LEWIS NUMBER EFFECTS ON TURBULENT PREMIXED FLAME STRUCTURE

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 anal ysis of the multiscale flame edges is performed and the distribution o f local flame front curvature is determined. Lean H-2/Air and C3H8/Air mixtures with similar laminar burning rates were investigated with Le wis numbers of 0.33 and 1.85 respectively. At the conditions studied t he laminar H-2/Air mixture is unstable and a cellular structure is obs erved. Turbulence in the reactant stream is generated by a perforated plate and the turbulent length scale (3mm) and intensity (7%) at the n ozzle exit are fixed. The equivalence ratio is set so that the laminar burning velocity is the same for all the cases. The results show clea rly that the turbulent flame surface area is dependent on the Lewis nu mber. For a Lewis number less than unity surface area production is ob served. The shape of the flame front curvature distribution is not fou nd to be very sensitive to the Lewis number. For the H-2/Air mixture t he distribution is skewed toward the positive values indicating the pr esence of cusps while for the C3H8/Air mixture the distribution is mor e symmetrical. In both cases the average curvature is found to be zero , and if the local burning speed varies linearly with curvature, the l ocal positive and negative burning velocity variations due to curvatur e will balance.