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1.
  • Abou-Taouk, Abdallah, 1982, et al. (författare)
  • Optimized Reduced Chemistry and Molecular Transport for Large Eddy Simulation of Partially Premixed Combustion in a Gas Turbine
  • 2016
  • Ingår i: Combustion Science and Technology. - : Informa UK Limited. - 0010-2202 .- 1563-521X. ; 188:1, s. 21-39
  • Tidskriftsartikel (refereegranskat)abstract
    • A methodology is discussed to automatically determine the parameters of closed budget equations for chemical species mass fractions and energy, in order to simulate spatially filtered flames as required in large eddy simulation (LES). The method accounts for the effects of LES filtering on chemistry and transport by simultaneously optimizing, for a reduced number of species, the Arrhenius reaction rates and a correction to mixture-averaged molecular diffusion coefficients. The objective is to match, for a given filter size, spatially filtered canonical one-dimensional flames simulated with detailed chemistry solutions. This approach is designed for quite well-resolved LES, in which most of the unresolved fluctuations result from flame thickening due to spatial filtering, thus featuring weak levels of sub-grid scale flame wrinkling. Methane-air partially premixed combustion is addressed. A four-step reduced reaction mechanism involving seven species is developed along with mass and heat molecular transport properties. The optimization is performed at atmospheric pressure and at 3 bar, for ranges of fresh gas temperatures [300–650 K] and equivalence ratios [0.4–1.2]. Comparisons with the filtered detailed chemistry solution of a planar propagating front show that the laminar flame speed, the adiabatic flame temperature, the species profiles in the reaction zone, and the flow chemical composition and temperature at equilibrium are adequately predicted. The new sub-grid scale modeling approach is then applied to three-dimensional LES of an industrial gas turbine burner. Good agreement is found between the quantities predicted with LES and experimental data, in terms of flow and flame dynamics, axial velocities, averaged temperatures, and some major species concentrations. Results are also improved compared to previous simulations of the same burner.
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2.
  • Alekseev, Vladimir, et al. (författare)
  • Experimental Uncertainties of the Heat Flux Method for Measuring Burning Velocities
  • 2016
  • Ingår i: Combustion Science and Technology. - : Informa UK Limited. - 1563-521X .- 0010-2202. ; 188:6, s. 853-894
  • Tidskriftsartikel (refereegranskat)abstract
    • The laminar burning velocity is a fundamental property of combustiblemixtures important for kinetic model validation as well as for practicalapplications. Many efforts are directed towards its accurate determination.The heat flux method is one of the commonly recognized methodsfor measuring laminar burning velocity, however, the information on theaccuracy of the method is scattered in the literature. In the present work,an attempt wasmade to summarize and extend the available informationon the different factors contributing to the experimental uncertainty ofthe heat flux method. Experimental setup of the Lund University group,typical for the heat flux community, and the procedures used to determinethe burning velocity are described. Furthermore, the influence ofdifferent uncertainty factors, originating from each part of the setup, isanalyzed. Asymmetric heat fluxes and the method for determining flamesurface area were found to give an important contribution to the totalerror. As a result of this, some of the previously published data have beenre-evaluated. Finally, recommendations are presented on how to controlor reduce the uncertainties in the heat flux measurements, and possibledirections for future research, aimed at improvement of the accuracy andunderstanding of the method, are outlined.
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3.
  • Bengtsson, Per Erik, et al. (författare)
  • C2 Production and Excitation in Sooting Flames using Visible Laser Radiation : Implications for Diagnostics in Sooting Flames
  • 1991
  • Ingår i: Combustion Science and Technology. - : Informa UK Limited. - 0010-2202 .- 1563-521X. ; 77:4-6, s. 307-318
  • Tidskriftsartikel (refereegranskat)abstract
    • Swan band emission of C2. which had been produced and excited (d3∏x←a 3∏u, Δ v = 0) by focused, tunable, visible laser radiation, was studied in sooting ethylene flames. The C2signal from the sooting regions of the flames was found to be proportional to the measured volume fractions of soot, indicating laser-vaporized soot to be the source of the signal, and as such it can be used as a diagnostic tool for point measurements of the volume fraction of soot. C2emission was detected for all incident wavelengths between 485 and 540 nm, with a C2signal 1–2 orders of magnitude higher when the wavelength was tuned to a C2 resonance than when tuned to a wavelength between the Swan bands. High concentrations of laser-produced C2 and emission from C2Swan bands can, through interference effects, limit the applicability of laser diagnostic techniques to sooting flames.
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4.
  • Bengtsson, Per Erik, et al. (författare)
  • Rotational cars thermometry in sooting flames
  • 1992
  • Ingår i: Combustion Science and Technology. - : Informa UK Limited. - 0010-2202 .- 1563-521X. ; 81:1-3, s. 129-140
  • Tidskriftsartikel (refereegranskat)abstract
    • Coherent anti-Stokes Raman scattering of pure rotational transitions, rotational CARS, is demonstrated as an efficient method for temperature determination in sooting flames. The dual broadband CARS approach was used to measure temperature profiles in premixed, sooting ethylene flames at atmospheric pressure by probing the nitrogen gas. The recorded spectra were of equally high quality in non-sooting and sooting flames with volume fractions of soot of up to 7 x 10 7 cm3 soot/cm3The advantages of rotational CARS in comparison with several other techniques for the measurement of temperatures in sooting flames, and the general applicability of the technique to different combustion conditions, are discussed. Potential limitations in the application of rotational CARS to sooting flames that are more heavily sooting than the ones investigated in this study, are outlined.
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5.
  • Berard, L., et al. (författare)
  • Compounds Produced by the Pyrolysis of Powders and Dusts Present in the Alimentary Industry
  • 2023
  • Ingår i: Combustion Science and Technology. - : Taylor and Francis Ltd.. - 0010-2202 .- 1563-521X. ; 195:3, s. 419-
  • Tidskriftsartikel (refereegranskat)abstract
    • Under certain conditions dust explosions occur in the alimentary industry. Following ATEX and other guidelines have not eliminated accidents. Therefore, more knowledge is needed. The current work delivers experimental results describing phase transitions and decomposition of dusts. Dusts from wheat flour, chili powder, corn starch, milk powder, cocoa powder, and by-product of grain are investigated. The temperature of pyrolisation has been identified using TGA to be in the range [250°C, 600°C] in air and [300°C, 450°C] in nitrogen. It was found that the compositions of the pyrolysis gases depend on temperature. Carbon monoxide, carbon dioxide, methane, and hydrogen were the main contributors to the pyrolysis gases. The distributions are described with a polynomial or Gaussian fit. The current paper proposes coefficients for Gaussian polynomials expressing the concentration for the four primary pyrolysis gases. 
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6.
  • Brackmann, Christian, et al. (författare)
  • Quantitative measurements of species and temperature in a DME-air counterflow diffusion flame using laser diagnostic methods
  • 2006
  • Ingår i: Combustion Science and Technology. - : Informa UK Limited. - 1563-521X .- 0010-2202. ; 178:6, s. 1165-1184
  • Tidskriftsartikel (refereegranskat)abstract
    • A diffusion flame of dimethyl ether (DME) and air in a counterflow burner has been investigated experimentally by means of different laser-based methods. Quantitative measurements of temperature, O-2 concentration, and OH concentration have been carried out using dual broadband rotational coherent anti-Stokes Raman Spectroscopy (CARS), Rayleigh scattering and laser-induced fluorescence (LIF). Furthermore a qualitative formaldehyde profile has been measured using LIF. The measured profile has been compensated for fluorescence signal quenching and the Boltzmann distribution to obtain a representative qualitative formaldehyde profile for the flame.
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7.
  • Brackmann, C., et al. (författare)
  • Strategies for Quantitative Planar Laser-Induced Fluorescence of NH Radicals in Flames
  • 2016
  • Ingår i: Combustion Science and Technology. - : Informa UK Limited. - 0010-2202 .- 1563-521X. ; 188:4-5, s. 529-541
  • Tidskriftsartikel (refereegranskat)abstract
    • Laser-induced fluorescence imaging of the NH radical using excitation in the (0-0) and (1-0) vibrational bands of the A3Π-X3Σ– electronic transition is characterized in premixed NH3-air flames. Filtered detection for excitation of the (1-0) band is found to be beneficial for measurement conditions with a challenging background and/or interferences. Concentration evaluation for excitation in the (0-0) and (1-0) bands is feasible by means of fully or partially saturated fluorescence, respectively. Detection limits of a few tens of ppm for averaged data and 100 ppm on a single-shot basis was achieved for 3-cm imaging size using both excitation alternatives. Thinning and broadening of the NH layer in a turbulent flame indicate turbulence/flame chemistry coupling, which occurred at lower turbulence levels compared with hydrocarbon combustion. Fluorescence imaging of the NH flame-front marker in such flames thus provide an experimental configuration for detailed studies of turbulence/flame chemistry interactions.
