Effects of turbulent mixing on soot sheet dimensions

Abstract

The dimensions of soot sheets have been analysed using two dimensional laser-induced incandescence (LII) imaging in a simple jet, precessing jet and bluff body flames firing commercial propane. These flames span a wide range of global mixing rates and sooting characteristics. The length and thickness of the soot sheets were characterised based on the major and minor axes of an ellipse of equivalent moment of area. Probability density functions (pdf's) were constructed for these characteristic values of length and width of the soot sheets at each axial height in the three flames. A bi-modal distribution of the length and width of the simple and precessing jet flames was found that is not evident in the bluff body flame. The second peak for the sheet length distribution in the precessing jet extends over the flame region where the average soot volume fraction, measured previously, is the highest (30-100 burner diameters), while this peak spans from about 40 burner diameters and reaches to the tip of the flame in the simple jet. The probability of occurrence of short soot sheets in the precessing jet flame increases sharply with axialheight from about 65 burner diameters towards the tip, whereas it remains fairly constant in the simple jet. The soot sheets in the bluff body flame are shorter and narrower in general than for the simple and precessing jet flames, and the probability distribution remains almost constant with axial height.