Dissertations / Theses on the topic 'Conditional moment closure'

This work presents a novel algorithm for supersonic combustion modelling. The method involved coupling the Conditional Moment Closure (CMC) model to a fully compressible, shock capturing, high-order flow solver, with the intent of modelling a reacting hydrogen-air, supersonic jet. Firstly, a frozen chemistry case was analysed to validate the implementation of the algorithm and the ability for CMC to operate at its frozen limit. Accurate capturing of mixing is crucial as the mixing and combustion time scales for supersonic flows are on the order of milliseconds. The results of this simulation were promising even with an unexplainable excess velocity decay of the jet core. Hydrogen mass fractions however, showed fair agreement to the experiment. The method was then applied to the supersonic reacting case of ONERA. The results showed the method was able to successfully capture chemical non-equilibrium effects, as the lift-off height and autoignition time were reasonably captured. Distributions of reactive scalars were difficult to asses as experimental data was deemed to be very inaccurate. As a consequence, published numerical results for the same test case were utilised to aid in analysing the results of the presented simulations. Due to the primary focus of the study being to assess non-equilibrium effects, the clustering of the computational grid lent itself to smeared and lower magnitude wall pressure distributions. Nevertheless, the wall pressure distributions showed good qualitative agreement to experiment. The primary conclusions from the study were that the CMC method is feasible to model supersonic combustion. However, a more detailed analysis of sub-models and closure assumptions must be conducted to assess the feasibility on a more fundamental level. Also, from the results of both the frozen chemistry and the reacting case, the effects of assuming constant species Lewis number was visible.

Lean premixed combustion is at present one of the most promising methods to reduce emissions and to maintain high efficiency in combustion systems. As the emission legislation becomes more stringent, modelling of turbulent premixed combustion has become an important tool for designing efficient and environmentally friendlier combustion systems. However, in order to predict these emissions reliable predictive models are required. One of the methods used for predicting pollutants is the conditional moment closure (CMC), which is suitable to predict pollutants with slow time scales. Despite the fact that CMC has been successfully applied to various non-premixed combustion systems, its application to premixed flames is not fully tested and validated. The main difficulty is associated with the modelling of the conditional scalar dissipation rate (CSDR) of the conditioning scalar, the progress variable. In premixed CMC, this term is an important quantity and represents the rate of mixing at small scales of relevance for combustion. The numerical accuracy of the CMC method depends on the accuracy of the CSDR model. In this study, two different models for CSDR, an algebraic model and an inverse problem model, are validated using two different DNS data sets. The algebraic model along with standard k-ε turbulence modelling is used in the computations of stoichiometric and very lean pilot stabilized Bunsen flames using the RANS-CMC method. A first order closure is used for the conditional mean reaction rate. The computed nonreacting and reacting scalars are in reasonable agreement with the experiments and are consistent with earlier computations using flamlets and transported PDF methods for the stoichiometric flames, and transported PDF methods for the very lean flames. Sensitivity to chemical kinetics mechanism is also assessed.

The accurate description of the turbulence chemistry interactions that can determine chemical conversion rates and flame stability in turbulent combustion modelling is a challenging research area. This thesis presents the development and implementation of a model for the treatment of fluctuations around the conditional mean (i.e., the auto-ignition and extinction phenomenon) of realistic turbulence-chemistry interactions in computational fluid dynamics (CFD) software. The wider objective is to apply the model to advanced combustion modelling and extend the present analysis to larger hydrocarbon fuels and particularly focus on the ability of the model to capture the effects of particulate formation such as soot. A comprehensive approach for modelling of turbulent combustion is developed in this work. A direct quadrature conditional moment closure (DQCMC) method for the treatment of realistic turbulence-chemistry interactions in computational fluid dynamics (CFD) software is described. The method which is based on the direct quadrature method of moments (DQMOM) coupled with the Conditional Moment Closure (CMC) equations is in simplified form and easily implementable in existing CMC formulation for CFD code. The observed fluctuations of scalar dissipation around the conditional mean values are captured by the treatment of a set of mixing environments, each with its pre-defined weight. In the DQCMC method the resulting equations are similar to that of the first-order CMC, and the “diffusion in the mixture fraction space” term is strictly positive and no correction factors are used. Results have been presented for two mixing environments, where the resulting matrices of the DQCMC can be inverted analytically. Initially the DQCMC is tested for a simple hydrogen flame using a multi species chemical scheme containing nine species. The effects of the fluctuations around the conditional means are captured qualitatively and the predicted results are in very good agreement with observed trends from direct numerical simulations (DNS). To extend the analysis further and validate the model for larger hydrocarbon fuel, the simulations have been performed for n-heptane flame using detailed multi species chemical scheme containing 67 species. The hydrocarbon fuel showed improved results in comparison to the simple hydrogen flame. It suggests that higher hydrocarbons are more sensitive to local scalar dissipation rate and the fluctuations around the conditional means than the hydrogen. Finally, the DQCMC is coupled with a semi-empirical soot model to study the effects of particulate formation such as soot. The modelling results show to predict qualitatively the trends from DNS and are in very good agreement with available experimental data from a shock tube concerning ignition delays time. Furthermore, the findings suggest that the DQCMC approach is a promising framework for soot modelling.

