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

An experimental and modeling study was performed to investigate drop d rag 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 re ferred to as bag, shear or boundary-layer stripping, and ''catastrophi c'' breakup regimes. In the experiments the injected diesel fuel drop' s initial diameter,vas 189 mu m, and its velocity was Id m/s. The tran sverse air 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 condi tions tested correspond to drop Weber numbers (based on gas density an d drop-gas relative velocity)from 56 to 463. Double-pulse, high-magnif ication photography was used to study the temporal progress of the bre akup of the drops. The experiments gave information about the microsco pic structure of the liquid breakup process, drop breakup regimes, dro p trajectories and drag coefficients, and the acceleration of atomizin g drops. The results show that the breakup mechanism consists of a ser ies of processes in which dynamic pressure effects deform the drop int o 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 f or drop distortion. The flattened drop subsequently breaks up into sma ll droplets. At high relative velocities, in the ''catastrophic'' brea kup regime, drops are flattened and fragmented by relatively large-wav elength waves whose wavelengths and growth rates are consistent with e stimates from Rayleigh-Taylor instability theory. The minute drops tha t are also produced at these high relative velocities appear to origin ate from short-wavelength Kelvin-Helmholtz waves growing on the larger liquid fragments. At high gas temperatures the decreased surface tens ion and increased vaporization causes the bag to disappear earlier in the bag breakup regime, and causes shortened ligament lengths in the o ther breakup regimes.