Publications
141 results found
Hůlek T, Lindstedt RP, 1996, Computations of steady-state and transient premixed turbulent flames using pdf methods, Combustion and Flame, Vol: 104, Pages: 481-504, ISSN: 0010-2180
Premixed propagating turbulent flames are modeled using a one-point, single time, joint velocity-composition probability density function (pdf) closure. The pdf evolution equation is solved using a Monte Carlo method. The unclosed terms in the pdf equation are modeled using a modified version of the binomial Langevin model for scalar mixing of Valino and Dopazo, and the Haworth and Pope (HP) and Lagrangian Speziale-Sarkar-Gatski (LSSG) models for the viscous dissipation of velocity and the fluctuating pressure gradient. The source terms for the presumed one-step chemical reaction are extracted from the rate of fuel consumption in laminar premixed hydrocarbon flames, computed using a detailed chemical kinetic mechanism. Steady-state and transient solutions are obtained for planar turbulent methane-air and propane-air flames. The transient solution method features a coupling with a Finite Volume (FV) code to obtain the mean pressure field. The results are compared with the burning velocity measurements of Abdel-Gayed et al. and with velocity measurements obtained in freely propagating propane-air flames by Videto and Santavicca. The effects of different upstream turbulence fields, chemical source terms (different fuels and strained/unstrained laminar flames) and the influence of the velocity statistics models (HP and LSSG) are assessed.
Lindstedt RP, Skevis G, 1996, Benene formation chemistry in premixed 1,3-butadiene flames, Symposium (International) on Combustion, Vol: 26, Pages: 703-709, ISSN: 0082-0784
Premixed laminar 1,3-butadiene flames have been studied using detailed kinetic modeling. The results obtained have been compared against molecular beam mass spectrometry (MBMS) data obtained by Cole and co-workers in stoichiometric (=1.0) and rich (=2.4) near-sooting flames. Predictions of burning velocities and concentrations of major species and key radicals are satisfactory with agreement close to experimental uncertainties. Particular attention has been given to the formation of the first aromatic ring with alternative benzene formation paths discussed in the context of an extensive sensitivity analysis. The kinetic model also contains a comprehensive benzene oxidation mechanism, which has been independently validated in previous work. The developed mechanism accurately predicts benzene levels and the near 100-fold increase in benzene concentrations due to the change in stoichiometry. The present study thus extends past work on benzene formation in flames to include a C4 fuel. Butadiene flames are particularly interesting in this aspect since they provide an early branching among the C2, C3, and C4 chains. The results obtained show that several benzene formation paths are of importance. Thus, given experimental and modeling uncertainties, propargyl radical recombination, vinyl radical addition to 1,3-butadiene, and vinyl radical addition to vinyl acetylene must be considered. Furthermore, acetylene addition to 1,3-C4H5 becomes more competitive in the stoichiometric flame. © 1996 Combustion Institute.
Popat NR, Catlin CA, Arntzen BJ, et al., 1996, Investigations to improve and assess the accuracy of computational fluid dynamic based explosion models, Journal of Hazardous Materials, Vol: 45, Pages: 1-25, ISSN: 0304-3894
A summary is given of part of the CEC co-sponsored project MERGE (Modelling and Experimental Research into Gas Explosions). The objective of this part of the project was to provide improved Computational Fluid Dynamic explosion models with the potential for use in hazard assessments. Five organisations with substantial experience in both theoretical and experimental explosion modelling contributed to this model assessment study; British Gas, Christian Michelsen Institute, Imperial College, Telemark Technological Research and Development Centre and TNO Prins Maurits Laboratory. The theoretical and numerical basis of the models are described. Results are given of a comparison exercise of model predictions against calculations which were chosen to test the accuracy of the various physical sub-models embodied within the overall explosion model. The development phase of the study is also described in which further extensions to the models were made to provide the best achievable agreement with small- and medium-scale experiments also conducted as part of the project. The models were finally used to simulate large-scale explosion experiments prior to the experiments being conducted. The overall capabilities of the models are reviewed and areas of uncertainty in the physics highlighted.
