bluff body acting as a flame holder. Using high fidelity numerical simulations, we reproduce a phenomenon observed experimentally, showing that in this configuration steady hydrogen–methane flames can exist at equivalence ratios below the flammability limit associated with planar unstrained flames with the same hydrogen–methane proportion. These ultra–lean hydrogen–enriched mixtures exhibit a distinct stabilization mechanism compared to pure methane flames: they stabilize in the form of inverted closed V or U flames farther away from the flame holder as the inflow reactants velocity is reduced, leading eventually to blow-off for sufficiently small velocities. Conversely, as the reactants flow rate is increased, the flames anchor closer to the flame holder, and surprisingly no blow-off is observed at high velocities. This response is shown to be linked to the presence of hydrogen in the fuel mixture and its large diffusivity, which results in locally richer mixtures in the strained, curved flame base.

      PubDate: 2018-02-05T09:13:59Z
       
  • Numerical simulations of microgravity ethylene/air laminar boundary layer
           diffusion flames
    • Abstract: Publication date: May 2018
      Source:Combustion and Flame, Volume 191
      Author(s): Jorge Contreras, Jean-Louis Consalvi, Andrés Fuentes
      Microgravity ethylene/air laminar boundary layer diffusion flames were studied numerically. Two oxidizer velocities of 250 and 300 mm/s and three fuel injection velocities of 3, 4, and 5 mm/s were considered. A detailed gas-phase reaction mechanism, which includes aromatic chemistry up to four rings, was used. Soot kinetics was modeled by using a pyrene-based model including the mechanisms of nucleation, heterogeneous surface growth and oxidation following the hydrogen-abstraction acetylene-addition (HACA) mechanism, polycyclic aromatic hydrocarbon (PAH) surface condensation and soot particle coagulation. Radiative heat transfer from CO, CO2, H2O and soot was calculated using the discrete ordinate method (DOM) coupled to a wide-band correlated-k model. Model predictions are in quantitative agreement with the available experimental data. Model results show that H and OH radicals, responsible for the dehydrogenation of sites in the HACA process, and pyrene, responsible for soot nucleation and PAH condensation, are located in a thin region that follows the stand-off distance. Soot is produced in this region and, then, is transported inside the boundary layer by convection and thermophoresis. The combustion efficiency is significantly lower than 1 and is reduced as the flow residence time increasing, confirming that these sooting micro-gravity diffusion flames are characterized by radiative quenching at the flame trailing edge. In particular, this quenching phenomenon explains the increase in flame length with the oxidizer velocity observed in previous experimental studies. The effects of using approximate radiative-property models, namely the optically-thin approximation and gray approximations for soot and combustion gases, were assessed. It was found that the re-absorption and the spectral dependence of combustion gases and soot must be taken into account to predict accurately temperature, soot volume fraction, flame geometry and flame quenching.

      PubDate: 2018-02-05T09:13:59Z
       
  • Exploring enhanced reactivity of nanosized titanium toward oxidation
    • Abstract: Publication date: May 2018
      Source:Combustion and Flame, Volume 191
      Author(s): Nikita V. Muravyev, Konstantin A. Monogarov, Alexey N. Zhigach, Mikhail L. Kuskov, Igor V. Fomenkov, Alla N. Pivkina
      Oxidation of nanosized titanium (nano-Ti), a promising component of energetic compounds, was studied using thermogravimetry and differential scanning calorimetry. To obtain more comprehensive insight into the kinetics and mechanism of oxidation, a variety of complementary non-isothermal and isothermal thermoanalytical experiments were performed. In sharp contrast to micron-sized titanium, oxidation of nano-Ti commences at much lower temperatures (150 °C instead of 650 °C) with profoundly lower activation energies (152 ± 3 kJ mol−1 and 220 ± 3 kJ mol−1, respectively). Moreover, reaction kinetics for nano-Ti obeys the logarithmic law, while in the case of micron-sized Ti kinetics is described by the 2D-diffusion model. At the microscopic level, the observed kinetics of nano-Ti oxidation is explained by switching of the limiting reaction stage to short-circuit diffusion of oxygen through the titanium oxide. This process is promoted by the increase of porosity upon initial water loss and the blocking of pores in the course of oxidation. The kinetic model proposed for oxidation of nano-Ti was independently benchmarked against the isothermal kinetics (zero heating rate limit) and ignition data (high heating rates). Our model provides reliable kinetics of the nano-Ti oxidation, which is valid for both storage and application conditions.

      PubDate: 2018-02-05T09:13:59Z
       
  • Concurrent flame spread over discrete thin fuels
    • Abstract: Publication date: May 2018
      Source:Combustion and Flame, Volume 191
      Author(s): JeanHyuk Park, Jared Brucker, Ryan Seballos, Byoungchul Kwon, Ya-Ting T. Liao
      An unsteady two-dimensional numerical model was used to simulate concurrent flame spread over paper-like thin solid fuels of discrete configurations in microgravity (0 g with 20 cm/s) and in normal gravity (1 g). An array of ten 1 cm-long fuel segments was uniformly distributed in the flow direction (0 g) or in the vertical direction (1 g). A hot spot ignition source was applied at the upstream leading edge of the first fuel segment. The separation distance between the fuel segments was a parameter in this study, ranging from 0 (corresponding to a continuous fuel) to 3 cm. Using this setup, the burning characteristics, spread rate of the flame base, and the solid burning rate were examined. The flame base spread rates in both 1 g and 0 g cases increase with the separation distance. This is due to the flame jumping across the gaps. For the solid burning rate, the dependency on the separation distance is different in 1 g and 0 g cases. At a flow velocity of 20 cm/s in 0 g, the flame reaches a limiting length and the flame length is approximately the same for all fuel configurations. As the separation distance increases, the heating length (the fuel area exposed to the flame) decreases, resulting in a decreasing total heat input and a decreasing solid burning rate. In 1 g, the flame is long and extends to the last fuel segment before the first fuel segment burns out. This suggests that the heating length is approximately the same in all simulated cases (∼total fuel length). However, the flame standoff distance decreases when the separation distance increases. This results in an increasing total heat input and an increasing solid burning rate. Terrestrial experiments were conducted to validate the 1 g model. The experimental results agreed reasonably with the model predictions of burning characteristics, burn durations, and flame spread rates.

