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1.
  • Bai, Xue-Song, et al. (författare)
  • Laminar flamelet structure at low and vanishing scalar dissipation rate
  • 2000
  • Ingår i: Combustion and Flame. - 0010-2180 .- 1556-2921. ; 120:3, s. 285-300
  • Tidskriftsartikel (refereegranskat)abstract
    • The laminar flamelet structures of methane/air, propane/air, and hydrogen/air nonpremixed combustion at low and vanishing scalar dissipation rates are investigated, by numerical calculations of a system of conservation equations in a counterflow diffusion flame configuration, together with a transport equation defining the mixture fraction and scalar dissipation rate. The chemical reaction mechanisms consist of 82 elementary reactions up to C-3 species. In the limit of vanishing scalar dissipation rate, two types of structures are shown to appear. In one structure fuel and oxygen are consumed in a thin layer located near the stoichiometric mixture fraction, Z(st), where the temperature and the major products reach their peaks. This is similar to the so-called Burke-Schumann single layer flame sheet structure. One example is the hydrogen/air diffusion flame. The second structure consists of multilayers. Fuel and oxygen are consumed at different locations. Oxygen is consumed at Z(l) (near Z(st)), where the temperature and the major products reach their peaks. Fuel is consumed at Z(r) (> Z(st)). Between Z(l) and Z(r) some intermediate and radical species are found in high concentrations. Hydrocarbon/air nonpremixed flames are of this type. It is shown that for methane/air diffusion flames, some chemical reactions which are negligible at large scalar dissipation rate near flame quenching conditions, play essential roles for the existence of the multilayer structure. Examples of such reactions are, CH4 --> CH3 + H, H2O + O-2 --> HO2 + OH, H2O + M --> H + OH + M and CHO + H-2 --> O + H. The sensitivity of the species distributions in the flamelet to the scalar dissipation rate varies for different species. The most sensitive species are the intermediates and radicals at the fuel-rich side. At low scalar dissipation rate the radiative heat transfer can significantly move the fuel consumption layer to the oxygen consumption layer, increase the oxygen leakage to fuel side, and even quench the flame. Differential diffusion modifies the species and temperature profiles in the flamelet, but does not affect the multilayer nature of the flamelet. This result is used to successfully explain the high CO emissions in a turbulent methane/air diffusion flame.
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2.
  • Andrae, Johan, et al. (författare)
  • Kinetic and Transport Effects of Pressurized Methane Flames in a Boundary Layer
  • 2003
  • Ingår i: Combustion and Flame. - 0010-2180 .- 1556-2921. ; 133:4, s. 503-506
  • Tidskriftsartikel (refereegranskat)abstract
    • We have recently modeled combustion of lean methane-air [1] mixtures in a boundary layer flow using the program CRESLAF [2, 3 and 4]. A uniform fuel-air mixture above the auto-ignition temperature was introduced at the inlet edge of the wall, which gave an arrested flame zone, propagating in the upstream direction with a local flame speed that is equal and opposite to the local flow velocity. We compared the interaction of this flame with three model wall materials representing three idealized cases from a chemical point of view, a completely inert wall, a radical recombining wall, and a wall supporting catalytic combustion. Here we report on an analogous study of a flame geometry that may be considered a combination of a one-dimensional flame propagating towards a wall and the combustion of a uniform fuel-air mixture in a boundary layer flow. In contrast to our previous work [1] where we had a uniform inlet flow composition consisting of unburnt gas, here there is only unburnt gas close to the walls while there is burnt gas in the center of the channel. The present study concerns lean pressurized methane flames propagating toward hot isothermal walls where chemistry on the wall is considered important. The main purpose is to compare the results with those obtained in Ref. [1], which enables us, for the same flow field (boundary layer flow), to compare the effect of flame geometry on the wall effects. We have found and have been able to explain theoretically that such subtle changes of the flame geometry, which would be rather difficult to study experimentally, may have surprisingly significant effects on the combustion process.
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3.
  • Andrae, Johan, 1973-, et al. (författare)
  • Numerical studies of wall effects with laminar methane flames
  • 2002
  • Ingår i: Combustion and Flame. - 0010-2180 .- 1556-2921. ; 128:1-2, s. 165-180
  • Tidskriftsartikel (refereegranskat)abstract
    • Wall effects in the combustion of lean methane mixtures have been studied numerically using the CHEMKIN software. To gain a deeper understanding of the flame-wall interaction in lean burn combustion, and in particular the kinetic and thermal effects, we have simulated lean and steady methane/air flames in a boundary layer flow. The gas-phase chemistry is modeled with the GRI mechanism version 1.2. Boundary conditions include an inert wall, a recombination wall and catalytic combustion of methane. Different pressures, wall temperatures and fuel-air ratios are used to address questions such as which part of the wall effects is most important at a given set of conditions. As the results are analyzed it can be seen that the thermal wall effects are more significant at the lower wall temperature (600 K) and the wall can essentially be modeled as chemical inert for the lean mixtures used. At the higher wall temperature (1,200 K), the chemical wall effects become more significant and at the higher pressure (10 atm) the catalytic surface retards homogeneous combustion of methane more than the recombination wall because of product inhibition. This may explain the increased emissions of unburned fuel observed in engine studies, when using catalytic coatings on the cylinder walls. The overall wall effects were more pronounced for the leaner combustion case (phi = 0.2). When the position of the reaction zone obtained from the boundary layer calculations is compared with the results from a one-dimensional premixed flame model, there is a small but significant difference except at the richer combustion case (phi = 0.4) at atmospheric pressure, where the boundary layer model may not predict the flame position for the given initial conditions.
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4.
  • Stenberg, Jenny, et al. (författare)
  • Measurements of Gas Concentrations in a Fluidized Bed Combustor Using Laser-Induced Photoacoustic Spectroscopy and Zirconia Cell Probes
  • 1998
  • Ingår i: Combustion and Flame. - 1556-2921 .- 0010-2180. ; 113:4, s. 477-486
  • Tidskriftsartikel (refereegranskat)abstract
    • The dynamic combustion behavior of a circulating, fluidized bed boiler (CFB) was studied using two high-speed gas analysis systems during the combustion of coal, pear, and wood chips. Time-resolved concentrations of SO2 and NO were measured by laser-induced photoacoustic spectroscopy (LIPS). A zirconia-cell based probe (lambda-probe), synchronized with the LIPS probe, measured fluctuations between reducing and oxidizing conditions. The two probes were positioned in the same measurement volume on the centerline of the CFB combustion chamber. The purpose of the work was to investigate the behavior of the LIPS in a combustion chamber containing reacting gases in order to extend the previous h-probe measurements to other gas components. Correlations between oxidizing and reducing conditions and gas species concentrations in three locations in the combustion chamber are presented. The best correlations were found in the upper part of the CFB combustion chamber. In some cases the correlation between reducing conditions and the LIPS signal was caused by unburnt hydrocarbons. Average values of [NO] and [SO2] obtained by the LIPS system were compared with the results from a sampling probe connected to on-line analyzers. The measurements of [NO] and [SO2] were disturbed by interfering gases during reducing conditions. During a sufficiently long time of oxidizing conditions, however, reasonable agreement was obtained between LIPS measurements of [NO] and [SO2] and those of the on-line analyzers. On some occasions (low SO2 concentration) the concentration of the OH radical was also measured.
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5.
  • Ahmed, Ahfaz, 1985, et al. (författare)
  • A comprehensive combustion chemistry study of n-propylcyclohexane
  • 2021
  • Ingår i: Combustion and Flame. - : Elsevier BV. - 1556-2921 .- 0010-2180. ; 233
  • Tidskriftsartikel (refereegranskat)abstract
    • Alkylated cycloalkanes are vital components in gasoline, aviation, and diesel fuels; however, their combustion chemistry has been less investigated compared to other hydrocarbon classes. In this work, the combustion kinetics of n-propylcyclohexane (n-Pch) was studied across a range of experiments including pressurized flow reactor (PFR), jet stirred reactor (JSR), shock tube (ST), and rapid compression machine (RCM). These experiments cover a wide range of conditions spanning low to intermediate to high temperatures, low to high pressures at lean to rich equivalence ratios. Stable intermediate species were measured in PFR over a temperature range of 550–850 K, pressure of 8.0 bar, equivalence ratio (φ) of 0.27, and constant residence time of 120 ms. The JSR was utilized to measure the speciation during oxidation of n-Pch at φ of 0.5–2.0, at atmospheric pressure, and across temperature range of 550–800 K. Ignition delay times (IDTs) for n-Pch were measured in the RCM and ST at temperatures ranging from 650 to 1200 K, at pressures of 20 and 40 bar, at φ=0.5,1.0. In addition, a comprehensive detailed chemical kinetic model was developed and validated against the measured experimental data. The new kinetic model, coupled with the breadth of data from various experiments, provides an improved understanding of n-Pch combustion.
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6.
  • Akkerman, V., et al. (författare)
  • Numerical study of turbulent flame velocity
  • 2007
  • Ingår i: Combustion and Flame. - : Elsevier BV. - 1556-2921 .- 0010-2180. ; 151:3, s. 452-471
  • Tidskriftsartikel (refereegranskat)abstract
    • A premixed flame propagating through a combination of vortices in a tube/channel is studied using direct numerical simulations of the complete set of combustion equations including thermal conduction, diffusion, viscosity, and chemical kinetics. Two cases are considered, a single-mode vortex array and a multimode combination of vortices obeying the Kolmogorov spectrum. It is shown that the velocity of flame propagation depends strongly on the vortex intensity and size. The dependence on the vortex intensity is almost linear in agreement with the general belief. The dependence on the vortex size may be imitated by a power law (proportional to D-2/3. This result is different from theoretical predictions, which creates a challenge for the theory. In the case of the Kolmogorov spectrum of vortices, the velocity of flame propagation is noticeably smaller than for a single-mode vortex array. The flame velocity depends weakly on the thermal expansion of burning matter within the domain of realistically large expansion factors. Comparison to the experimental data indicates that small-scale turbulence is not the only effect that influences the flame velocity in the experimental flows. Large-scale processes, such as the Darrieus-Landau instability and flame-wall interaction, contribute considerably to the velocity of flame propagation. Still, on small scales, the Darrieus-Landau instability becomes important only for a sufficiently low vortex intensity. (C) 2007 The Combustion Institute. Published by Elsevier Inc. All rights reserved.
