SOOTING BEHAVIOR DYNAMICS OF A NON-BUOYANT LAMINAR DIFFUSION FLAME

Abstract

Local soot concentrations in non-buoyant laminar diffusion flames have been demonstrated to be the outcome of two competitive processes, soot formation and soot oxidation. It was first believed that soot formation was the controlling mechanism and thus soot volume fractions could be scaled with a global residence time. Later studies showed that this is not necessarily the case and the local ratio of the soot formation and oxidation residence times is the prime variable controlling the ultimate local soot volume fractions. This ratio is a strong function of geometry and flow field, thus a very difficult variable to properly quantify. This study presents a series of microgravity, low oxidizer flow velocity, experiments where soot volume fraction measurements have been conducted on a laminar, flat plate boundary layer type diffusion flame. The objective of the study is to determine if the above observations apply to this type of diffusion flames. The fuel is ethylene and is injected through a flat plate porous burner into an oxidizer flowing parallel to the burner surface. The oxidizer consists of different mixtures of oxygen and nitrogen, flowing at different velocities. These experiments have been complemented with numerical simulations that emphasize resolution of the flow field to simulate the trajectory of soot particles and to track their history from inception to oxidation. The results validate that local soot volume fractions are a function of the local formation and oxidation residence times and are not necessarily a function of the global residence time. For this particular geometry, an increase in oxidizer velocity leads to local acceleration that reduces the oxidation residence time, leading to higher soot concentrations. It was also observed that the flames become longer as the flow velocity is increased in contrast with the reversed trend observed in flames at higher flow velocities. This result is important because it seems to indicate the presence of a maximum in the flame length and luminosity below those encountered in natural convection. The result would have implications for fire safety in spacecrafts since the ambient gas velocities are below those observed in natural convection, and longer and more luminous flames represent a higher hazard.