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A technique called the Direct Simulation Monte Carlo (DSMC) method is used for simulating laminar one-dimensional hydrogen-air flames with an aim to establish the feasibility of molecular-level simulations for combustion using the Quantum-Kinetic (QK) reaction model. In DSMC, simulation particles are employed which represent many molecules with similar properties. The DSMC method effectively solves the Boltzmann equation by decoupling the motion of molecules into two phases: a Move phase and a Collision phase. Chemistry is treated using the Quantum-Kinetic model in which the total energy exchange resulting from molecular collision determines the outcome of the chemical reactions. A major advantage of DSMC is that it avoids using continuum Arrhenius reaction rates or simplified gradient laws for the diffusivities of mass, momentum and heat. Instead, any such laws and their parameters emerge naturally from the DSMC results. Simulations of one-dimensional hydrogen-air flames using the DSMC method with a detailed chemical scheme show promising results for molecular-level simulations of combustion. An adjustment to the activation energy of a key reaction is made in order to achieve the correct flame speed. The species and temperature profiles from DSMC show reasonable agreement with those obtained from Direct Numerical Simulation (DNS) using a conventional continuum approach. Likewise, the molecular diffusivity of hydrogen and oxygen also show reasonable agreement with the DNS results, although differences in the results still exist, especially for oxygen diffusivity. These are expected to improve with further development in the DSMC method for combustion. [ABSTRACT FROM AUTHOR]

The performance of algebraic flame surface density (FSD) models has been assessed for flames with nonunity Lewis number (Le) in the thin reaction zones regime, using a direct numerical simulation (DNS) database of freely propagating turbulent premixed flames with Le ranging from 0.34 to 1.2. The focus is on algebraic FSD models based on a power-law approach, and the effects of Lewis number on the fractal dimension D and inner cut-off scale ηi have been studied in detail. It has been found that D is strongly affected by Lewis number and increases significantly with decreasing Le. By contrast, ηi remains close to the laminar flame thermal thickness for all values of Le considered here. A parameterisation of D is proposed such that the effects of Lewis number are explicitly accounted for. The new parameterisation is used to propose a new algebraic model for FSD. The performance of the new model is assessed with respect to results for the generalised FSD obtained from explicitly LES-filtered DNS data. It has been found that the performance of the most existing models deteriorates with decreasing Lewis number, while the newly proposed model is found to perform as well or better than the most existing algebraic models for FSD. [ABSTRACT FROM AUTHOR]

A direct numerical simulation (DNS) database of freely propagating statistically planar turbulent premixed flames with a range of different turbulent Reynolds numbers has been used to assess the performance of algebraic flame surface density (FSD) models based on a fractal representation of the flame wrinkling factor. The turbulent Reynolds number Ret has been varied by modifying the Karlovitz number Ka and the Damköhler number Da independently of each other in such a way that the flames remain within the thin reaction zones regime. It has been found that the turbulent Reynolds number and the Karlovitz number both have a significant influence on the fractal dimension, which is found to increase with increasing Ret and Ka before reaching an asymptotic value for large values of Ret and Ka. A parameterisation of the fractal dimension is presented in which the effects of the Reynolds and the Karlovitz numbers are explicitly taken into account. By contrast, the inner cut-off scale normalised by the Zel'dovich flame thickness ηi/δz does not exhibit any significant dependence on Ret for the cases considered here. The performance of several algebraic FSD models has been assessed based on various criteria. Most of the algebraic models show a deterioration in performance with increasing the LES filter width. [ABSTRACT FROM AUTHOR]

The effects of global Lewis number on the statistical behaviour of turbulent kinetic energy transport in turbulent premixed flames are analysed using three-dimensional direct numerical simulation (DNS) data for freely propagating statistically planar flames with Lewis number ranging from 0.34 to 1.2. For flames with Lewis number significantly smaller than unity, it is observed that the turbulent kinetic energy is significantly augmented within the flame brush due to flame-generated turbulence. In these flames, it is demonstrated that effects of the mean pressure gradient and pressure dilatation are sufficient to overcome the effects of viscous dissipation. By contrast, for flames with Lewis number close to unity, it is found that the turbulent kinetic energy decays monotonically through the flame brush. In these flames, the effects of the mean pressure gradient and pressure dilatation terms are relatively much weaker than those of viscous dissipation. The modelling of the various unclosed terms of the turbulent kinetic energy transport equation has been analysed in detail. The predictions of existing models are compared with corresponding quantities extracted from DNS data. Based on this a-priori DNS assessment, either appropriate models are identified or new models are proposed where necessary. It is shown that the turbulent flux of turbulent kinetic energy exhibits counter-gradient transport for the low Lewis number flames where the turbulent scalar flux is also counter-gradient in nature. However, for flames with Lewis number close to unity, the turbulent flux of turbulent kinetic energy exhibits predominantly gradient type transport. A new model has been proposed for the flux of turbulent kinetic energy in premixed flames and is found to capture the qualitative and quantitative behaviour obtained from DNS data for all the different Lewis numbers considered. [ABSTRACT FROM AUTHOR]

