George Kosaly

A laminar flamelet approach to subgrid-scale chemistry in turbulent flows

Abstract A method is presented whereby filtered chemical species may be modeled in large eddy simulations (LES) of nonpremixed, turbulent combustion. The model is based on the concept of laminar flamelets, and assumes that a filtered mixture-fraction, as well as its subgrid-scale variance and filtered dissipation rate, are known at each grid cell of an LES. The model makes use of the subgrid-scale or “large-eddy” probability density function of the mixture-fraction, which is assumed to follow a beta-distribution. Also, an assumed functional form for the scalar dissipation rate is employed. The model is evaluated by filtering data from direct numerical simulations (DNS) of homogeneous, isotropic, decaying turbulence. Results show that the model, termed the large-eddy, laminar flamelet model (LELFM), is reasonably accurate and that the accuracy improves with increasing Damkohler number.

Investigation of closure models for nonpremixed turbulent reacting flows

The stationary laminar flamelet model and the conditional moment closure are two salient approaches to the decoupling of the chemistry problem from the turbulence problem. These two models are investigated using direct numerical simulations. The results and their analyses show that a full understanding of the validity of the stationary laminar flamelet model cannot be reached based upon the reaction zone thickness in Z space. A new condition regarding the quasisteadiness of the chemical reaction is derived and applied to the interpretation of the data. The conditional moment closure leads to excellent data predictions provided the average scalar dissipation rate conditioned on the mixture fraction is properly modeled.

Extinction and reignition in a diffusion flame: a direct numerical simulation study

The goal of this study is to provide a window into the physics of extinction and reignition via three-dimensional simulations of non-premixed combustion in isotropic decaying turbulence using one-step global reaction and neglecting density variations. Initially non-premixed fields of fuel and oxidant are developing in a turbulent field. Due to straining, the scalar dissipation rate is initially increasing and its fluctuations create extinguished regions on the stoichiometric surface. Later in the process, the stoichiometric surface again becomes uniformly hot. Besides using Eulerian data, this research applies flame element tracking and investigates the time history of individual points (‘flame elements’) along the stoichiometric surface. The main focus of the study is the discussion of the different scenarios of reignition. This paper identifies three major scenarios: independent flamelet scenario, reignition via edge (triple) flame propagation, and reignition through engulfment by a hot neighbourhood. The results give insight into the role different scenarios play in the reignition process, reveal the physical processes associated with each scenario, and provide the relative frequency of reignition for each scenario.

Modeling extinction and reignition in turbulent nonpremixed combustion using a doubly-conditional moment closure approach

The scalar dissipation rate is introduced as a second conditioning variable into the first-moment, singly conditional moment closure model to describe extinction and reignition effects in turbulent, nonpremixed combustion. A priori testing of the combustion model using direct numerical simulation experiments exhibiting local extinction/reignition events is described. The singly conditional moment closure model is either unable to describe the extinction seen in the numerical experiments or predicts global extinction when it does not occur. The new doubly conditional moment closure approach is able to describe the extinction seen on average, but predicts the onset of reignition too early.

Testing of mixing models for Monte Carlo probability density function simulations

Testing of mixing models widely used for Monte Carlo probability density function simulations of turbulent diffusion flames is performed using the data obtained from direct numerical simulations (DNS) that are specifically designed for the study of local flame extinction and reignition. In particular, the interaction by exchange with the mean (IEM) [J. Villermaux and J. C. Devillon, “Representation de la coalescence et de la redispersion des domaines de segregation dans un fluide per modele d’interaction phenomenologique,” in Proceedings of the Second International Symposia on Chemical Reaction Engineering (ISCRE, Netherlands, 1972), p. B1], the modified Curl [J. Janicka, W. Kolbe, and W. Kollmann, J. Non-Equilib. Thermodyn. 4, 47 (1979)], and the Euclidean minimum spanning tree (EMST) [S. Subramaniam and S. B. Pope, Combust. Flame 115, 487 (1998)] mixing models are tested. The tests are designed to examine the mixing model performance when implemented in both Reynolds-averaged simulations and large-eddy ...

