Two-phase pressure drop and flow regime of refrigerants and refrigerant-oil mixtures in small channels

As microchannel heat exchangers have become more sophisticated in their design, more exact understanding of the flow inside them is necessary. A decrease in diameter enhances the heat transfer (which takes place at the inner walls of the tubes), but also increases the pressure drop (as the diameter decreases, it becomes like drinking a milkshake through a coffee stirrer). The inclusion of even small amounts of oil in circulation can have a significant effect as well. Historical correlations and studies of two-phase flow have been shown to be insufficient for predicting pressure drops in the smaller channels, due to the different fluid physics that are relevant in flows of small diameter. This study is aimed at understanding the fluid property effects that contribute to pressure drop and flow regime. Two-phase pressure drop data for four refrigerants (R134a, R410A, R290 and R717) were measured in a channel with hydraulic diameter of 148 mum. These data were combined with previous two-phase data of R134a in small channels (hydraulic diameters ranging from 70 to 300 mum) to generate a separated flow model that spans a wide variety of fluid properties. Refrigerant was then mixed with two different viscosities of oil at concentrations ranging from 0.5 to 5% oil, and two-phase pressure drop measurements were taken of those mixtures. Flow visualizations of three of these refrigerants (R134a, R290 and R717) and several concentrations of a R134a-oil mixture were made in a channel with 500 mum hydraulic diameter, and flow regime classifications and comparisons with previous flow maps were made. Finally, a mechanistic description of the two-phase flow that occurs in small channels is put forth, based on the pressure drop measurements and the flow visualizations.