Hůlek T, Lindstedt RP, 1996, Modelling of unclosed nonlinear terms in a PDF closure for turbulent flames, Mathematical and Computer Modelling, Vol: 24, Pages: 137-147, ISSN: 0895-7177
Turbulent combustion modelling based on the probability density function (pdf) approach has proven to offer certain significant advantages over other solution methods. In the pdf formulation, important nonlinear terms appearing in the mathematical description of the flow are closed. The most prominent examples are in the case of the joint velocity-scalar pdf, the effects of chemical reactions and mean pressure gradient. Established models used in Monte Carlo particle pdf modelling to approximate the unclosed terms in the pdf equation are briefly reviewed. The models for the molecular mixing of scalars, viscous dissipation of momentum, and fluctuating pressure gradient perform well in constant density flows. However, modifications of these models are necessary if variable-density combusting flows are to be modelled accurately. This problem is here examined with particular reference to the flamelet regime of combustion, where the effects of combustion-induced acceleration of a fluid may be quantified. Simple approximations which result in a stochastic model for the effect of volume generation in flamelets are proposed.
Lindstedt RP, Lockwood FC, Selim MA, 1995, A Detailed Kinetic Study of Ammonia Oxidation, Combustion Science and Technology, Vol: 108, Pages: 231-254, ISSN: 0010-2202
A comprehensive study of ammonia oxidation in laminar flames and flow reactors using detailed chemical kinetic modelling is reported. A flat premixed NH3/NO flame and counterflow CO/O2/N2 and CH4/O2/N2 diffusion flames doped with ammonia have been studied along with NH3/NO/O2 and NH3/NO/O2/C2H6 flow reactors. Available kinetic data has been reviewed and a detailed reaction mechanism for the C/N system is proposed and results from computations are compared with experimental data. The destruction and formation of NO is found to be dominated by NH2, NH and N radicals in most of the cases studied. It is shown that the relative significance of the different NH, radicals in the various NO formation channels depends entirely on the flame conditions and that the role of the amidogen is crucial in nitric oxide reduction. © 1995, Taylor & Francis Group, LLC. All rights reserved.
Leung KM, Lindstedt RP, 1995, Detailed kinetic modeling of C<inf>1</inf> - C<inf>3</inf> alkane diffusion flames, Combustion and Flame, Vol: 102, Pages: 129-160, ISSN: 0010-2180
A study of detailed chemical kinetics in coflow and counterflow diffusion flames is presented. The chemistry of diffusion flames is of fundamental importance from a practical as well as a mechanistic viewpoint. The present study uses systematic reaction path flux and sensitivity analyses to determine the crucial reaction paths in methane and propane diffusion flames. The formation of benzene and intermediate hydrocarbons via C3 and C4 species has been given particular attention and the relative importance of reaction channels has been assessed. The developed mechanism considers singlet and triplet CH2, isomers of C3H4, C3H5, C4H3,H5 and C4H6. Computational results show that benzene in methane - air diffusion flames is formed mainly via reactions involving propargyl radicals and that reaction paths via C4 species are insignificant. It is also shown that uncertainties in thermodynamic data may significantly influence predictions and that the reaction of acetylene with the hydroxyl radical to produce ketene may be an important consumption path for acetylene in diffusion flames. Quantitative agreement has been achieved between computational results and experimental measurements of major and minor species profiles, including benzene, in methane-air and propane-air flames. It is also shown that the mechanism correctly predicts laminar burning velocities for stoichiometric C1-C3 flames. Finally, results for a two-dimensional methane-air flame on a Wolfhard-Parker burner obtained with full detailed chemistry are presented along with flamelet computations and the accuracy of the latter are discussed. © 1995.
