A condensation heat transfer correlation for millimeter-scale tubing with flow regime transition

This study documents local convection heat transfer and flow regime measurements for HFC-134a condensing inside a horizontal rectangular multi-port aluminum condenser tube of 1.46 mm hydraulic diameter. The data is compared with condensation heat transfer correlations and flow regime maps from the literature. Existing correlations are found to overpredict both heat transfer and the stratified-to-annular flow regime transition velocity. Results of the experiments suggest that liquid drawn into the corners of the tube alter the phase distribution in the annular flow regime as well as stabilizing the annular flow regime at lower vapor velocities. To predict the heat transfer data, two correlations, each representing the physics of the specific phase distributions, are developed. A boundary layer analysis is applied for annular flow, in which the friction multiplier and dimensionless boundary layer temperature are evaluated specifically for this tube configuration. For stratified flow, documented film condensation and single-phase forced convection correlations are combined with straightforward void fraction weighting. Finally, a weighting correlation is successfully proposed to account for the all data regardless of the mix of flow regimes experienced. This weighting applies the result of a modified flow regime map developed from the flow visualizations. The final result is a practical correlation for the design of a condenser with millimeter-scale tubes.