Investigation of Modeling for Non-Premixed Turbulent Combustion

A method for predicting filtered chemical species concentrations and filtered reaction rates in Large-Eddy Simulations of non-premixed, non-isothermal, turbulent reacting flows has been demonstrated to be quite accurate for higher Damköhler numbers. This subgrid-scale model is based on flamelet theory and uses presumed forms for both the dissipation rate and subgrid-scale probability density function of a conserved scalar. Inputs to the model are the chemistry rates, the Favre-filtered scalar, and its subgrid-scale variance and filtered dissipation rate. In this paper, models for the filtered dissipation rate and subgrid-scale variance are evaluated by filtering data from 5123-point Direct Numerical Simulations of a single-step, isothermal reaction developing in the isotropic, incompressible, decaying turbulence field of Comte-Bellot and Corrsin. Both the subgrid-scale variance and the filtered dissipation rate models (the ”sub-models”) are found to be reasonably accurate. The effect of the errors introduced by the sub-models on the overall model is found to be small, and the overall model is shown to accurately predict the spatial average of the filtered species concentrations over a wide range of times.

Differential diffusion in turbulent reacting flows

Results obtained from direct numerical simulations are presented to examine effects due to differential diffusion on reacting scalars in isotropic, decaying turbulence. In the simulations fuel and oxidant react via a one-step, isothermal reaction (activation energy is set to zero). The results demonstrate that effects due to differential diffusion decrease with increasing Reynolds numbers and increase with increasing Damkohler number values. A principal issue investigated in this paper is whether a conditional moment closure-flamelet approach can be applied to accurately account for differential diffusion effects. The results show that the neglect of the conditional fluctuations in the modeling amplifies the influence of differential diffusion and leads to incorrect dependence on the Reynolds number. The paper investigates the different terms representing the conditional fluctuations in the model equations. Also investigated is the influence of differential diffusion on the validity of the boundary conditions set in conserved scalar space.

Differentially diffusing scalars in turbulence

Direct numerical simulation (DNS) data are presented to study differentially diffusing conserved scalars in isotropic turbulence. The two scalars are initially identical but subsequently decorrelate because one is diffusing faster than the other. The main focus of the paper is the study of the time evolution and possible modeling of quantities that characterize differential diffusion. The Reynolds number dependence of the mean square of the difference of the conserved scalars (α=Z−Zf, 〈α2〉) is investigated and compared to the theoretical scaling predicted by Kerstein et al. [Phys. Fluids A 7, 1999 (1995)]. The behavior and possible modeling of the conditional average of α is studied. The recent closure method of Kronenburg and Bilger [Phys. Fluids A 5, 1435 (1977)] is further corroborated and a novel approach to the prediction of the conditional average of α is discussed.

Direct Numerical Simulation Investigation of the Conditional Moment Closure Model for Nonpremixed Turbulent Reacting Flows

Abstract By using direct numerical simulations (DNS), the accuracy of the Conditional Moment Closure Model (CMC) is tested for a nonpremixed reaction in decaying, spatially homogeneous turbulence. Simple one step, second order, irreversible, isothermal chemistry is used. The model equation is solved in two ways:(a) assuming that the mixture fraction and its dissipation are statistically independent, (b) accounting for their statistical dependence via a shape factor. In both cases (a) and (b) the predicted mean of the fuel mass fraction agreed well with the DNS data. Predictions obtained using approximation (b) were somewhat better than those using approximation (a).

Direct numerical solution of turbulent nonpremixed combustion with multistep hydrogen-oxygen kinetics

Results are reported from three-dimensional direct numerical simulations of nonpremixed hydrogen-oxygen combustion using a reduced kinetic mechanism in low Mach number, decaying, variable density, isotropic turbulence. The reduced chemical kinetics scheme is based on a seven-species, ten-reaction hydrogen-oxygen mechanism. Realistic kinetic parameters were used. The speed of the entire chemical process was scaled relative to the mixing process by varying an appropriately chosen Damkohler number. The simulation results were compared to predictions of the Conditional Moment Closure model. Next, predictions for the radical species concentrations based on steady-state and partial equilibrium assumptions were compared to simulation results. A model is proposed which gives the thermochemical state as a function of the mixture fraction and a reaction progress variable. The thermochemical states are derived and tabulated from homogeneous premixed calculations. The predictions of this model are compared to simulation data. An expression is derived for the evolution of the error in a manifold approximation.