Lindstedt RP, Maurice LQ, 1995, Detailed kinetic modelling of n-heptane combustion, COMBUSTION SCIENCE AND TECHNOLOGY, Vol: 107, Pages: 317-353, ISSN: 0010-2202
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- Citations: 114
Lindstedt PR, 1994, Simplified soot nucleation and surface growth steps for non-premixed flames
Simplified reaction steps for the formation and growth of soot particles in laminar non-premixed flames are outlined. The resulting models are combined with detailed gas phase chemistry and incorporate simplified steps for nucleation, surface growth and particle agglomeration. The soot nucleation and surface growth reactions are linked to the gas phase chemistry by the simplifying assumptions that benzene and acetylene are indicative of the locations in the flame structure where nucleation and soot mass growth occurs. The reaction mechanisms are applied to a range of ethylene and propane counterflow diffusion flames and the sensitivity of soot predictions to different nucleation and surface growth formulations are investigated. The formation paths of benzene in flames of this type are also discussed due to the importance of aromatic species in soot nucleation. It is shown that good qualitative and quantitative agreement with measured data for soot volume fraction, particle growth and number density can be obtained using simplified reaction steps.
Lindstedt RP, Selim MA, 1994, Reduced reaction mechanisms for ammonia oxidation in premixed laminar flames, Combustion Science and Technology, Vol: 99, Pages: 277-298, ISSN: 0010-2202
Reduced reaction mechanisms are essential for the inclusion of direct kinetic effects into turbulent flame computations for practical problems. In the present paper reduced seven step, five step and four step reaction mechanisms for ammonia oxidation have been developed by the study of premixed laminar flames. A systematic reduction technique using steady state assumptions for intermediaries and radicals has been used to obtain the reduced schemes from a comprehensive detailed kinetic mechanism. Special emphasis has been given to the formation and destruction of NO and N20. The reduced mechanisms have been tested by comparing calculations performed with the reduced mechanisms and the detailed starting mechanism. Results for an extensive set of flat laminar premixed H2/O2flames doped with ammonia or ammonia and nitric oxide diluted with argon and NH3/O2flames are presented. The reduced mechanisms generally provide excellent agreement with the temperature, major species and nitric-oxide profiles for all flame conditions covered in the present study. However, the 4-step mechanism is not recommended for calculations where accurate N2O profiles are required, as the steady state approximation for N2O is not well founded. © 1994, Taylor & Francis Group, LLC. All rights reserved.
Lindstedt RP, Lockwood FC, Selim MA, 1994, Detailed kinetic modelling of chemistry and temperature effects on ammonia oxidation, Combustion Science and Technology, Vol: 99, Pages: 253-276, ISSN: 0010-2202
An extensive set of flat laminar premixed NH3/H2/O2, NH3/NO/H2/O2and NH3/O2flames have been investigated by detailed chemical kinetic modelling to facilitate the construction of a reaction mechanism capable of satisfactory predictions for a wide range of flames. Available information for the rate coefficients of all the reactions in the detailed mechanism has been reviewed. An extensive sensitivity analysis has been performed to distinguish the reactions of greatest importance to the formation and destruction of nitric oxide. The relative significance of the different NO formation channels is found to depend entirely on the flame conditions: (i) for all flames the reaction of NH2with the O radical is found to be significant; (ii) in pure ammonia flames the reaction of the NH radical with OH becomes important; (iii) in hydrogen flames with ammonia and ammonia with nitric oxide dopants the Zel’dovich mechanism becomes increasingly significant with increasing fuel concentrations. The conversion of NO to N2is dominated by reactions involving the NH2and N radicals with NH providing a secondary path. In pure ammonia and doped lean hydrogen flames the reaction of NO with NH2becomes the major NO conversion path. In doped stoichiometric and rich hydrogen flames the reaction of NO with N is dominant. © 1994, Taylor & Francis Group, LLC. All rights reserved.