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7.
  • Akkerman, V'yacheslav, et al. (författare)
  • Accelerating flames in cylindrical tubes with nonslip at the walls
  • 2006
  • Ingår i: Combustion and Flame. - : Elsevier BV. - 0010-2180 .- 1556-2921. ; 145:1-2, s. 206-219
  • Tidskriftsartikel (refereegranskat)abstract
    • An analytical theory of flame acceleration in cylindrical tubes with one end closed is developed. It is shown that all realistic flames with a large density drop at the front accelerate exponentially because of the nonslip at the tube walls. Such acceleration mechanism is not limited in time and, eventually, it may lead to detonation triggering. It is found that the acceleration rate decreases with the Reynolds number of the flow. On the contrary, the acceleration rate grows with the thermal expansion of the burning matter. It is shown that the flame shape and the velocity profile remain self-similar during the flame acceleration. The theory is validated by extensive direct numerical simulations. The simulations are performed for the complete set of combustion and hydrodynamic equations including thermal conduction, diffusion, viscosity, and chemical kinetics. The simulation results are in very good agreement with the analytical theory.
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8.
  • Akkerman, V'yacheslav, et al. (författare)
  • Flame oscillations in tubes with nonslip at the walls
  • 2006
  • Ingår i: Combustion and Flame. - : Elsevier BV. - 0010-2180 .- 1556-2921. ; 145:4, s. 675-687
  • Tidskriftsartikel (refereegranskat)abstract
    • A laminar premixed flame front propagating in a two-dimensional tube is considered with nonslip at the walls and with both ends open. The problem of flame propagation is solved using direct numerical simulations of the complete set of hydrodynamic equations including thermal conduction, diffusion, viscosity, and chemical kinetics. As a result, it is shown that flame interaction with the walls leads to the oscillating regime of burning. The oscillations involve variations of the curved flame shape and the velocity of flame propagation. The oscillation parameters depend on the characteristic tube width, which controls the Reynolds number of the flow. In narrow tubes the oscillations are rather weak, while in wider tubes they become stronger with well-pronounced nonlinear effects. The period of oscillations increases for wider tubes, while the average flame length scaled by the tube diameter decreases only slightly with increasing tube width. The average flame length calculated in the present work is in agreement with that obtained in the experiments. Numerical results reduce the gap between the theory of turbulent flames and the experiments on turbulent combustion in tubes.
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9.
  • Andrae, Johan C. G., et al. (författare)
  • Autoignition of toluene reference fuels at high pressures modeled with detailed chemical kinetics
  • 2007
  • Ingår i: Combustion and Flame. - : Elsevier BV. - 0010-2180 .- 1556-2921. ; 149:02-jan, s. 2-24
  • Tidskriftsartikel (refereegranskat)abstract
    • A detailed chemical kinetic model for the autoignition of toluene reference fuels (TRF) is presented. The toluene submechanism added to the Lawrence Livermore Primary Reference Fuel (PRF) mechanism was developed using recent shock tube autoignition delay time data under conditions relevant to HCCI combustion. For two-component fuels the model was validated against recent high-pressure shock tube autoignition delay time data for a mixture consisting of 35% n-heptane and 65% toluene by liquid volume. Important features of the autoignition of the mixture proved to be cross-acceleration effects, where hydroperoxy radicals produced during n-heptane oxidation dramatically increased the oxidation rate of toluene compared to the case when toluene alone was oxidized. Rate constants for the reaction of benzyl and hydroperoxyl radicals previously used in the modeling of the oxidation of toluene alone were untenably high for modeling of the mixture. To model both systems it was found necessary to use a lower rate and introduce an additional branching route in the reaction between benzyl radicals and O-2. Good agreement between experiments and predictions was found when the model was validated against shock tube autoignition delay data for gasoline surrogate fuels consisting of mixtures of 63-69% isooctane, 14-20% toluene, and 17% n-heptane by liquid volume. Cross reactions such as hydrogen abstractions between toluene and alkyl and alkylperoxy radicals and between the PRF were introduced for completion of chemical description. They were only of small importance for modeling autoignition delays from shock tube experiments, even at low temperatures. A single-zone engine model was used to evaluate how well the validated mechanism could capture autoignition behavior of toluene reference fuels in a homogeneous charge compression ignition (HCCI) engine. The model could qualitatively predict the experiments, except in the case with boosted intake pressure, where the initial temperature had to be increased significantly in order to predict the point of autoignition.
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10.
  • Andrae, Johan C. G., et al. (författare)
  • HCCI experiments with toluene reference fuels modeled by a semidetailed chemical kinetic model
  • 2008
  • Ingår i: Combustion and Flame. - : Elsevier BV. - 0010-2180 .- 1556-2921. ; 155:4, s. 696-712
  • Tidskriftsartikel (refereegranskat)abstract
    • A semidetailed mechanism (137 species and 633 reactions) and new experiments in a homogeneous charge conic pression ignition (HCCI) engine on the autoignition of toluene reference fuels are presented. Skeletal mechanisms for isooctane and n-heptane were added to a detailed toluene submechanism. The model shows generally good agreement with ignition delay times measured in a shock tube and a rapid compression machine and is sensitive to changes in temperature, pressure, and mixture strength. The addition of reactions involving the formation and destruction of benzylperoxide radical was crucial to modeling toluene shock tube data. Laminar burning velocities for benzene and toluene were well predicted by the model after some revision of the high-temperature chemistry. Moreover, laminar burning velocities of a real gasoline at 353 and 500 K Could be predicted by the model using a toluene reference fuel as a surrogate. The model also captures the experimentally observed differences in combustion phasing of toluene/n-heptane mixtures, compared to a primary reference fuel of the same research octane number, in HCCI engines as the intake pressure and temperature are changed. For high intake pressures and low intake temperatures, a sensitivity analysis at the moment of maximum heat release rate shows that the consumption of phenoxy radicals is rate-limiting when a toluene/n-heptane fuel is used, which makes this fuel more resistant to autoignition than the primary reference fuel. Typical CPU times encountered in zero-dimensional calculations were on the order of seconds and minutes in laminar flame speed calculations. Cross reactions between benzylperoxy radicals and n-heptane improved the model prediction,,; of shock tube experiments for phi = 1.0 and temperatures lower than 800 K for an n-heptane/toluene fuel mixture, but cross reactions had no influence on HCCI Simulations.
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11.
  • Andrae, Johan C. G., et al. (författare)
  • HCCl experiments with gasoline surrogate fuels modeled by a semidetailed chemical kinetic model
  • 2009
  • Ingår i: Combustion and Flame. - : Elsevier BV. - 0010-2180 .- 1556-2921. ; 156:4, s. 842-851
  • Tidskriftsartikel (refereegranskat)abstract
    • Experiments in a homogeneous charge compression ignition (HCCI) engine have been conducted with four gasoline surrogate fuel blends. The pure components in the Surrogate fuels consisted of n-heptane, isooctane, toluene, ethanol and diisobutylene and fuel sensitivities (RON-MON) in the fuel blends ranged from two to nine. The operating conditions for the engine were p(in) = 0.1 and 0.2 MPa, T-in = 80 and 250 degrees C, phi = 0.25 in air and engine speed 1200 rpm. A semidetailed chemical kinetic model (142 species and 672 reactions) for gasoline surrogate fuels, validated against ignition data from experiments conducted in shock tubes for gasoline Surrogate fuel blends at 1.0 <= p <= 5.0 MPa, 700 <= T <= 1200 K and 0 = 1.0, was successfully used to qualitatively predict the HCCI experiments using a single zone modeling approach. The fuel blends that had higher fuel sensitivity were more resistant to autoignition for low intake temperature and high intake pressure and less resistant to autoignition for high intake temperature and low intake pressure. A sensitivity analysis shows that at high intake temperature the chemistry of the fuels ethanol, toluene and diisobutylene helps to advance ignition. This is consistent with the trend that fuels with the least Negative Temperature Coefficient (NTC) behavior show the highest octane sensitivity, and become less resistant to autoignition at high intake temperatures. For high intake pressure the sensitivity analysis shows that fuels in the fuel blend with no NTC behavior consume OH radicals and acts as a radical scavenger for the fuels with NTC behavior. This is consistent with the observed trend of an increase in RON and fuel sensitivity. With data from shock tube experiments in the literature and HCCI modeling in this work, a correlation between the reciprocal pressure exponent oil the ignition delay to the fuel sensitivity and volume percentage of single-stage ignition fuel in the fuel blend was found. Higher fuel sensitivity and single-stage fuel content generally gives a lower value of the pressure exponent. This helps to explain the results obtained while boosting the intake pressure in the HCCI engine.
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12.
