Carbon dioxide-lubricant two-phase flow patterns in small horizontal wetted wall channels: The effects of refrigerant/lubricant thermophysical properties

Microchannel heat exchangers are gaining popularity due to their ability to handle high pressures, reduce refrigerant charge, and reduce heat exchanger package size. These heat exchanger designs provide better heat exchange performance due to increased refrigerant side heat transfer coefficients and geometries that allow for a denser packing volume for the air-side fins, resulting in an overall smaller package. Unfortunately, most currently developed working models focus on developing two-phase flow patterns for the pure refrigerant flow in the tubes while ignoring the thermophysical properties of the actual working stream, a mixture of refrigerant with co-existing lubricant. Researchers have been looking into the effect of refrigerant/lubricant flows in small channels; however, to our knowledge, none of these organizations has become proficient in measuring the refrigerant-lubricant mixture thermophysical properties to report as variable parameters in their studies.;Past researchers involved with flow mapping channels sizes less than 1 0 mm ID have often postulated that the interfacial tension plays an important role. Unfortunately, these authors either have left the phenomena as a hypothesis or have performed experiments with only a single fluid (constant interfacial tension and viscosity). While viscous and interfacial tension capillary effects are well known as important parameters in the medical, space and thermosiphon specialties for water and air-water systems through the Capillary number, Ca = We/Re = U□□□□, this phenomenon has only recently come under investigation in HVAC applications.;This research developed measurement capabilities for measuring the solubility, miscibility, liquid density, liquid viscosity, and interfacial tension of lubricant-refrigerant mixtures. For the first time, interfacial tension measurements for refrigerant-lubricant mixtures over the range of concentrations from neat lubricant to pure refrigerant have been measured and published. The research has also lead to the development of a new prediction and modeling method for liquid viscosity and extends the historic lubricant only mixing rules to systems of lubricants with dissolved liquefied gases. A high-speed, high-pressure (< 100 bar, 1500 psia) video system has been constructed to capture the two-phase flow patterns of CO2 flows near the critical point with and without coexisting lubricant inside 0.5 mm tubes. A new flow map based on the thermophysical properties of the flowing fluid has been developed to account for the viscous and interfacial tensions effects on two-phase flows in channels below the criteria for interfacial tension affected flows.