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8.
  • Bychkov, Vitaly, et al. (författare)
  • Gas compression moderates flame acceleration in deflagration-to-detonation transition
  • 2012
  • Ingår i: Combustion Science and Technology. - : Informa UK Limited. - 0010-2202 .- 1563-521X. ; 184:7-8, s. 1066-1079
  • Tidskriftsartikel (refereegranskat)abstract
    • The effect of gas compression at the developed stages of flame acceleration in smooth-wall and obstructed channels is studied. We demonstrate analytically that gas compression moderates the acceleration rate, and we perform numerical simulations within the problem of flame transition to detonation. It is shown that flame acceleration undergoes three distinctive stages: (1) initial exponential acceleration in the incompressible regime, (2) moderation of the acceleration process due to gas compression, so that the exponential acceleration state goes over to a much slower one, (3) eventual saturation to a steady (or statistically steady) high-speed deflagration velocity, which may be correlated with the Chapman-Jouguet deflagration speed. The possibility of deflagration-to-detonation transition is demonstrated.
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9.
  • Bychkov, Vitaly, et al. (författare)
  • On the Theory of Turbulent Flame Velocity
  • 2007
  • Ingår i: Combustion Science and Technology. - : Informa UK Limited. - 0010-2202 .- 1563-521X. ; 179:1&2, s. 137-151
  • Tidskriftsartikel (refereegranskat)abstract
    • The renormalization ideas of self-similar dynamics of a strongly turbulent flame front are applied to the case of a flame with realistically large thermal expansion of the burning matter. In that case a flame front is corrugated both by external turbulence and the intrinsic flame instability. The analytical formulas for the velocity of flame propagation are obtained. It is demonstrated that the flame instability is of principal importance when the integral turbulent length scale is much larger than the cutoff wavelength of the instability. The developed theory is used to analyze recent experiments on turbulent flames propagating in tubes.
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10.
  • Chan, Y. L., et al. (författare)
  • The Comparative and Combined Effects of Nitric Oxide and Higher Alkanes in Sensitizing Methane Oxidation
  • 2012
  • Ingår i: Combustion Science and Technology. - : Informa UK Limited. - 1563-521X .- 0010-2202. ; 184:1, s. 114-132
  • Tidskriftsartikel (refereegranskat)abstract
    • The comparative and combined effects of nitric oxide (NO) and higher alkanes on methane oxidation were examined by experimentation and kinetic modeling. Experiments were conducted using fuel-lean, lower-alkanes mixtures with NO (0-400 ppm v/v) in an atmospheric flow reactor at residence time of 2 s over the temperature range of 820-950 K. NO was found to greatly promote methane conversion, and its sensitizing effect strengthened with the increasing concentration that was added to the system. The promoting effect of higher alkanes on methane conversion was also evident, particularly at zero or low NO concentrations. A strong dependency of the sensitizing effect on the concentration of higher alkanes present was also observed. The kinetic mechanism from Le Cong et al., performed reasonably well in reproducing the experimental trends. However, the sensitizing propensity of higher alkanes in the presence of NO could not be unambiguously ranked under all conditions. Modification to the kinetic mechanism of Le Cong et al., was attempted. Specifically, the submechanism of C(3) peroxy radicals was added, but the modeling results indicated a lower impact on methane conversion than was initially expected. The most sensitive reactions were revealed, and the generalized reaction pathways for methane oxidation sensitized by higher alkanes, with or without the presence of NO, were also proposed, following detailed sensitivity analysis.
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11.
  • Chomiak, Jerzy, 1934, et al. (författare)
  • Flame kernel growth in a rotating gas
  • 2008
  • Ingår i: Combustion Science and Technology. - : Informa UK Limited. - 0010-2202 .- 1563-521X. ; 180:2, s. 391-399
  • Tidskriftsartikel (refereegranskat)abstract
    • The communication deals with an ignition kernel development in uniformly rotating mixture. A simple model is presented in order to predict the time evolution of the kernel length and its diameter under the assumption of ignition on the axis of rotation, which is preferred mode for rapid flame development. The analytic expressions for flame radius and length are compared with experimental results. The predicted radius growth rate is in good agreement with experimental data, whereas the length evolution rate predictions deviate substantially from measurements due to flame propagation effects involving quenching and perturbation of the surrounding flow by the growing kernel. An interesting general result supported by the theory and experiment is that the diameter growth rate of the cylindrical part of the kernel is about half the growth rate of the spherical kernel in a quiescent mixture and is independent of the rotation rate. Wall effects start to reduce the kernel development rate when the distance from the wall is less than double the kernel diameter. The effects are quite strong.
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12.
  • Chukalovsky, A. A., et al. (författare)
  • Study of singlet delta oxygen O 2 ( 1 δg) impact on H 2-O 2 mixture ignition in flow reactor : 2D modeling
  • 2012
  • Ingår i: Combustion Science and Technology. - : Informa UK Limited. - 0010-2202 .- 1563-521X. ; 184:10-11, s. 1768-1786
  • Tidskriftsartikel (refereegranskat)abstract
    • Influence of electron excited singlet delta oxygen (SDO) molecules produced in direct current (DC) glow discharged plasma on the induction length decrease in the H2-O2 mixture has been studied via comprehensive two-dimensional (2D) numerical simulations. The sensitivity analysis of the induction length on SDO mole fractions and residual mole fractions of odd oxygen was carried out. The influence of the reaction H 2+O 2( 1g)H+HO2 on the ignition length was found to be negligible due to the presence of residual odd oxygen in the oxidizer flow. Two stages specify the ignition time decrease for the studied conditions: (1) chain initiation in reactions with residual odd oxygen and (2) the following chain-branching enhancement due to reaction H 2+O 2( 1g)O+OH. The last reaction is the key process for studied conditions when the concentration of SDO exceeds 4%. The estimated rate constant of this reaction was found to be about 2.510 13cm 3/s at 780K. The quenching reaction H 2+O 2( 1g)H 2+O 2 does not affect the ignition length.
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13.
  • Dahl, Daniel, 1982, et al. (författare)
  • The Origin of Pressure Waves in High Load HCCI Combustion: A High-Speed Video Analysis
  • 2011
  • Ingår i: Combustion Science and Technology. - : Informa UK Limited. - 0010-2202 .- 1563-521X. ; 183:11, s. 1266-1281
  • Tidskriftsartikel (refereegranskat)abstract
    • Homogeneous charge compression ignition is an alternative combustion strategy for spark-ignited gasoline engines that improves engine efficiency and thus reduces CO2 emissions, which is crucial to meet targets set by legislation. However, this combustion strategy is limited to low loads, mainly due to pressure oscillations that arise if combustion is too rapid. The aim of the work described in this article was to record and identify these pressure oscillations and correlate them with the preceding combustion. This was performed using an image-intensified high-speed video camera with a sampling rate of 111 kHz filming inside the combustion chamber of an engine with optical access through a quartz piston. A strategy is described whereby video analysis is used to extract the acoustic modes resulting from combustion. In this work it was possible to detect four different acoustic modes. It is shown that the type and magnitude of these modes can be correlated to the size and position of combustion chamber zones with rapid combustion and the light intensity development (combustion speed) in these zones. It is also shown that the highest combustion rates occur in regions where combustion starts late.
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14.
  • Dakshnamurthy, Shanmugasundaram, et al. (författare)
  • Experimental Study and a Short Kinetic Model for High-Temperature Oxidation of Methyl Methacrylate
  • 2019
  • Ingår i: Combustion Science and Technology. - : Informa UK Limited. - 0010-2202 .- 1563-521X. ; 191:10, s. 1789-1814
  • Tidskriftsartikel (refereegranskat)abstract
    • Synthetic and natural polymeric esters find applications in transport and construction sectors, where fire safety is an important concern. One polymer that is widely used is poly (methyl methacrylate) (PMMA), which almost completely undergoes thermal decomposition into methyl methacrylate (its monomer) CH2=C(CH3) - C(= O) - O - CH3 (MMA) at ~250–300°C. In order to analyze the high-temperature gas-phase oxidation of PMMA, and thereby predict its fire behavior (such as burning rate, temperature of the material, and heat fluxes) with less computational effort, a compact kinetic model for the oxidation of its primary decomposition product, MMA, is most essential. This is accomplished in the present work by obtaining a reduced mechanism for MMA oxidation from a detailed mechanism from the Lawrence Livermore National Laboratories group. To extend the available data base for model validation and present validation data at atmospheric pressure conditions, for the first time, (i) detailed measurements of species profiles have been performed in stoichiometric laminar flat flames using flame sampling molecular beam mass spectrometry (MBMS) technique and (ii) laminar burning velocities have been obtained using the heat flux method for various unburnt mixture temperatures. Evaluating the model against these data sets point to the need to revise the kinetic model, which is achieved by adopting rate constants of key reactions among analogous molecules from recent literature. The updated compact kinetic model is able to predict the major species in the flat flame as well as the burning velocity of MMA satisfactorily. The final “short MMA mechanism” consists of 88 species and 1084 reactions.