  • Andrae, Johan, et al. (författare)
  • Cooxidation in the auto-ignition of primary reference fuels and n-heptane/toluene blends
  • 2005
  • Ingår i: Combustion and Flame. - : Elsevier BV. - 0010-2180 .- 1556-2921. ; 140:4, s. 267-286
  • Tidskriftsartikel (refereegranskat)abstract
    • Auto-ignition of fuel mixtures was investigated both theoretically and experimentally to gain further understanding of the fuel chemistry. A homogeneous charge compression ignition (HCCI) engine was run under different operating conditions with fuels of different RON and MON and different chemistries. Fuels considered were primary reference fuels and toluene/n-heptane blends. The experiments were modeled with a single-zone adiabatic model together with detailed chemical kinetic models. In the model validation, co-oxidation reactions between the individual fuel components were found to be important in order to predict HCCI experiments, shock-tube ignition delay time data, and ignition delay times in rapid compression machines. The kinetic models with added co-oxidation reactions further predicted that an n-heptane/toluene fuel with the same RON as the corresponding primary reference fuel had higher resistance to auto-ignition in HCCI combustion for lower intake temperatures and higher intake pressures. However, for higher intake temperatures and lower intake pressures the n-heptane/toluene fuel and the PRF fuel had similar combustion phasing.
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13.
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14.
  • Bu, Changsheng, 1987, et al. (författare)
  • Devolatilization of a single fuel particle in a fluidized bed under oxy-combustion conditions. Part A: Experimental results
  • 2015
  • Ingår i: Combustion and Flame. - : Elsevier BV. - 1556-2921 .- 0010-2180. ; 162:3, s. 797-808
  • Tidskriftsartikel (refereegranskat)abstract
    • Devolatilization of a single fuel particle and the related flame combustion were studied in a two-dimensionalfluidized bed with a quartz wall, allowing visual observation of the conversion process. The aimwas to evaluate the devolatilization behavior (ignition, flame temperature, flame life-time, devolatilizationtime) of different fuels (4 ranks of coal from lignite to anthracite and wood) when replacing O2/N2 byO2/CO2 in O2 volume concentrations from 0% to 40%, at a fixed bed temperature of 1088 K using 6 mmspherical fuel particles. The volatiles’ flame was recorded by a color video camera to analyze ignitionand extinction. The flame temperature was estimated by two-color pyrometry. Two thermocouples wereinserted in the fuel particle to measure the temperature at the center and near the surface. Homogeneousand heterogeneous ignition modes, times of devolatilization, and flame duration (flame-life) under differentgas atmospheres were analyzed. Results indicate that the mode of ignition of bituminous coal, lignitecoal and wood changes when N2 is replaced by CO2. The ignition-delay time is much longer, and the flametemperature is lower in the O2/CO2 atmosphere than in an O2/N2 atmosphere for all the tested fuels. Thedevolatilization time of the anthracite particle is almost unaffected by the surrounding atmosphere,while for the other fuels this time is generally longer in O2/CO2 than in O2/N2 at the same O2 concentration.The presence of a flame during the volatiles combustion did not accelerate the particle heating, noteven at the highest O2 concentration tested (40 vol%), however, after the extinction of the flame, the rateof particle heating is significantly affected by the oxygen concentration.
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15.
  • Bu, Changsheng, 1987, et al. (författare)
  • Devolatilization of a single fuel particle in a fluidized bed under oxy-combustion conditions. Part B: Modeling and comparison with measurements
  • 2015
  • Ingår i: Combustion and Flame. - : Elsevier BV. - 1556-2921 .- 0010-2180. ; 162:3, s. 809-818
  • Tidskriftsartikel (refereegranskat)abstract
    • A detailed one-dimensional transient model is developed to describe the conversion of a single fuelparticle in O2/N2 and O2/CO2 atmospheres in a fluidized bed (FB). The model takes into account the mainrelevant phenomena occurring from the addition of a particle to the FB up to the instant when most of thevolatiles have been released. The model accounts for the rates of drying, fuel devolatilization, homogeneouscombustion of volatiles in a thin flame, heterogeneous combustion of char, and mass and heattransfer, the latter involving the heat transfer from the FB reactor and flame to the particle. The modelis used to simulate and explain the experiments given in Part A of the present work, which includes testswith four ranks of coal (from anthracite to lignite) and one type of wood in O2/N2 and O2/CO2 atmosphereswith the O2 volume concentration varying in the range of 0–40% at a fixed bed temperature of1088 K. The predicted history of the temperature of the fuel particle and of the volatiles flame agrees wellwith the measurements. The simulated results indicate that the heat transfer processes at the particlescale are similar in pure N2 and CO2. The model reveals that only a small amount of heat from the flameis transferred to the fuel particle, explaining why the rate of particle heating is hardly affected by theflame. The decrease in the devolatilization time measured at higher O2 concentration is explained by heterogeneous(char) combustion, which is seen to be significant during the last stages of devolatilization.The model shows that the char combustion is limited by the rate of diffusion of O2 to the particle andjustifies the lower heating rate observed in O2/CO2 compared to in O2/N2. A sensitivity analysis showsthat the thermal capacity and conductivity of the fuel, as well as the convective heat transfer coefficient,are the most influencing parameters affecting the time of devolatilization.
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16.
  • Bu, C. S., et al. (författare)
  • The effect of H2O on the oxy-fuel combustion of a bituminous coal char particle in a fluidized bed: Experiment and modeling
  • 2020
  • Ingår i: Combustion and Flame. - : Elsevier BV. - 1556-2921 .- 0010-2180. ; 218, s. 42-56
  • Tidskriftsartikel (refereegranskat)abstract
    • Oxy-fuel fluidized bed (FB) combustion is considered one of the promising ways to control CO2 emission from coal-fired power plants. The effect of H2O on the char conversion during wet flue-gas recycle, in which the H2O concentration could be around 40%, is still not well understood. To this end, experiments and modeling were performed in this work. Combustion tests with bituminous coal char were carried out in an electrically heated fluidized bed in O2/CO2, O2/H2O and O2/CO2/H2O for various O2, CO2 and H2O concentrations at the bed temperature of 850 °C. At the same time, the influence of the bed temperature and the char size on char combustion was investigated in O2/CO2/H2O atmosphere. A thermocouple was inserted into the center of the char particle to measure the particle temperature, from which the char combustion characteristics were determined and analyzed. The results indicate that the participation of H2O in the combustion atmosphere enhances the carbon conversion, and it also reduces the particle temperature. A transient char-particle conversion model, taking into account heat and mass transfer from the bed to the particle and heterogeneous combustion and gasification of char, was developed to quantitatively examine the role of H2O. The model shows a good ability to predict the measured char-temperature history. Simulations were carried out to establish the role of H2O in O2/H2O and O2/CO2/H2O as in the FB experiments. The model was used to analyze the peak temperature and the burnout time of a char particle, as well as the relative contributions to the consumption of the carbon in the char by O2 (combustion), and CO2 and H2O (gasification). The results indicate that the endothermic char-H2O reaction is the main reason for the prolongation of the burnout time of char and the decrease in the particle temperature in O2/CO2/H2O as compared in O2/CO2. During wet flue-gas recycle, char-O2 still accounts for a major part of the total carbon consumption, but the contribution of char-H2O to the overall carbon consumption increases with the H2O concentration and cannot be ignored (i.e. when the H2O concentration attains 30%, the contribution of the char-H2O reaction to the overall carbon consumption is 14%). However, the contribution of char-CO2 to the char conversion is limited.
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17.
  • Bychkov, Vitaly, et al. (författare)
  • Flame acceleration in the early stages of burning in tubes
  • 2007
  • Ingår i: Combustion and Flame. - : Elsevier BV. - 1556-2921 .- 0010-2180. ; 150:4, s. 263-276
  • Tidskriftsartikel (refereegranskat)abstract
    • Acceleration of premixed laminar flames in the early stages of burning in long tubes is considered. The acceleration mechanism was suggested earlier by Clanet and Searby [Combust. Flame 105 (1996) 225]. Acceleration happens due to the initial ignition geometry at the tube axis when a flame develops to a finger-shaped front, with surface area growing exponentially in time. Flame surface area grows quite fast but only for a short time. The analytical theory of flame acceleration is developed, which determines the growth rate, the total acceleration time, and the maximal increase of the flame surface area. Direct numerical simulations of the process are performed for the complete set of combustion equations. The simulations results and the theory are in good agreement with the previous experiments. The numerical simulations also demonstrate flame deceleration, which follows acceleration, and the so-called '' tulip flames.'' (c) 2007 Published by Elsevier Inc. on behalf of The Combustion Institute.
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18.
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19.
  • Bäckström, Daniel, 1985, et al. (författare)
  • Measurement of the size distribution, volume fraction and optical properties of soot in an 80 kW propane flame
  • 2017
  • Ingår i: Combustion and Flame. - : Elsevier BV. - 0010-2180 .- 1556-2921. ; 186, s. 325-334
  • Tidskriftsartikel (refereegranskat)abstract
    • This work presents measurements of the size distribution, volume fraction, absorption and scattering coefficients of soot in an 80 kW swirling propane-fired flame. Extractive measurements were performed in the flame using an oil-cooled particle extraction probe. The particle size distribution was measured with a Scanning Mobility Particle Sizer (SMPS) and the optical properties were measured using a Photo Acoustic Soot Spectrometer (PASS-3). A detailed radiation model was used to examine the influence of the soot volume fraction on the particle radiation intensity. The properties of the gas were calculated with a statistical narrow-band model and the particle properties were calculated using Rayleigh theory with four different complex indices of refraction for soot particles. The modelled radiation was compared with measured total radiative intensity, the latter which was measured with a narrow angle radiometer. The results show that the measured soot volume fraction yields particle radiation that corresponds well with the determined difference between gas and total radiation. This indicates that the presented methodology is capable of quantifying both the particle and gaseous radiation in a flame of technical size. (C) 2017 The Combustion Institute. Published by Elsevier Inc. All rights reserved.
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20.