This thesis describes the implementation of the conditional moment closure (CMC) combustion model in a numerical scheme and its application to the modelling of enclosure fires. Prediction of carbon monoxide (CO) in the upper smoke layer of enclosure fires is of primary interest because it is a common cause of death. The CO concentration cannot be easily predicted by empirical means, so a method is needed which models the chemistry of a quenched, turbulent fire plume and subsequent mixing within an enclosed space. CMC is a turbulent combustion model which has been researched for over a decade. It has provided predictions of major and minor species in jet diffusion flames. The extension to enclosure fires is a new application for which the flow is complex and temperatures are well below adiabatic conditions. Advances are made in the numerical implementation of CMC. The governing combustion equations are cast in a conserved, finite volume formulation for which boundary conditions are uniquely defined. Computational efficiency is improved through two criteria which allow the reduction in the size of the computational domain without any loss of accuracy. Modelling results are compared to experimental data for natural gas fires burning under a hood. Comparison is made in the recirculating, post-flame region of the flow where temperatures are low and reactions are quenched. Due to the spatial flux terms contained in the governing equations, CMC is able to model the situation where chemical species are produced in the high temperature fire-plume and then transported to non-reacting regions. Predictions of CO and other species are in reasonable agreement with the experimental data over a range of lean and rich hood-fire conditions. Sensitivity of results to chemistry, temperature and modelling closures is inves- tigated. Species predictions are shown to be quite different for the two detailed chemical mechanisms used. Temperature conditions within the hood effect the for- mation of species in the plume prior to quenching and subsequently species predic- tions in the post-flame region are also effected. Clipped Gaussian and ß-function probability density functions (PDFs) are used for the stochastic mixture fraction. Species predictions in the plume are sensitive to the form of the PDF but in the post-flame region, where the ß-function approaches a Gaussian form, predictions are relatively insensitive. Two models are used for the conditional scalar dissipation: a uniform model, where the conditional quantity is set equal to the unconditional scalar dissipation across all mixture fraction space; and a model which is consistent with the PDF transport equation. In the plume, predictions of minor species are sensitive to the modelling used, but in the recirculating, post-flame region species are not significantly effected.

Mathematical modelling of the turbulent combustion process is becoming increasingly applied in calculations to assist in the design and analysis of practical combustion devices for efficiency-improvement and emission reduction. The current requirement to accurately predict pollutant emissions in many applications has increased the need for linking turbulent flow calculations and finite- rate chemistry effects in a rigorous way. Several methodologies are available for modelling such interactions, including the transported probability density function (PDF) approach and the conditional moment closure (CMC) method. Although in the early stages of its development, the CMC method has been shown to be a promising technique for predicting a wide range of practical problems. These include both premixed and non-premixed combustion, relatively slow chemistry effects, and ignition and extinction phenomena. This study concerns the CMC approach, and addresses the application of a number of models to a wide range of flows displaying varied compositions and geometries, including hydrogen and methane, and rim-stable and lifted jets. The impact of the choice of chemistry mechanism is considered for all the flows, and a higherorder CMC chemistry closure is investigated for the hydrogen flames. Analysis is made as to the ability of a parabolic CMC model to predict such flows, and the performance of the sub-model interactions is also reported on. The method of coupling the turbulent mixing field and the chemical kine tics is also investigated, and the effects of Reynolds stress and k - E turbulence closures upon subsequent CMC calculations are compared in all the flows considered. Overall, the results shown and conclusions drawn are very promising with respect to the possible future development of CMC. Requirements essential for this step forward of CMC methodologies for use in modelling practical geometries are specified, and an outline for the continuation of these studies is presented.

