A numerical investigation of multiple flame configurations in convective droplet gasification

Multiple flame configurations and gasification rates of a convective fuel droplet are numerically investigated at various Damköhler and Reynolds numbers. The gas flow field is predicted by solving the quasi-steady conservation equations of mass, momentum, and energy, in which gas-phase combustion is modeled by a one-step global finite-rate chemical reaction. Droplet internal motion is predicted by solving the momentum equations. Both gas- and liquid-phase solutions are obtained through an iterative process that satisfied the interfacial conditions. Numerical results reveal that plots of droplet gasification rates versus the Damköhler number exhibit multivalued curves for Reynolds numbers smaller than 80. Similarly, plots of droplet gasification rates versus the Reynolds number also exhibit multivalued curves for low Damköhler numbers. However, monotonically increasing curves are obtained for high Damköhler numbers. Multiple flame configurations are obtained under the present quasi-steady predictions, depending upon the branch solution examined. As the Damköhler number increases or the Reynolds number decreases, flame structure first exists as pure vaporization, then as a wake flame, a transition flame, and finally as an envelope flame for the lower branch solution, while for the upper branch solution, flame structure exists as a wake flame and then an envelope flame. Both lower and upper branch solutions are in qualitative agreement with previous unsteady calculation (Dwyer and Sanders 1988) and experimental studies (Gollahalli and Brzustowski 1973 and 1975), respectively. Based on whether the gasified fuel vapor acts as an oxidizer sink or as a fuel vapor source, convective droplet ignition and extinction points are suggested according to the flame structure rather than the gasification rate.