      PubDate: 2018-02-05T09:13:59Z
       
  • Experimental and kinetic modeling investigation on pyrolysis and
           combustion of n-butane and i-butane at various pressures
    • Abstract: Publication date: May 2018
      Source:Combustion and Flame, Volume 191
      Author(s): Wei Li, Guoqing Wang, Yuyang Li, Tianyu Li, Yan Zhang, Chuangchuang Cao, Jiabiao Zou, Chung K. Law
      Butane is the smallest alkane with normal and branched isomers. To obtain insight into the effects of fuel structure and pressure on its intermediate-to-high temperature combustion chemistry, flow reactor pyrolysis and laminar burning velocities of the butane isomers were investigated at various pressures. In the pyrolysis experiments, species profiles were measured as function of the heating temperature at 0.04, 0.2 and 1 atm using synchrotron vacuum ultraviolet photoionization mass spectrometry. Laminar burning velocities of both n-butane/air and i-butane/air mixtures were measured at 298 K and 1–10 atm using spherically expanding flames. It was observed that both the pyrolysis and combustion behaviors of the butane isomers were influenced by the fuel structures. A detailed kinetic model of butane isomers was developed and validated against the new experimental data. Both rate of production analysis and sensitivity analysis were performed to give insight into the chemistry of butane pyrolysis and combustion. In the flow reactor pyrolysis, the weaker primary-tertiary CC bond than the primary-secondary and secondary-secondary CC bonds leads to lower initial decomposition temperatures of i-butane than n-butane. Under both pyrolysis and combustion conditions, the reaction pathways towards C2 and C3 species pool are emphasized for the decomposition of n-butane and i-butane, respectively. The more abundant production of C3 precursors explains the higher concentrations of benzene in the i-butane pyrolysis, while the higher laminar burning velocities of n-butane/air mixtures at all investigated pressures are mainly attributed to the easy production of H atom from n-butane decomposition and the dominance of the reactive C2 chemistry. Moreover, the i-butane/air flames exhibit stronger pressure dependence than the n-butane/air flames. The model was further validated against a wide range of experimental data in the literature, including ignition delay times and species profiles in flow reactor pyrolysis and oxidation, shock tube pyrolysis and oxidation, jet-stirred reactor oxidation and laminar premixed flames.

      PubDate: 2018-02-05T09:13:59Z
       
  • Experimental and modeling studies of small typical methyl esters
           pyrolysis: Methyl butanoate and methyl crotonate
    • Abstract: Publication date: May 2018
      Source:Combustion and Flame, Volume 191
      Author(s): Yitong Zhai, Beibei Feng, Wenhao Yuan, Chengcheng Ao, Lidong Zhang
      In order to examine in detail the effect of C  C bond on the combustion chemistry of biodiesel fuels, two C4 fatty acid methyl esters (FAMEs) were investigated, namely methyl butanoate (MB) and methyl crotonate (MC). Pyrolysis experiments of these two FAMEs at 30, 150 and 780 Torr were conducted in a flow reactor over the temperature range of 773–1323 K, using gas chromatography-mass spectrometry. A number of pyrolysis species including C1 to C4 hydrocarbons, oxygenated products, esters and aromatics were observed and identified. A comprehensive kinetic model for MB and MC combustion was developed, and applied to validate against the new experimental data. In this work, peak mole fraction of benzene, as well as other unsaturated hydrocarbons in MC pyrolysis, was found to be in slightly higher amounts in comparison with that of MB. Kinetic modeling analysis revealed that the dominant formation pathway of benzene was the self-combination reaction of propargyl radical. Furthermore, the model was also validated against the previous experimental data on MB and MC combustion, including oxidation in jet stirred reactor, pyrolysis in shock tube and laminar premixed flame. This study suggests that the effect of C  C double bond in FAMEs might give rise to a growing tendency of initial PAH and soot precursors in the whole thermal decomposition process.

      PubDate: 2018-02-05T09:13:59Z
       
  • Cool diffusion flames of butane isomers activated by ozone in the
           counterflow
    • Abstract: Publication date: May 2018
      Source:Combustion and Flame, Volume 191
      Author(s): Adamu Alfazazi, Abdullah Al-Omier, Andrea Secco, Hatem Selim, Yiguang Ju, S. Mani Sarathy
      Ignition in low temperature combustion engines is governed by a coupling between low-temperature oxidation kinetics and diffusive transport. Therefore, a detailed understanding of the coupled effects of heat release, low-temperature oxidation chemistry, and molecular transport in cool flames is imperative to the advancement of new combustion concepts. This study provides an understanding of the low temperature cool flame behavior of butane isomers in the counterflow configuration through the addition of ozone. The initiation and extinction limits of butane isomers’ cool flames have been investigated under a variety of strain rates. Results revealed that, with ozone addition, establishment of butane cool diffusion flames was successful at low and moderate strain rates. iso-Butane has lower reactivity than n-butane, as shown by higher fuel mole fractions needed for cool flame initiation and lower extinction strain rate limits. Ozone addition showed a significant influence on the initiation and sustenance of cool diffusion flames; as ozone-less cool diffusion flame of butane isomers could not be established even at high fuel mole fractions. The structure of a stable n-butane cool diffusion flame was qualitatively examined using a time of flight mass spectrometer. Numerical simulations were performed using a detailed chemical kinetic model and molecular transport to simulate the extinction limits of the cool diffusion flames of the tested fuels. The model qualitatively captured experimental trends for both fuels and ozone levels, but over-predicted extinction limits of the flames. Reactions involving low-temperature species predominantly govern extinction limits of cool flames. The simulations were used to understand the effects of methyl branching on the behavior of n-butane and iso-butane cool diffusion flames.

      PubDate: 2018-02-05T09:13:59Z
       
  • Combustion of sonochemically-generated Ti−Al−B nanopowders in a
           premixed methane/air dust flame
    • Abstract: Publication date: May 2018
      Source:Combustion and Flame, Volume 191
      Author(s): Michael R. Weismiller, Zachary J. Huba, Albert Epshteyn, Brian T. Fisher
      Sonochemically-generated Ti−Al−B reactive mixed-metal nanopowders were tested by seeding them into a premixed, fuel-lean (Φ = 0.63), methane/air flame to investigate their combustion characteristics. Tests were conducted on powders with and without cryogenic milling. The attenuation of a diode laser beam was measured to calculate the time-resolved concentration of the powder in the flame. Radiant heat flux was measured with three gauges at different heights (1 cm, 6 cm, and 12 cm) along the axis of the flame. Flame spectra were collected to monitor chemiluminescence of intermediate species, and a multi-wavelength pyrometry method was applied to the spectra to calculate the temperature of the hot particulates in the flame. Commercially available metal powders were tested as a benchmark. These included micron-scale aluminum, nano-scale aluminum, micron-scale boron, and inert nano-scale alumina powders. The spectra from flames seeded with the sonochemically-generated Ti−Al−B powder show strong chemiluminescence from the BO2, an indicator of boron oxidation. Peak temperatures measured with flame pyrometry were approximately 2100 K, which is below the vaporization point of B2O3. The radiant heat flux from the seeded flame increased with concentration faster for the Ti−Al−B material than for any of the commercial powders, suggesting a greater gravimetric power density. Based on these results, the Ti−Al−B powders show promising combustion and heat-release characteristics, and therefore warrant further examination as a high-performance solid fuel.

