Numerical Investigation and Comparison of Bubble Approximation and Coalescence in Two-Phase Flows

Abstract

The coalescence of bubbles and droplets has an important role in many industrial processes such as petroleum refining, heat exchangers of nuclear reactors, and even in food and beverage production, where multiphase flows are frequently encountered [8]. Understanding the behavior of bubbles-particularly bubble approximation and coalescence-is crucial for accurately predicting flow characteristics, such as phase distribution, pressure, and velocity fields. Experimental research has been made to reproduce the coalescence phenomena in lab conditions for droplets [4, 11], and bubbles [5, 6], to understand their behavior and how the film thickness reaches the rupture state. Numerical approaches and simulations were also developed in order to simulate the phenomena using different methods such as moving mesh interface tracking (MMIT) [9] and a five-equation model to simulate the interfaces between compressible fluids [1]. To develop a wider understanding of the phenomena, a numerical approach was used to model the coalescence of multiple bubbles in a vertical liquid flow to simulate situations commonly encountered in industry. The numerical simulation of bubble coalescence often relies on capturing the interface dynamics between phases, as the merging process involves thin film drainage, rupture, and interface reconnection. Traditional numerical approaches such as volume of fluid (VOF), level set (LS), and front tracking methods have been widely used to simulate two-phase flows. Among these, the VOF method, introduced by Hirt and Nichols (1981) [2], remains a popular choice due to its inherent mass conservation properties. In recent years, significant advancements have been made to improve the accuracy and robustness of VOF-based methods, including OpenFOAM's implementation of the VOF solver. OpenFOAM, an open-source computational fluid dynamics (CFD) toolbox, has gained prominence for simulating two-phase flows due to its flexible framework and efficient algorithms. Specifically, OpenFOAM's interFoam solver is widely used for incompressible, isothermal, immiscible two-phase flows, using a VOF-based interface capturing approach [3, 7, 10]. The OpenFOAM VOF-based approach whas used to compare the results with our simulations.