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15.
  • Danon, B., et al. (författare)
  • EMISSION AND EFFICIENCY COMPARISON OF DIFFERENT FIRING MODES IN A FURNACE WITH FOUR HiTAC BURNERS
  • 2011
  • Ingår i: Combustion Science and Technology. - : Informa UK Limited. - 0010-2202 .- 1563-521X. ; 183:7, s. 686-703
  • Tidskriftsartikel (refereegranskat)abstract
    • Combustion in a furnace equipped with two HiTAC burner pairs, with a thermal power of 100kW(th) each, has been investigated experimentally and computationally. The objective of this study is (1) to observe differences in the performance of the furnace operating in two different firing modes, parallel and staggered, and (2) to explain these differences using detailed CFD simulations. Besides the permanent measurements of temperature, flow and pressure, in-furnace probe measurements of temperature, oxygen and emissions (NO and CO) have been performed. Experimental results show that the efficiency of the furnace was higher in parallel mode compared to staggered mode, 48% and 41% respectively. The values of CO emitted were equal for both firing modes. However, in parallel mode the NOx production was 39 ppm(v)@3%O-2, whereas in staggered mode 53 ppm(v)@3%O-2 NOx was produced. Considering both efficiency and emissions, parallel firing mode performs better than staggered mode. Next, CFD simulations of the furnace were performed in order to explain the observed differences. The simulations were validated with the in-furnace measurements. It was confirmed that the furnace firing in parallel mode achieved a higher efficiency. The radiative heat transfer was higher due to formation of a larger zone with gases with improved radiative properties. In addition, higher velocities along the cooling tubes, due to lower momentum destruction, led to higher convective heat transfer. Also, the lower production of NOx in parallel mode was reproduced by the simulations. This is due to the fact that in parallel mode the fuel jets are merging slower with the combustion air jet, leading to less intense combustion zones. Thus, lower peak temperatures and radical concentrations are achieved, and the NOx production via the thermal and N2O pathways was lower.
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16.
  • De Paepe, Ward, et al. (författare)
  • Experimental Investigation of the Effect of Steam Dilution on the Combustion of Methane for Humidified Micro Gas Turbine Applications
  • 2016
  • Ingår i: Combustion Science and Technology. - : Informa UK Limited. - 0010-2202 .- 1563-521X. ; 188:8, s. 1199-1219
  • Tidskriftsartikel (refereegranskat)abstract
    • Water introduction in the micro gas turbine (mGT) cycle is considered the optimal route for waste heat recovery and flexibility increase of such a small-scale combined heat and power (CHP) unit. However, humidification of the combustion air in a mGT affects combustion stability, efficiency, and exhaust gas emissions. This can lead to a non-stable, incomplete combustion, which will affect the global efficiency negatively. Additionally, CO emissions will increase. The non-stable, incomplete combustion might result in an engine shutdown due to a flameout. To study the impact of humidification on the combustion of methane in a humidified mGT, we performed combustion experiments in an atmospheric, variable-swirl, premixed combustion chamber. The results of these experiments are summarized in this article. The effect of the humidification of the combustion air was simulated by adding steam to the combustion air. The impact of the steam injection on methane combustion has been studied at variable swirl number and steam fraction. Experimental results showed a linearly increasing lean blowout (LBO) equivalence ratio for methane combustion with increasing steam fraction. In addition, CO emission levels started to rise at higher equivalence ratio for higher steam fractions compared to combustion under dry conditions. The CO emission levels at stable combustion were however still the same order of magnitude as for the dry combustion. The swirl number has little effect on the LBO limit. Final results indicated the possibility to maintain complete and stable combustion under humidified conditions with low CO emissions at higher equivalence ratio compared to the dry combustion.
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17.
  • Delattin, Frank, et al. (författare)
  • A Comparison Between the Combustion of Natural Gas and Partially Reformed Natural Gas in an Atmospheric Lean Premixed Turbine-Type Combustor
  • 2008
  • Ingår i: Combustion Science and Technology. - : Informa UK Limited. - 0010-2202 .- 1563-521X. ; 180:8
  • Tidskriftsartikel (refereegranskat)abstract
    • A small-scale combustor was set up to analyze the combustion of natural gas and two mixtures of partially reformed natural gas. The partially reformed mixtures can be formed using biomass to feed the endothermic reforming reactions. Before combusting these mixtures in a gas turbine, experimental work was done on a primary zone combustion chamber to examine the combustor behavior when switching from natural gas to the wet and dry hydrogen-rich mixtures. Temperature profiles, flame location and ignition limits have been investigated for a variety of stoichiometries and several air temperatures. Possible problems concerning blow-off, flashback, increased pollutant products and excessive liner wall temperatures were analyzed. It was concluded that the switch in operation from natural gas to these wet and/or dry partially reformed natural gas mixtures lowers the blow-off limits while maintaining similar liner wall temperature profiles. Furthermore, no significant changes in pollutant production were observed. Flame area, shape and position display considerable differences in combustion regime for the three tested fuel types.
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18.
  • Du, Chengjun, 1985, et al. (författare)
  • Two-dimensional measurements of soot in a turbulent diffusion diesel flame: the effects of injection pressure, nozzle orifice diameter, and gas density
  • 2018
  • Ingår i: Combustion Science and Technology. - : Informa UK Limited. - 0010-2202 .- 1563-521X. ; 190:9, s. 1659-1688
  • Tidskriftsartikel (refereegranskat)abstract
    • © 2018, Published with license by Taylor & Francis Group, LLC © 2018 [Chengjun Du, Sven Andersson, Mats Andersson]. Two-dimensional light extinction, flame luminosity, and OH* chemiluminescence images were captured at a constant ambient temperature of 823 K and two gas densities (20 and 26 kg/m3), with injection pressures of 800–2500 bar using nozzle orifices with diameters of 0.19 and 0.10 mm. Soot volume fraction and OH distribution images were obtained using the Abel inversion method, and the local equivalence ratio in the lift-off length region was predicted. The results show that the equivalence ratio along the jet’s center at the lift-off length (ϕCL) was found to play a critical role in soot formation. Reductions in ϕCLthickened the OH zone in the upstream region of the jet, reducing the volume corresponding to the maximum soot volume fraction. The expansion of the OH zone also helped reduce the sooting zone’s width. Under high sooting conditions (e.g., ϕCL>3.5), the sooting zone width in the downstream jet was independent of ϕCL.
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19.
  • Duwig, Christophe, et al. (författare)
  • Experimental and numerical study of flameless combustion in a model gas turbine combustor
  • 2008
  • Ingår i: Combustion Science and Technology. - : Informa UK Limited. - 0010-2202 .- 1563-521X. ; 180:2, s. 279-295
  • Tidskriftsartikel (refereegranskat)abstract
    • Flameless combustion is an attractive solution to address existing problems of emissions and stability when operating gas turbine combustors. Theoretical, numerical and experimental approaches were used to study the flameless gas turbine combustor. The emissions and combustion stability were measured and the limits of the flameless regime are discussed. Using experimental techniques and Large Eddy Simulation (LES), detailed knowledge of the flow field and the oxidation dynamics was obtained. In particular the relation between the turbulent coherent structures dynamics and the flameless oxidation was highlighted. A model for flameless combustion simulations including detailed chemistry was derived. The theoretical analysis of the flameless combustion provides 2 non-dimensional numbers that define the range of the flameless mode. It was determined that the mixture that is ignited and burnt is composed of similar to 50% of fresh gases and similar to 50% vitiated gases.
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20.
  • Duwig, Christophe, et al. (författare)
  • Large eddy simulation of a H-2/N-2 lifted flame in a vitiated co-flow
  • 2008
  • Ingår i: Combustion Science and Technology. - : Informa UK Limited. - 0010-2202 .- 1563-521X. ; 180:3, s. 453-480
  • Tidskriftsartikel (refereegranskat)abstract
    • A lifted turbulent H-2/N-2 flame in a vitiated co-flow is studied using Large Eddy Simulation together with a closure based on perfectly stirred reactors. A part of the closure, chemical look-up tables, are generated to close the filtered temperature equations and to compute local radical concentrations throughout the computational domain. The approach has been used to simulate a lifted turbulent flame. The results have been found to be insensitive to the combustion model employed and to the grid resolution. However, the results are very sensitive to the temperature of the co-flow stream and this result is well in line with previous findings. The numerical predictions were further compared to detailed experimental data obtained by Cabra et al. (2002). The agreement between the two sets of data is very good, indicating that the present approach predicts successfully the combustion process including the OH mass fractions. Finally, the LES data were used to study the flame dynamics and stabilization mechanisms.
