Hydrodynamic effects on soot formation in laminar hydrocarbon-fueled diffusion flames.

Abstract

An experimental and numerical study of hydrodynamic effects on soot formation in laminar hydrocarbon-fueled diffusion flames was conducted. The main issues addressed were hydrodynamic suppression of soot formation in laminar diffusion flames, fundamental processes of soot nucleation and growth in laminar coflowing-jet diffusion flames, and the structure of soot-free (permanently-blue) opposed-jet diffusion flames. The existence of state relationships in permanently-blue diffusion flames was also examined. The experiments involved acetylene, propylene and 1,3-butadiene coflowing-jet diffusion flames, burning in air with air/fuel-stream velocity ratios in the range 0.4-6.7 and pressures in the range 0.19-0.50 atm; and hydrocarbon-fueled opposed-jet diffusion flames, burning at atmospheric pressure with stoichiometric mixture fractions in the range 0.06-0.78 and strain rates ranging from 30 s$\sp{-1}$ to flame extinction conditions. Measurements included laminar smoke-point flame lengths, soot volume fractions, soot and gas temperatures, soot structure, gas compositions and flow velocities. Numerical simulations were based on the CHEMKIN algorithm, using the OPPDIF code and both the GRI-mech and Peters' mechanisms, for permanently-blue ethylene opposed-jet diffusion flames with a stoichiometric mixture fraction of 0.7 and strain rates of 60, 120 and 240 s$\sp{-1}$. The measurements showed that laminar smoke-point flame lengths can be increased and soot emissions possibly suppressed entirely, by increasing the air/fuel-stream velocity ratios for coflowing-jet diffusion flames; in addition, the critical strain rates at soot-particle inception can be decreased and soot emissions can also possibly be suppressed entirely, by increasing the stoichiometric mixture fractions for opposed-jet diffusion flames. The mechanism of these effects involves the manipulating direction of the flow velocities relative to the flame sheet and the soot residence times available for soot nucleation and growth. Soot nucleation and growth rates exhibit first-order behavior with respect to acetylene concentrations, with an activation energy of 39 kcal/gmole and a collision efficiency of 0.0030. The structure of permanently-blue ethylene diffusion flames can be predicted quite well using the GRI-mech and Peters' mechanisms; in addition, reasonably universal state relationships for temperature and major gas species are observed for the permanently-blue flames over a wide range of strain rates.