. In this case earlier work suggests that turbulent premixed ames have increasing ame brush width controlled in the model only by turbulence and independent from the counter-gradient transport phenomenon which has gasdynamics nature, and a turbulent ame speed which quickly adapts to a local equilibrium value, i.e. Intermediate Steady Propagation (ISP) ames. According to the present analysis transport in turbulent premixed ames is in fact composed by two contributions: real physical gradient turbulent di usion, which is responsible for the growth of ame brush thickness, and counter-gradient pressure-driven convective transport related to the di erential acceleration of burnt and unburnt gases subject to the average pressure variation across the turbulent ame. The novel gas dynamics model for the pressuredriven transport which is developed here, shows that in open turbulent premixed ames the overall transport may be of gradient or counter-gradient nature according to which of these two contributions is dominant and that along the ame a transformation from gradient to counter-gradient transport takes place. Reasonable agreement with the mentioned laboratory experimental data, strongly support the validity of the present modelling ideas. Finally, the model predicts existence of this phenomenon also in large-scale industrial burners at much higher Reynolds numbers.

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Numerical simulation and comparison with standard experimental data of turbulent premixed combustion occurring at large Reynolds and moderately large Damk ohler 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 Intermediate Steady Propagatioin (ISP) regime of combustion in real combustors, i.e. when the turbulent ame propagates with equilibrium turbulent ame speed but has ame brush thickness growing according to the turbulent dispersion law. These ISP ames precede usually analysed 1-D stationary ames 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 ames and 1-D stationary ames. 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 ame 9]. Finally, it has been explained in the framework of the TFC combustion model that "countergradient di usion", i.e. the necessity to use a negative e ective di usion coe cient to describe experimental heat 1 and progress variable uxes inside the ame, is an inherent feature of turbulent premixed ames 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 di usion phenomenon is not in contradiction with the actual increasing of the ame brush width.

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