LINDSTEDT RP, 1994, FORMATION AND DESTRUCTION OF AROMATIC-COMPOUNDS AND SOOT IN FLAMES, ABSTRACTS OF PAPERS OF THE AMERICAN CHEMICAL SOCIETY, Vol: 207, Pages: 22-FUEL, ISSN: 0065-7727
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Lindstedt RP, Skevis G, 1994, Detailed kinetic modeling of premixed benzene flames, Combustion and Flame, Vol: 99, Pages: 551-561, ISSN: 0010-2180
A detailed kinetic mechanism capable of modeling aromatic ring breakdown in benzene flames has been developed through reaction path and systematic sensitivity analyses. The model incorporates benzene oxidation and pyrolysis steps, a C5 chemistry submechanism and some novel features such as linearization reactions of cyclic and aromatic species. Model predictions are compared with the MBMS data of Bittner and Howard obtained in a rich, near-sooting, laminar premixed C6H6/O2/Ar flame. Excellent agreement is observed for major species and key intermediates. However, the C5 chemistry is still tentative, though the influence of the uncertainties on the main reaction paths is small to moderate. It is shown that aromatic ring destruction occurs via linearization to C6 species and subsequent breakdown to the C3 and C4 chains. It has been shown that a direct decomposition route for phenyl, or benzene, to propargyl leads to poor agreement with measurements. The present work provides a basis for further study of the formation of PAH in flames. © 1994.
Fairweather M, Jones WP, Ledin HS, et al., 1992, Predictions of soot formation in turbulent, non-premixed propane flames, Symposium (International) on Combustion, Vol: 24, Pages: 1067-1074, ISSN: 0082-0784
Predictions of soot formation in turbulent jet flames have been made by solving the fluid dynamic equations in conjunction with a second-order closure model for the turbulent transport of momentum and scalar fluxes. The gas-phase, non-premixed combustion process is modelled via the conserved scalar/prescribed probability density function (p.d.f.) approach using the laminar flamelet concept. Soot formation, growth and consumption is included through balance equations for soot mass fraction and particle number density which admit finite-rate kinetic effects. Both flamelet and sooting prescriptions are derived from a detailed gas-phase chemistry scheme coupled to a global reaction scheme for soot formation, the latter involving soot nucleation, surface growth, particle coagulation and oxidation steps. Solutions of the complete model are compared with experimental data obtained by Nishida and Mukohara in co-flowing turbulent propane flames. For two air preheat temperatures studied experimentally, predictions of the model are shown to be in good qualitative and quantitative agreement with measured soot concentrations. In particular, the model faithfully reproduces the significant influence of air preheat on the sooting characteristics of the flames. © 1992 Combustion Institute.
Fairweather M, Jones WP, Lindstedt RP, 1992, Predictions of radiative transfer from a turbulent reacting jet in a cross-wind, Combustion and Flame, Vol: 89, Pages: 45-63, ISSN: 0010-2180
Predictions of the structure and received thermal radiation around a turbulent reacting jet discharging into a cross-flow have been made using a finite-difference scheme for solving the fluid dynamic equations. The model employs a two-equation, k-ε{lunate} turbulence model. The gas-phase, non-premixed combustion process is modeled via the conserved scalar/prescribed probability density function approach using the laminar flamelet concept, whilst soot formation and consumption is included through balance equations for mass fraction and particle number density which admit finite-rate kinetic effects. Both flamelet and sooting prescriptions are derived from a global reaction scheme for hydrocarbon combustion. Levels of radiation received around a flame are obtained using the discrete transfer method coupled to a narrow band model of radiative transfer. In order to assess the usefulness of the model for predicting the consequences associated with atmospheric venting and flaring operations, solutions are compared with experimental data from laboratory and field scale studies of natural gas flames. Predictions are shown to be in good agreement with measurements of received radiation made around all the flames examined. In particular, results for a number of sooting strain rates indicate that a single rate suffices for predicting the radiation received about a wide range of flame sizes. © 1992.
