Effect of the ambient pressure on the explosive burning of emulsified and multicomponent fuel droplets

Abstract

A simplified model to study the mechanism controlling the growth of vapor bubbles in superheated pure and multicomponent liquids is formulated. The model is used to analyze the effect of ambient pressure and of the presence of solid particles or gas pockets on the nature and character of the liquid-phase disruption which may occur during the vaporization and burnng of emulsified and multicomponent fuel droplets. The analysis is in good qualitative agreement with existing atmospheric pressure experimental results. The “micro-explosive” buring of water-in-fuel emulsion droplets is caused by the very fast growth of superheated water-vapor bubbles. It is shown that the growth rate of these bubbles is primarily governed by the pressure difference between the superheated vapor and the liquid and by the inertia imparted to the liquid by the motion of the bubble surface (“Inertia controlled growth”). For the multicomponent fuel cases the model shows that the disruption of the droplets results from a much slower vapor-bubble growth which is governed by heat diffusion from the liquid to the bubble rather than by inertial and pressure effects (“Diffusion controlled growth”). Furthermore, it is predicted that any emulsion or multicomponent fuel droplet for which liquid-phase disruption is observed at atmospheric pressure will also exhibit disruption at higher pressures. However, as the ambient-pressure is increased the bubble growth rate will decrease in both solution and emulsion cases, resulting in a less effective and slower disruption. Finally, it is shown that through a reduction in the super-heat limit temperature, the presence of solid particles or dissolved gases in the liquid may also result in a less effective and slower disruption.