Gravitational effects on sooting diffusion flames

Abstract

Simulations of a laminar ethylene-air diffusion flame burning in quiescent air are conducted to gain a better understanding of the effects of buoyancy on the dynamics and behavior of heavily sooting flames under normal-, partial-, micro-, zero-, and negative-gravity conditions and under conditions of gravitational jitter. The simulations solve the time-dependent reactive-flow Navier-Stokes equations coupled with submodels for soot formation and multidimensional radiation transport. Results from the computations follow many of the trends that have been experimentally observed in nonbuoyant diffusion flames. Due to the significant reduction in buoyancy-induced convection, diffusion becomes the dominant mechanism of transport. Microgravity flames are much longer and wider than their earth gravity counterparts due to the reduction in axial velocity and the thicker diffusion layers. In microgravity, flame flicker disappears due to the lack of a buoyancy-induced instability and the entire sooting region is much larger. The reduction in the axial velocity results in significantly longer residence times, allowing more time for soot particle surface growth, and resulting in greatly enhanced soot volume fraction. The enhanced soot production results in increased radiative heat losses, resulting in reduced flame temperatures. By tracing the path lines along which a soot parcel travels, the simulations show significant differences in the local environments through which soot passes between earth-gravity and microgravity flames.