Catlin CA, Lindstedt RP, 1991, Premixed turbulent burning velocities derived from mixing controlled reaction models with cold front quenching, Combustion and Flame, Vol: 85, Pages: 427-439, ISSN: 0010-2180
The Eddy Breakup model has been investigated in two modified forms in which the reaction rate is quenched at the "cold front" of the flame. One quench criterion is based on the reaction progress variable (RPVQC) and the other on temperature (TQC). Burning velocities for the RPVQC have been calculated from the steady conservation equations by the use of apparently novel numerical and analytical eigenvalue techniques. Both RPVQC and TQC criteria were also studied in numerical simulations of transient one-dimensional flame propagation. The latter study also determined essential constraints on grid and time step size required to ensure accurate predictions. For the RPVQC the eigenvalue analysis and transient computations are in good agreement, provided the quench point is not close to the cold front. As quenching approaches the cold front, both analyses indicate that the reaction zone becomes infinitely thick, a behavior that is not observed in experiments on accelerating flames. These results are also consistent with the transient computations using the TQC, where good agreement was obtained for slow flame speeds in which the cold front reactant temperature is close to ambient. However, at higher flame speeds the burning velocity predicted by the TQC increased more rapidly relative to that of the RPVQC. This is because adiabatic compression raises the cold front temperature closer to that of the quench temperature. At sufficiently high flame speeds the burning velocity and flame thickness grow continuously with time ultimately leading to an unphysically propagated detonation wave. Predicting experimentally observed flame behavior with this type of modified reaction model is apparently not possible without significant cold front quenching. © 1991.
Fairweather M, Jones WP, Lindstedt RP, et al., 1991, Predictions of a turbulent reacting jet in a cross-flow, Combustion and Flame, Vol: 84, Pages: 361-375, ISSN: 0010-2180
The article presents an application of a finite-difference scheme for solving the fluid dynamic equations of three-dimensional elliptic flow to the problem of a turbulent reacting jet discharging perpendicularly into an unconfined cross-flow. The mathermatical model employs a standard two-equation, k-ε{lunate} model to calculate the distribution of Reynolds stresses, with the turbulent nonpremixed combustion process being modeled via the conserved scalar/prescribed probability density function approach. The laminar flamelet concept is used to specify the instantaneous thermochemical state of the combusting mixture. In order to assess the ultimate usefulness of the model for predicting the consequences associated with atmospheric venting and flaring of flammable gases, solutions of the model are compared with experimental data for natural gas flames obtained from wind tunnel studies by Birch et al. Over a range of ratios of cross-flow to jet velocity, predictions of flame trajectory and length are in reasonable qualitative agreement with experimental data. At one particular velocity ratio, for which detailed measurements of the mean temperature field of the flame are available, close agreement between theory and experiment is obtained provided the effects of flame liftoff and radiative heat loss are incorporated into the turbulent flow calculation. © 1991.
Leung KM, Lindstedt RP, Jones WP, 1991, A simplified reaction mechanism for soot formation in nonpremixed flames, Combustion and Flame, Vol: 87, Pages: 289-305, ISSN: 0010-2180
The present article outlines a simplified reaction mechanism for the formation, growth, and combustion of soot particles in laminar nonpremixed flames. The model can be combined with detailed chemistry descriptions for the gas phase, as in the present case, or with reduced chemical reaction mechanisms. The reaction mechanism involves nucleation, surface growth, particle coagulation, and combustion steps. The model outlined has been created with the intention of being applicable to the prediction of turbulent flames via different approaches. The soot nucleation and surface growth reactions are linked to the gas phase by presuming that pyrolysis products, in the present case acetylene, and not the fuel itself, are of primary importance in the soot formation process. The deduced reaction mechanism is applied to counterflow ethylene and propane flames burning with a range of oxygen-enriched and -depleted air streams. The results obtained show excellent qualitative and quantitative agreement with measured data for soot volume fraction, particle growth, and number density. © 1991.
