Soot formation in diffusion flames of fuel/oxygen mixtures

Abstract

Sooting trends have been measured as a function of equivalence ratio of fuel/oxygen mixtures over the range of infinity (pure diffusion flames) to low values (double flames; an inner premixed flame and an outer diffusion flame combination). All interesting observations have been explained on the basis of changes due to oxygen addition in the fuel pyrolysis chemistry, flame structure, and interaction of double flames. This effect of oxygen addition to the fuel on soot formation has been studied in coflow and counterflow diffusion flames of ethene and propane by performing smoke, height, laser light extinction, temperature, and velocity measurements. In counterflow diffusion flames (CFDF's) established around a porous cylindrical burner, the equivalence ratio (φ) of the fuel/oxygen mixtures ranged from infinity to 2.60. Oxygen addition increased the, soot formed in ethene flames to anof about 6.0. Greater oxygen addition resulted in a sharp reduction of the soot loadings and no soot was detected in flames with an φ < 3.0. In contrast to ethene, oxygen addition to, propane CFDF's first decreased the soof loadings until anof about 6.0. The soot loadings then increased and subsequently decreased sharply with the peak occurring at anof 3.5 and no soot was present in flames with φ < 2.60. The smoke height measurements in coflow diffusion flames (CDF's) are in complete agreement with the CFDF results. Whereas oxygen addition to ethene decreased the sooting fuel flow rate substantially, an increase was measured for propane and isobutane. Moreover, CDF's with φ's less than about 3.6 for propane and 6.7 for ethene could not be stabilized due to flashback, and just before flashback propane flames were observed to soot copiously.