      PubDate: 2018-02-05T09:13:59Z
       
  • Advancements on the propagation mechanism of a detonation wave in an
           obstructed channel
    • Abstract: Publication date: May 2018
      Source:Combustion and Flame, Volume 191
      Author(s): Mark Kellenberger, Gaby Ciccarelli
      Utilising a recently developed technique, involving high-speed schlieren photography shot through a soot-coated glass sheet, new details of the propagation of combustion waves in obstructed channels have been revealed. In this study, a channel equipped with 50% blockage ratio obstacles was used to examine the repeated detonation initiation and failure processes responsible for the large CJ detonation velocity deficits observed in the quasi-detonation regime. Using a combination of simultaneous schlieren images, soot foil records, and average velocity measurements, experiments were carried out in mixtures of stoichiometric hydrogen–oxygen at initial pressures between 9 kPa and 30 kPa in a 3.66 m long, 7.62 cm by 2.54 cm rectangular cross-section channel. Results indicate continuous detonation propagation through the core of the channel for sufficiently reactive mixtures, while fast-flame propagation occurred for weaker mixtures which do not exhibit detonation initiation at the obstacle face. Two unique propagation modes, one symmetrical and one asymmetrical about the channel centreline, were found to occur at the DDT limit that resulted in average combustion wave velocities between that of the fast-flame and product speed of sound. Local detonation initiation at the obstacle face, following shock reflection, was found to be governed by both the incident shock strength and the distance between the lead shock and trailing flame. For sufficient shock strength and shock-flame spacing, the resulting detonation waves produce a shock interaction at the channel centreline that results in the formation of an axially propagating overdriven detonation that decays in strength with distance. For these quasi-detonations, the average wave velocity over many obstacles is governed by the frequency of these detonation initiation events. These centreline detonation initiation events were typically symmetrical across the channel width, producing a bell-shaped cellular region on the soot foil. However, asymmetrical detonation initiation events, originating at one sidewall, were also observed that produced a narrow vertical band of fine cells resulting from the head-on collision of the detonation wave propagating transversely through the compressed gas region between the lead shock and flame.

      PubDate: 2018-02-05T09:13:59Z
       
  • Laminar flame characteristics and chemical kinetics of
           2-methyltetrahydrofuran and the effect of blending with isooctane
    • Abstract: Publication date: May 2018
      Source:Combustion and Flame, Volume 191
      Author(s): Xibin Wang, Xiangshan Fan, Kangkang Yang, Jingshan Wang, Ximing Jiao, Zhiyi Guo
      2-methyltetrahydrofuran (MTHF) has been considered as a potential biofuel candidate for the renewable feedstock and attractive properties. In this study, the spherically propagating flames of the MTHF/isooctane-air mixtures in different blending ratios were investigated at elevated temperatures and pressures over equivalence ratios of 0.8–1.5 in a constant volume chamber using high-speed photography technique. Laminar burning velocities were calculated through nonlinear method and correlated by mixing rules as a function of initial temperature, pressure, equivalence ratio, and blending ratio. To investigate the influence of the potential interaction between MTHF and isooctane, a detailed model was established by merging the models of the two fuels and validated against laminar burning velocity, ignition delay times and flame structure. The laminar flame velocities of MTHF were observed to increase with initial temperature while the flame velocities decrease with increase in pressure. Kinetic analysis indicates that the major consumption pathways for MTHF are through H abstraction reactions at 2 and 5 sites. The H abstraction reactions at methyl group are less competitive. Unsaturated hydrocarbons and aldehydes are produced in high concentration and are the main stable intermediates of MTHF combustion. The laminar burning velocity is sensitive to the reactions of small radicals and some intermediates such as propylene and ethylene. The laminar burning velocities of MTHF/isooctane blends increase with MTHF blending ratio. The influence of blending ratio on laminar burning velocity should attribute to the chemical factors, rather than thermal or diffusion factors. MTHF oxidation generates less proportion of propylene and higher fractions of H, OH than isooctane, reflecting positive chemical effects on laminar burning velocity as MTHF blended. According to the instability analysis, the Markstein length and critical radius shows similar flame instability among different blending ratios.

      PubDate: 2018-02-05T09:13:59Z
       
  • A new post-processing technique for analyzing high-dimensional combustion
           data
    • Abstract: Publication date: May 2018
      Source:Combustion and Flame, Volume 191
      Author(s): Ehsan Fooladgar, Christophe Duwig
      This paper introduces a novel post-processing technique for analyzing high dimensional combustion data. In this technique, t-Distributed Stochastic Neighbor Embedding (t-SNE) is used to reduce the dimensionality of the combustion data with no prior knowledge while preserving the similarity of the original data. Multidimensional combustion datasets are from premixed and non-premixed laminar flame simulations and measurements of a series of well documented piloted flames with inhomogeneous inlets. The resulting reduced manifold is visualized by scatter plots to reveal the global and local structure of the data (manual labeling). Unsupervised clustering algorithms are then utilized for post-processing the t-SNE manifold in order to develop an automatic labeling process.

      PubDate: 2018-02-05T09:13:59Z
       
  • Coupling heat transfer and large eddy simulation for combustion
           instability prediction in a swirl burner
    • Abstract: Publication date: May 2018
      Source:Combustion and Flame, Volume 191
      Author(s): Christian Kraus, Laurent Selle, Thierry Poinsot
      Large eddy simulations (LES) of combustion instabilities are often performed with simplified thermal wall boundary conditions, typically adiabatic walls. However, wall temperatures directly affect the gas temperatures and therefore the sound speed field. They also control the flame itself, its stabilization characteristics and its response to acoustic waves, changing the flame transfer functions (FTFs) of many combustion chambers. This paper presents an example of LES of turbulent flames fully coupled to a heat conduction solver providing the temperature in the combustor walls. LES results obtained with the fully coupled approach are compared to experimental data and to LES performed with adiabatic walls for a swirled turbulent methane/air burner installed at Engler-Bunte-Institute, Karlsruhe Institute of Technology and German Aerospace Center (DLR) in Stuttgart. Results show that the fully coupled approach provides reasonable wall temperature estimations and that heat conduction in the combustor walls strongly affects both the mean state and the unstable modes of the combustor. The unstable thermoacoustic mode observed experimentally at 750 Hz is captured accurately by the coupled simulation but not by the adiabatic one, suggesting that coupling LES with heat conduction solvers within combustor walls may be necessary in other configurations in order to capture flame dynamics.

      PubDate: 2018-02-05T09:13:59Z
       
  • A model of tetrahydrofuran low-temperature oxidation based on
           theoretically calculated rate constants
    • Abstract: Publication date: May 2018
      Source:Combustion and Flame, Volume 191
      Author(s): Yann Fenard, Adrià Gil, Guillaume Vanhove, Hans-Heinrich Carstensen, Kevin M. Van Geem, Phillip R. Westmoreland, Olivier Herbinet, Frédérique Battin-Leclerc
      The first detailed kinetic model of the low-temperature oxidation of tetrahydrofuran has been developed. Thermochemical and kinetic data related to the most important elementary reactions have been derived from ab initio calculations at the CBS-QB3 level of theory. A comparison of the rate constants at 600 K, obtained from these calculations with values estimated using recently published rate rules for alkanes, sometimes show differences of several orders of magnitude for alkylperoxy radical isomerizations, HO2-eliminations, and oxirane formations. The new model satisfactorily reproduces previously published ignition delay times obtained in a rapid-compression machine and in a shock tube, as well as numerous product mole fractions measured in a jet-stirred reactor at low to intermediate temperatures and in a low-pressure laminar premixed flame. To highlight the most significant reaction pathways, flow-rate and sensitivity analyses have been performed with this new model.

