Experimental analysis of flame surface density models for premixed turbulent combustion

Flame surface density (FSD) balance equations are now used widely to model turbulent combustion. The FSD transport equation may be derived from first principles but needs closure assumptions. These closure rules may be deduced from detailed analysis, direct numerical simulations, and experiments. The last approach is exploited in this work by performing detailed measurements on a two-dimensional turbulent premixed propane-air V flame. Velocity profiles are obtained from laser Doppler velocimetry. The flame front is visualized by high-speed laser sheet imaging. Results are then processed to extract flame front characteristics (flame surface density, flame front normal vector, curvature). Emission from CH radicals provides an estimate of the reaction rate. First, the flame front dynamics, modified by the thermal expansion, are analyzed in terms of turbulent transport showing the occurrence of counter-gradient turbulent dimension. Important terms of the transport equation for flame surface density are then examined. The strain rate acting on the fame surface is controlled mainly by thermal expansion. The flame orientation factors may be closed from Reynolds shear stresses as proposed by Mantel and Borghi A new model is devised for the curvature and propagation terms that act as source terms on the fresh gas side and sink terms in the burned stream. The stabilization region behind the flame holder needs a specific description. In this region, the surface density takes large values but the reaction rate remains low because of the chemical induction time.