BREAKUP MECHANISMS AND DRAG COEFFICIENTS OF HIGH-SPEED VAPORIZING LIQUID DROPS

Abstract

An experimental and modeling study was performed to investigate drop drag and breakup mechanisms of liquid drops injected into a transverse high-velocity air jet at room and elevated temperature conditions. The range of conditions included three drop breakup regimes previously referred to as bag, shear or boundary-layer stripping, and "catastrophic" breakup regimes. In the experiments the injected dieselfuel drop's initial diameter was 189 μm, and its velocity was 16 m/s. The transverse cur jet velocity was varied from 70 to 200 m/s, and the air jet's temperature was varied from room temperature up to 450 K. The conditions tested correspond to drop Weber numbers (based on gas density and drop-gas relative velocity) from 56 to 463. Double-pulse, high-magnification photography was used to study the temporal progress of the breakup of the drops. The experiments gave information about the microscopic structure of the liquid breakup process, drop breakup regimes, drop trajectories and drag coefficients, and the acceleration of atomizing drops. The results show that the breakup mechanism consists of a series of processes in which dynamic pressure effects deform the drop into a thin liquid sheet. This drop flattening significantly affects the drop's drag coefficient. It was found that drop trajectories could be modeled adequately using a modified dynamic drag model that accounts for drop distortion. The flattened drop subsequently breaks up into small droplets. At high relative velocities, in the "catastrophic" breakup regime, drops are flattened and fragmented by relatively large-wavelength waves whose wavelengths and growth rates are consistent with estimates from Rayleigh-Taylor instability theory. The minute drops that are also produced at these high relative velocities appear to originate from short-wavelength Kelvin-Helmholtz waves growing on the larger liquid fragments. At high gas temperatures the decreased surface tension and increased vaporization causes the bag to disappear earlier in the bag breakup regime, and causes shortened ligament lengths in the other breakup regimes.