SUMMARY

Microchannel heat exchangers are widely used in air-conditioning and refrigeration systems, high heat flux electronics cooling, and are also being considered for biological devices. Heat transfer and pressure drop in microchannels with single-phase flow have been studied in greater detail compared to two-phase flow. Heat transfer and pressure drop in two-phase flow inside tubes are closely related to the structure of the flow. In convective condensation, the fluid flows in a variety of flow regimes as it changes from vapor to liquid. The flow patterns formed in microchannels differ in type and extent from those seen in conventional tubes. Wavy and stratified flows are virtually absent at microchannel dimension, while intermittent and annular flows predominate. The subject research focuses on understanding the flow physics in microchannels during condensation. The extensive condensation flow-regime database of a previous study is employed for this purpose. This database comprises the flow-regime observations in tubes of hydraulic diameter ranging from 1-4.91 mm during condensation of refrigerant R-134a. The mass flux ranges from 150-750 kg/m2-s over a vapor quality range of 0 to 1. The results from this previous experimental study are used to understand the physical mechanisms and the governing influences for each of the identified flow regimes. Using this understanding and data, criteria for transitions between the different regimes have been developed. These criteria developed in non-dimensional form can be utilized to identify the flow regimes and transitions for various fluids, operating conditions and channel sizes, thereby generalizing the applicability of these results. This mechanistic determination of condensation flow regimes in different operating conditions and geometries will assist in the choice of the appropriate models for the evaluation of heat transfer and pressure drop, and therefore enable the development of optimum microchannel heat exchangers.