The influence of adjustable parameters based on VOF method on the computational accuracy and cost of bubble coalescence process

Bubble coalescence affects the interphase heat and mass transfer by changing the interfacial area of gas and liquid phases. The coalescence time and liquid film thickness are important physical parameters to describe the bubble coalescence process. When simulating the bubble coalescence with volume of fluid(VOF) method, reasonable settings of adjustable parameters(such as mesh size, maximum Courant number and equation cycle
times, etc.) can improve the convergence of the solution and save the computational time. The aim of this paper is to investigate the coalescence process of two coaxial bubbles combining the VOF method with the adaptive mesh refinement technology based on OpenFOAM. The influence of the maximum Courant number Comax, the cycle times of phase equation nα and the cycle times of governing equation npimple on the computational efficiency and accuracy of numerical simulation of bubble coalescence process was explored. Meanwhile, the time evolution of the liquid film thickness among bubbles was obtained for different adjustable parameters. The results show that the thinning speed of the liquid film between two bubbles is proportional to nα and npimple, while is inversely proportional to Comax. This is because the computational accuracy is promoted by the decrease of Comax and the increase of nα and npimple. The transport lag of the fluid on the grid element is therefore improved, and the bubble coalescence and liquid film thinning is accelerated. Considering the computational accuracy and cost, a set of better adjustable parameters for simulating bubble coalescence with VOF method is obtained, i. e., (Comax, nα,npimple)=(0.05, 3, 8).