Experimental Investigation of Viscous Two-Phase Flow in Microchannels

Multi-port microchannel tubes are increasingly popular for use in a variety of heat transfer applications, primarily for automotive condensers and radiators, but also in a variety of refrigeration and air conditioning applications. These channels offer a greater surface area to volume ratio, providing for enhanced heat transfer over a conventional tube in many applications. Previous research has focused on characterizing the performance of such tubes for two-phase refrigerant flow. Most studies have focused on pure refrigerant flow, but in most applications, as a third viscous " phase " will be present in the form of lubricating oil. Much research has been done to account for the effects of increased viscosity due to the presence of oil in the flow, but the effects of viscosity in microchannels rather than larger conventional tubes remain largely unexplored. The goal of this study is to investigate the qualitative and quantitative effects of the presence of oil within the refrigerant for two-phase flow in multi-port, extruded aluminum microchannel tubes. Three techniques are used to characterize these effects. Flow visualization experiments, using a transparent test section, demonstrate the flow configuration between the ports and flow regime within individual ports. Additionally, experimental adiabatic pressure drop and void fraction measurements – performed for a variety of fluids and flow conditions – quantitatively characterize the behavior of the refrigerant-oil mixture in two-phase flow. Experimental results demonstrate a stark change in the flow when viscosity of the liquid phase is increased. These are noted by a change in the observed flow patterns, increased pressure drop, and depressed void fraction as compared to less viscous conditions. These trends cause significant departures from the behaviors characterized in many existing predictive correlations, and present a challenge to incorporate viscosity into modified correlations.