Numerical simulation of convective fuel droplet vaporization and combustion in a low pressure zero -gravity environment

A quasi-steady numerical model has been developed to study fuel droplet evaporation and combustion in a low pressure, zero-gravity environment. Combustion is modeled using finite-rate chemical kinetics and a one-step overall reaction. The gas-phase solution is obtained using the quasi-steady equations of mass, momentum, species, and energy conservation. Droplet circulation is accounted for by solving the quasi-steady equations for mass and momentum conservation in the liquid-phase. The evaporation and combustion of n-heptane droplets in air at atmospheric pressure is numerically investigated. A vortex forms in the wake of an evaporating droplet at a lower Reynolds number than for the case of a solid sphere. An increase in ambient temperature results in a longer vortex at a given Reynolds number. Results for extinction velocity as a function of droplet diameter and freestream temperature are presented. Experimental results available in the literature compare well with the numerical predictions. A linear dependence of the extinction velocity on droplet diameter constitutes the present state of knowledge. This study predicts a nonlinear dependence for small diameters (d \u3c 1 mm), and a linear dependence only for large diameters (d \u3e 2 mm). A detailed description of envelope, transition, and wake flame configurations is provided. Transition flames cause an enhancement in the local surface mass flux at the point along the droplet surface near the start of the flame front. The location of the minimum surface pressure moves toward the rear of the droplet as the flame changes from a wake to an envelope flame. As the flame changes from a transition to an envelope flame, the friction drag decreases and the thrust drag increases with the net result being a slight reduction in overall drag. Multiple flame configurations are predicted at lower ambient temperatures. The velocity which results in a change from an envelope flame to a different flame configuration exhibits an exponential dependence on the ambient temperature