Modelling turbulent premixed combustion in the intermediate steady propagation regime

Numerical simulation and comparison with standard experimental data of turbulent premixed combustion occurring at large Reynolds and moderately large Damkohler numbers (a situation which is typical in industrial burners) have been presented. The simulation has been performed in the framework of the Turbulent Flame-speed Closure (TFC) combustion model, developed in [1-4], which makes use of a theoretical expression for the turbulent combustion velocity for the closure of the progress variable transport equation. This model is based on the concept of the Intermediate Steady Propagation (ISP) regime of combustion in real combustors, i.e. when the turbulent flame propagates with equilibrium turbulent flame speed but has flame brush thickness growing according to the turbulent dispersion law. These ISP flames precede usually analysed 1D stationary flames, and from the theoretical point of view they are in fact intermediate asymptotic of the combustion process between the period of formation of developed turbulent flames and 1D stationary flames. Numerical results of turbulent premixed combustion in a two-dimensional planar channel at parameters that correspond to real industrial combustors have been compared with corresponding standard experimental data on a high speed turbulent premixed flame [9]. Finally, it has been explained in the framework of the TFC combustion model that "countergradient transport", i.e. the necessity to use a negative effective diffusion coefficient to describe experimental heat and progress variable fluxes inside the flame, is an inherent feature of turbulent premixed flames, and is connected with direct dependence of the second order velocity-scalars correlation on combustion. It has been shown that the existence of the countergradient transport phenomenon is not in contradiction with the actual increasing of the flame brush width.