This thesis aims at improving our knowledge on swirl combustors. The work presented here is based on Large Eddy Simulations (LES) coupled to an advanced combustion model: the Conditional Moment Closure (CMC). Numerical predictions have been systematically compared and validated with detailed experimental datasets. In order to analyze further the physics underlying the large numerical datasets, Proper Orthogonal Decomposition (POD) has also been used throughout the thesis. Various aspects of the aerodynamics of swirling flames are investigated, such as precession or vortex formation caused by flow oscillations, as well as various combustion aspects such as localized extinctions and flame lift-off. All the above affect flame stabilization in different ways and are explored through focused simulations. The first study investigates isothermal air flows behind an enclosed bluff body, with the incoming flow being pulsated. These flows have strong similarities to flows found in combustors experiencing self-excited oscillations and can therefore be considered as canonical problems. At high enough forcing frequencies, double ring vortices are shed from the air pipe exit. Various harmonics of the pulsating frequency are observed in the spectra and their relation with the vortex shedding is investigated through POD. The second study explores the structure of the Delft III piloted turbulent non-premixed flame. The simple configuration allows to analyze further key combustion aspects of combustors, with further insights provided on the dynamics of localized extinctions and re-ignition, as well as the pollutants emissions. The third study presents a comprehensive analysis of the aerodynamics of swirl flows based on the TECFLAM confined non-premixed S09c configuration. A periodic component inside the air inlet pipe and around the central bluff body is observed, for both the inert and reactive flows. POD shows that these flow oscillations are due to single and double helical vortices, similar to Precessing Vortex Cores (PVC), that develop inside the air inlet pipe and whose axes rotate around the burner. The combustion process is found to affect the swirl flow aerodynamics. Finally, the fourth study investigates the TECFLAM configuration again, but here attention is given to the flame lift-off evident in experiments and reproduced by the LES-CMC formulation. The stabilization process and the pollutants emission of the flame are investigated in detail.

Cette thèse propose une modélisation avancée des transferts thermiques dans les écoulements turbulents pour tous les types de conditions aux limites sur la température aux parois. Ces travaux reposent sur un double constat : d'une part, les modèles de turbulence traitant la thermique de l'écoulement dans la plupart des applications industrielles sont basés sur de simples relations algébriques incapables de représenter des physiques complexes, comme la convection naturelle et la thermique de la zone proche-paroi. D'autre part, la condition aux limites sur la température à la paroi (température fixée, flux de chaleur imposé ou transfert thermique conjugué) influence le comportement proche-paroi des variables thermiques turbulents. La formulation du modèle bas-Reynolds du second ordre des flux thermiques turbulents EBDFM (Elliptic Blending Differential Flux Model), développée à l'origine pour traiter des cas où une température est fixée à la paroi, a été étendue à des cas de flux de chaleur imposé et de transfert thermique conjugué. Cette nouvelle formulation se fonde sur des analyses asymptotiques rigoureuses des termes des équations de transport des flux thermiques turbulents pour chaque condition aux limites sur la température. Un des éléments essentiels de la nouvelle formulation de l'EBDFM est le ratio des échelles de temps thermique et dynamique R. Le comportement asymptotique de ce ratio dépend fortement de la condition aux limites : R tend vers le nombre de Prandtl à la paroi lorsqu'une température est imposée, et vers l'infini sinon. Ainsi, dans le but de reproduire fidèlement ce comportement, il s'est avéré nécessaire de résoudre des équations de transport pour la variance de température ¯(θ^'2 )et pour son taux de dissipation ε_θ puisque ces deux variables pilotent le comportement asymptotique de R. Par conséquent, cette thèse propose des modèles bas-Reynolds pour les variables ¯(θ^'2 )et ε_θ valables pour toutes les conditions aux limites thermiques. La nouvelle formulation du modèle EBDFM ainsi que les modèles de ¯(θ^'2 )et ε_θ ont été validées par des simulations réalisées avec le logiciel de CFD Code_Saturne pour des écoulements dans un canal plan en convection forcée
Advanced modeling of turbulent heat transfer for all thermal boundary conditions is proposed. This work was motivated by two facts: first, the thermal turbulent models used in most of the industrial computations are based on eddy-viscosity models which cannot deal with complex physics such as natural convection or heat transfer in the near-wall region. Then, the thermal boundary condition at the wall (imposed temperature, imposed heat flux, conjugate heat transfer) influences the near-wall behavior of the turbulent thermal variables. The formulation of the low-Reynolds number second moment closure EBDFM (Elliptic Blending Differential Flux Model), which was originally developed for an imposed temperature at the wall, has been extended to an imposed heat flux and a conjugate heat transfer condition. This new formulation is based on rigorous asymptotic analysis of the terms of the transport equation of the turbulent heat flux for all thermal boundary conditions. One of the key elements is the thermal-to-mechanical time-scale ratio R. Its asymptotic behavior highly depends on the thermal boundary condition: R goes to the Prandtl number at the wall for an imposed temperature and tends to infinity otherwise. Thus, solving a transport equation for the temperature variance ¯(θ^'2 ) and for its dissipation rate ε_θ is necessary to reproduce the asymptotic behavior of R. Indeed, these two variables drive the behavior of R in the near-wall region. Therefore, low-Reynolds number models for ¯(θ^'2 ) and ε_θ, valid for all thermal boundary conditions, are proposed. The new formulation of the EBDFM and the models for ¯(θ^'2 ) and ε_θ have been validated by performing Code_Saturne computations of channel flows in the forced convection regime