      PubDate: 2018-02-05T09:13:59Z
       
  • Impact of direct integration of Analytically Reduced Chemistry in LES of a
           sooting swirled non-premixed combustor
    • Abstract: Publication date: May 2018
      Source:Combustion and Flame, Volume 191
      Author(s): Anne Felden, Eleonore Riber, Benedicte Cuenot
      Large-eddy simulation (LES) of a swirl-stabilized non-premixed ethylene/air aero-engine combustor experimentally studied at DLR is performed, with direct integration of Analytically Reduced Chemistry (ARC). Combined with the Dynamic Thickened Flame model (DTFLES), the ARC-LES approach does not require specific flame modeling assumptions and naturally adapts to any flow or geometrical complexity. To demonstrate the added value of the ARC methodology for the prediction of flame structures in various combustion regimes, including formation of intermediate species and pollutants, it is compared to a standard tabulation method (FPI). Comparisons with available measurements show an overall good agreement with both chemistry approaches, for the velocity and temperature fields. However, the flame structure is shown to be much improved by the inclusion of explicitly resolved chemistry with ARC. In particular, the ability of ARC to respond to strain and curvature, and to intrinsically contain CO/O2 chemistry greatly influences the flame shape and position, as well as important species production and consumption throughout the combustion chamber. Additionally, since both chemistry descriptions are able to account for intermediate species such as OH and C2H2, soot formation is also investigated using a two-equations empirical soot model with C2H2 as the sole precursor. It is found that, in the present configuration, this precursor is strongly impacted by differential diffusion and partial premixing, not included in the FPI approach. This leads to a strong under-prediction of soot levels by about one order of magnitude with FPI, while ARC recovers the correct measured soot concentrations.

      PubDate: 2018-02-05T09:13:59Z
       
  • Regression rates and burning characteristics of boron-loaded paraffin-wax
           solid fuels in ducted rocket applications
    • Abstract: Publication date: May 2018
      Source:Combustion and Flame, Volume 191
      Author(s): Syed Alay Hashim, Srinibas Karmakar, Arnab Roy, Suneel Kumar Srivastava
      Boron based solid fuels have remained attractive for solid fuel ducted rocket (SFDR) applications since long due to their potential to release high energy on combustion. However, boron's energetic potential has not been successfully harnessed even till date in any practical combustion system. In view of this, present investigation is focused on utilizing boron nanoparticles embedded in paraffin-wax in various proportions (5–20% by weight) as solid fuel. To evaluate its performance an opposed flow burner (OFB) is used in presence of gaseous oxygen (GOX) and the oxidizer mass flux (Gox) varies from 5 to 30 kg/m2 s to estimate its regression rates relative to paraffin-wax. Burning process of boron loaded samples was captured using a high-speed camera to understand the ejection trajectory of the particles/agglomerates. Further, different material characterization techniques were employed on the pre- and post-burnt samples to understand the chemical and morphological changes and interlink that with overall burning characteristics. The active boron content determined from thermogravimetric analysis of pre-burnt sample, ejected agglomerates and post-burnt residue were 78.8, 34 and 18.7% respectively which confirm the combustion of boron nanoparticles up to a significant extent in the present OFB configuration. The present study may be helpful in futuristic application of hybrid propellant-based gas generator for SFDR systems.

      PubDate: 2018-02-05T09:13:59Z
       
  • Elucidating the flame chemistry of monoglyme via experimental and modeling
           approaches
    • Abstract: Publication date: May 2018
      Source:Combustion and Flame, Volume 191
      Author(s): Wenyu Sun, Tao Tao, Ruzheng Zhang, Wei Li, Jiuzhong Yang, Bin Yang
      This study is concerning the flame chemistry of monoglyme (CH3OCH2CH2OCH3), an oxygenated compound recognized as a clean diesel additive and an ignition improver. Speciation diagnosis was performed for two low-pressure premixed flames fueled by monoglyme with different equivalence ratios (ϕ = 1.0 and 1.5) using the technique of photoionization molecular-beam mass spectrometry (PI-MBMS). Dozens of flame intermediates including some reactive species were quantitatively probed from the monoglyme flames. A kinetic model was proposed for the first time for the combustion of this fuel and validated against the flame structure measurements. By combining experimental observations and modeling interpretations, it has been revealed that under flame conditions, the fuel consumption is dominated by hydrogen abstractions from the central (–CH2CH2-) moiety of monoglyme. Subsequent β-scissions of the resulting fuel radical lead to the formation of fuel-specific intermediates, methyl vinyl ether and methoxy acetaldehyde. The species pool detected in monoglyme flames differs much from that of dimethyl ether (DME, CH3OCH3) flames, though a monoglyme molecule is symmetrically composed of two DME fuel radicals. This could be attributed to the presence of the central carbon-to-carbon (CC) bond in monoglyme. Further modeling analyses suggest that the CC contents together with the stoichiometry of fuel mixtures can impact the concentrations of benzene precursors under premixed flame conditions.

      PubDate: 2018-02-05T09:13:59Z
       
  • Oxidation of cyclopentadienyl radical with molecular oxygen: A theoretical
           study
    • Abstract: Publication date: May 2018
      Source:Combustion and Flame, Volume 191
      Author(s): Artem D. Oleinikov, Valeriy N. Azyazov, Alexander M. Mebel
      The potential energy surface for the reaction of cyclopentadienyl radical with O2 has been studied using ab initio calculations at the CCSD(T)-F12/cc-pVTZ-f12//B3LYP/6-311G(d,p) level and the RRKM-Master Equation approach has been employed to compute reaction rate constants and product branching ratios at various temperatures and pressures pertinent to combustion. The results show that at low temperatures from 500 to 800–1250 K (depending on pressure), the reaction predominantly forms a collisionally-stabilized C5H5 OO complex and then, the thermalized complex rapidly decomposes back to the reactants establishing a C5H5 + O2/C5H5 OO equilibrium. At higher temperatures, typically above 1000 K, the mechanism is different and the C5H5 + O2 reaction proceeds to form various bimolecular products. Cyclopentadienone C5H4O + OH are predicted to be the predominant product (63.5–83.3%). Relatively minor products include H2CCHCHC(H)O + CO (20-3%), vinylketene + HCO (12-2%), and OC(H)CHCHCHCO + H (3-5%), which are formed via the OC(H)CHCHCHC(H)O intermediate residing in a deep potential well, and highly endothermic C5H5O + O (up to 6.5% at 2500 K) produced directly by the OO bond cleavage in the initial complex. The calculated rate constants for the formation of C5H4O + OH and C5H5O + O are shown to be independent of pressure above 800 K, but the rate constants for the reaction channels resulting in CO, HCO, and H eliminations show some pressure dependence in the low end of the high-temperature regime and decrease with the pressure growing from 10 to 100 atm. The CO2 loss channel leading to the formation of 1,3-butadien-1-yl C4H5 is shown to be negligible. The total reaction rate constants at all considered pressures from 0.03 to 100 atm merge at 1375 K and show no pressure dependence at higher temperatures, as only the bimolecular products are formed. Overall, the rate constant of the C5H5 + O2 reaction at combustion-relevant temperatures is predicted to be very slow, 10−16-10−15 cm3 molecule−1 s−1, that is typically ∼5 orders of magnitude lower than those for the oxidation reactions of cyclopentadienyl with OH and O(3P). A comparison of the rates of the C5H5 + O2/OH/O reactions allowed us to conclude that molecular oxygen can play only a small role in oxidation and removal of five-member rings in combustion and only when the concentration of O2 is orders of magnitude higher than the concentrations of O and OH.

