Dynamic Response of Superheated Liquid Droplets in Steady Supersonic Airflow
AIR FORCE INST OF TECH WRIGHT-PATTERSON AFB OH
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Individual 70 micrometers diameter droplets of ethanol, 1- propanol, and hexanol were smoothly accelerated to supersonic conditions relative to the droplets in a convergent, free-expansion nozzle. The velocities of the droplets were measured, from which the acceleration of the droplets was determined in a supersonic airflow of known properties. Droplets were illuminated by a double-pulsed laser and imaged by an ICCD camera. Dividing the distance between the resulting two images of the drop by the known time step gave a velocity, at a given down-stream distance. The measured results were compared to those of a dynamic droplet acceleration model based on a rigid sphere of the same diameter. The droplets achieved a peak Mach number relative to the surrounding air flow of approximately 3.5. The three test fluids had similar physical characteristics except for vapor pressure, allowing for variation in the degree of superheat the droplets of loss until they were finally completely disrupted. Population histograms as a function of downstream distance confirm progressive disruption of the droplets downstream of the nozzle throat. The histograms also indicate that there is a small effect of the degree of superheat on the rate at which the populations of droplets decrease.