  • Cracknell, R. F., et al. (författare)
  • The chemical origin of octane sensitivity in gasoline fuels containing nitroalkanes
  • 2009
  • Ingår i: Combustion and Flame. - : Elsevier BV. - 0010-2180 .- 1556-2921. ; 156:5, s. 1046-1052
  • Tidskriftsartikel (refereegranskat)abstract
    • Experimental octane measurements are presented for a standard gasoline to which has been added various quantities of nitromethane, nitroethane and 1-nitropropane. The addition of nitroalkanes was found to suppress the Motor Octane Number to a much greater extent than the Research Octane Number. in other words addition of nitroalkanes increases the octane sensitivity of gasoline. Density Functional Theory was used to model the equilibrium thermodynamics and the barrier heights for reactions leading to the break-up of nitroethane. These results were used to develop a chemical kinetic scheme for nitroalkanes combined with a surrogate gasoline (for which a mechanism has been developed previously). Finally the chemical kinetic simulations were combined with a quasi-dimensional engine model in order to predict autoignition in octane rating tests. Our results suggest that the chemical origin of octane sensitivity in gasoline/nitroalkane blends cannot be fully explained on the conventional basis of the extent to which NTC behaviour is absent. Instead we have shown that the contribution of the two pathways leading to autoignition in gasoline containing nitroalkanes becomes much more significant under the more severe conditions of the Motor Octane method than the Research Octane method.
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21.
  • Fatehi, Hesameddin, et al. (författare)
  • Gas phase combustion in the vicinity of a biomass particle during devolatilization : model development and experimental verification
  • 2018
  • Ingår i: Combustion and Flame. - : Elsevier. - 0010-2180 .- 1556-2921. ; 196, s. 351-363
  • Tidskriftsartikel (refereegranskat)abstract
    • A numerical and experimental study on the devolatilization of a large biomass particle is carried out to quantify the effect of homogeneous volatile combustion on the conversion of the particle and on the temperature and species distribution at the particle vicinity. A global chemical kinetic mechanism and a detailed reaction mechanism are considered in a one dimensional numerical model that takes into account preferential diffusivity and a detailed composition of tar species. An adaptive moving mesh is employed to capture the changes in the domain due to particle shrinkage. The effect of gas phase reactions on pyrolysis time, temperature and species distribution close to the particle is studied and compared to experiments. Online in situ measurements of average H2O mole fraction and gas temperature above a softwood pellet are conducted in a reactor using tunable diode laser absorption spectroscopy (TDLAS) while recording the particle mass loss. The results show that the volatile combustion plays an important role in the prediction of biomass conversion during the devolatilization stage. It is shown that the global reaction mechanism predicts a thin flame front in the vicinity of the particle deviating from the measured temperature and H2O distribution over different heights above the particle. A better agreement between numerical and experimental results is obtained using the detailed reaction mechanism, which predicts a wider reaction zone.
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22.
  • Feng, Ruixue, et al. (författare)
  • Influence of gas expansion on the propagation of a premixed flame in a spatially periodic shear flow
  • 2021
  • Ingår i: Combustion and Flame. - : Elsevier. - 0010-2180 .- 1556-2921. ; 227, s. 421-427
  • Tidskriftsartikel (refereegranskat)abstract
    • It has been previously demonstrated that thermal gas expansion might have a role in boundary layer flashback of premixed turbulent flames [Gruber et al., J Fluid Mech 2012], inducing local flow-reversal in the boundary layer's low-velocity streaks on the reactants’ side of the flame and facilitating its upstream propagation. We perform a two-dimensional numerical investigation of the interaction between a periodic shear flow and a laminar premixed flame. The periodic shear is a simplified model for the oncoming prolonged streamwise velocity streaks with alternating regions of high and low velocities found in turbulent boundary layers in the vicinity of the walls. The parametric study focuses on the amplitude and wavelength of the periodic shear flow and on the gas expansion ratio (unburnt-to-burnt density ratio). With the increase of the amplitudes of the periodic shear flow and of the gas expansion, the curved flame velocity increases monotonically. The flame velocity dependence on the periodic shear wavelength is non-monotonic, which is consistent with previous theoretical studies of curved premixed flame velocity. The flame shape that is initially formed by the oncoming periodic shear appears to be metastable. At a later stage of the flame propagation, the flame shape transforms into the stationary one dominated by the Darrieus-Landau instability.
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23.
  • Fiorina, B., et al. (författare)
  • Challenging modeling strategies for LES of non-adiabatic turbulent stratified combustion
  • 2015
  • Ingår i: Combustion and Flame. - : Elsevier BV. - 0010-2180 .- 1556-2921. ; 162:11, s. 4264-4282
  • Tidskriftsartikel (refereegranskat)abstract
    • Five different low-Mach large eddy simulations are compared to the turbulent stratified flame experiments conducted at the Technical University of Darmstadt (TUD). The simulations were contributed by TUD, the Institute for Combustion Technology (ITV) at Aachen, Lund University (LUND), the EM2C laboratory at Ecole Centrale Paris, and the University of Duisburg-Essen (UDE). Combustion is modeled by a premixed flamelet tabulation with local flame thickening (TUD), a premixed flamelet progress variable approach coupled to a level set method (ITV), a 4-steps mechanism combined with implicit LES (LUND), the F-TACLES model that is based on filtered premixed flamelet tabulation (EM2C), and a flame surface density approach (UDE). An extensive comparison of simulation and experimental data is presented for the first two moments of velocity, temperature, mixture fraction, and major species mass fractions. The importance of heat-losses was assessed by comparing simulations for adiabatic and isothermal boundary conditions at the burner walls. The adiabatic computations predict a flame anchored on the burner lip, while the non-adiabatic simulations show a flame lift-off of one half pilot diameter and a better agreement with experimental evidence for temperature and species concentrations. Most simulations agree on the mean flame brush position, but it is evident that subgrid turbulence must be considered to achieve the correct turbulent flame speed. Qualitative comparisons of instantaneous snapshots of the flame show differences in the size of the resolved flame wrinkling patterns. These differences are (a) caused by the influence of the LES combustion model on the flame dynamics and (b) by the different simulation strategies in terms of grid, inlet condition and numerics. The simulations were conducted with approaches optimized for different objectives, for example low computational cost, or in another case, short turn around. (C) 2015 The Combustion Institute. Published by Elsevier Inc. All rights reserved.
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24.
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25.
  • Fooladgar, Ehsan, et al. (författare)
  • A new post-processing technique for analyzing high-dimensional combustion data
  • 2018
  • Ingår i: Combustion and Flame. - : ELSEVIER SCIENCE INC. - 0010-2180 .- 1556-2921. ; 191, s. 226-238
  • Tidskriftsartikel (refereegranskat)abstract
    • This paper introduces a novel post-processing technique for analyzing high dimensional combustion data. In this technique, t-Distributed Stochastic Neighbor Embedding (t-SNE) is used to reduce the dimensionality of the combustion data with no prior knowledge while preserving the similarity of the original data. Multidimensional combustion datasets are from premixed and non-premixed laminar flame simulations and measurements of a series of well documented piloted flames with inhomogeneous inlets. The resulting reduced manifold is visualized by scatter plots to reveal the global and local structure of the data (manual labeling). Unsupervised clustering algorithms are then utilized for post-processing the t-SNE manifold in order to develop an automatic labeling process. 
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26.
  • Fooladgar, Ehsan, et al. (författare)
  • Characterization of flameless combustion in a model gas turbine combustor using a novel post-processing tool
  • 2019
  • Ingår i: Combustion and Flame. - : Elsevier Inc.. - 0010-2180 .- 1556-2921. ; , s. 356-367
  • Tidskriftsartikel (refereegranskat)abstract
    • Flameless combustion is a very promising technology for the future gas turbines. It is clean and stable—without large oscillations, noise and flashback. To facilitate the adoption of this technology in gas turbines, advanced design tools are needed. In this paper, a recently developed unsupervised post-processing tool is used to analyze the large amount of high-dimensional data produced in a series of Large Eddy Simulations (LES) of a model gas turbine operating in flameless mode. Simulations are performed using Finite Rate Chemistry (FRC) combustion modeling and a detailed description of chemistry. The automatic post-processing reveals important features of the combustion process that are not easily recognizable by other methods, making it a complementary step for the already established FRC–LES approach, and a potential design tool for advanced combustion systems.
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27.
  • Ganji, Hamed F., et al. (författare)
  • Thermoacoustic stability analysis and robust design of burner-deck-anchored flames using flame transfer function composition
  • 2024
  • Ingår i: Combustion and Flame. - : Elsevier BV. - 0010-2180 .- 1556-2921. ; 269
  • Tidskriftsartikel (refereegranskat)abstract
    • Thermoacoustic instabilities in combustion systems are influenced by the thermoacoustic properties, such as the transfer function (TF) of the burner with flame. One promising approach to address these instabilities is by targeting the burner's thermoacoustic properties. The approach developed in this contribution is based on an idea of modifying or designing a targeted composite flame transfer function (TF) and involves the heuristic concept that the acoustic response of a particular flame can be counterbalanced by the corresponding response of other flames. For premixed conical flames anchored on the burner deck, at the fixed gas composition the TF mostly depends on such parameters as the diameter of the perforations and the flame spacing (pitch). This suggests the concept of combining different sizes and shapes of perforations in one burner deck. In this study, we investigate the acoustic response of burners made of sintered ceramic fibers with multiple patterns of perforation using the TF composition strategy. This approach allows us to represent the cumulative flame TF as a weighted sum of the elemental TF of the flame groups, based on the additive nature of the individual heat release rate of the flames. We first show how this approach can be used to design composite burners that operate thermo-acoustically stable in a given system. Then, we mark the critical frequency range for the designed composite burners in the frequency domain using the so-called direct conservative stability (DCS) criterion. Following this, a stability map representing the complete picture of safe values of gain and phase of the flame TF is introduced that can serve as a designing target. Finally, we use stability margin and uncertainty analysis based on Monte-Carlo simulation to check the robustness of designed composite burner. Novelty and significance statement center dot We have introduced a systematic flame stabilization framework centered around flame modification. center dot This framework allows for the utilization of various characterized basic/elemental burners, each with their associated flames, to design complex burners capable of achieving thermoacoustic stability. center dot Leveraging the DCS framework for stability analysis, we have generated a comprehensive stability map for the thermoacoustic system. This map provides a design target and guidance for systematic flame stabilization. center dot Recognizing the presence of uncertainties in the simulation and measurement of subsystems (including upstream and downstream reflection coefficients and flame transfer functions), we have demonstrated that stability margin can serve as a robustness indicator for the design. We have included the minimum acceptable stability margin along with uncertainties in the stability map, making it a valuable tool for assessing robustness in burner development within the context of thermoacoustics. center dot A pressure drop composition model, designed to estimate the pressure drop of a complex burner based on the pressure drop of its individual segments, has been proposed and tested. center dot We have introduced the concept of flame transfer function composition for segmented/partitioned burners, which has been proposed, validated, and elaborated upon for designing intricate industrial composite burners.