      PubDate: 2018-02-05T09:13:59Z
       
  • Experimental and modeling study on the effects of dimethyl
           methylphosphonate (DMMP) addition on H2, CH4, and C2H4 ignition
    • Abstract: Publication date: May 2018
      Source:Combustion and Flame, Volume 191
      Author(s): Olivier Mathieu, Waruna D. Kulatilaka, Eric L. Petersen
      Dimethyl methylphosphonate (C3H9O3P, DMMP) is considered both a surrogate of Sarin, a chemical weapon of mass destruction, and a fire suppressant. In this study, new ignition delay time measurements were taken in a heated shock tube for a mixture of DMMP/O2 in 99% Ar and for diluted mixtures of H2, CH4, and C2H4 doped with DMMP. These data can be used to validate and refine a detailed kinetics model for DMMP. Results showed that the DMMP addition did not modify the ignition delay times obtained with the C2H4 mixture, while a strong promoting effect was observed with CH4. Results were compared to detailed kinetics models from the literature, and it was shown that these models need to be significantly improved. A tentative model was assembled based on a modern hydrocarbon mechanism from the group of Curran and on updated phosphorus chemistry and thermochemistry recently proposed. Computations using this tentative model exhibited the need to revise several aspects of the DMMP oxidation chemistry, such as the rate coefficients of the reactions describing the rapid thermal fragmentation of DMMP and potentially the interactions between the phosphorus chemistry and radicals and molecules in the C2 chemistry.

      PubDate: 2018-02-05T09:13:59Z
       
  • Performance of iodine oxides/iodic acids as oxidizers in thermite systems
    • Abstract: Publication date: May 2018
      Source:Combustion and Flame, Volume 191
      Author(s): Tao Wu, Xizheng Wang, Peter Y. Zavalij, Jeffery B. DeLisio, Haiyang Wang, Michael R. Zachariah
      Iodine oxides are of interest as biocidal components in energetic application such as thermites due to their high energy release and biocidal agent delivery. In this study, various iodine oxides/iodic acids, including I2O5, HI3O8 and HIO3, were employed as oxidizers in thermite systems. Their decomposition behaviors were studied using a home-made time resolved temperature-jump/time-of-flight mass spectrometer (T-Jump/TOFMS), which identified a single step decomposition for all oxides at high heating rates (∼105 K/s). In addition, both nano-aluminum (nAl, ∼80 nm) and nano-tantalum (nTa, <50 nm) were adopted as the fuel in order to fully understand how iodine containing oxidizers react with the fuel during ignition. The ignition and reaction process of nAl-based and nTa-based thermites were characterized with T-Jump/TOFMS, and their combustion properties were evaluated in a constant-volume combustion cell and compared to a traditional thermite system (nAl/CuO). The ignition temperatures of nAl-based thermites using these oxidizers were all very close to the melting point of aluminum (∼660 °C), which suggests that the mobility of the aluminum core dominats the ignition/reaction and the gaseous oxygen released from the decomposition of the oxidizer does not participate in the ignition until the molten aluminum is available. Unlike nAl-based thermites, the ignition temperatures of nTa-based thermites are lower than the oxygen release temperatures from the corresponding bare oxidizers. All nTa-based thermites ignited prior to the release of gas phase oxygen. In this case, a condensed phase reaction mechanism is proposed to dominate the ignition process. Moreover, combustion cell tests results show that nAl/a-HI3O8 has the highest pressurization rate and peak pressure and shortest burn time, and since it also has an iodine content of ∼75% as high as I2O5 on a per mass basis, this material may be a very promising candidate in biocidal application.

      PubDate: 2018-02-05T09:13:59Z
       
  • Hydrogen abstraction ratios: A systematic iPEPICO spectroscopic
           investigation in laminar flames
    • Abstract: Publication date: May 2018
      Source:Combustion and Flame, Volume 191
      Author(s): Dominik Krüger, Patrick Oßwald, Markus Köhler, Patrick Hemberger, Thomas Bierkandt, Yasin Karakaya, Tina Kasper
      The radicals produced by hydrogen abstraction in the initial fuel decomposition step are essential in combustion kinetics, but their experimental detection is very challenging. Imaging photoelectron photoion coincidence spectroscopy enables the detection and identification of even these isomeric radicals. Laminar low-pressure (40 mbar) hydrogen flames doped with different alkanes and alkenes are investigated systematically with the goal to identify the formation pathways and the fate of fuel radicals formed in hydrogen abstraction reactions. The abstraction reactions of primary, secondary, tertiary, and vinylic H atoms were never target of a systematic, direct semiquantitative investigation in a flame environment and this paper describes such a study for the first time. Performing the measurements at the vacuum ultraviolet beamline located at the Swiss Light Source enables isomer-selective detection of reactive radical species by imaging photoelectron photoion coincidence spectroscopy. For unambiguous identification of several isomeric radicals, threshold photoelectron spectra were compared with reference photoelectron spectra. H-abstraction ratios of isomeric radicals were determined and compared to literature reaction barriers and rate coefficients. In addition to the quantitative information, the peak positions of the profiles of radicals formed by hydrogen abstraction or addition to the fuel molecules as function of distance from the burner show faster H-abstraction for unbranched alkanes and alkenes than for branched fuels and faster H-addition than H-abstraction, respectively.

