Flow regime transitions during condensation in microchannels

•Extensive microchannel condensation flow visualization database analyzed.•Data used to understand governing influences of microchannel flow transitions.•Dimensionless transition criteria developed for condensation in microchannels.•Criteria compared to large and small diameter tube data and flow maps.

Heat transfer and pressure drop for two-phase flow inside tubes are closely related to the corresponding flow mechanisms. The flow patterns formed in microchannels during condensation differ from those observed in conventional tubes. Using an extensive R134a condensation flow-regime database (1 < Dh < 4.91 mm, 150 < G < 750 kg m−2 s−1), new flow regime transition criteria are proposed. The data are used to understand the physical mechanisms and the governing influences for each of the identified flow regimes and develop dimensionless transition criteria. These criteria can be utilized to identify the flow regimes and transitions for various fluids, operating conditions and channel sizes, thereby generalizing their applicability. This mechanistic determination of condensation flow regimes in different operating conditions and geometries will assist in the development of models for predicting condensation heat transfer and pressure drop, enabling the development of optimized microchannel heat exchangers.