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21.
  • Duwig, Christophe, et al. (författare)
  • Large eddy simulation of turbulent premixed combustion using a marker field
  • 2007
  • Ingår i: Combustion Science and Technology. - : Informa UK Limited. - 0010-2202 .- 1563-521X. ; 179:10, s. 2135-2152
  • Tidskriftsartikel (refereegranskat)abstract
    • A new laminar flamelet model is presented for Large Eddy Simulation ( LES) of premixed turbulent combustion. The model uses a marker field S+ to capture the flame. Compared to previous work on marker fields, a modification was introduced to simplify the boundary conditions. Although the scalar S+ is smooth in space, it allows describing steep flame fronts. The S+ equation captures the laminar or turbulent flame propagation via a reactive diffusive balance. The present LES model was used to simulate a turbulent premixed flame stabilized behind a sudden expansion. The LES predictions agree well with available experimental data in term of reproducing the time averaged velocity field as well as for capturing the flame dynamics and flame response to a perturbation.
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22.
  • Duwig, Christophe, et al. (författare)
  • Study of flame stabilization in a swirling combustor using a new flamelet formulation
  • 2005
  • Ingår i: Combustion Science and Technology. - : Informa UK Limited. - 0010-2202 .- 1563-521X. ; 177:8, s. 1485-1510
  • Tidskriftsartikel (refereegranskat)abstract
    • The dynamics in a swirl-stabilized flame is studied using large eddy simulation (LES). We account for the effect of turbulence on the flame through a model based on a filtered flamelet technique. The model provides a consistent and robust reaction-diffusion expression for simulating the correct propagation of premixed flames. The filtered flamelet formulation has been implemented into a high-order-accurate LES code and used to study the flame stabilization and the combustion dynamics in a gas-turbine combustion chamber. The effects of inlet boundary conditions, in terms of velocity and equivalence ratio radial profiles, have been studied. The flow is found to be very sensitive to small changes in terms of flame shapes and anchoring position. The sensitivity of the results to the subgrid-scale flame thickness has also been investigated. The influence on the flame position is not significant. However, a too-large subgridscale flame thickness leads to different flame dynamics.
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23.
  • Fatehi, Hesameddin, et al. (författare)
  • Effect of In-cylinder Flow Structures on Late Cycle Soot Oxidation in a Quiescent Heavy-duty Diesel Engine
  • 2022
  • Ingår i: Combustion Science and Technology. - : Informa UK Limited. - 0010-2202 .- 1563-521X. ; 194:2, s. 316-336
  • Tidskriftsartikel (refereegranskat)abstract
    • This paper reports on CFD simulations of in-cylinder flow and combustion in an open-bowl heavy duty diesel engine at high load. The focus of the study is to unravel the effect of swirl motion on the soot formation and on the late cycle soot oxidation. To incorporate detailed kinetic mechanism while maintaining a manageable computational time, Representative Interactive Flamelets (RIF) method is employed to describe the chemical reactions, ignition, flame propagation, and emissions in the engine. In modeling the soot, a phenomenological model is employed where soot formation, nucleation, coagulation, and oxidation with O2 and OH are considered. Four values for swirl numbers, SN = 0, 0.5, 1.7, and 3.4, at three injection pressures and three injection timings are considered. It is shown that increasing the swirl number leads to an increase in both the amount of soot in the exhaust gas and the peak value of soot in the engine cylinder. The mechanisms of soot formation and oxidation and swirl/in-cylinder flow interaction in the given engine configuration are investigated based on the numerical results. It is shown that in-cylinder turbulence is the key parameter affecting the formation and oxidation of soot. Despite the higher mean flow kinetic energy in the high swirl cases than in the no-swirl case, the no-swirl case has a higher turbulent kinetic energy at late cycle. This is due to the presence of coherent structures created from the spray-wall interaction, which can survive longer time after the end of injection (EOI).
  •  
24.
  • Golovitchev, Valeri, 1945 (författare)
  • CFD Modeling of Diesel Oil and DME Performance in a Two-Stroke Free Piston Engine
  • 2007
  • Ingår i: Combustion Science and Technology. - 0010-2202 .- 1563-521X. ; 179, s. 417-436
  • Tidskriftsartikel (refereegranskat)abstract
    • The paper presents the CFD model and numerical results of combustion process simulations in a two-stroke, uniflow scavenging dual free piston engine, FPE, designed for electricity generation. Two fuels, diesel oil and dimethyl ether (DME),were studied in order to achieve HCCI-like combustion. It is illustrated that by varying the direc injection timing, a conparably efficient, low emission operation has been predicted for both fuel.
  •  
25.
  • Goswami, M., et al. (författare)
  • Numerical Simulations of Flat Laminar Premixed Methane-Air Flames at Elevated Pressure
  • 2014
  • Ingår i: Combustion Science and Technology. - : Informa UK Limited. - 1563-521X .- 0010-2202. ; 186:10-11, s. 1447-1459
  • Tidskriftsartikel (refereegranskat)abstract
    • Two-dimensional axisymmetric simulation of stoichiometric methane-air flames stabilized on flat burners at elevated pressure is reported in the present work. Such flames, in practice, are experimentally obtained using the heat flux method for measurement of laminar burning velocity of fuel-oxidizer mixtures (Bosschaart and de Goey, 2004; Goswami et al., 2013). The method makes use of a burner with a perforated brass burner plate. The dimensions of such a plate play an important role in creating flat flames. The present investigation is focused on studying laminar premixed flame structure numerically at elevated pressure up to 15 bar using a one-step and a detailed chemical reaction mechanism. Three burner plate models (of varying hole diameter and porosity) are used in the simulations for pressures up to 7 bar with a one-step mechanism. The surface area increase of the flame was evaluated based on an isotherm at 900 K and the net reaction rate of methane compared to a flat flame. The comparison of these models shows that the surface area increase can significantly be reduced by choosing a smaller hole diameter and larger porosity. The results of the detailed simulations using an appropriate chemical reaction mechanism up to 15 bar using a burner plate model, which is similar to the ones used in experiments (mentioned above), show a nonlinear increase of the flame curvature with elevating pressure. A hole diameter of 0.25 mm and a pitch of 0.29 mm is suggested for a burner plate in such experiments. Flame structure at elevated pressure is also analyzed further based on species profiles obtained.
  •  
26.
  • Hesameddin, Fatehi, et al. (författare)
  • Comprehensive mathematical model for biomass combustion
  • 2014
  • Ingår i: Combustion Science and Technology. - : Informa UK Limited. - 1563-521X .- 0010-2202. ; 186:4-5, s. 574-593
  • Tidskriftsartikel (refereegranskat)abstract
    • This article reports on an investigation of a comprehensive mathematical model for biomass combustion within the one-dimensional model framework. The model takes into account different thermochemical processes, e.g., moisture evaporation, pyrolysis, heterogeneous char reactions, intra-particle heat and mass transfer, and changes in thermo-physical properties. Different approaches to model the various processes involved in the thermochemical conversion of biomass are discussed, and a sensitivity study is carried out to investigate the performance of sub-models for the drying process. The comprehensive model is used to investigate the effect of moisture diffusion and vapor condensation inside the particle pores. The model is evaluated under different conditions, and satisfactory comparison of the model results with experimental data and model results from other researchers is observed.
  •  
27.
  • Hodzic, Erdzan, et al. (författare)
  • A Large Eddy Simulation Study of Bluff Body Flame Dynamics Approaching Blow-Off
  • 2017
  • Ingår i: Combustion Science and Technology. - : Taylor & Francis. - 0010-2202 .- 1563-521X. ; 189:7, s. 1107-1137
  • Tidskriftsartikel (refereegranskat)abstract
    • The mechanisms leading to blowoff were investigated numerically by analyzing bluff body stabilized flame at two conditions: a condition far from blowoff to a condition just prior to blowoff. Large eddy simulations have been used to capture the time dependent, three-dimensional evolution of the field. The results were first validated to available experimental data, showing very good agreement for the flow and overall good agreement for the flame. Changes in the large-scale structures are investigated by means of proper orthogonal decomposition and the wavelet method, elucidating the underlying dynamics of the complex flow-flame interaction of a flame approaching blowoff. Our results reveal that, when the flame approaches blowoff conditions, significant changes are found in the large-scale structures responsible for entrainment of species into the recirculation zone located downstream of the bluff body. Possible causes of this shift in large-scale structures are also discussed, which may be useful for extending the blowoff limits of bluff body stabilized burners.
