Unsteady Combustion Model of Nonmetalized Organic Gel Fuel Droplet

Abstract

It is widely accepted that nonmetalized organic gel fuel droplets exhibit characteristic and complicated oscillations during both evaporation and combustion, in contrast to the quasi-steady combustion of conventional liquid droplets. In this paper, an unsteady combustion model for the gel droplet is developed, taking into consideration the interactions between oscillating evaporation and flame structures. This model is first validated against experimental results for droplets of an organic gel based on unsymmetrical dimethylhydrazine (UDMH) burning in various nitrogen tetroxide (NTO) oxidizing atmosphere. Using this newly derived model and a detailed UDMH/NTO gas phase chemical reaction mechanism, the combustion characteristics of spray-sized UDMH gel droplets are numerically simulated and discussed in detail. The simulation and experimental results for droplet diameter and temperature both exhibit the same trend during the combustion process. The simulation error for droplet lifetime increases with both droplet size and ambient pressure as a consequence of neglecting heat conduction of the suspending thread and nonideal gas behavior in the model. Numerical simulation results for the detailed flow field parameter variations of the spray-size gel droplet combustion show obvious oscillations in both droplet radius and UDMH vapor mass fraction at the droplet surface following the formation of a gellant film. The oscillation frequency gradually decreases as the film thickness increases and the fuel is depleted. Although the double-flame temperatures and their standoff ratios also oscillate as the gellant film swells and ruptures, their relative oscillation amplitudes are smaller than those of the droplet radius and the vapor mass fraction at the droplet surface. The temperature of the gas phase flow field is higher when fuel vapor at the droplet surface is more abundant, but there is usually a time lag between the gas phase temperature reaching its maximum and the gellant film bursting within each swell-burst period because of the finite fuel vapor diffusion velocity. The temperature of the droplets continues to rise even after reaching the boiling point of liquid UDMH, since the boiling of the relatively small amount of liquid UDMH in a spray droplet cannot absorb all the heat flux from the flame.