      PubDate: 2018-02-05T09:13:59Z
       
  • The effects of nozzle geometry and equivalence ratio on a premixed
           reacting jet in vitiated cross-flow
    • Abstract: Publication date: May 2018
      Source:Combustion and Flame, Volume 191
      Author(s): Matthew D. Pinchak, Vincent G. Shaw, Ephraim J. Gutmark
      The effects of nozzle geometry, jet equivalence ratio (ϕj), and momentum flux ratio (J) on the flow field, stability characteristics, and flame topology of a premixed ethylene–air jet injected transverse to a vitiated cross-flow are investigated experimentally. Cross-flow conditions of 900 K and 100 m/s were chosen to simulate the environment of a secondary combustor in a staged combustion system. The dependence of the flame liftoff height on J suggests that at these conditions the flame stabilization process is flame-propagation controlled rather than autoignition assisted. A circular nozzle and high aspect ratio slotted nozzle of identical exit area were investigated for jet to cross-flow momentum flux ratios ranging from 5 to 65 for jet equivalence ratios of up to ϕj = 5.0. High-speed particle image velocimetry was utilized to study the time-averaged flow field and OH* chemiluminescence was used to capture time-averaged and instantaneous features of the flame behavior. The nozzle geometry was determined to have a significant effect on RJICF flame stability, with substantially expanded blow-out limits for the slotted nozzle. Enhanced operability of the high aspect ratio slotted nozzle was shown to be attributable to the substantially larger and stronger recirculation zone on the leeward side of the jet when compared to the circular nozzle. This area is characterized by a more disperse region of elevated vorticity levels, resulting in the entrainment of more hot combustion products with a longer residence time in the recirculation zone, which in turn provides a stronger and more stable ignition source to the oncoming, unburned reactants. A correlation for the isothermal JICF trajectory was modified to account for gas expansion effects and found to satisfactorily capture the jet trajectory for both the non-reacting and reacting slotted nozzle. The jet trajectory was demonstrated to be independent of ϕj, whereas the jet flame penetration decreases as ϕj increases, indicating that where the flame situates itself is determined by the local mixture concentration rather than changes in the flow field. Nevertheless, ϕj was also found to have an effect on the mean flow field, with slightly higher magnitudes of reversed flow velocity and an increase in mean recirculation zone length observed as the fuel content of the jet is increased.

      PubDate: 2018-02-05T09:13:59Z
       
  • Precise measurement of the length-scale effects on the flame propagation
           velocity using a compact annular-stepwise-diverging-tube (ASDT)
    • Abstract: Publication date: May 2018
      Source:Combustion and Flame, Volume 191
      Author(s): Hun Young Kim, Nam II Kim
      Recently, an annular-stepwise-diverging-tube (ASDT) was developed to investigate the length-scale effects on the flame propagation velocity (FPV), and the overall FPV of a specific fuel was described as a continuous surface in the physical domain of the concentration-length-velocity. In this study, the ASDT burner was improved in three aspects. First, all flame images taken during the experiment were analyzed to improve the experimental resolution and reliability. Second, an appropriate condition for the flow rate reduction was determined. Third, the thickness of the step-unit was reduced from 10 mm to 3 mm. Precise FPV values depending on the length-scale could be obtained using a compact ASDT burner.

      PubDate: 2018-02-05T09:13:59Z
       
  • Flame propagation and combustion modes in end-gas region of confined space
    • Abstract: Publication date: April 2018
      Source:Combustion and Flame, Volume 190
      Author(s): Lei Zhou, Lijia Zhong, Jianfu Zhao, Dongzhi Gao, Haiqiao Wei
      Flame propagation is investigated in a designed experimental apparatus equipped with a perforated plate in a constant volume chamber. The effect of the perforated plate is to generate a rapidly accelerating flame based on Bychkov work (Bychkov et al. 2008), in which the flame across the obstacle will becomes a strong jet flame. The experiment was conducted with a hydrogen–air mixture at different conditions. In this work, six different turbulent flame propagation and combustion modes were clearly observed at various conditions in our designed experiment. In the presence of perforated plate, the turbulent flame formed through the perforated plate may perform six types of turbulent propagations at the end gas regime. These types form through the interaction between the flame and the shock or acoustic wave and because of the limited effect of the wall in confined space. The six forms are as follows: (1) a normal flame propagation with a low flame front tip velocity and combustion rate; (2) a weak pulsation propagation with weak fluctuation due to the acoustic wave; (3) a pulsation propagation only with a visible reflected shock wave; (4) a strong pulsation propagation with a forward shock wave and shock reflection; (5) a continuously accelerating flame propagation due to auto-ignition of the unburned mixture between flame front and shock wave, which also leads to strong pressure oscillation; and (6) a violent pulsation propagation with a multi-shock wave leading to end gas auto-ignition with large pressure oscillation.

      PubDate: 2018-02-05T09:13:59Z
       
  • Effect of suboxides on dynamics of combustion of aluminum nanopowder in
           water vapor: Numerical estimate
    • Abstract: Publication date: April 2018
      Source:Combustion and Flame, Volume 190
      Author(s): Vladimir B. Storozhev, Alexander N. Yermakov
      The model of dynamics of combustion of a pre-prepared mixture of aluminum nanoparticles and water vapor under adiabatic conditions is considered and the results of numerical calculations are presented. The formation dynamics of the condensed phase of aluminum oxide (c-phase) in the form of aerosol particles is modeled with allowance for the homogeneous nucleation of Al2O3 molecules, and the processes of condensation, evaporation and coagulation. The condensation process is considered with participation of unstable Al2O3 gas molecules, and heterogeneous processes involving gaseous and adsorbed suboxides of aluminum: AlO, AlO2 Al2O2, and atomic oxygen. The participation of aluminum suboxides has been found to accelerate the combustion of aluminum. Besides, it increases the depth of aluminum burning and rises the final temperature of the process. The changes of temperature, phase composition, and concentrations of gaseous and adsorbed components during the combustion are reported. The mechanism of combustion and c-phase formation is detailed, and the mutual influence of reactions in the gas phase, heterogeneous reactions and the process of aerosol particle formation during combustion are analyzed.

      PubDate: 2017-12-26T18:01:06Z
       
  • Laser initiation of RDX crystal slice under ultraviolet and near-infrared
           irradiations
    • Abstract: Publication date: April 2018
      Source:Combustion and Flame, Volume 190
      Author(s): Zhonghua Yan, Wei Liu, Yong Jiang, Yongyong Xie, Chuanchao Zhang, Jingxuan Wang, Guorui Zhou, Liang Yang, Xia Xiang, Xiaoyang Li, Wei Liao, Haijun Wang, Jinshan Li, Bisheng Tan, Ming Huang, Zongwei Yang, Zhijie Li, Li Li, Miao Li, Xiaodong Yuan, Xiaotao Zu
      Interactions between energetic materials and laser beams of specific frequency are of both scientific and engineering importance for understanding and manipulating the laser-induced ignition of energetic crystals. In this work, we investigate the effects of laser irradiation of variable energy densities from ultraviolet (355 nm) to near-infrared (1064 nm) on the ignition properties of a well-treated cyclotrimethylenetrinitramine (RDX) crystal slice (9.3 mm × 9.1 mm × 2.9 mm). The laser-induced damage and initiation dynamics were characterized in detail by using optical microscopy as well as an ultrafast pump-probe imaging technique at nanoseconds. It discloses that both ignition probabilities (p) of 355 and 1064 nm change exponentially with increasing laser fluence (H, J/cm2): p ( 355 ) = 1 − e − 1 . 72 * ( H − 3.981 ) , p ( 1064 ) = 1 − e − 0 . 74 * ( H − 7.898 ) . RDX crystals can be more easily ignited under ultraviolet laser irradiation than by near-infrared one due to its different absorptions (α(355) = 1.6306 cm−1, and α(1064) = 0.5313 cm−1) and the effects of photochemical initiation mechanism. The damage induced by either 355 nm or 1064 nm laser exhibits three typical morphologies varied with laser exposure. In addition, damage generated by ultraviolet laser appears on the incident surface of the crystal slice, while it is mainly located on the exit surface when being ignited by near-infrared laser. Our work sheds light on the dedicated interaction mechanism between energetic crystals and laser beams of various frequency and energy density.
      Graphical abstract image

