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  • Experimental study of corner fires—Part I: Inert panel tests
    Combust. Flame (IF 3.663) Pub Date : 2017-11-06
    Davood Zeinali, Steven Verstockt, Tarek Beji, Georgios Maragkos, Joris Degroote, Bart Merci

    Corner fires are known to spread more intensely in comparison with single wall fires. In view of the challenges associated with prediction of such fire behavior, the fire growth in a corner configuration of Medium Density Fiberboard (MDF) panels is investigated to provide a set of experimental data, performing Single Burning Item (SBI) tests. First, though, test results with inert calcium silicate panels are discussed for three values of HRR (10, 30 and 55 kW), allowing to address the main physics involved. The experimental data for 30 kW, the default SBI HRR, is used for detailed discussion of the observations. The SBI testing methodology, materials, and set-up are described. The results of total Heat Release Rates (HRR) and Smoke Production Rates (SPR), as well as the panel temperatures and total heat fluxes at several characteristic locations are analyzed. Moreover, the puffing frequency of the corner fire is characterized thanks to Video Fire Analysis (VFA) of the experimental footage. Additionally, flame heights are discussed, including the concept of mirroring. A new correlation for mean flame height is introduced, using the hypotenuse of the triangle as characteristic length for entrainment of air into the fire plume, and expressing that the flame height increases proportional to the square root of the fire heat release rate. The 30 kW propane burner of the standard SBI test is shown to feature a mean flame height of nearly 0.9 m and a puffing frequency of 2 ± 0.3 Hz, and an average total heat flux exceeding 44 kW/m2 near the burner early on in the test. The completeness of the dataset is expected to be useful for testing and development of CFD codes for corner fire scenarios.

    更新日期:2017-12-14
  • Experimental study of corner fires—Part II: Flame spread over MDF panels
    Combust. Flame (IF 3.663) Pub Date : 2017-11-04
    Davood Zeinali, Steven Verstockt, Tarek Beji, Georgios Maragkos, Joris Degroote, Bart Merci

    Having explained the characteristics of a corner fire in the configuration of Single Burning Item (SBI) test in Part I [1], the results of three flame spread experiments conducted with Medium Density Fiberboard (MDF) panels are discussed. The fire growth, in terms of flame heights and spread, is examined from two different angles visually and through Video Fire Analysis (VFA) with a flame detection algorithm. Total Heat Release Rates (HRR) and Smoke Production Rates (SPR), as well as total heat fluxes at several characteristic locations, are presented. Moreover, temperature evolutions are discussed for multiple locations and through the thickness of the panels. Also the backside temperatures, important as boundary condition for numerical simulations, are reported. The corner fire tests with MDF panels yield an average peak HRR of 151 kW and an average total heat flux exceeding 60 kW/m2 close to the burner, with average flame heights surpassing 1.5 m in about 60 s.

    更新日期:2017-12-14
  • Premixed flames subjected to extreme levels of turbulence part I: Flame structure and a new measured regime diagram
    Combust. Flame (IF 3.663) Pub Date : 2017-09-29
    Aaron W. Skiba, Timothy M. Wabel, Campbell D. Carter, Stephen D. Hammack, Jacob E. Temme, James F. Driscoll

    This paper presents high-fidelity flame structure measurements of premixed methane–air Bunsen flames subjected to extreme levels of turbulence. Specifically, 28 cases were studied with longitudinal integral length scales (Lx) as large as 43 mm, turbulence levels (u′/SL) as high as 246, and turbulent Reynolds (ReT,0) and Karlovitz (KaT) numbers up to 99,000 and 533, respectively. Two techniques were employed to measure the preheat and reaction layer thicknesses of these flames. One consisted of planar laser-induced fluorescence (PLIF) imaging of CH radicals, while the other involved taking the product of simultaneously acquired PLIF images of formaldehyde (CH2O) and hydroxyl (OH) to produce “overlap-layers.” The average preheat layer thicknesses are found to increase with increasing u′/SL and with axial distance from the burner (x/D). In contrast, average reaction layer (i.e. CH- and overlap-layer) thicknesses did not increase appreciably even as u′/SL increased by a factor of ∼ 60. Furthermore, the reaction layer thicknesses (based on the CH images only) did not increase with increasing x/D. The reaction layers are also observed to remain continuous; that is, local extinction events are rarely observed. Although based on a sequence of combined CH–OH PLIF images acquired at a rate of 10 kHz, it is apparent that when instances of local extinction do occur they are the result of cool gas entrainment. The results presented here, as well as those from 12 prior experimental and 9 numerical investigations, do not agree with the predicted Klimov–Williams boundary on the theoretical Borghi Diagram. Thus, a new Measured Regime Diagram is proposed wherein the Klimov–Williams criterion is replaced by a metric that relates the turbulent diffusivity ( D T = u ′ L x ) to the molecular diffusivity within the preheat layer ( D * = S L δ F , L ). Justification for this replacement is based on physical reasoning and the fact that the line defined by DT/D* ≈ 180 accurately separates cases with thin flamelets from those with broadened preheat yet thin reaction layers (i.e. BP-TR flames). Additionally, the results suggest that the BP-TR regime extends well beyond what was previously theorized since neither broken nor broadened reaction layers were observed under conditions with Karlovitz numbers as high as 533, which is five times higher than the theoretical boundary.