  •  
28.
  • Hodzic, Erdzan, et al. (författare)
  • Large Eddy Simulation of Bluff-Body Flame Approaching Blow-Off : A Sensitivity Study
  • 2019
  • Ingår i: Combustion Science and Technology. - : TAYLOR & FRANCIS INC. - 0010-2202 .- 1563-521X. ; 191:10, s. 1815-1842
  • Tidskriftsartikel (refereegranskat)abstract
    • As almost all combustion processes of practical interest take place in the presence of turbulence, the development of the increasingly refined turbulence-chemistry interaction (TCI) models has led to highly sophisticated approaches. Nearly all of the studies comparing different models focus on stable premixed/non-premixed flame configurations. In this work, the focus is on well-documented, lean premixed bluff-body stabilized flames approaching blow-off and on the blow-off sequence itself. Large Eddy Simulations (LES) have been used to capture the time-dependent, three-dimensional flow-field using Transported Probability Density Function (TPDF), Partially Stirred Reactor Model (PaSR), and Implicit LES (ILES) models. Furthermore, the influence of finite-rate chemistry and different chemical mechanisms is evaluated to determine the limitation and capability of the different TCI approaches for modeling flames just prior to and during the transient blow-off process. While the average flow-fields do not reveal any significant differences between modeling approaches, detailed analysis of the flame reveals that there are differences in the predicted flame thickness and composition. The ability of the considered TCI models to predict local as well as full-flame extinction during the blow-off is investigated as well. It is demonstrated that such a blow-off sequence is not always governed by complex chemistry.
  •  
29.
  • Horvat, Andrej, et al. (författare)
  • Numerical and experimental investigation of backdraft
  • 2008
  • Ingår i: Combustion Science and Technology. - : Informa UK Limited. - 1563-521X .- 0010-2202. ; 180:1, s. 45-63
  • Tidskriftsartikel (refereegranskat)abstract
    • The article describes full-scale backdraft experiments in a shipping container using methane as a fuel. Numerical modelling has followed the experimental setup. The numerical simulations show the initial gravity current, the ignition, the spreading of flame in the enclosure, the external fireball, and the subsequent decay. The Detached Eddy Simulation (DES) approach has been used to model turbulence. In order to describe the combustion process of the mixture from the local ignition to progressive deflagration, three separate combustion models have been implemented for laminar, low- and high-intensity turbulence flow regimes. The calculated ignition time is slightly shorter than the average ignition time observed in the experiments. The fire front progresses through the combustible mixture, generating a cloud of hot gases that are accelerated from the container into the external environment. The velocity increases up to 20 m/s. When the fire front reaches the door, combustion continues outside the enclosure as the fuel has been pushed through the door. The comparison between the calculated time history of relative pressure and the pressure sensor record shows that the numerical simulations slightly overpredict the flame front speed, with a stronger pressure pulse and higher temperatures than the observations.
  •  
30.
  • Huang, Chen, 1981, et al. (författare)
  • Application of Flame Speed Closure Model to RANS Simulations of Stratified Turbulent Combustion in a Gasoline Direct-Injection Spark-Ignition Engine
  • 2016
  • Ingår i: Combustion Science and Technology. - : Informa UK Limited. - 0010-2202 .- 1563-521X. ; 188:1, s. 98-131
  • Tidskriftsartikel (refereegranskat)abstract
    • © 2016 Taylor & Francis. The present work aims at development and validation of a tool for numerically modeling stratified turbulent combustion in a gasoline direct injection (GDI) engine. For this purpose, an open source code called OpenFOAM ® , which has been attracting growing interests from both industries and academies due to an opportunity to access the source code and to test new models without paying license fees, is further developed by implementing advanced models relevant to stratified turbulent burning. In particular, first, the Flame Speed Closure model of premixed turbulent combustion is implemented in order to simulate flame propagation through inhomogeneously premixed reactants. Second, a newly calculated approximation of the laminar flame speed of gasoline-air mixtures as a function of the equivalence ratio, pressure, and temperature is implemented in order to simulate dependence of burning rate on the local mixture composition. Third, a newly calculated approximation of the combustion temperature of gasoline-air mixtures as a function of the equivalence ratio, pressure, and product enthalpy is implemented in order to allow for dissociation of combustion products and heat losses. Fourth, a presumed mixture-fraction probability density function (PDF) approach is implemented in order to simulate the influence of turbulent fluctuations in the mixture fraction on the local burning rate. In addition to commonly used mass-weighted mixture-fraction PDF, a more consistent model that deals also with the canonical mixture-fraction PDF is developed and the two approaches are compared. Numerical results that show the influence of the aforementioned implementations on computed global characteristics of stratified combustion in a research GDI engine are discussed. The developed numerical tool is quantitatively validated by comparing computed pressure traces in the GDI engine with experimental data obtained in three different cases associated with two different loads, late injection timings, and short time intervals between the injection and spark ignition.
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31.
  • Ji, Liang, et al. (författare)
  • Rate-Ratio Asymptotic Analysis of Strained Premixed Laminar Methane Flame Under Nonadiabatic Conditions
  • 2023
  • Ingår i: Combustion Science and Technology. - : Informa UK Limited. - 0010-2202 .- 1563-521X. ; 195:15, s. 3810-3834
  • Tidskriftsartikel (refereegranskat)abstract
    • Motivated by the pioneering activation-energy asymptotic analysis of strained laminar premixed flames in counterflow by Libby and his coworkers, a rate-ratio asymptotic analysis is carried out to elucidate the structure and predict the critical conditions of extinction of strained premixed methane flames. Steady, axisymmetric, laminar flow of two counterflowing streams: a reactive mixture stream and a product stream toward a stagnation plane is considered. The temperature of the reactive mixture stream is (Formula presented.) and it is made up of methane (CH4), oxygen (O2) and nitrogen (N2), while the temperature of the product stream is (Formula presented.), and it is made up of O2, carbon dioxide, water vapor and N2. The asymptotic flame structure is presumed to be made up of a thin reaction zone where all chemical reactions take place. On one side of the reaction-zone is an inert, preheat zone containing the reactants and on the other side a post-flame zone made up of products. Analysis of the preheat zone gives matching conditions that is required to analyze the structure of the reaction zone. A four-step, reduced mechanism is used to describe the chemical reactions. The reaction zone is presumed to be made up of an inner layer, where CH4 is consumed. The hydrogen (H2) and carbon monoxide that are formed in this layer are consumed in an oxidation layer that is made up of two layers: an H2-oxidation layer and a CO-oxidation layer. The results of the analysis are used to predict the flame location, (Formula presented.), flame temperature, (Formula presented.), and the speed of the convective flow, (Formula presented.), in the reaction zone as a function of the strain-rate, (Formula presented.). Classical C-shaped curves were obtained when (Formula presented.), (Formula presented.) and (Formula presented.) are plotted as a function of (Formula presented.) and they were used to predict extinction. A key finding of this work is that (Formula presented.) is proportional to (Formula presented.), where (Formula presented.) is the crossover temperature predicted by the rate-ratio asymptotic analysis. Whether abrupt extinction will take place or not was found to depend on the value of (Formula presented.) relative to (Formula presented.), which is different from the predictions of activation-energy asymptotic analysis where (Formula presented.) must be compared with the value of the adiabatic temperature. Similar to the analysis of Libby and his coworkers, the rate-ratio asymptotic analysis predicts the existence of “negative flame speeds,” where the convective flow and the diffusive flow of reactants in the reaction zone, are in opposite directions. The predictions of the rate-ratio asymptotic analysis were found to agree with the results of computations with detailed chemistry and previous experimental data.
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32.
  • Joelsson, Tobias, et al. (författare)
  • Large eddy simulation of turbulent combustion in a spark-assisted homogenous charge compression ignition engine
  • 2012
  • Ingår i: Combustion Science and Technology. - : Informa UK Limited. - 1563-521X .- 0010-2202. ; 184:7-8, s. 1051-1065
  • Tidskriftsartikel (refereegranskat)abstract
    • A large eddy simuation (LES) model is presented for simulation of spark-assisted homogeneous charge compression ignition combustion. The model is based on tabulated chemical kinetic rate for ignition and flame surface density for flame propagation, taking into account interaction between flame propagation introduced by the spark and auto-ignition due to charge compression. The model is used to simulate the combustion process in an experimental HCCI engine, with operation conditions ranging from spark-ignition controlled flame propagation to auto-ignition controlled HCCI combustion. The model is shown to be able to predict the combustion behavior observed in previous engine experiments. With low initial temperature, the SI flame mode prevails; with high initial temperature, the HCCI mode prevails. With moderate initial temperature, the SI flame and HCCI ignition interact more closely, which results in higher sensitivity to the initial temperature and turbulence conditions. This may be the reason of the observed high cyclic variation in the experiments.