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28.
  • Gonzalo-Tirado, C., et al. (författare)
  • Comparative study of four alternative models for CO oxidation around a burning coal char particle
  • 2014
  • Ingår i: Combustion and Flame. - : Elsevier BV. - 1556-2921 .- 0010-2180. ; 161:4, s. 1085-1095
  • Tidskriftsartikel (refereegranskat)abstract
    • The steady state combustion of a quiescent char particle is investigated by means of a detailed model which accounts for heterogeneous oxidation and gasification, as well as homogeneous reactions (GRI-Mech 3.0) in the particle's boundary layer. First, the way and extent in which the mass and energy transfer are altered due to the oxidation of CO are examined by comparison with the predictions of a single-film model in the case of anthracite particles within 60-1000 mu m. Then, four alternative descriptions of the gas phase (single-film, double-film, global kinetics and detailed kinetics) are evaluated for two coals of very different reactivity towards O-2 and CO2, at low and high O-2 concentration and in the same broad range of sizes. The overall influence of the CO conversion modeling on the particle burning rate and temperature (i.e. whether the reduction in O-2 surface concentration or the heat and CO2 provided by the flame dominate over each other) is found to depend on the conditions considered. The single-film approach reasonably fits the predictions of the most complete model in all cases (and especially in the pulverized-coal size range), whereas the double-film hypothesis and the global kinetics generally overestimate the effects of the flame on the consumption rates and the particle temperatures. (C) 2013 The Combustion Institute. Published by Elsevier Inc. All rights reserved.
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29.
  • Gustavsson, Lennart, 1956, et al. (författare)
  • Modeling of chemical reactions in afterburning for the reduction of N2O
  • 1996
  • Ingår i: Combustion and Flame. - 1556-2921 .- 0010-2180. ; 106:3, s. 345-358
  • Tidskriftsartikel (refereegranskat)abstract
    • Afterburning involves burning a secondary fuel in the flue gases from a fluidized bed combustor to raise the temperature, and thereby decrease the emission of N2O. Tests in a 12-MW circulating fluidized bed boiler used the cyclone as an afterburning combustor. The results from these tests are analyzed by chemical kinetic calculations with homogeneous hydrocarbon and nitrogen chemistry. Furthermore, a study is made of the influence on the calculation of particles in the flue gases. The deviation between calculated and measured data is small at high temperatures, but increases at the lower temperatures investigated. The influence of particles is predicted to be small under conditions prevailing in the cyclone.
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30.
  • Hansson, Karl-Martin, 1973, et al. (författare)
  • Formation of HNCO, HCN, and NH3 from the pyrolysis of bark and nitrogen-containing model compounds
  • 2004
  • Ingår i: Combustion and Flame. - 1556-2921 .- 0010-2180. ; 137, s. 265-277
  • Tidskriftsartikel (refereegranskat)abstract
    • Bark pellets have been pyrolyzed in a fluidized bed reactor at temperatures between 700 and 1000C. Identifiednitrogen-containing species were hydrogen cyanide (HCN), ammonia (NH3), and isocyanic acid (HNCO). Quantification of HCN and to some extent of NH3 was unreliable at 700 and 800C due to low concentrations. HNCO could not be quantified with any accuracy at any temperature for bark, due to the low concentrations found. Since most of the nitrogen in biomass is bound in proteins, various protein-rich model compounds were pyrolyzed with the aim of finding features that are protein-specific, making conclusions regarding the model compounds applica-ble for biomass fuels in general. The model compounds used were a whey protein isolate, soya beans, yellow peas,and shea nut meal. The split between HCN and NH3 depends on the compound and temperature. It was found that the HCN/NH3 ratio is very sensitive to temperature and increases with increasing temperature for all compounds, including bark. Comparing the ratio for the different compounds at a fixed temperature, the ratio was found to decrease with decreasing release of volatile nitrogen. The temperature dependence implies that heating rate andthereby particle size affect the split between HCN and NH3. For whey, soya beans, and yellow peas, HNCO was also quantified. It is suggested that most HCN and HNCO are produced from cracking of cyclic amides formed as primary pyrolysis products. The dependence of the HNCO/HCN ratio on the compound is fairly small, but the temperature dependence of the ratio is substantial, decreasing with increasing temperature. The release of nitrogen-containing species does not seem to be greatly affected by the other constituents of the fuel, and proteins appear to be suitable model compounds for the nitrogen in biomass.
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31.
  • Hemdal, Stina, 1974 (författare)
  • Characterization of stratified fuel distribution and charge mixing in a DISI engine using Rayleigh scattering
  • 2018
  • Ingår i: Combustion and Flame. - : Elsevier BV. - 1556-2921 .- 0010-2180. ; 193, s. 218-228
  • Tidskriftsartikel (refereegranskat)abstract
    • The stratified fuel distribution and early flame development in a firing spray-guided direct-injection spark-ignition (DISI) engine were characterized applying optical diagnostics. The objectives were to compare effects of single and double injections on the stratified air-fuel mixing and early flame development. Vaporized in-cylinder fuel distributions resulting from both single and double injections before, during and after ignition were selectively visualized applying Rayleigh scattering. The optical investigation of the in-cylinder fuel distributions and early flame propagation corroborated the better mixing, showing that double injections were associated with more evenly distributed fuel, fewer local areas with high fuel concentrations, faster initial flame spread and more even flame propagation (more circular flame spreading). The results support the hypothesis that delivering fuel in closely coupled double injections results in better mixing than corresponding single injections. The improved mixing is believed to stem from the longer time available for mixing of the air and fuel in double injection events.
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32.
  • Hermansson, Sven, 1978, et al. (författare)
  • CFD modelling of bed shrinkage and channelling in fixed-bed combustion
  • 2011
  • Ingår i: Combustion and Flame. - : Elsevier BV. - 1556-2921 .- 0010-2180. ; 158:5, s. 988-999
  • Tidskriftsartikel (refereegranskat)abstract
    • Combustion of fixed fuel beds in grate furnaces is common within production of heat and power fromsolid fuels. Available theoretical and experimental experience provides a solid base of knowledge onhow a conversion model of a fuel bed, using Computational Fluid Dynamics (CFD), needs to be structuredand solved. Most existing models, however, handle the conversion in one single dimension of constantbed properties; when observing a burning fuel bed in a grate furnace it becomes apparent that the fuelbed is neither homogeneous nor uni-dimensional. In this study, a two-dimensional model of the combustionof fixed fuel beds has been developed for the purpose of studying the influence of heterogeneousfuel-bed properties on the conversion. In the model, the available experience from fuel-bed modellingby means of the sub-models for fixed-bed conversion was structured into a fluid-flow scale and into afuel-particle scale, in which new formulations describing the shrinkage of the fuel bed on a multi-particlescale was introduced. Both available and new sub-models were introduced into a pre-existing CFD-platform,in which the framework for simulating fluid flow in porous media was used to solve also the conversionrelated processes acting within the particle scales as well as within the multi-particle scales. Thecomplete model was validated with good correspondence between available measurements of temperatureand species concentration in a wood-char combustor. In addition, the modelled shrinkage was foundto well describe the observed shrinkage of the fuel bed in a combustion experiment. Results of modelsimulations by using heterogeneous bed porosity show that a porous passage through the bed risks causingchannelling in the fuel bed – a phenomenon common in modern grate furnaces and suspected tocause increased emissions of nitric oxides and unburned carbon compounds. The channelling tendencycould, however, to a large extent be reduced by grates of higher flow resistance. The natural porosityincrease attributable to the packing of particles onto a wall was shown to concentrate combustion disturbancesclose to the surface of the grate. Thus, larger changes in the porosity than caused by natural fuelpacking against a wall are needed to give rise to channels that emerge through the fuel bed.
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33.
  • Hoeijmakers, Maarten, et al. (författare)
  • Flame dominated thermoacoustic instabilities in a system with high acoustic losses
  • 2016
  • Ingår i: Combustion and Flame. - : Elsevier. - 0010-2180 .- 1556-2921. ; 169, s. 209-215
  • Tidskriftsartikel (refereegranskat)abstract
    • The thermoacoustic stability behaviour of a flame is experimentally investigated in the presence of large acoustic losses. Recently it has become clear that under such conditions an instability can occur due to an intrinsic local feedbackloop at the heat source. The experimental results confirm that despite significant acoustic losses, thermoacoustic instabilities can still be present. These findings imply that the effectiveness of passive thermoacoustic damping devices is limited by the intrinsic stability properties of the flame. (C) 2016 The Combustion Institute. Published by Elsevier Inc. All rights reserved.
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34.
