Bubble coalescence in turbulent flows: A mechanistic model for turbulence-induced coalescence applied to microgravity bubbly pipe flow

A mechanistic model for bubble coalescence in turbulent flow is presented. The model is developed in two steps, which are essentially separable. In th e first, expressions put forward earlier for the collision frequency and co alescence probability of equal bubbles during turbulence-driven, high-Reyno lds-number collisions are extended to unequal bubbles and to take account o f bubble-turbulence and bubble-bubble interactions. In the second, the resu lting expression for the coalescence rate is used to derive source terms in the transport equations for the moment densities of the bubble-diameter di stribution, which can readily be evaluated locally within a CFD code. The r esult is an extremely compact framework capable of providing predictions of the evolution of bubble size distributions in space and time at the expens e of only two additional scalar transport equations. To provide an experime ntal validation of the model, some data on the bubble size evolution along a pipe flow under microgravity conditions have been used. Microgravity expe riments on gas-liquid bubbly pipe flows have been carried out during parabo lic flights in aircraft. Bubble diameter distributions have been determined from high speed video recording and image processing. In the absence of gr avity, collisions between bubbles smaller than the integral length scale of turbulence are primarily due to turbulence. The results from the calculati on are in good agreement with the experimental data. The model is then used to predict the influence of the void fraction, the bubble size at the pipe inlet and the liquid mean velocity on the coalescence rate. (C) 2001 Elsev ier Science Ltd. All rights reserved.