WORCESTER POLYTECHNIC INSTITUTE
The goal of this project is to examine the breakup and vaporization of liquid droplets in supersonic flow using numerical simulations and to complement an experimental study funded by a companion award (to the University of Washington). The simulations will help quantify the role of several physical mechanisms operative for vaporizing droplets in compressible flows. These mechanisms include the deformation of the droplet due to aerodynamic forces, the inertial instability associated with droplet acceleration, and shear instability due to the high-speed flow across the droplet surface. An additional instability can result from the rapid evaporation that can result from the droplet fluid becoming superheated due to the low static pressure. A varying degree of droplet superheat will result from the rapid depressurization in the supersonic flow, depending on the liquid composition and vapor pressure and the Mach number of the flow relative to the droplets. These results will provide detailed knowledge of the droplet deformation/disruption, the dispersion of the expelled vapor, the droplet acceleration, the compressible flow field near the droplet, and the features of the interfacial instabilities. The possible existence of optimum combinations of parameters, such as droplet size, vapor pressure, and compressible free-stream conditions for the most rapid droplet disruption and vaporization will be explored. The original and potentially transformative aspects of this research stem from the combination of locally supersonic conditions with potential liquid superheating, which explores a practically important regime of droplet disruption that has not been examined in depth or in a systematic fashion to date. The simulations will be done using a finite volume/front tracking method capable of simulating droplet deformation and explosive evaporation under compressible flow conditions. The simulations will be conducted synergistically with the experiments, using flow information from the experiments to guide the development and implementation of the numerical modeling. In turn, the numerical simulations will serve both to guide the conduct of the experiments as well as to help interpret the experimental results by providing key information not readily accessible by the experiments, such as the pressure variation in the vicinity of the droplets and the rate of vaporization. The main tasks under the present funding will be: (1) computations of the breakup of axisymmetric compressible drops without evaporation and detailed comparisons with experimental results. This task builds on an already developed numerical method, although some developmental work is needed, including the addition of surface tension. Further extension of this work will include the addition of evaporation to the method used in the first task and a detailed comparison with experimental results, as well as computations of the fully three-dimensional breakup to examine the effect of more complex disturbances.
This research topic has applications to a number of practically important problems involving the injection of liquids in high-speed flows, including supersonic combustion ramjets (scramjets), pulsed detonation engines, re-entry body cooling, and surface erosion in high-speed flows. Such applications are impacted critically by the nature of droplet disruption and vaporization mechanisms of the liquid fuel droplets to be studied in the proposed research. The proposed research will also directly impact the education and recruiting programs at WPI in several ways, including providing classroom examples and research opportunities for undergraduates.