  • Hoeijmakers, Maarten, et al. (författare)
  • Intrinsic instability of flame-acoustic coupling
  • 2014
  • Ingår i: Combustion and Flame. - : Elsevier BV. - 0010-2180 .- 1556-2921. ; 161:11, s. 2860-2867
  • Tidskriftsartikel (refereegranskat)abstract
    • This paper shows that a flame can be an intrinsically unstable acoustic element. The finding is clarified in the framework of an acoustic network model, where the flame is described by an acoustic scattering matrix. The instability of the flame acoustic coupling is shown to become dominating in the limit of no acoustic reflections. This is in contrast to classical standing-wave thermoacoustic modes, which originate from the positive feedback loop between system acoustics and the flame. These findings imply that the effectiveness of passive thermoacoustic damping devices is limited by the intrinsic stability properties of the flame.
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35.
  • Hosseini, N., et al. (författare)
  • Evaluating thermoacoustic properties of heating appliances considering the burner and heat exchanger as acoustically active elements
  • 2018
  • Ingår i: Combustion and Flame. - : Elsevier. - 0010-2180 .- 1556-2921. ; 191, s. 486-495
  • Tidskriftsartikel (refereegranskat)abstract
    • Heat exchangers are an essential constituent part of many combustion systems. The thermoacoustic instability in such systems is a common problem and it has been studied extensively. However, the heat exchanger has not gained much attention in the field of combustion thermoacoustics, leading to a lack of knowledge about the thermoacoustic interactions between the burner and the heat exchanger. In this paper, a modeling approach is introduced to study these interactions in an academic representation of a heating appliance, comprised of a perforated slit burner and a tube heat exchanger. Both elements are considered thermally and acoustically active. A CFD model is used in a two-dimensional domain to simulate the response of the system to small amplitude broadband velocity perturbations. The thermochemical and acoustic coupling between the burner and the heat exchanger is investigated and a method is introduced to decouple their effects and study them separately. The extents to which this method is valid are addressed by varying the distance between the elements. Results show that as long as the flames do not impinge on the heat exchanger surface, a linear network modeling approach can be applied to construct the acoustic response of the composed configuration from the responses of its constituting elements. This approach requires registering the average velocity on a properly chosen intermediate plane between the burner and heat exchanger. Choosing this plane may be to some point difficult, i.e. when the burner and heat exchanger are close and cannot be considered independent. Moreover, when flame impingement occurs, the interactions between the flame and heat exchanger affect their individual thermoacoustic behaviors and the burner plus heat exchanger assembly needs to be considered as one coupled acoustic element. Particularly, flame impingement changes the phase of the heat absorption response of the heat exchanger and it may significantly alter the acoustic properties of the coupled assembly. The physics lying behind the effects of such interactions on the thermoacoustics of the system is discussed. The obtained results signify that a correct stability prediction of an appliance with burner and heat exchangers requires considering active thermoacoustic behavior of both elements as well as their interactions.
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36.
  • Ivanov, Mikhail F., et al. (författare)
  • Ignition of deflagration and detonation ahead of the flame due to radiative preheating of suspended micro particles
  • 2015
  • Ingår i: Combustion and Flame. - : Elsevier BV. - 0010-2180 .- 1556-2921. ; 162:10, s. 3612-3621
  • Tidskriftsartikel (refereegranskat)abstract
    • We study a flame propagating in the gaseous combustible mixture with suspended inert solid micro particles. The gaseous mixture is assumed to be transparent for thermal radiation emitted by the hot combustion products, while particles absorb and reemit the radiation. Thermal radiation heats the particles, which in turn transfer the heat to the surrounding unburned gaseous mixture by means of thermal heat transfer, so that the gas phase temperature lags that of the particles. We consider different scenarios depending on the spatial distribution of the particles, their size and the number density. In the case of uniform spatial distribution of the particles the radiation causes a modest increase of the temperature ahead of the flame and corresponding modest increase of the combustion velocity. In the case of non-uniform distribution of the particles (layered dust cloud), such that the particles number density is relatively small in the region just ahead of the flame front and increases in the distant regions ahead of the flame, the preheating caused by the thermal radiation may trigger additional independent source of ignition. Far ahead of the flame, where number density of particles increases forming a dense cloud of particles, the radiative preheating results in the formation of a temperature gradient with the maximum temperature sufficient for ignition. Depending on the steepness of the temperature gradient formed in the unburned mixture, either deflagration or detonation can be initiated via the Zel'dovich's gradient mechanism. The ignition and the resulting combustion regimes depend on the number density profile and, correspondingly, on the temperature profile (temperature gradient), which is formed in effect of radiation absorption and gas-dynamic expansion. The effect of radiation preheating as stronger as smaller is the normal flame velocity. The effect of radiation heat transfer in the case of coal dust flames propagating in layered particle-gas deposits cloud can result in the spread of combustion wave with velocity up to 1000 m/s and it is a plausible explanation of the origin of dust explosion in coal mines.
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37.
  • Johansson, Robert, 1977, et al. (författare)
  • Account for variations in the H2O to CO2 molar ratio when modelling gaseous radiatve heat transfer with the weighted-sum-of-grey-gases model
  • 2011
  • Ingår i: Combustion and Flame. - : Elsevier BV. - 1556-2921 .- 0010-2180. ; 158:5, s. 893-901
  • Tidskriftsartikel (refereegranskat)abstract
    • This work focuses on models suitable for taking into account the spectral properties of combustion gasesin computationally demanding applications, such as computational fluid dynamics. One such model,which is often applied in combustion modelling, is the weighted-sum-of-grey-gases (WSGG) model.The standard formulation of this model uses parameters fitted to a wide range of temperatures, but onlyfor specific ratios of H2O to CO2. Then, the model is limited to gases from fuels with a given compositionof hydrogen and carbon, unless several sets of fitted parameters are used. Here, the WSGG model is modifiedto account for various ratios of H2O to CO2 concentrations. The range of molar ratios covers both oxyfuelcombustion of coal, with dry- or wet flue gas recycling, as well as combustion of natural gas. The nongreyformulation of the modified WSGG model is tested by comparing predictions of the radiative sourceterm and wall fluxes in a gaseous domain between two infinite plates with predictions by a statisticalnarrow-band model. Two grey approximations are also included in the comparison, since such modelsare frequently used for calculation of gas radiation in comprehensive combustion computations. It isshown that the modified WSGG model significantly improves the estimation of the radiative source termcompared to the grey models, while the accuracy of wall fluxes is similar to that of the grey models orbetter.
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38.
  • Kassman, Håkan, 1962, et al. (författare)
  • The effect of oxygen and volatile combustibles on the sulphation of gaseous KCl
  • 2013
  • Ingår i: Combustion and Flame. - : Elsevier BV. - 1556-2921 .- 0010-2180. ; 160:10, s. 2231-2241
  • Tidskriftsartikel (refereegranskat)abstract
    • Sulphur/sulphate containing additives, such as elemental sulphur (S) and ammonium sulphate (NH4)(2)SO4), can be used for sulphation of KCl during biomass combustion. These additives convert KCl to an alkali sulphate and a more efficient sulphation is normally achieved for ammonium sulphate compared to sulphur. The presence of SO3 is thus of greater importance than that of SO2. Oxygen and volatile combustibles could also have an effect on the sulphation of gaseous KCl. This paper is based on results obtained during co-combustion of wood chips and straw pellets in a 12 MW circulating fluidised bed (CFB) boiler. Ammonium sulphate was injected at three positions in the boiler i.e. in the upper part of the combustion chamber, in the cyclone inlet, and in the cyclone. The sulphation of KCl was investigated at three air excess ratios (lambda = 1.1, 1.2 and 1.4). Several measurement tools were applied including IACM (on-line measurements of gaseous alkali chlorides), deposit probes (chemical composition in deposits collected) and gas analysis. The position for injection of ammonium sulphate had a great impact on the sulphation efficiency for gaseous KCl at the different air excess ratios. There was also an effect of oxygen on the sulphation efficiency when injecting ammonium sulphate in the cyclone. Less gaseous KCl was reduced during air excess ratio lambda = 1.1 compared to the higher air excess ratios. The optimal position and conditions for injection of ammonium sulphate were identified by measuring KCl with IACM. A correlation was observed between the sulphation of gaseous KCl and reduced chlorine content in the deposits. The experimental observations were evaluated using a detailed reaction mechanism. It was used to model the effect of volatile combustibles on the sulphation of gaseous MCI by SO3. The calculations supported the proposition that the presence of combustibles at the position of SO3 injection (i.e. AS) causes reduction of SO3 to SO2.
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39.
  • Kassman, Håkan, 1962, et al. (författare)
  • The importance of SO2 and SO3 for sulphation of gaseous KCl - An experimental investigation in a biomass fired CFB boiler
  • 2010
  • Ingår i: Combustion and Flame. - : Elsevier BV. - 1556-2921 .- 0010-2180. ; 157:9, s. 1649-1657
  • Tidskriftsartikel (refereegranskat)abstract
    • This paper is based on results obtained during co-combustion of wood pellets and straw in a 12 MW circulating fluidised bed (CFB) boiler. Elemental sulphur (S) and ammonium sulphate ((NH4)(2)SO4) were used as additives to convert the alkali chlorides (mainly KCl) to less corrosive alkali sulphates. Their performance was then evaluated using several measurement tools including, IACM (on-line measurements of gaseous alkali chlorides), a low-pressure impactor (particle size distribution and chemical composition of extracted fly ash particles), and deposit probes (chemical composition in deposits collected). The importance of the presence of either SO2 or SO3 for gas phase sulphation of KCl is also discussed. Ammonium sulphate performed significantly better than elemental sulphur. A more efficient sulphation of gaseous KCl was achieved with (NH4)(2)SO4 even when the S/Cl molar ratio was less than half compared to sulphur. Thus the presence of gaseous SO3 is of greater importance than that of SO2 for the sulphation of gaseous KCl. (C) 2010 The Combustion Institute. Published by Elsevier Inc. All rights reserved.
