The mechanics of vapor bubble collapse under spherically symmetrical conditions is examined to ascertain the relative importance of the effects of liquid inertia and heat transfer on the collapse rate. A dimensionless parameter, Beff, is identified to characterize the mode of collapse. Discriminating values of this parameter are suggested for the simple case where the collapse is initiated by a step change in pressure or temperature. For heat transfer controlled collapse, a model is also proposed to account for the influence of a permanent gas present in the bubble. Experimental results for bubbles with initial radii ranging from 0.3 cm to 0.9 cm collapsing in water and ethyl alcohol at atmospheric pressure levels and under free fall conditions are presented. The pressure difference ranges from 12 cm Hg to 63 cm Hg and the corresponding degrees of subcooling are 5 deg C to 45 deg C. Data are also given for water vapor bubbles containing significant amounts of nitrogen, helium, and xenon. When compared with theory, reasonable agreements are obtained. For slowly collapsing bubbles, the significance of small translational velocities is brought to attention. Photographic evidences are also given for bubble instability under suitable conditions.

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