Dispersion (Electrostatic/Mechanical) and Fuel Properties Effects on Soot Propensity in Clusters of Drops

Abstract

Soot propensity is studied numerically for an initially binary size, axisymmetric cluster of evaporating drops by defining it as the propensity for nucleation reactions to occur; the study does not address physical or chemical processes ensuing after soot nucleation, such as soot oxidation effects resulting from the fuel molecular structure. The relative magnitude of the fuel vapor partial density is taken as an indication of the soot nucleation magnitude; thus, the effect of drop dispersion on soot (precursor) formation is isolated from that of soot production resulting from formation/destruction by oxidation. The cluster is embedded in an inviscid vortex and exchanges mass, momentum, species, and energy with its surroundings. The vortical motion disperses the drops and the initial cluster evolves into a cylindrical shell with an inner and an outer boundary. In addition to the forces resulting from the vortical motion, an electrostatic force acts on the cluster when the drops are charged; in this situation, the drops might become small enough to reach the Rayleigh limit. Results are obtained for typical vortical motion times having the same order of magnitude as the drop lifetime. Analysis of the results shows that the motion of uncharged drops is determined primarily by centrifugation, whereas for charged drops the electrostatic dispersion becomes the dominant influence in the outer part of the cluster. In the range of parameters investigated, mechanical dispersion cannot rival electrostatically induced dispersion for decreasing the fuel vapor partial density. An additional feature of drop charging is the maintenance of a finite slip velocity in the outer part of the cluster, thereby compounding the advantage of increased dispersion to enhanced evaporation. The results also show that mechanical dispersion combined with electrostatic dispersion does not have a substantial advantage over electrostatic dispersion alone. For uncharged drops it has been found that the latent heat governs soot propensity at small drop dispersion, whereas the liquid density becomes increasingly important with increasing drop dispersion. Drop charging does not affect the influence of fuel physical properties on soot propensity.