Transient CFD Modelling of Air–Water Two-Phase Annular Flow Characteristics in a Small Horizontal Circular Pipe

The liquid film formed around the inner walls of a small horizontal circular pipe often exhibits non-uniform distributions circumferentially, where the film is thinner at the top surface than the bottom one. Even with this known phenomenon, the problem remains a challenging task for Computational Fluid Dynamics (CFD) to predict the liquid film formation on the pipe walls, mainly due to inaccurate two-phase flow models that can induce an undesirable ‘dry-out’ phenomenon. Therefore, in this study, a user-defined function subroutine (ANNULAR-UDF) is developed and applied for CFD modelling of an 8.8 mm diameter horizontal pipe, in order to capture transient flow behaviour, flow pattern formation and evolving process and other characteristics in validation against experiments. It is found that CFD modelling is able to capture the liquid phase friction pressure drop about maximum of 30% in deviation, consistent to the correlated experimental data by applying an empirical correlation of Chisholm. Due to the gravity effect, the liquid film is generally thicker at the bottom wall than at the top wall and this trend can be further enhanced by increasing the superficial air–water velocity ratios. These findings could be valuable for HVAC industry applications, where some desirable annular flow features are necessary to retain to achieve high efficiency of heat transfer performance.