This thesis describes the application of the first-order Conditional Moment Closure (CMC) to the autoignition of high-pressure fuel jets, and to piloted and lifted turbulent jet flames using classical and advanced CMC submodels. A Doubly-Conditional Moment Closure (DCMC) formulation is further proposed. In the first study, CMC is applied to investigate the impact of C₂H₆, H₂ and N₂ additives on the autoignition of high-pressure CH₄ jets injected into lower pressure heated air. A wide range of pre-combustion air temperatures is considered and detailed chemical kinetics are employed. It is demonstrated that the addition of C₂H₆ and H₂ does not change the main CH₄ oxidisation pathways. The decomposition of these additives provides additional ignition-promoting radicals, and therefore leads to shorter ignition delays. N₂ additives do not alter the CH₄ oxidisation pathways, however, they reduce the amount of CH₄ available for reaction, causing delayed ignition. It is further shown that ignition always occurs in lean mixtures and at low scalar dissipation rates. The second study is concerned with the modelling of a piloted CH₄/air turbulent jet flame. A detailed assessment of several Probability Density Function (PDF), Conditional Scalar Dissipation Rate (CSDR) and Conditional Velocity (CV) submodels is first performed. The results of two β-PDF-based implementations are then presented. The two realisations differ by the modelling of the CSDR. Homogeneous (inconsistent) and inhomogeneous (consistent) closures are considered. It is shown that the levels of all reactive scalars, including minor intermediates and radicals, are better predicted when the effects of inhomogeneity are included in the modelling of the CSDR. The two following studies are focused on the consistent modelling of a lifted H₂/N₂ turbulent jet flame issuing into a vitiated coflow. Two approaches are followed to model the PDF. In the first, a presumed β-distribution is assumed, whereas in the second, the Presumed Mapping Function (PMF) approach is employed. Fully consistent CV and CSDR closures based on the β-PDF and the PMF-PDF are employed. The homogeneous versions of the CSDR closures are also considered in order to assess the effect of the spurious sources which stem from the inconsistent modelling of mixing. The flame response is analysed over a narrow range of coflow temperatures (Tc). The stabilisation mechanism is determined from the analysis of the transport budgets in mixture fraction and physical spaces, and the history of radical build-up ahead of the stabilisation height. The β-PDF realisations indicate that the flame is stabilised by autoignition irrespective of the value of Tc. On the other hand, the PMF realisations reveal that the stabilisation mechanism is susceptible to Tc. Autoignition remains the controlling stabilisation mechanism for sufficiently high Tc. However, as Tc is decreased, stabilisation is achieved by means of premixed flame propagation. The analysis of the spurious sources reveals that their effect is small but non-negligible, most notably within the flame zone. Further, the assessment of several H₂ oxidation mechanisms show that the flame is very sensitive to chemical kinetics. In the last study, a DCMC method is proposed for the treatment of fluctuations in non-premixed and partially premixed turbulent combustion. The classical CMC theory is extended by introducing a normalised Progress Variable (PV) as a second conditioning variable beside the mixture fraction. The unburnt and burnt states involved in the normalisation of the PV are specified such that they are mixture fraction-dependent. A transport equation for the normalised PV is first obtained. The doubly-conditional species, enthalpy and temperature transport equations are then derived using the decomposition approach and the primary closure hypothesis is applied. Submodels for the doubly-conditioned unclosed terms which arise from the derivation of DCMC are proposed. As a preliminary analysis, the governing equations are simplified for homogeneous turbulence and a parametric assessment is performed by varying the strain rate levels in mixture fraction and PV spaces.

