Position, thickness and transport properties of turbulent premixed flames in stagnating flows

Abstract

The stabilization mechanism of turbulent premixed flames in stagnation flows is analysed in the framework of a turbulent burning rate closure. It is shown that the mean flame brush thickness depends in this kind of flame on the balance between turbulent dispersion of the flame brush and the adverse gradient of the mean axial mass flux at the combustor axis. The flame position is determined in terms of the characteristic turbulent burning rate, the axial velocity distribution and the radial curvature of the flame at the combustor axis, the last pushing a flame curved toward the stream of reactants closer to the stagnation point. The flame curvature at the axis is related by simple mass conservation considerations to the radial curvature of the axial velocity which in turn is related to the shape of the stagnation body. The transport properties of turbulent premixed flames in stagnation flows are also analysed. In particular, a model developed by Zimont and Biagioli (2002 Combust. Theory Modelling 6 79) to account for the pressure-driven, typically counter-gradient, component ofin one-dimensional freely propagating flames and extended by Biagioli and Zimont (2003 29th Int. Symp. Combustion p 2087) to the case of stagnation-type flames is further reconsidered here to account for the effect of pressure-driven transport in radial direction and for buoyancy. This model, whose key element is the conservation of reactants total pressure, gives the pressure-driven part of in algebraic closed form. The model is successfully applied to recent experimental data for stagnation-type flames showing that scalar transport can have a gradient or counter-gradient nature depending on the intensity of turbulent velocity fluctuations. The idea of flame thickness is also successfully validated with these experiments.