Fully compressible direct numerical simulation (DNS) is used to study the behaviour of a stratified turbulent premixed flame in the wrinkled flamelet regime (ReΛ = 38, u'/SL = 0.7, Da = 2.3) with reduced multi-step chemistry. A laminar premixed flame in an initially quiescent domain propagates into a turbulent flow so that the scalar and velocity fields are correctly coupled. Comparison of a stratified scenario (0.4 < φ < 1.2) with an otherwise identical homogeneous case indicates that in this regime the turbulent flame speed is increased by enhanced flame front wrinkling as a result of differential propagation speeds. The preferential propagation into more combustible mixture results in a correlation between the mixture fraction and progress variable, particularly at the leading edge of the flame brush. In front of the flame, the composition gradient preferentially aligns with the most compressive strain. Within the flame front, the progress variable gradient preferentially aligns with the most extensive turbulent strain, and in the latter portion of the flame the alignment between the mixture fraction gradient and the most compressive principal strain deteriorates. Despite this, the convenient approximation of the joint pdf by assuming statistical independence of the mixture fraction and progress variable does not introduce a significant error in the approximation of the reaction rates. [ABSTRACT FROM AUTHOR]

Spark ignition in inhomogeneous mixtures is numerically studied using three-dimensional compressible Direct Numerical Simulations (DNS) with simplified chemistry. The thermal effect of the spark is represented by a Gaussian power distribution in the energy transport equation. Success of the spark ignition and subsequent self-sustained flame propagation is shown to be heavily dependent on the turbulent velocity fluctuation. It has been found that high levels of turbulent velocity fluctuation have detrimental effects on the spark ignition event, which ultimately may lead to misfire. It is observed that the flame shows a tribrachial structure following successful ignition. The local flame propagation speed may attain values equal to several times the laminar burning velocity of an unstrained premixed stoichiometric flame SL, but may also be negative, in agreement with previous findings. The mean flame propagation speed decreases with increasing turbulent intensity. [ABSTRACT FROM AUTHOR]

An understanding of the flame normal vector and the relationships between its components is important in flamelet modeling of premixed turbulent combustion. The product of flame normal components N i N j is closely related to the magnitude of the tangential strain rate acting on the flame. In the context of flame surface density (FSD)-based modeling, an accurate estimate of the mean or filtered values is necessary for the evaluation of the mean tangential strain rate in Reynolds-average Navier-Stokes (RANS) calculations and the resolved tangential strain rate in large-eddy simulation (LES). In the present work, the mutual interaction between local flame normal components is studied using three-dimensional compressible direct numerical simulations (DNS) of statistically planar turbulent premixed flames. Detailed statistics of the flame normal vector are presented. The probability density functions (pdfs) of combinations of the flame normal components reveal the predominance of locally cylindrical curvature on the three-dimensional wrinkled flame surface. This is confirmed by the pdf of the curvature shape factor, which is found to be consistent with previous DNS studies. The fluctuations of the normal components are shown to be anisotropic in nature, which is in accordance with previous experimental results obtained in two dimensions. Three different approaches currently in use for modeling < N i N j > s in the RANS context are extended in order to provide new models for in LES, and the predictions of these new models are assessed in comparison to values of obtained from LES-filtered DNS data. [ABSTRACT FROM AUTHOR]

Measurements of 3D turbulent flame surface area and burnt gas were carried out for spherically expanding flames in different methane-air and hydrogen-air mixtures using a high frequency swinging laser sheet technique based on Mie scattering. The corresponding turbulent burning velocities were measured simultaneously using the rate of pressure rise, at turbulence rms velocities between 0.3 and 2.0 m/s. The ratio of turbulent burning velocity enhancement u t m / u l to flame surface area enhancement A 3 D / a 3 D was measured as a function of turbulence rms velocity. For the methane-air flames, the turbulent burning velocity enhancement is close to that of the flame surface area enhancement. For the hydrogen-air flames, the former can exceed the latter by a factor of up to 6 at the largest values of turbulence rms velocity tested. The large discrepancy suggests that in the case of hydrogen-air flames, the measured rate of burning per unit flame area is significantly enhanced by the turbulence. For the reconstructed A 3 D / a 3 D , the corresponding A 2 D / a 2 D are also discussed. [ABSTRACT FROM AUTHOR]