A three-dimensional CFD study of the hydrodynamic behavior of equal and unequal-sized in-line methane bubbles at high pressure

The hydrodynamic behavior of multiple bubbles rising upward is a field of ongoing research since various aspects of their interaction require further analysis. Shape deformation, rise velocity, and drag coefficient are some of the uncertainties to be determined in a bubble upward flow. For this study the predictions of the three-dimensional numerical simulations of the volume of fluid (VOF) CFD model were first compared with experimental results available in the literature, serving as benchmark cases. Next, 28 cases of pairs of equal and unequal-sized in-line pairs of bubbles moving upwards were simulated. The bubble size varied between 2.0–10 mm. Breakthrough of the present study is the small initial distance of 2.5 R between the center of the bubbles. To provide a more practical nature in this study material properties were selected to match methane gas and seawater properties at deepsea conditions of 15 MPa and 4 °C, thus yielding a fluid-to-bubble density ratio λ = 7.45 and viscosity ratio n = 100.46. This is one of the few studies to report results of the coalescence procedure in this context. The hydrodynamic behavior of the leading and trailing bubbles was thoroughly studied. Simulation results of the evolution of the rise velocity and the shape deformation with time indicate that the assumption that the leading bubble is rising as a free rising single one is not valid for bubbles between 2.0–7.0 mm. Finally, results of the volume of the daughter bubble exhibited an oscillating nature.