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33.
  • Kaldvee, Billy, et al. (författare)
  • Room-Fire Characterization Using Highly Range-Resolved Picosecond Lidar Diagnostics and CFD Simulations
  • 2013
  • Ingår i: Combustion Science and Technology. - : Informa UK Limited. - 1563-521X .- 0010-2202. ; 185:5, s. 749-765
  • Tidskriftsartikel (refereegranskat)abstract
    • In fire safety engineering, the use of computational fluid dynamics (CFD) allows for detailed multidimensional calculations of important parameters, for example, temperature. However, increasing use of CFD models puts requirements on experimental validation, a challenge for many measurement techniques under harsh fire conditions. In this work, laser-based picosecond light detection and ranging (ps-lidar) was used for remote measurements in a 1/2-scale ISO 9705 room containing either a methanol pool fire or a methane diffusion flame. Spatially resolved Rayleigh thermometry was conducted in the vertical door plane and in a horizontal plane inside the room. Temperatures obtained by ps-lidar are compared with values from thermocouples located in the doorway as well as results from CFD simulations. The technique allows for quantitative thermometry provided that minimal particle scattering interferences are present. Measurements of detailed distributions of temperature and particulates clearly demonstrate the potential of ps-lidar for diagnostics in large-scale combustion.
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34.
  • Kong, Chengdong, et al. (författare)
  • Non-thermal gliding arc discharge assisted turbulent combustion (up to 80 kW) at extended conditions : phenomenological analysis
  • 2024
  • Ingår i: Combustion Science and Technology. - : Informa UK Limited. - 0010-2202 .- 1563-521X. ; 196:2, s. 161-176
  • Tidskriftsartikel (refereegranskat)abstract
    • Plasma assisted combustion has been proposed as an efficient technique to enhance combustion, especially under the extreme conditions. For shedding light on the interactions between plasma and turbulent flame at extended conditions, a burner design with integrated electrodes was used to couple a non-thermal gliding arc (GA) discharge to a turbulent flame. The morphology and dynamic behaviors of the GA assisted flame under extended flow rates and gas temperatures were investigated by high-speed video imaging. It is found that two distinct types of flame (named as Flame A and Flame B) can be sustained by the GA discharge depending on the local flow conditions. Flame A was sustained by the GA on stable anchor points, while Flame B moved together with the thin plasma volume of the gliding arc, behaving as an unstable flame. When the fed air gas temperature was increased, Flame A became more stable while Flame B became fragile and extinguished easily. Furthermore, the phenomenological findings under different flow conditions imply typical four flame types for the GA discharge assisted combustion system, including the self-sustained flame at relatively low Reynolds number (Re), the GA sustained stable flame at moderate Re number, the GA sustained unstable flame and the GA assisted auto-ignited and propagating flame at relatively large Re number. In all, the GA discharge seems to provide various effects on combustion depending on the overall turbulence as well as the local equivalence ratio, the gas temperature, etc.
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35.
  • Kreitzberg, Thobias, et al. (författare)
  • A Shortcut Method to Predict Particle Size Changes during Char Combustion and Gasification under regime II Conditions
  • 2022
  • Ingår i: Combustion Science and Technology. - : Taylor & Francis. - 0010-2202 .- 1563-521X. ; 194:2, s. 272-291
  • Tidskriftsartikel (refereegranskat)abstract
    • In most industrial applications, combustion and gasification of char progresses under regime II conditions. Unlike in other regimes, both particle size and density change simultaneously in regime II due to non-uniform consumption of carbon inside the particles. In this work, mathematical predictions of diameter changes in regime II were made by a one-dimensional simulation tool, where transient species balances are resolved locally inside the particle. This simulation is computationally expensive and usually not appropriate for the implementation in comprehensive CFD simulations of combustion or gasification processes. To overcome this restraint, an alternative shortcut method with affordable computation time has been developed and validated against the detailed model. This method allows the calculation of diameter changes during combustion and gasification from precalculated effectiveness factors. Additionally, the change of particle size has been investigated experimentally in a single particle converter setup. Therein, particles are fixed on a sample holder placed in the hot flue gas of a flat flame burner. Size and temperature trends are optically assessed by a 3CCD camera.
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36.
  • Kuznetsov, M., et al. (författare)
  • Experimental Study of the Preheat Zone Formation and Deflagration to Detonation Transition
  • 2010
  • Ingår i: Combustion Science and Technology. - : Informa UK Limited. - 0010-2202 .- 1563-521X. ; 182:11-12, s. 1628-1644
  • Tidskriftsartikel (refereegranskat)abstract
    • The authors present experimental studies of the deflagration-to-detonation transition (DDT) in tubes with smooth and rough walls in stoichiometric hydrogen-oxygen and ethylene-oxygen mixtures. On the basis of experimental evidence, it is shown that formation of the preheat zone, where reaction is chemically frozen, promotes the transition to detonation if temperature and width of the preheat zone are above certain critical values. A sequence of high-speed Schlieren records permits an accurate determination of the minimal values of temperature and width of the preheat zone, leading to transition to detonation. The experimentally measured critical temperatures and widths of the preheat zone initiating restructuring of the flame and transition to detonation in hydrogen-oxygen and ethylene-oxygen mixtures are consistent with the developed theory.
  •  
37.
  • Lehtiniemi, Harry, et al. (författare)
  • Modeling diesel spray ignition using detailed chemistry with a progress variable approach
  • 2006
  • Ingår i: Combustion Science and Technology. - : Informa UK Limited. - 1563-521X .- 0010-2202. ; 178:10-11, s. 1977-1997
  • Tidskriftsartikel (refereegranskat)abstract
    • In this work, a progress variable approach is used to model diesel spray ignition with detailed chemistry. The flow field and the detailed chemistry are coupled using the flamelet assumption. A flamelet progress variable is transported by the computational fluid dynamics (CFD) code. The progress variable source term is obtained from an unsteady flamelet library that is evaluated in each grid cell. The progress variable chosen is based on sensible enthalpy. By using an unsteady flamelet library for the progress variable, the impact of local effects, for example variations in the turbulence field, effects of wall heat transfer etc. on the autoignition chemistry can be considered on a cell level. The coupling between the unsteady flamelet library and the transport equation for total enthalpy follows the ideas of the representative interactive flamelet (RIF) approach. The method can be compared to having an interactive flamelet in each computational cell in the CFD grid. The results obtained using the proposed model are compared to results obtained using the RIF model. Differences are exhibited during the autoignition process. After ignition, the results obtained using the proposed model and RIF are virtually identical. The model was used to study lift-off lengths in sprays as function of nozzle diameter and injection pressure. A good agreement between model predictions and experimental trends was found.
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38.
  • Liberman, Michael, et al. (författare)
  • Dynamics of flames in tubes with no-slip walls and the mechanism of tulip flame formation1
  • 2023
  • Ingår i: Combustion Science and Technology. - : Informa UK Limited. - 0010-2202 .- 1563-521X. ; 195:7, s. 1637-1665
  • Tidskriftsartikel (refereegranskat)abstract
    • A hydrogen/air flame propagation and the development of tulip-shaped flame in 2D tubes of different aspect ratios with both closed ends and in a half-open rectangular channel were studied using high resolution direct numerical simulations of the fully compressible Navier–Stokes equations coupled with a detailed chemistry. Flame propagation in a 3D rectangular channel was studied using large eddy simulations and compared with the results of direct numerical simulations of flame propagation in a 2D rectangular channel with the same aspect ratio. It is shown that the interaction of the rarefaction wave generated by the flame at the deceleration stage with the “positive” flow of unburned gas generated by the flame at the previous accelerating stage leads to a significant decrease of the velocity of the unburned gas flow in the near field zone ahead of the flame front. As a result, the thickness of the boundary layer in the near-field zone ahead of the flame increases significantly, and the profile of the axial velocity of the unburned gas in the near-field zone ahead of the flame front takes the form of a tulip or an inverted tulip, which leads to corresponding changes in the velocities of different parts of the flame front, the flame front inversion, and the formation of a tulip-shaped flame.
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39.
