Disruption and Vaporization of Simulated Fuel Droplets under Locally Supersonic Conditions
Hermanson, James C.
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The disruption of droplets under supersonic conditions was studied in a draw-down supersonic wind tunnel. The test liquids included 2-propanol, tetraethylene glycol dimethyl ether, and a hexanol-pentane 50/50 mixture by volume. The hexanol-pentane mixture has similar physical properties to 2-propanol, but a considerably higher vapor pressure. The droplets were accelerated in the supersonic flow, achieving supersonic velocities relative to the surrounding air. The droplets reached a relative Mach number of as high as 1.8 and Weber number of 320. The droplets were imaged by direct close-up single-and multiple-exposure imaging and by Laser-Induced Fluoresecence (LIF) imaging. The low static pressure in the supersonic stream had the potential to give rise to superheating of the droplet fluid as the local static pressure became significanlty lower than the vapor pressure of the droplet liquid, depending on the test liquid employed. Droplet lifetimes for the more volatile hexanol/pentane mixture appeared to be shorter due to accelerated vaporization consistent with superheating, though little impact was observed on the droplet velocity and relative Mach number. LIF imaging of the expelled vapor indicated that the more volatile liquid droplets had a higher vaporization rate than non-volatile droplets at all downstream locations, suggesting that droplet superheating does play some role in accelerating the vaporization of supersonic droplets under these conditions.