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40.
  • Keller, Martin, 1985, et al. (författare)
  • Gasification inhibition in chemical-looping combustion with solid fuels
  • 2011
  • Ingår i: Combustion and Flame. - : Elsevier BV. - 1556-2921 .- 0010-2180. ; 158:3, s. 393-400
  • Tidskriftsartikel (refereegranskat)abstract
    • Chemical-looping combustion (CLC) is a novel technology that can be used to meet growing demands on energy production without CO2 emissions. The CLC process includes two reactors, an air and a fuel reactor. Between these two reactors oxygen is transported by an oxygen carrier, which most often is a metal oxide. This arrangement prevents mixing of N-2 from the air with CO2 from the combustion giving combustion gases that consist almost entirely of CO2 and H2O. The technique reduces the energy penalty that normally arises from the separation of CO2 from other flue gases, hence, CLC could make capture of CO2 cheaper. For the application of CLC to solid fuels, the char remaining after devolatilization will react indirectly with the oxygen carrier via steam gasification. It has been suggested that H-2, and possibly CO, has an inhibiting effect on steam gasification in CLC. In this work experiments were conducted to investigate this effect. The experiments were conducted in a laboratory fluidized-bed reactor that was operating cyclically with alternating oxidation and reduction periods. Two different oxygen carriers were used as well as an inert sand bed. During the reducing period varying concentrations of CO or H-2 were used together with steam while the oxidation was conducted with 10% O-2 in N-2. The temperature was constant at 970 degrees C for all experiments. The results show that CO does not directly inhibit the gasification whereas the partial pressure of H-2 had a significant influence on fuel conversion. The results also suggest that dissociative hydrogen adsorption is the predominant hydrogen inhibition mechanism under the laboratory conditions, thus explaining why char conversion is much faster in a bed of oxygen carrying material, compared to an inert sand bed. (C) 2010 The Combustion Institute. Published by Elsevier Inc. All rights reserved.
  •  
41.
  • Kojourimanesh, Mohammad, et al. (författare)
  • Evaluation of the thermo-acoustic instability frequency and growth rate via input reflection coefficient measurement for central heating equipment
  • 2024
  • Ingår i: Combustion and Flame. - : Elsevier BV. - 0010-2180 .- 1556-2921. ; 263
  • Tidskriftsartikel (refereegranskat)abstract
    • Experimental measurement methods and theoretical evaluations based on low-order modeling approaches for both the growth rate and frequency at the onset of thermo-acoustic combustion instability are proposed, and their performance is evaluated. The developed techniques are demonstrated through a systematic measurement of the linear growth rate and frequency of evolving oscillations in the laboratory setup and also applied for the thermo-acoustic qualification of an industrial domestic boiler and a heat cell unit (combination of a burner with a heat-exchanger). Generic measurements have been done for a burner deck with premixed surface-stabilized Bunsen-type flames. The industrial domestic boiler and the heat cell unit are equipped with burners of a similar type but differ by their perforation pattern. They have been tested at different conditions and the experimental and theoretical results are compared. Two modeling strategies are tested: 1- in Laplace domain, with estimating a rational function in the complex domain to fit the measured frequency response, 2- exclusively in frequency domain, without estimating a rational function. Both methods include measurement of the frequency response of two reflection coefficients from i) the upstream part of the system, Rup, and ii) burner with flame completed by the downstream part of the appliance, Rin. Within the first approach, a procedure for an analytic continuation of the measured frequency response to the complex domain is applied and complex eigenfrequencies are calculated by solving the corresponding dispersion equation. An alternative approach was proposed by Kopitz and Polifke and allows estimating both the frequency of oscillation and the growth rate from the analysis of the polar plot of the system's characteristic equation in the frequency domain. The comparison shows that the unstable frequencies can be predicted accurately by both tested modeling strategies. This conclusion holds also for the tested industrial applications. The prediction of the instability growth rates is closer to the measured one when the modeling method in the complex domain is used. However, the frequency domain analysis provides less accurate, but still reasonable estimates of the growth rates and frequencies. Moreover, a good overview of thermo-acoustic performance of each industrial boiler/burner at different conditions is obtained via Rin measurements.
  •  
42.
  • Kojourimanesh, M., et al. (författare)
  • Thermo-acoustic flame instability criteria based on upstream reflection coefficients
  • 2021
  • Ingår i: Combustion and Flame. - : Elsevier BV. - 0010-2180 .- 1556-2921. ; 225, s. 435-443
  • Tidskriftsartikel (refereegranskat)abstract
    • A prospective method to assess thermo-acoustic instabilities based on two reflection coefficients measured from the upstream side of the burner is presented and experimentally validated. In order to compose a model which allows predicting the onset of thermo-acoustic instability of combustion in a practical appliance, one has to characterize the thermo-acoustic properties of the burner including the flame as an acoustically active element and acoustic properties of all other (usually passive) components of the combustion appliance both upstream as well as downstream of the burner. This kind of modeling strategy usually faces serious practical problems related to the need of measurements/modeling at the hot downstream part of the system. In the present work, we propose a measurement and a system modeling approach which relies on two acoustic measurements, namely reflection coefficients, only at the cold (burner upstream) part of the combustion appliance. Both reflection coefficients, termed upstream and input, can be readily measured using standard acoustic techniques. The need to measure the input reflection coefficient of an acoustically active subsystem may impose difficulties related to the acoustic instability of the measurement setup itself. The approach and technical solution to handle this problem via a special modification of the excitation source (loudspeaker box) is proposed. The dispersion relation to search for system eigen frequencies is represented in a form that couples the reflection coefficients of the upstream part of the appliance and input reflection coefficient from the downstream part as observed through the burner with flame. This form of the dispersion relation is commonly used in the theory of radio-frequency circuits and recently introduced for thermo-acoustic problems. The proposed method is applied to burners with premixed burner-stabilized Bunsen-type flames. The observed instability conditions and oscillation frequencies are compared with predictions of the proposed modeling approach and reveal good correspondence.
  •  
43.
  • Lackmann, Tim, 1983, et al. (författare)
  • A representative linear eddy model for simulating spray combustion in engines (RILEM)
  • 2018
  • Ingår i: Combustion and Flame. - : Elsevier BV. - 1556-2921 .- 0010-2180. ; 193, s. 1-15
  • Tidskriftsartikel (refereegranskat)abstract
    • The design of new combustion concepts for low emission, high efficiency internal combustion engines often leads to combustion under low temperature conditions. Under those conditions, the assumption of fast chemistry, which has been the cornerstone of many turbulent combustion models, is not strictly valid anymore and the validity and applicability of classical combustion models such as flamelet models might be limited. In this paper we present an updated version of a recently developed regime independent modeling approach for turbulent non-premixed combustion with an emphasis on applications to internal combustion engines. The model utilizes the mode-and regime-independent linear eddy model (LEM) as a combustion and micro-mixing model in a representative way. This is achieved by time advancing only one LEM realization representing the combustion process in the whole engine domain and coupling it to a RANS simulation with a presumed 9-function PDF approach for the mixture fraction. The use of LEM rather than flamelet combustion closure has several benefits, an important one being regime independence. Additionally, LEM incorporates a physically based representation of the stochastic variability of turbulent eddy motions, implying an intrinsic representation of scalar dissipation rate fluctuations. In order to capture key features of engine spray-combustion environments, the LEM methodology is extended by introducing a conical LEM domain to approximate spray spatial development, fuel vapor input based on CFD-prescribed spray evaporation, and a representation of large scale turbulent motions distinct from the inertial-range turbulence that develops at smaller scales. The representative character of LEM states is evaluated by comparing mixture fraction statistics and scalar dissipation rates generated by LEM and the CFD. The performance and predictive capability of the model for typical engine applications is evaluated by simulating a standard test case-Spray B of the Engine Combustion Network (ECN)-and comparing the results with experimental data. The results demonstrate the capability of the model to represent the spray combustion process with reasonable accuracy but also reveal some limitations. The limitations and shortcomings of the model are discussed and an outlook for further development of the approach into a regime-and mode-independent combustion model for internal engine applications is given. (C) 2018 The Combustion Institute. Published by Elsevier Inc. All rights reserved.
  •  
44.
  • Leckner, Bo G, 1936 (författare)
  • Spectral and total emissivity of water vapor and carbon dioxide
  • 1972
  • Ingår i: Combustion and Flame. - 1556-2921 .- 0010-2180. ; 19:1, s. 33-48
  • Tidskriftsartikel (refereegranskat)abstract
    • Data on the infrared radiation characteristics of carbon dioxide and water vapor in the form of absorption coefficients and line spacings averaged over narrow spectral intervals have been compiled from various sources. These data are to be used in heat transfer calculations from hot gases. In order to investigate the accuracy of the data, the simplest case possible is chosen: a comparison with the total emissivity charts of water vapor and carbon dioxide. It appears however that the charts are not entirely reliable as standards for comparison: it seems probable that Hottel's chart for water vapor gives too low values at temperatures above 900°C and that the partial pressure correction is temperature dependent. With the exception of some regions where judgment is difficult, the calculations using spectral data seem to represent total emissivities with a maximum error which is estimated to around 10%. Sources of error in the spectral data and in Hottel's total emissivity charts are discussed. Total emissivity charts, pressure and overlap corrections based on calculations with spectral data are presented.
  •  
45.
