Dynamics of Supersonic Droplets of Volatile Liquids

Abstract

T HE breakup and vaporization of liquid droplets in supersonic flow is an interesting research problem with potentially important implications for supersonic combustion ramjets (scramjets). Noncryogenic, liquid hydrocarbons have substantial benefits as scramjet fuel [1], including higher energy density, lower cost, and ease of handling compared to liquid hydrogen fuel. The complications and time associated with the atomization and vaporization of liquid hydrocarbon scramjet fuel can, in principle, be avoided by injecting the fuel in the vapor phase. In some situations, however, such as a “cold start” in which the fuel is not preheated, hydrocarbon fuel may necessarily be injected while still in liquid phase. In this case, the rates and physical mechanisms associated with the disruption and vaporization of liquid droplets under supersonic flow conditions become critical issues to scramjet combustor performance. One possible technique to increase the dispersion of liquid fuels is to exploit the accelerated vaporization made possible by superheating the liquid. Investigation of the vaporization of superheated droplets and sprays have to date been largely confined to incompressible flows [2,3], with the physics of superheated liquid droplet disruption and vaporization in supersonic flow not yet well established. Studies of droplet disruption in compressible flows that have been conducted [4–6] have generally not considered the effects of superheating. The numerical study of Joseph et al. [7] did suggest a flash vaporization mechanism in the disruption of liquid drops in steady, supersonic flow, arguing that superheating may occur over small regions of the droplet surface due to a local combination of low pressure and frictional heating. The disruption of droplets in high-speed flow has often been studied by the sudden application of aerodynamic loads though the use of shock tubes [4,6,8–10]. Though this technique can produce liquid droplets under locally supersonic conditions, this is accomplished only after the passage of a shock wave through the droplets. Other droplet disruption studies have been conducted at subsonic speeds [11,12], for example, by droplets falling across a high-speed gas jet [13], droplet-bearing jets in cross flow [14], and in drop tubes [15]. In any case, these techniques do not typically result in the droplets achieving a significant degree of superheating. The research presented here investigated the dynamics of droplets consisting of volatile fluid smoothly accelerated to supersonic Mach numbers without passage of shock waves through the droplet. This was accomplished over a range of liquid vapor pressures using a compact, underexpanded supersonic jet configuration. This is an extension of a previous study of superheated droplet disruption with compressible, but subsonic, flow relative to the droplets [16]. One challenging aspect in the study of liquid drops in high-speed airstreams is the measurement of the drop velocity and acceleration [17]. Double-pulsed, planar laser imaging was employed here to determine the velocity of the droplets for various test fluids.