A one-way coupled, Euler-Lagrangian simulation of bubble coalescence in a turbulent pipe flow

A bubble coalescence model is developed using an Euler-Lagrangian approach for unstructured grids. The Eulerian carrier fluid is solved using large-eddy simulation (LES) and the Lagrangian particle motion is solved with equations relating the turbulent motion of the carrier fluid to forces on each discrete bubble. The collision process is deterministic; bubble-bubble collisions are assumed to be binary and are modeled using a hard-sphere approach. A stochastic approach is used to model coalescence, with the probability of coalescence being a function of the bubble-bubble interaction timescale and the time to drain fluid between the colliding bubbles. The intention is to develop and validate the collision and coalescence model, neglecting two-way coupling turbulent subgrid stress transport of bubbles and the modification of the subgrid stress by the bubbles. Coalescence in a bubbly, turbulent pipe flow in microgravity is simulated with conditions similar to experiments by Colin et al. (Colin, C., Fabre, J., Dukler, A.E., 1991. Gas-liquid flow at microgravity conditions - I. Dispersed bubble and slug flow. Int. J. Multiphase Flow 17 (4), 533-544.) and excellent agreement of bubble size distribution is obtained between simulation and experiment. With increasing downstream distance, the number density of bubbles decreases due to coalescence and the average probability of coalescence decreases slightly due to an increase in overall bubble size.