On the analysis of bubble growth and detachment at low Capillary and Bond numbers using Volume of Fluid and Level Set methods

Abstract

The growth and detachment of an isolated air bubble injected through a single orifice is considered using four different interface capturing methods, An algebraic Volume of Fluid (VOF) method developed in OpenFOAM®, a geometric VOF method implemented in ANSYS-Fluent®-v13, a Level Set (LS) method in TransAT© software, and a geometric coupled VOF with LS (CLSVOF) recently implemented in ANSYS-Fluent.13. These can be distinguished in terms of the way the interface is tracked and surface tension forces at the bubble interface are modeled. The numerical simulations were carried out using an axi-symmetrical domain with orifice radius 0.8mm and four different flow rates (50, 100, 150, and 200mlph) chosen to satisfy the quasi-static bubble growth condition and to assess the methods at low Capillary and Bond numbers. Under such conditions, the capillary force at the gas/liquid interface, the interaction with the solid wall where a triple contact line exists, and the rapid geometrical changes during the bubble break-up present significant challenges for the numerical methods. The full formation process was studied using the bubble geometrical properties (bubble center of gravity, aspect ratio, and contact angle). The numerical data have been assessed by comparison against experimental observations. All methods have provided generally similar trends during the growth and pinch-off but significant bubble oscillations, which were not observed experimentally, have been modeled by the CLSVOF method. The tests under different flow rates have highlighted the sensitivity of the bubble behavior to the surface tension implementation in particular at the low flow rates with all methods except the LS method which gave the most consistent and stable results.