      PubDate: 2017-12-26T18:01:06Z
       
  • Ab initio kinetics on low temperature oxidation of iso-pentane: The first
           oxygen addition
    • Abstract: Publication date: April 2018
      Source:Combustion and Flame, Volume 190
      Author(s): Lili Ye, Lidong Zhang, Fei Qi
      The chemistry of R + O2 reaction in the low temperature oxidation of iso-pentane has been investigated by using quantum chemical calculations coupled with RRKM/master-equation simulations. All the four independent C5H11 radicals of the iso-pentane molecule were included in the investigation, i.e., 2-methylbutyl, 3-methylbutyl, 1,2-dimethylpropyl and 1,1-dimethylpropyl. The major reaction channels were explored in great detail, and the rate constants of key reactions were provided at high pressure limit and the falloff region. The QOOH radicals generated in R + O2 reactions, which are of particular importance to subsequent chain-branching, are explicitly identified. For these QOOH radicals, the buildup of concentration is typically attributed to the combined effect of the rapid equilibration with preceding alkylperoxy radical and the absence of fast dissociation channels. In addition, these QOOH radicals are all formed from isomerization of RO2 via six-member ring transition states, demonstrating the great significance of 1,5 H-transfers in the low temperature oxidation of alkanes. This work provides useful data for kinetic modeling of low temperature combustion for surrogate fuels and can be used as a starting point for the study about the reaction kinetics of the second oxygen addition in higher alkanes.

      PubDate: 2017-12-26T18:01:06Z
       
  • Analysis of premixed flame propagation between two closely-spaced parallel
           plates
    • Abstract: Publication date: April 2018
      Source:Combustion and Flame, Volume 190
      Author(s): Daniel Fernández-Galisteo, Vadim N. Kurdyumov, Paul D. Ronney
      Motivated by experimental observations on premixed-gas flame propagation in Hele-Shaw cells, this work analyzes quasi-isobaric flame propagation between two adiabatic parallel plates using a simple quasi-2D formulation based on averaging the flow properties across the cell gap. Instabilities associated with thermal expansion, buoyancy, viscosity change across the front and differential diffusion of thermal energy and reactants are investigated with one-step chemistry, constant heat capacity and variable transport coefficients through time-dependent computations of the flame front evolution in large domains. These instabilities are found to induce flame wrinkling which increases flame surface area and thus propagation speeds in ways different from those associated with freely propagating flames. The simulations are compared with experiments in Hele-Shaw cells; very good qualitative and (in some cases) quantitative agreement is found.

      PubDate: 2017-12-26T18:01:06Z
       
  • Shared low-dimensional subspaces for propagating kinetic uncertainty to
           multiple outputs
    • Abstract: Publication date: April 2018
      Source:Combustion and Flame, Volume 190
      Author(s): Weiqi Ji, Jiaxing Wang, Olivier Zahm, Youssef M. Marzouk, Bin Yang, Zhuyin Ren, Chung K. Law
      Forward propagation of kinetic uncertainty in combustion simulations usually adopts response surface techniques to accelerate Monte Carlo sampling. Yet it is computationally challenging to build response surfaces for high-dimensional input parameters and expensive combustion models. This study uses the active subspace method to identify low-dimensional subspace of the input space, within which response surfaces can be built. Active subspace methods have previously been developed only for single (scalar) model outputs, however. This paper introduces a new method that can simultaneously approximate the marginal probability density functions of multiple outputs using a single low-dimensional shared subspace. We identify the shared subspace by solving a least-squares system to compute an appropriate combination of single-output active subspaces. Because the identification of the active subspace for each individual output may require a significant number of samples, this process may be computationally intractable for expensive models such as turbulent combustion simulations. Instead, we propose a heuristic approach that learns the relevant subspaces from cheaper combustion models. The performance of the active subspace for a single output, and of the shared subspace for multiple outputs, is first demonstrated with the ignition delay times and laminar flame speeds of hydrogen/air, methane/air, and dimethyl ether (DME)/air mixtures. Then we demonstrate extrapolatory performance of the shared subspace: using a shared subspace trained on the ignition delays at constant volume, we perform forward propagation of kinetic uncertainties through zero-dimensional HCCI simulations – in particular, single-stage ignition of a natural gas/air mixture and two-stage ignition of a DME/air mixture. We show that the shared subspace can accurately reproduce the probability of ignition failure and the probability density of ignition crank angle conditioned on successful ignition, given uncertainty in the kinetics.

      PubDate: 2017-12-26T18:01:06Z
       
  • Fire spread across a sloping fuel bed: Flame dynamics and heat transfers
    • Abstract: Publication date: April 2018
      Source:Combustion and Flame, Volume 190
      Author(s): Frédéric Morandini, Xavier Silvani, Jean-Luc Dupuy, Arnaud Susset
      The complex interactions between the inclined terrain and the flow generated by the fire make the slope one of the most influencing factors on fire spread. In order to gain a deeper understanding of the mechanisms involved in wildfires spreading upslope, the investigation of flow dynamics and heat transfers is fundamental. This paper reports a series of fire spread experiments conducted across a porous bed of excelsior in a large-scale facility, under both no-slope and 30° up-slope conditions. The coupling of particle image velocimetry and video imaging allowed characterizing the flow pattern with respect to the fire front. Simultaneous heat flux measurements with high scan rate were also performed at the edge of the fuel bed. From the collected data, the increase of the rate of spread with increasing slope is attributed to a major change in fluid dynamics surrounding the flame. For horizontal fire spread, flame fronts exhibit quasi-vertical plume resulting from the buoyancy forces generated by the fire. These buoyancy effects induce an inward flow of ambient air that is entrained laterally into the fire from both sides. Flame radiation is the dominant fuel preheating mechanism. Under upslope conditions, the fire plume is tilted toward the unburnt vegetation, increasing radiation levels. The air entrainment at the burnt side of the fire strongly influences the downstream flow, which becomes attached to the surface over a characteristic length scale. Ahead of the flame front, the induced wind blows away from the fire rather than toward it, enhancing convective heating. Periodical forward bursts of flame combined with distant fuel ignitions were also observed. The heat flux measurements confirmed the existence of such convective mechanisms.

