Lewis Number Effects on Turbulent Premixed Flame Structure

ABSTRACT

The influence of the Lewis number on turbulent flame front geometry is investigated in a premixed turbulent stagnation point flame. A laser tomography technique is used to obtain the flame shape, a fractal analysis of the multiscale flame edges is performed and the distribution of local flame front curvature is determined. Lean H₂/Air and C₃H₈/Air mixtures with similar laminar burning rates were investigated with Lewis numbers of 0·33 and 1·85 respectively. At the conditions studied the laminar H₂/Air mixture is unstable and a cellular structure is observed. Turbulence in the reactant stream is generated by a perforated plate and the turbulent length scale (3 mm) and intensity (7%) at the nozzle exit are fixed. The equivalence ratio is set so that the laminar burning velocity is the same for all the cases. The results show clearly that the turbulent flame surface area is dependent on the Lewis number. For a Lewis number less than unity surface area production is observed. The shape of the flame front curvature distribution is not found to be very sensitive to the Lewis number. For the H₂/Air mixture the distribution is skewed toward the positive values indicating the presence of cusps while for the C₃H₈/Air mixture the distribution is more symmetrical. In both cases the average curvature is found to be zero, and if the local burning speed varies linearly with curvature, the local positive and negative burning velocity variations due to curvature will balance.