  • Lipatnikov, Andrei, 1961, et al. (författare)
  • Modelling of the influence of mixture fraction fluctuations on burning rate in partially premixed turbulent flames
  • 2015
  • Ingår i: Combustion Science and Technology. - : Informa UK Limited. - 0010-2202 .- 1563-521X. ; 187:4, s. 594-626
  • Tidskriftsartikel (refereegranskat)abstract
    • A common approach to modeling the influence of turbulent fluctuations in the mixture fraction f on the burning rate in a partially premixed flame consists of invoking a presumed Favre (mass-weighted) probability density function (PDF) . In the present work, two issues relevant to such an approach are addressed. First, averaging of a dependence q(f), where q is an arbitrary quantity, e.g., the laminar flame speed SL, requires modeling of the canonical PDF P(f) if q is not proportional to the density . Second, because the shape of is not known a priori in a typical case, the presumed PDF approach can be a predictive tool only if the mean quantities are weakly sensitive to the PDF shape. To study the two issues, dependencies of and , computed for gasoline surrogate-air mixtures under elevated temperatures and pressures, associated with the conditions in a gasoline direct injection spark ignition engine, are averaged invoking either beta function or double-Dirac delta function Favre or canonical PDFs. Moreover, a simpler approach is proposed to evaluate . The approach consists of expanding in Taylor series with respect to , followed by averaging. The mean quantities and obtained for various Favre first and second moments using the aforementioned alternative methods are compared with each other and with and , respectively. The following conclusions are drawn. First, when averaging under conditions of the present study, the difference between the Favre and canonical beta-function PDFs may be disregarded for simplicity. Second, is sensitive to the shape of presumed PDF if the magnitude of turbulent fluctuations in the mixture fraction is sufficiently large. Third, if the magnitude of turbulent fluctuations in the mixture fraction is sufficiently low in order for the mean laminar flame speeds obtained invoking the beta-function and double-Dirac delta-function PDFs to be approximately equal to one another, then, can also be evaluated using the Taylor-expansion approach.
  •  
40.
  • Lipatnikov, Andrei, 1961 (författare)
  • Simulations of scalar transport in developing turbulent flames solving a conditioned balance equation
  • 2010
  • Ingår i: Combustion Science and Technology. - : Informa UK Limited. - 0010-2202 .- 1563-521X. ; 182:7, s. 405-421
  • Tidskriftsartikel (refereegranskat)abstract
    • A balance equation for the difference in the conditioned velocities (u) over bar (b) and (u) over bar (u), derived and validated recently (Lipatnikov, 2008a, 2008b), is numerically solved in a statistically planar, one-dimensional case in order to (a) highlight the influence of premixed turbulent flame development on the direction of the mean scalar flux and (b) assess the equation by comparing computed trends with available experimental and DNS data. Numerical results show that (a) the flux (rho u '' c '') over bar gradient during an early stage of flame development followed by a transition to countergradient scalar transport (i. e., (rho u '' c '') over bar center dot del(c) over bar >0) at certain instant t(tr); (b) the transition time t(tr) is increased by the rms turbulent velocity and decreases when the density ratio or the laminar flame speed increases; and (c) even after the transition from gradient to countergradient scalar transport, the mean flame brush thickness grows because the mean rate of product creation overwhelms the transport term in the combustion progress variable balance equation and serves to not only control the turbulent burning rate, but also cause the growth of the thickness.
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41.
  • Marklund, Magnus, et al. (författare)
  • Determination of the influence of uncertain model parameters in pressurized gasification of black liquor using a factorial design
  • 2005
  • Ingår i: Combustion Science and Technology. - : Informa UK Limited. - 0010-2202 .- 1563-521X. ; 177:3, s. 435-453
  • Tidskriftsartikel (refereegranskat)abstract
    • Introduction of pressurized gasification of black liquor in the pulping industry has the potential to give a significant increase in energy efficiency. However, uncertainties about the reliability and robustness of the technology are preventing large-scale market introduction. One important step toward a greater trust in the process reliability is the development of a better understanding of the sensitivity of the process to parameter variations. A computational fluid dynamics model for pressurized gasification of black liquor in an entrained-flow gasifier is presented and used for investigation of the effects of uncertainties in the specific heat capacity of black liquor, the radiation absorption coefficient, and the volatile devolatilization rate using factorial design methodology. It is found that all main factor effects, but none of the interaction effects, influence the considered responses: char conversion, maximum temperature, and outlet temperature. However, the main effects are found to be relatively small and the uncertainties in the examined model parameters would not invalidate the results from a design optimization with the presented model. Copyright © Taylor & Francis Inc.
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42.
  • Mauss, Fabian, et al. (författare)
  • Aspects of modeling soot formation in turbulent diffusion flames
  • 2006
  • Ingår i: Combustion Science and Technology. - : Informa UK Limited. - 1563-521X .- 0010-2202. ; 178:10-11, s. 1871-1885
  • Tidskriftsartikel (refereegranskat)abstract
    • Soot formation in a turbulent jet diffusion flame is modeled using an unsteady flamelet approach. In the present work, we study the effects of the choice of the dependence of scalar dissipation rate on mixture fraction and agglomeration processes on the predicted soot volume fraction. It is found that good predictions of soot volume fraction can be obtained without considering preferential diffusion effects.
  •  
43.
  • Moréac, Gladys, et al. (författare)
  • Automatic generation of a detailed mechanism for the oxidation of n-decane
  • 2006
  • Ingår i: Combustion Science and Technology. - : Informa UK Limited. - 1563-521X .- 0010-2202. ; 178:10-11, s. 2025-2038
  • Tidskriftsartikel (refereegranskat)abstract
    • A single detailed kinetic mechanism for the oxidation and combustion of n-decane and n-heptane has been written by means of an automatic mechanism generator ( REACTION) developed in our laboratory. It shows a good prediction of the ignition delay time versus temperature for the oxidation of n-decane at 13 and 50 bar and n-heptane at 13 and 40 bar for different equivalence ratios. The n-decane/n-heptane mechanism consists of a validated and recently published O-2=H-2=C-1-C-4 mechanism produced manually and a generated C-5-C-10 set of sub-mechanisms. The mechanism includes a complete description of both n-decane and n-heptane chemistry. This mechanism has a reasonable size, 506 species and 3684 reactions, but nevertheless it has an extensive range of chemistry. This paper represents not only the validation of a specific mechanism but also a validation of the rate constants of the reaction classes used to model the oxidation of alkanes at low and high temperature.
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44.
  • Mörtberg, Magnus, et al. (författare)
  • Combustion of normal and low calorific fuels in high temperature and oxygen deficient environment
  • 2006
  • Ingår i: Combustion Science and Technology. - : Informa UK Limited. - 0010-2202 .- 1563-521X. ; 178:7, s. 1345-1372
  • Tidskriftsartikel (refereegranskat)abstract
    • Combustion characteristics of two different gaseous fuels (a low calorific value fuel and methane fuel) have been examined using high temperature and low oxygen concentration combustion air. The momentum flux ratio between the fuel jet and the combustion airflow was kept constant to provide similarity in mixing between the different experimental cases to understand the role of fuel jet property on combustion. Direct flame photography, 2-D Particle image velocimetry (PIV), Light Emission Spectroscopy and chemiluminescent NOX analyzer was used as the diagnostics. These diagnostics allowed information on global flame features, mean and rms components of axial and radial velocity, axial strain rates and vorticity, the spatial distribution of combustion intermediate species, such as, OH and CH, and overall NOX emission levels. The results indicate a slower mixing during high temperature air combustion with low calorific value fuel as compared to methane fuel. The results showed higher turbulence levels and higher axial strain rates for low calorific fuel jets as compared to methane fuel jet during the high temperature air combustion condition. This results in less intense (or mild) combustion conditions with the result of increased flame length and volume and lower NOX emissions. Even for the normal methane fuel high temperature and oxygen deficient combustion conditions provided lower NOX emission. Furthermore, the high temperatures obtained for methane combustion provided lower vorticity and axial strain rates than the low calorific value fuel due to the suppression of vortical structure formation from the stronger heat release. In the case of low calorific value fuel, higher fuel jet velocity into low-density high temperature air leads to longer jet length. This jet causes a local stagnation to the upstream cross-flow to create local higher value of turbulence levels immediately upstream of the jet. The spatial distribution of the flame generated radicals (OH and CH) revealed significant ignition delay of the LCV fuel jet and a far more uniform distribution of the intermediate species. The methane fuel jet showed a prolonged reaction zone and faster ignition at high temperature and oxygen deficient conditions when compared to normal temperature air combustion of methane.
  •  
45.
