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