  • Leckner, Bo G, 1936 (författare)
  • The spectral and total emissivity of carbon dioxide
  • 1971
  • Ingår i: Combustion and Flame. - 1556-2921 .- 0010-2180. ; 17:1, s. 37-44
  • Tidskriftsartikel (refereegranskat)abstract
    • The calculation of radiative heat transfer from flames and combustion gases requires, in the general case of non-homogeneous gases, a knowledge of the spectral characteristics of the gases. In this paper an analysis is made of available spectral data on carbon dioxide, extended with an approximative description of the 15-μm, 10-μm, and 2-μm bands, in the form of a comparison with measured total emissivities. The contribution of the bands to the total emissivity is shown at various temperatures and path lengths. The spectral emissivity is expressed as mean-values over wave-number intervals by means of the statistical band model. The agreement between Hottel's diagrams of total emissivities and the total emissivities calculated on the basis of spectral data is good for optical path lengths greater than 1.0 cm atm. If Hottel's values are corrected in the small-path length region for the possible influence of carbon dioxide in the air layer between the measuring instrument and the gas container, there remains a discrepancy of less than 15% around path lengths of 0.1 cm atm.
  •  
46.
  • Lee, Hsu Chew, et al. (författare)
  • A DNS study of extreme and leading points in lean hydrogen-air turbulent flames – Part I: Local thermochemical structure and reaction rates
  • 2022
  • Ingår i: Combustion and Flame. - : Elsevier BV. - 1556-2921 .- 0010-2180. ; 235
  • Tidskriftsartikel (refereegranskat)abstract
    • A Direct Numerical Simulation (DNS) study of statistically one-dimensional and planar, lean complex-chemistry hydrogen-air flames characterized by a low Lewis number Le and three different Karlovitz numbersKa ranging from 3 to 33 is performed, with the same complex-chemistry flames being also simulated by setting molecular diffusivities of all species equal to the heat diffusivity of the mixture. The simulations predict a significant increase in a ratio of turbulent burning velocity to the laminar flame speed in the former (Le<1) flames when compared to the latter (equidiffusive) flames. Extreme points characterized by the peak (over the computational domain) Fuel Consumption Rate (FCR) or Heat Release Rate (HRR) are found at each instant. In the equidiffusive flames, such extreme FCR and HRR are close to their peak values in the unperturbed laminar flame. If Le is low, the former rates are significantly higher than the latter ones due to an increase in the local temperature, equivalence ratio, and radical mass fractions, caused by diffusive-thermal effects. While the studied extreme points may appear sufficiently far from the leading edge of the instantaneous flame brush, leading points characterized by a lower, but still high (Le<1) FCR or HRR are observed close to the leading edge at each instant. Various local characteristics (temperature, equivalence ratio, species mass fractions and their gradients, reaction rates, etc.) of the extreme and leading points are explored and significant differences between zones characterized by high FCR or HRR are revealed. For instance, in the latter zones, major chemical pathways are changed. Moreover, while the extreme HRRs strongly fluctuate in time, with their mean and rms values being significantly increased by Ka, the extreme FCRs fluctuate weakly and are close at different Ka, thus, implying that almost the same extreme FCR can be reached in substantially different local burning structures.
  •  
47.
  • Lee, Hsu Chew, et al. (författare)
  • A DNS study of extreme and leading points in lean hydrogen-air turbulent flames - part II: Local velocity field and flame topology
  • 2022
  • Ingår i: Combustion and Flame. - : Elsevier BV. - 1556-2921 .- 0010-2180. ; 235
  • Tidskriftsartikel (refereegranskat)abstract
    • Data obtained in recent direct numerical simulations (Lee et al.) of statistically one-dimensional and planar, lean complex-chemistry hydrogen-air flames characterized by three different Karlovitz numbers Ka ranging from 3 to 33 are further analyzed in order to explore local characteristics and structure of (i) extreme points characterized by the peak (over the computational domain) Fuel Consumption Rate (FCR) or Heat Release Rate (HRR) and (ii) leading points that are also characterized by a high FCR or HRR, but advance furthest into unburned reactants. Results show that, on the one hand, common characteristics of flame perturbations (curvature, strain and stretch rates, displacement speed) fluctuate significantly in the extreme or leading, FCR or HRR points and are different in different flames. Moreover, other two-point local quantities such as the local gradients of combustion progress variables or species (e.g., the radical H) mass fractions are different in different flames. Therefore, a common simple configuration of a perturbed laminar flame cannot be used as a catchall model of the entire local structure of zones surrounding the discussed points at various Ka. On the other hand, single-point local characteristics (temperature, species mass fractions, rates of their production) of the FCR extreme points are comparable in all three turbulent flames and in the critically strained planar laminar flame. In particular, the FCRs in the extreme points fluctuate weakly and are approximately equal to each other and to the peak FCR in the critically strained laminar flame. The latter finding implies that (i) the maximum FCR evaluated in the critically strained laminar flame could be used to characterize, in a first approximation, the local FCR in the extreme or leading points in turbulent flames, thus, supporting the leading point concept, and (ii) almost the same extreme FCR can be reached in substantially different local burning structures.
  •  
48.
  • Li, Jun, et al. (författare)
  • Flame characteristics of pulverized torrefied-biomass combusted with high-temperature air
  • 2013
  • Ingår i: Combustion and Flame. - : Elsevier BV. - 0010-2180 .- 1556-2921. ; 160:11, s. 2585-2594
  • Tidskriftsartikel (refereegranskat)abstract
    • In this work, the flame characteristics of torrefied biomass were studied numerically under high-temperature air conditions to further understand the combustion performances of biomass. Three torrefied biomasses were prepared with different torrefaction degrees after by releasing 10%, 20%, and 30% of volatile matter on a dry basis and characterized in laboratory with standard and high heating rate analyses. The effects of the torrefaction degree, oxygen concentration, transport air velocity, and particle size on the flame position, flame shape, and peak temperature are discussed based on both direct measurements in a laboratory-scale furnace and CFD simulations. The results primarily showed that the enhanced drag force on the biomass particles caused a late release of volatile matter and resulted in a delay in the ignition of the fuel-air mixture, and the maximum flame diameter was mainly affected by the volatile content of the biomass materials. Furthermore, oxidizers with lower oxygen concentrations always resulted in a larger flame volume, a lower peak flame temperature and a lower NO emission. Finally, a longer flame was found when the transport air velocity was lower, and the flame front gradually moved to the furnace exit as the particle size increased. The results could be used as references for designing a new biomass combustion chamber or switching an existing coal-fired boiler to the combustion of biomass.
  •  
49.
  • Li, Tian, et al. (författare)
  • A fast-solving particle model for thermochemical conversion of biomass
  • 2020
  • Ingår i: Combustion and Flame. - : Elsevier BV. - 1556-2921 .- 0010-2180. ; 213, s. 117-131
  • Tidskriftsartikel (refereegranskat)abstract
    • Computational fluid dynamics (CFD) simulations of large-scale furnaces or reactors for thermal conversion of solid fuels remains challenging partially due to the high computational cost related to the particle sub-models. Owing to the thermally thick nature, it is particularly expensive to simulate the conversion of large fuel particles such as biomass particles. To address this issue, a fast-solving particle model was developed in this work with special attention to the computational efficiency. The model spatially discretizes a fuel particle in one homogenized dimension. The conversion process of the fuel particle is treated as a reactive variable-volume one-dimensional transient heat conduction problem. The model also utilizes several features that are typically found in sharp interphase-based models to reduce the computational cost. Validation of the model was carried out by comparing with experimental results under both pyrolysis and combustion conditions. The accuracy and computational efficiency of the model was thoroughly examined by varying the degrees of temporal and spatial discretization. It was found that the model well predicted pyrolysis and combustion of a single biomass particle within a broad range of temporal and spatial discretization. The time used to simulate the conversion of a biomass particle using the developed model can be more than one order of magnitude smaller than the conversion process itself. It was also revealed that a well-predicted conductive heat transfer inside the particle is essential for a precise simulation of the drying and devolatilization process. The char conversion process, however, is less sensitive to the external heat transfer as it is mainly controlled by the mass diffusion process. Further studies showed that a time step of 1×10−3 s and a spatial discretization of 20 cells were sufficient for simulating the conversion of typical fuel particles in grate-fired and fluidized-bed furnaces. We also demonstrated that when the particle model was implemented in a CFD solver, only 2.2% of computational overhead was introduced by the model. As the model can efficiently employ fixed time stepping, optimal load balancing during parallel computing of many simultaneous conversion processes becomes trivial. This performance opens up new possibilities for treating fuel polydispersity in Eulerian CFD simulations of biomass conversion.
  •  
50.
  • Lin, Leteng, et al. (författare)
  • Aerosol-based method for investigating biomass char reactivity at high temperatures
  • 2011
  • Ingår i: Combustion and Flame. - : Elsevier BV. - 0010-2180 .- 1556-2921. ; 158:7, s. 1426-1437
  • Tidskriftsartikel (refereegranskat)abstract
    • An aerosol-based method was proposed and developed to characterize particles fragmented from biomass chars during oxidation. The chars were prepared from both wood and miscanthus pellets under various pyrolysis conditions. Char fragments with aerodynamic diameters in the range of 0.5–10 μm were suspended and transported in a reactive gas through an aerosol reactor, which was heated by an electric oven. The oxidation of char particles in the reactor was investigated by determining on-line the particle size distributions before and after passage through the reactor using an aerodynamic particle sizer (APS) spectrometer. The interpretation of APS data was evaluated by both experiments and models in which the fine char particles were assumed to keep either constant density or constant diameter during the oxidation process. The results indicate that the aerosol-based method can be used to determine reaction kinetics of char particles in the high temperature range, where oxidation is normally controlled by diffusion limitation if measuring with the conventional techniques. The application of the aerosol method indicated that high pyrolysis temperature and prolonged retention time will reduce the char reactivity.
  •  
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