Lindstedt RP, Michels HJ, 1989, Deflagration to detonation transitions and strong deflagrations in alkane and alkene air mixtures, Combustion and Flame, Vol: 76, Pages: 169-181, ISSN: 0010-2180
Deflagration to detonation transitions and strong deflagrations for a large range of stoichiometric alkane (CH4, C2H6, C3H8, C4H10) and air mixtures and alkene (C2H4, C3H6, C4H8) and air mixtures have been studied systematically by varying surface roughness via the introduction of Shchelkin spirals. The use of such spirals, rather than repeated obstacles such as orifice plates, is preferred, as improved acceleration and higher flame speeds are obtained due to reduced momentum losses. The results obtained indicate that deflagration to detonation transitions (DDT) and quasi-detonations can be obtained for all fuels tested. Using short obstacles it is demonstrated that quasi-stable strong deflagrations of considerable duration, supported only by a smooth-walled tube, can easily be established and typically serve as a basis for further acceleration and transition to detonation. The effects of different obstacle exit velocities on the duration of the strong deflagration phase and on DDT parameters have been investigated. By further increasing the obstacle length, DDT via quasi-detonations has been investigated and it is demonstrated that the relative detonability of the fuels under strongly turbulent conditions differs systematically from that obtained in smooth tube experiments. © 1989.
Jones WP, Lindstedt RP, 1988, Global reaction schemes for hydrocarbon combustion, Combustion and Flame, Vol: 73, Pages: 233-249, ISSN: 0010-2180
Global reaction schemes for the combustion of alkane hydrocarbons up to butane in mixtures with air in premixed and diffusion flames have been derived using analysis of flame structures. The schemes include two competing fuel breakdown reactions, and equilibrium assumptions have been used to derive initial estimates of the forms of the rate expressions. The deduced four-step reaction mechanism is CnH2n+2+ n 2O2{curly arrow to the right}nCO+(n+1)H2CnH2n+nH2O{curly arrow to the right}nCO+(2n+1)H2H2+ 1 2⇌H2OCO+H2O⇌CO2+H2. The final kinetic parameters for the resulting rate equations have been determined by comparisons with experimental data for premixed methane and propane flames, along with diffusion flame data for a methane-air flame. The resulting schemes have been found to combine mathematic tractability with good agreement for a range of flame parameters such as flame speed, flame thickness, and species profiles. © 1988.
Lindstedt RP, Michels HJ, 1988, Deflagration to detonation transition in mixtures of alkane LNG/LPG constituents with O<inf>2</inf> N<inf>2</inf>, Combustion and Flame, Vol: 72, Pages: 63-72, ISSN: 0010-2180
Deflagration to detonation transitions (DDT) of methane, methane ethane, ethane, propane, and butane in mixtures with increasing dilution of nitrogen have been studied in a smooth 2″ id detonation tube with a length/diameter ratio of 220. The results obtained demonstrate the presence of two regimes of DDT depending on the reactivity of the mixture. The first regime displays rapid acceleration to a stable detonation, while the second regime contains a quasistable, strong deflagration of rapidly increasing duration with decreasing mixture reactivity. It is established that the order of DDT under the conditions employed follows the order of autoignition temperatures for the fuels considered. The conditional use of transition parameters for estimates of relative detonabilities based on a qualitative similarity between induction times and times to transition is demonstrated to hold only for the first transition regime. © 1988.
JONES WP, LINDSTEDT RP, 1988, THE CALCULATION OF THE STRUCTURE OF LAMINAR COUNTERFLOW DIFFUSION FLAMES USING A GLOBAL REACTION-MECHANISM, COMBUSTION SCIENCE AND TECHNOLOGY, Vol: 61, Pages: 31-49, ISSN: 0010-2202
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- Citations: 24
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