The conditional moment closure method using two conditioning scalar variables is applied to derive the transport equation of species mass fraction, temperature, and scalar dissipation in a decaying, isotropic, homogeneous turbulent methane-air flow. The strain tensor in the transport equation of scalar dissipation of the conditioning variables is simulated using stochastic processes. The results of this model are then compared to DNS and conditional moment closure with one variable for the same test case.

Autoignition of non-premixed methane-air mixtures is investigated using first-order Conditional Moment closure (CMC). In CMC, scalar quantities are conditionally averaged with respect to a conserved scalar, usually the mixture fraction. The conditional fluctuations are often of small order, allowing the chemical source term to be modeled as a function of the conditional species concentrations and the conditional enthalpy (temperature). The first-order CMC derivation leaves many terms unclosed such as the conditional scalar dissipation rate, velocity and turbulent fluxes, and the probability density function. Submodels for these quantities are discussed and validated against Direct Numerical Simulations (DNS). The CMC and the turbulent velocity and mixing fields calculations are decoupled based on the frozen mixing assumption, and the CMC equations are cross-stream averaged across the flow following the shear flow approximation. Finite differences are used to discretize the equations, and a two-step fractional method is implemented to treat separately the stiff chemical source term. The stiff ODE solver LSODE is used to solve the resulting system of equations. The recently developed detailed chemical kinetics mechanism UBC-Mech 1.0 is employed throughout this study, and preexisting mechanisms are visited. Several ignition criteria are also investigated. Homogeneous and inhomogeneous CMC calculations are performed in order to investigate the role of physical transport in autoignition. Furthermore, the results of the perfectly homogeneous reactor calculations are presented and the critical value of the scalar dissipation rate for ignition is determined. The results are compared to the shock tube experimental data of Sullivan et al. The current results show good agreement with the experiments in terms of both ignition delay and ignition kernel location, and the trends obtained in the experiments are successfully reproduced. The results were shown to be sensitive to the scalar dissipation model, the chemical kinetics, and the ignition criterion.

Autoignition of high pressure methane jets at engine relveant conditions within a shock tube is investigated using Conditional Moment Closure (CMC). The impact of two commonly used approximations applied in previous work is examined, the assumption of homogeneous turbulence in the closure of the micro-mixing term and the assumption of negligible radial variation of terms within the CMC equations. In the present work two formulations of an inhomogeneous mixing model are implemented, both utilizing the β -PDF, but differing in the respective conditional velocity closure that is applied. The common linear model for conditional velocity is considered, in addition to the gradient diffusion model. The validity of cross-stream averaging the CMC equations is examined by comparing results from two-dimensional (axial and radial) solution of the CMC equations with cross-stream averaged results. The CMC equations are presented and all terms requiring closure are discussed. So- lution of the CMC equations is decoupled from the flow field solution using the frozen mixing assumption. Detailed chemical kinetics are implemented. The CMC equations are discretized using finite differences and solved using a fractional step method. To maintain consistency between the mixing model and the mixture fraction variance equation, the scalar dissipation rate from both implementations of the inhomogeneous model are scaled. The autoignition results for five air temperatures are compared with results obtained using homogeneous mixing models and experimental data. The gradient diffusion conditional velocity model is shown to produce diverging be- haviour in low probability regions. The corresponding profiles of conditional scalar dis- sipation rate are negatively impacted with the use of the gradient model, as unphysical behaviour at lean mixtures within the core of the fuel jet is observed. The predictions of ignition delay and location from the Inhomogeneous-Linear model are very close to the homogeneous mixing model results. The Inhomogeneous-Gradient model yields longer ig- nition delays and ignition locations further downstream. This is influenced by the higher scalar dissipation rates at lean mixtures resulting from the divergent behaviour of the gradient conditional velocity model. The ignition delays obtained by solving the CMC equations in two dimensions are in excellent agreement with the cross-stream averaged values, but the ignition locations are predicted closer to the injector.

