Analysis of bubble growth and motion dynamics in superheated liquid during flash evaporation

Abstract

This study proposes a mathematical model for studying the growth and rise of a bubble in a superheated liquid during flash evaporation. The motion of the bubble is simulated based on force balance by considering various forces acting on the bubble, especially including the history force and the pressure force from a decreasing pressure field. The transformation of the rising bubble and its effect on interfacial heat and mass transfer are considered. Using this model, bubble growth and motion characteristics in the superheated liquid are analyzed in detail, and the effects of history and depressurization forces on both bubble motion and growth are examined. The results present a strong coupling of bubble growth with motion. A slow rise velocity of bubble at the late transition stage can significantly promote bubble growth, and the bubble growth even contributes more to the deceleration of bubble rise compared with the drag force. The history force is found insignificant for bubble rise and growth if the bubble has a relatively high growth rate in water at a superheated degree greater than 3.9 K in this study. The additional force from rapid depressurization provides a large pulse acceleration on the bubble. However, the bubble rise and growth are only sensitive to the depressurization rate for slow depressurizations. As the depressurization rate increases, the growth and rise curves of the bubble gradually converge to the case of instantaneous depressurization. In addition, the bubble rise velocity is lower for a higher overall pressure drop owing to a higher bubble growth rate. The proposed model is finally extended to simulate bubble growth and rise in superheated NaCl solution by revising the thermodynamic conditions at the bubble/solution interface based on the two-characteristic-parameter correlation model.