Soot formation in laminar jet diffusion flames.

The structure and soot properties of round laminar jet hydrocarbon diffusion flames burning in air were studied experimentally. Ten normal-gravity flames were investigated in detail, including four burning acetylene due to the widely recognized importance of acetylene in soot formation and one each burning ethane, propane, n-butane, ethylene, propylene and 1,3-butadiene. In order to maintain manageable soot concentrations, the ambient pressure was varied from 25-99 kPa and nitrogen dilution was added to some fuels. The measurements emphasized properties along the flame axes, where soot processes most resemble those in nonbuoyant diffusion flames. The measurements included: soot volume fractions, temperatures, soot structure, concentrations of major gas species, and velocities. Along the axes of the present test flames, soot was first observed when temperatures reached 1250 K, and soot formation ended when fuel-equivalence ratios decreased to 1.4. Observations of the four acetylene flames yielded soot growth rates similar to past observations of new soot in premixed flames and, after correction for soot oxidation, revealed first-order growth with respect to acetylene concentrations with a negligible activation energy and an acetylene/soot collision efficiency of 0.41%. Observations of the other hydrocarbon flames revealed higher soot growth rates at corresponding acetylene concentrations, which were attributed to a parallel ethylene growth channel with a collision efficiency of 1.41%. Present measurements of soot nucleation rates indicated first-order nucleation with respect to acetylene concentrations in all the flames with an activation energy of 35 kcal/gmole. In addition to the measurements in flames at normal gravity, laminar smoke points were determined for a number of low-gravity laminar jet diffusion flames using the NASA KC-135 flying laboratory. These involved propane- and ethylene-air flames at pressures of 0.5, 1.0 and 2.0 atm. The laminar smoke point luminosity lengths of these nonbuoyant flames were found to be independent of burner diameter and roughly one fourth as long as the smoke-point lengths of corresponding flames in normal gravity. The latter observation results from different soot paths for the buoyant and nonbuoyant jet diffusion flames.