    更新日期:2017-12-14
  • Effect of fuel composition on soot and aromatic species distributions in laminar, co-flow flames. Part 2. Partially-premixed fuel
    Combust. Flame (IF 3.663) Pub Date : 2017-09-11
    Anandkumar Makwana, Yefu Wang, Suresh Iyer, Milton Linevsky, Robert J. Santoro, Thomas A. Litzinger, Jacqueline O'Connor

    The goal of this work is to understand the effect of fuel molecular structure on soot precursors and soot in an axisymmetric, co-flow, laminar flame configuration at atmospheric pressure with partially-premixed fuel jets. Five fuels with varying molecular structure are investigated: methylcyclohexane/n-dodecane mixture, n-heptane/n-dodecane mixture, iso-octane/n-dodecane mixture, m-xylene/n-dodecane mixture and pure n-dodecane. The flames investigated have jet equivalence ratios of 24 and 6. The total carbon flow rate and carbon fraction of the two components are kept constant to facilitate comparison among fuels. A laser-induced fluorescence technique is used to obtain spatially-resolved polycyclic aromatic hydrocarbons, soot precursors, in the jet flames. The polycyclic aromatic hydrocarbons are identified into two classes: single/two ring aromatics (small) and aromatics having three to five rings (large). A laser-induced incandescence and laser extinction technique are applied to obtain two-dimensional soot volume fraction for all the flames. The experimental results indicate that the level of soot is highest for the m-xylene/n-dodecane fuel at approximately three times the peak soot levels in the paraffinic fuels. The data show the effects of premixing on the spatial distribution of aromatic species and soot, including the shift from a soot field that peaks near the flame front to one that has maximum soot volume fractions near the centerline in m-xylene/n-dodecane flame. The iso-octane/n-dodecane and methylcyclohexane/n-dodecane fuels show a similar transition of soot field that has an annular peak in the non-premixed flame to a more uniform soot field under premixed conditions. Normalizing the maximum soot volume fraction by the maximum for the n-dodecane base fuel, the data shows that, within measurement uncertainty, the effect of fuel structure on maximum soot volume fraction is independent of the equivalence ratio of the fuel jet.

    更新日期:2017-12-14
  • Effect of fuel composition on soot and aromatic species distributions in laminar, co-flow flames. Part 1. Non-premixed fuel
    Combust. Flame (IF 3.663) Pub Date : 2017-09-09
    Yefu Wang, Anandkumar Makwana, Suresh Iyer, Milton Linevsky, Robert J. Santoro, Thomas A. Litzinger, Jacqueline O’Connor

    The work described in this paper is part of Department of Defense-Industry-University collaboration to develop fundamental understanding of the effects of alternative fuels on emissions from military gas turbine engines. The study was conducted in an axisymmetric, co-flow, laminar flame configuration at atmospheric pressure to determine the effects of fuel structure on aromatic species and soot. Five fuels with varying molecular structure were investigated: pure n-dodecane and four binary mixtures: m-xylene/n-dodecane, n-heptane/n-dodecane, iso-octane/n-dodecane, and methylcyclohexane/n-dodecane. Flames with non-premixed and partially-mixed fuel jets were studied. This paper describes the results of flames with non-premixed fuel jets; a companion paper describes the results for the partially pre-mixed fuel jets. In all of the experiments, the total carbon flow rate and the fraction of carbon from the two components in the binary mixtures were kept constant to facilitate comparison among the fuels. A laser-induced fluorescence technique was used to obtain spatially-resolved information on the aromatic species. Signals from aromatic species were collected in two wavelength ranges corresponding to species with one or two rings and three–five rings. Laser-induced incandescence and laser extinction were applied to obtain two-dimensional soot volume fraction for all flames. The flames of the paraffin fuels are non-smoking and have similar spatial distributions of aromatic species and soot as well as maximum soot volume fractions. The flame from the m-xylene fuel is smoking and has spatial distributions of aromatic species and soot that are distinctly different than for the paraffin fuels. Maximum soot volume fraction produced by the m-xylene fuel is approximately three times that of the paraffin fuels.

    更新日期:2017-12-14
  • Detailed kinetic modeling of dimethoxymethane. Part I: Ab initio thermochemistry and kinetics predictions for key reactions
    Combust. Flame (IF 3.663) Pub Date : 2017-09-06
    Wassja A. Kopp, Leif C. Kröger, Malte Döntgen, Sascha Jacobs, Ultan Burke, Henry J. Curran, Karl Alexander Heufer, Kai Leonhard

    Despite the great interest in oxygenated methyl ethers as diesel fuel additives and as fuels themselves, the influence of their methylenedioxy group(s) (O–CH2–O) has never been quantified using ab initio methods. In this study we elucidate the kinetics and thermochemistry of dimethoxymethane using high-level ab initio (CCSD(T)/aug-cc-pV(D+T)Z//B2PLYPD3BJ/6-311++g(d,p)) and statistical mechanics methods. We model torsional modes as hindered rotors which has a large influence on the description of the thermal behavior. Rate constants for hydrogen abstraction by Ḣ and ĊH3 are computed and show that abstraction from the methylenedioxy group is favored over abstraction from the terminal methyl groups. β-scission and isomerization of the radicals are computed using master equations. The effect of rovibrationally excited radicals from preceding hydrogen abstraction reactions on subsequent hot β-scission is computed and has large influence on the decomposition of the formed dimethylether radical. The quantification of the effect of the dominant methylenedioxy group using ab initio methods can guide modeling of oxygenated methyl ethers that contain that group several times.

    更新日期:2017-12-14
Some contents have been Reproduced with permission of the American Chemical Society.
Some contents have been Reproduced by permission of The Royal Society of Chemistry.
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