In the present work, the processes of steady combustion and autoignition of hydrogen are investigated using the Conditional Moment Closure (CMC) model with a Reynolds Averaged Navier-Stokes (RANS) Computational Fluid Dynamics (CFD) code. A study of the effects on the flowfield of changing turbulence model constants, specifically the turbulent Schmidt number, Sct, and C epsilon 1 of the k − epsilon model, are investigated. The effects of two different mixing models are explored: the AMC model, which is commonly used in CMC implementations, and a model based on the assumption of inhomogeneous turbulence. The background equations required for implementation of the CMC model are presented, and all relevant closures are discussed. The numerical implementation of the CMC model, in addition to other techniques aimed at reducing computational expense of the CMC calculations, are provided. The CMC equation is discretised using finite volume (FV) method. The CFD and CMC calculations are fully coupled, allowing for simulations of steady flames or flame development after the occurrence of autoignition. Through testing of a steady jet flame, it is observed that the flowfield calculations follow typical k − epsilon model trends, with an overprediction of spreading and an underprediction of penetration. The CMC calculations are observed to perform well, providing good agreement with experimental measurements. Autoignition simulations are conducted for 3 different cases of turbulence constants and 7 different coflow temperatures to determine the final effect on the steady flowfield. In comparison to the standard constants, reduction of Sct results in a reduction of the centreline mixing intensity within the flowfield and a corresponding reduction of ignition length, while reducing C 1 results in an increase of centreline mixing intensity and an increase in the ignition length. All scenarios tested result in an underprediction of ignition length in comparison to experimental results; however, good agreement with the experimental trends is achieved. At low coflow temperatures, the effects of mixing intensity within the flowfield are seen to have the largest influence on ignition length, while at high coflow temperatures, the chemical source term in the CMC equation increases in magnitude, resulting in very little difference between predictions for different sets of turbulence constants. The inhomogeneous mixing model is compared using the standard turbulence constants. A reduction of ignition lengths in comparison to the AMC model is observed. In steady state simulation of the autoigniting flow, the inhomogeneous model is observed to predict both lifted flames and fully anchored flames, depending on coflow temperature.

This thesis is concerned with investigations on two related issues of turbulent jet diffusion flame, namely (a) stabilization at liftoff and (b) autoignition in a turbulent jet diffusion flame. The approach of Conditional Moment Closure (CMC) has been taken. Fully elliptic first order CMC equations are solved with detailed chemistry to simulate lifted H2/N2 flame in vitiated coflow. The same approach is further used to simulate transient autoignition process in inhomogeneous mixing layers. In Chapter 1, difficulties involved in numerical simulation of turbulent combustion problems are explained. Different numerical tools used to simulate turbulent combustion are briefly discussed. Previous experimental, theoretical and numerical studies of lifted jet diffusion flames and autoignition are reviewed. Various research issues related to objectives of the thesis are discussed. In Chapter 2, the first order CMC transport equations for the reacting flows are presented. Various closure models that are required for solving the governing equations are given. Calculation of mean reaction rate term for detailed chemistry is given with special focus on the reaction rates for pressure dependent reactions. In Chapter 3, starting with the laminar flow code, further extension is carried to include kε turbulence model and PDF model. The code is validated at each stage of inclusion of different model. In this chapter, the code is first validated for the test problem of constant density, 2D, axisymmetric turbulent jet. Further, validation of PDF model is carried out by simulating the problem of nonreacting jet of cold air issuing into a vitiated coflow. The results are compared with the published data from experiments as well as numerical simulations. It is shown that the results compare well with the data. In Chapter 4, numerical results of lifted jet diffusion flame are presented. Detailed chemistry is modelled using Mueller mechanism for H2/O2 system with 9 species and 21 reversible reactions. Simulations are carried out for different jet velocities and coflow stream temperatures. The predicted liftoff generally agrees with experimental data, as well as joint PDF results. Profiles of mean scalar fluxes in the mixture fraction space, for different coflow temperatures reveal that (1) Inside the flamezone, the chemical term balances the molecular diffusion term, and hence the structure is of a diffusion flamelet for both cases. (2) In the preflame zone, the structure depends on the coflow temperature: for low coflow temperatures, the chemical term being small, the advective term balances the axial diffusion term. However, for the high coflow temperature case, the chemical term is large and balances the advective term, the axial diffusion term being small. It is concluded that, liftoff is controlled (a) by turbulent premixed flame propagation for low cofflow temperature while (b) by autoignition for high coflow temperature. In Chapter 5, the numerical results of autoignition in inhomogeneous mixing layer are presented. The configuration consists of a fuel jet issued into hot air for which transient simulations are performed. It is found that the constants assumed in various modelling terms can severely influence the results, particularly the flame temperature. Hence, modifications to these constants are suggested to obtain improved predictions. Preliminary work is carried out to predict autoignition lengths (which may be defined by Tign × Ujet incase of jet- and coflowvelocities being equal) by varying the coflow temperature. The autoignition lengths show a reasonable agreement with the experimental data and LES results. In Chapter 6, main conclusions of this thesis are summarized. Possible future studies on this problem are suggested.