Refrigerant condensation flow regimes in enhanced tubes and their effect on heat transfer coefficients and pressure drops

Flow regimes influence the heat and mass transfer processes during two-phase flow, inferring that any statistically accurate and reliable prediction of heat transfer and pressure drop during flow condensation should be based on the analysis of the prevailing flow pattern. Many correlations for heat transfer coefficient and pressure drop during flow condensation completely ignored flow regime effects, and treated flows as either annular or non-stratified flow, or as stratified flow. This resulted in correlations of poor accuracy and limited validity and reliability. Current heat transfer coefficient, pressure drop, and void fraction models are, however, based on the local flow pattern, resulting in deviations of around 20% from experimental data. There are, however, several inconsistencies and anomalies regarding these models, which are discussed in this paper. A generalised solution methodology for two-phase flow problems still remains an elusive goal, mainly because gas-liquid flow systems combine the complexities of turbulence with those of deformable vapour-liquid interfaces. The paper focuses on the state-of-the-art in correlating flow condensation in micro-fin tubes, and proposes flow regime-based correlations of heat transfer coefficient and pressure drop for refrigerant condensation in smooth, helical micro-fin, and herringbone micro-fin tubes.