  • Olofsson, Nils-Erik, et al. (författare)
  • Are Sooting Premixed Porous-Plug Burner Flames One-Dimensional? A Laser-Based Experimental Investigation
  • 2013
  • Ingår i: Combustion Science and Technology. - : Informa UK Limited. - 1563-521X .- 0010-2202. ; 185:2, s. 293-309
  • Tidskriftsartikel (refereegranskat)abstract
    • The one-dimensional character of sooting premixed porous-plug (McKenna) burner flames has been examined, motivated by recent investigations where the one-dimensionality has been questioned. The examination employs laser diagnostic techniques to measure different characteristics: temperature using rotational coherent anti-Stokes Raman spectroscopy (CARS) and soot properties using laser-induced incandescence and elastic light scattering. Special attention has been paid to the influence of an outer shroud gas flow of either nitrogen or air. The nitrogen shroud gas leads to flame cooling at the outer edge, whereas the air shroud results in heating through the oxidation of CO/H-2/soot. This generally results in a more inhomogeneous spatial particle size profile for the nitrogen shroud flame, with smaller particle sizes at the flame edge. The present results emphasize the need to characterize burner and operating conditions accurately, and also, regarding parameters that at first glance seem less relevant, such as the shroud gas, to provide useful data for comparison between laboratories.
  •  
46.
  • Podbolotov, K. B., et al. (författare)
  • Solution Combustion Synthesis of Copper Nanopowders : The Fuel Effect
  • 2017
  • Ingår i: Combustion Science and Technology. - : Taylor and Francis Inc.. - 0010-2202 .- 1563-521X. ; 189:11, s. 1878-1890
  • Tidskriftsartikel (refereegranskat)abstract
    • Pure copper nanoparticles have previously been successfully produced by different combustion methods, but most of them require the use of an inert atmosphere (N2, Ar) during the synthesis process or the usage of addition post reducing of metal oxides. In this article, novel modification of solution combustion synthesis technique for one-step metallic Cu nanoparticles preparation was developed. The main unique feature of our approach is the use of microwave-assisted foam preparation. Also, the effect of different types of fuels (urea, citric acid, glycine, and hexamethylenetetramine) on the combustion process and characteristics of resultant solid products was investigated. It was shown that the combination of microwave-assisted foam preparation and use of hexamethylenetetramine as a fuel allows producing pure metallic Cu nanoparticles (~67 nm) by one-step solution combustion synthesis under normal air atmosphere without any post reduction.
  •  
47.
  • Robert, Etienne, et al. (författare)
  • Soot Formation in Unstrained Diffusion Flames
  • 2015
  • Ingår i: Combustion Science and Technology. - : Informa UK Limited. - 1563-521X .- 0010-2202. ; 187:4, s. 577-593
  • Tidskriftsartikel (refereegranskat)abstract
    • The formation of soot particles has been investigated in CH4/O-2 diffusion flames using a unique burner design, which allows the creation of a nearly unstrained planar reaction sheet. Spatially resolved soot volume fractions were obtained using laser-induced incandescence. These soot measurements and the sooting limits were obtained as a function of bulk flow across the flame and mixture strength. Samples were collected using thermophoretic sampling and analyzed using electron microscopy, revealing a broad range of microstructures including particles with unusually large primary diameters and carbon nanotubes. A theoretical model is presented, which confirms that under certain conditions the 1D nature of the flow field of the burner and the strong adverse temperature gradient on the fuel side of the flame result in the soot particles being held in place by thermophoretic forces and allowed to grow for very long time periods. Some of these so-called super aggregates reached sizes of tens of microns and became visible to the naked eye in the soot layer.
  •  
48.
  • Rocha, Rodolfo C., et al. (författare)
  • Combustion and Emission Characteristics of Ammonia under Conditions Relevant to Modern Gas Turbines
  • 2021
  • Ingår i: Combustion Science and Technology. - : Informa UK Limited. - 0010-2202 .- 1563-521X. ; 193:14, s. 2514-2533
  • Tidskriftsartikel (refereegranskat)abstract
    • Ammonia (NH3) is considered a promising alternative fuel, capable of producing energy with zero CO2 emissions. Its combustion, however, poses a series of challenges due to the low reactivity of NH3 and the formation of very high quantities of NOx. This work numerically investigates the combustion and emission characteristics of ammonia in three modern stationary gas turbine concepts, namely (a) lean-burn dry-low emissions (DLE); (b) rich-burn, quick-quench and lean-burn (RQL); and (c) moderate or intense low oxygen dilution (MILD), under operating conditions typical of commercial gas turbines (inlet temperatures of 500 K and pressure of 20 bar). Numerical simulations employing detailed chemical kinetic mechanisms are carried out to study the propagation speed of ammonia, the combustor temperatures, and the emissions of NOx and NH3. The simulations are first validated against literature NOx data and then the most accurate mechanism is selected. The performance of the different gas turbine engine concepts is subsequently compared based on the results from the selected mechanism. The results show that the lowest emissions are achieved with the RQL and MILD combustion concepts, while the DLE combustion concept only presents acceptable emission values under conditions deemed unstable, where the laminar flame speeds are below 3 cm/s.
  •  
49.
  • Sabel'nikov, Vladimir, 1946, et al. (författare)
  • A simple model for evaluating conditioned velocities in premixed turbulent flames
  • 2011
  • Ingår i: Combustion Science and Technology. - : Informa UK Limited. - 0010-2202 .- 1563-521X. ; 183:6, s. 588-613
  • Tidskriftsartikel (refereegranskat)abstract
    • A simple model is proposed to evaluate (a) the divergence of velocity vector conditioned on unburned mixture, and (b) the vector component normal to the mean flame brush in the flamelet regime of premixed turbulent combustion. The model involves a single constant and does not invoke an extra balance equation. To perform the first test of the model, six flames stabilized in impinging jets and experimentally investigated by 4 research groups were numerically simulated. In the computations, (a) approximations of the measured axial profiles of the mean combustion progress variable were invoked, (b) the well-known (Bray et al., 1998, and 2000) statistically steady and 1-dimensional Favre-averaged continuity and Euler equations were numerically integrated in order to approximate the measured axial profiles of the mean axial velocity, and, then, (c) the approximations were utilized in order to evaluate conditioned velocities and turbulent scalar flux using the proposed model supplemented with the BML approach and balance equation for the Favre-averaged combustion progress variable. The obtained agreement between the measured and computed axial profiles of the conditioned axial velocities or axial turbulent scalar flux was encouraging, thus, indicating that the proposed simple model is promising. Since the correlation between fluctuations of velocity and unity normal vectors, conditioned to flamelet surface, plays a key role in the model, the encouraging test results call for studying this correlation in future DNS. Moreover, further research into the difference in velocity conditioned on unburned mixture and velocity conditioned on the unburned side of flamelets is necessary for improving the model at the leading edge of a turbulent flame brush.
  •  
50.
  • Schneider, Adrian, et al. (författare)
  • Ignition and extinction in catalytic partial oxidation of methane-oxygen mixtures with large H2O and CO2 dilution
  • 2008
  • Ingår i: Combustion Science and Technology. - : Informa UK Limited. - 0010-2202 .- 1563-521X. ; 180:1, s. 89-126
  • Tidskriftsartikel (refereegranskat)abstract
    • The ignition and extinction in catalytic partial oxidation (CPO) of CH4/O-2 mixtures with large exhaust gas dilution (46.3% H2O and 23.1% CO2 vol.) has been investigated experimentally and numerically at 5 bar. Experiments were carried out in a short contact time Rh-coated honeycomb reactor and involved temperature measurements along the reactor and exhaust gas analysis. Numerical predictions were performed with a 2-D transient elliptic code that included detailed chemical reaction schemes and relevant heat transfer mechanisms in the solid. The employed heterogeneous reaction scheme reproduced the measured minimum inlet temperatures required for catalytic ignition (light-off), the elapsed times for the propagation of the reaction front, and the steady-state exhaust gas compositions at a fuel-to-air equivalence ratio of phi = 4.0. The chemical impact of the added H2O, although important already at the early light-off stages, was minimal on the ignition delay times because the latter were dominated by total oxidation and not by partial oxidation or reforming reactions. The key reaction controlling catalytic ignition was the surface oxidation of CO to CO2, which was the main exothermic heat release step in the induction zone. Measurements and predictions indicated that vigorous combustion could be sustained at inlet temperatures at least as low as 473K and 298K in CPO with and without exhaust gas dilution, respectively. The extended stability limits of CPO combustion were due to a shift from partial to total oxidation products, and hence to higher exothermicity, with decreasing inlet temperature. The key parameter controlling extinction was the CO(s) coverage, which led to catalyst poisoning. Finally, operation at non-optimal stoichiometries (phi = 2.5) was shown to be beneficial in CPO of power generation systems with large exhaust dilution, due to the moderating effect of dilution on the maximum reactor temperature.
  •  
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