Modeling temperature distribution inside an emulsion fuel droplet under convective heating: A key to predicting microexplosion and puffing

Abstract

Microexplosion/puffing is rapid disintegration of a water-in-oil emulsion droplet caused by explosive boiling of embedded superheated water sub-droplets. To predict microexplosion/puffing, modeling the temperature distribution inside an emulsion droplet under convective heating is a prerequisite, since the temperature field determines the location of nucleation (vapor bubble initiation from superheated water). In the first part of the present study, convective heating of water-in-oil emulsion droplets under typical combustor conditions is investigated using high-fidelity simulation in order to accurately model inner-droplet temperature distribution. The shear force due to the ambient air flow induces internal circulation inside a droplet. It has been found that for droplets under investigation in the present study, the liquid Peclet number PeL is in a transitional regime of 100 < PeL < 500. The temperature field is therefore somewhat distorted by the velocity field, but the distortion is not strong enough to form Hill's vortex for the temperature field. In the second part of the present study, a novel approach is proposed to model the temperature field distortion by introducing angular dependency of the thermal conductivity and eccentricity of the temperature field. The model can reproduce the main features of the temperature field inside an emulsion droplet, and can be used to predict the nucleation location, which is a key initial condition of microexplosion/puffing.