      PubDate: 2017-12-26T18:01:06Z
       
  • Relating microstructure, temperature, and chemistry to explosive ignition
           and shock sensitivity
    • Abstract: Publication date: April 2018
      Source:Combustion and Flame, Volume 190
      Author(s): W. Lee Perry, Brad Clements, Xia Ma, Joseph T. Mang
      An analysis is put forth that relates explosive properties including microstructure, temperature, and chemistry to explosive ignition and sensitivity. Unlike approaches that focus only on elucidating mesoscopic mechanisms important to ignition, the present analysis seeks a methodology directly applicable to continuum explosive burn models. Because the Scaled Uniform Reactive Front (SURF) burn model has a microstructural basis, it is chosen as the starting point of the present analysis. To build upon the SURF framework, a literal translation is performed of the high-level conceptual notions for which SURF is based, to concrete ignition–combustion parameters, a statistical description of the microstructure, and other thermo-chemical data acquired by non-shock experiments. The analysis requires a void volume fraction distribution acquired from ultra-small angle neutron scattering (USANS). The shock response of PBX 9502 is used to illustrate the theory. While the analysis requires calibration to a shock experiment (the pop plot, for example), the results are a self-consistent set of realistic physical quantities that contribute to the shock initiation process including, initial temperature, chemical kinetics, a statistical description of the microstructure, and the hot spot size, spacing, and temperature. The utility of having a mesoscale based theory is that once the critical hot spot temperature, as a function of shock pressure, is known for a given explosive and initial porosity distribution, the entire set of meso- and macro-scale results, including the pop plot can be calculated for any other porosity. Thus, one can understand changes in sensitivity at different densities (void size distributions), relying only on the assumption that the hot spot temperature curve would not differ significantly if the morphology of the hots spots were similar. Another important utility of the present analysis is to address the question of the role of initial temperature on the observed shock sensitivity of PBX 9502 and other explosives.

      PubDate: 2017-12-26T18:01:06Z
       
  • Gasoline direct injection engine soot oxidation: Fundamentals and
           determination of kinetic parameters
    • Abstract: Publication date: April 2018
      Source:Combustion and Flame, Volume 190
      Author(s): Maria Bogarra, Jose M. Herreros, Athanasios Tsolakis, Jose Rodríguez-Fernández, Andrew P.E. York, Paul J. Millington
      Current emissions legislation for road transport vehicles, including modern gasoline vehicle fleet limits the mass and the number of Particulate Matter (PM) emitted per kilometre. The introduction of a gasoline particulate filter (GPF) is expected to be necessary, as was the case for diesel vehicles, the traditionally recognised source of PM in transportation. Therefore, for the design of efficient GPFs and the regeneration strategies, soot oxidation characteristics in gasoline must be understood. Extensive research has been carried out mainly to investigate the oxidation of diesel soot, however, in the most cases soot were collected on microfiber filters and the activation energy was calculated with the logarithm method assuming mass and oxygen reaction orders equal to one. Identified limitations that lead to inconsistent and inaccurate trends and results are presented in this paper. As a consequence, a novel methodology to accurately obtain the oxidation kinetic parameters for soot emitted from a Gasoline Direct Injection (GDI) engine has been developed and presented in this paper. The particles collected in a silicon carbide wall-flow particulate filter are directly exposed to oxidation conditions in a thermogravimetric analysis (TGA) without the use of microfiber filter. The significance of more accurate and consistent calculations of soot oxidation kinetic parameters as a result of this methodology will aid modelling and experimental work of the aftertreatment systems and will lead in improving the GPF regeneration process in modern GDI vehicles. Avoiding high peak temperatures during regeneration and large thermal stress gradients and thus increasing the operating life of the filters is amongst the benefits can be seen.

      PubDate: 2017-12-26T18:01:06Z
       
  • Influence of potassium chloride and other metal salts on soot formation
           studied using imaging LII and ELS, and TEM techniques
    • Abstract: Publication date: April 2018
      Source:Combustion and Flame, Volume 190
      Author(s): Johan Simonsson, Nils-Erik Olofsson, Ali Hosseinnia, Per-Erik Bengtsson
      An experimental investigation has been performed where the influence of metal salts on soot formation has been studied. By combining two-dimensional laser-induced incandescence (LII) and elastic light scattering (ELS), two-dimensional information could be obtained on soot properties in the flames. For these studies, seven metal salts (NaCl, MgCl2, AlCl3, KCl, CaCl2, FeCl3 and ZnCl2) were dissolved in water and aspirated into a premixed ethylene/air flame. At lower flame heights, in the soot inception region, the LII signal (representing soot volume fraction) was marginally affected by all additives, whereas the ELS signal strongly decreased with increasing additive concentration for the alkali salts. At higher heights, in the soot growth region, the soot volume fractions were lowered for the addition of potassium, calcium and sodium chloride, in order of significance. Some of the salts (MgCl2, AlCl3 and FeCl3) resulted in negligible influence on LII signals and slightly higher ELS signals throughout the flames, and we relate the increased ELS signals to salt particles propagating through the flame. Main focus in our study was on the addition of potassium chloride for which several parameters were investigated. For example, soot primary particle sizes were evaluated using combined LII and ELS, showing decreasing particle sizes for increasing concentrations of potassium, in reasonable agreement with particle sizes evaluated using transmission electron microscopy. Also, CARS thermometry showed slightly higher flame temperature, ∼30 K, for the potassium-seeded flame compared to the reference flame.

      PubDate: 2017-12-26T18:01:06Z
       
  • Experimental and numerical investigations on propagating modes of
           detonations: Detonation wave/boundary layer interaction
    • Abstract: Publication date: April 2018
      Source:Combustion and Flame, Volume 190
      Author(s): Xiaodong Cai, Jianhan Liang, Ralf Deiterding, Yasser Mahmoudi, Mingbo Sun
      In the present work the propagating modes of detonation wave in supersonic hydrogen–air mixtures are investigated in narrow rectangular channels. To clarify the effect of the detonation wave interaction with the boundary layer on the evolution and propagation of detonation phenomenon, high-speed laser schlieren experiments and adaptive Navier–Stokes (NS) simulations (pseudo-DNS) combined with a detailed reaction model are performed. The experimental results show that after successful ignition, two propagating modes are observed and can be classified as Oblique shock-induced combustion/Mach stem-induced detonation (OSIC/MSID) and pure Oblique shock-induced combustion (OSIC). For the OSIC/MSID mode, a Mach stem induced overdriven detonation is generated in the middle of the main flow. For the pure OSIC mode, no detonation wave but two oblique shock-induced combustion regions are generated throughout the whole channel with the overall structure taking a thwartwise V shape. The OSIC/MSID and pure OSIC propagation modes are further confirmed by pseudo-DNS employing a detailed reaction model and dynamic adaptive mesh refinement for the same conditions as utilized in the experiments. The numerical results show that because of subsonic combustion near the walls induced by the boundary layers, the OSIC/MSID is not entirely symmetrical, while for the pure OSIC mode, larger fluctuations are observed along the oblique shock waves resulting from enhanced instabilities due to additional chemical heat release.

      PubDate: 2017-12-26T18:01:06Z
       
  • 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
       
 
 
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: 54.144.84.155
 
About JournalTOCs
API
Help
News (blog, publications)
JournalTOCs on Twitter   JournalTOCs on Facebook

JournalTOCs © 2009-