Experimental and Theoretical Investigation of Annular Film Flow Reversal In a Vertical Pipe: Application To Oil Return In Refrigeration Systems

Abstract

The work studied, both experimentally and theoretically, the fundamental mechanism of oil transport by vapor, liquid, and two-phase refrigerant through analysis of the characteristic behavior of several refrigerant-lubricant pairs. Extensive flow visualization experiments were conducted to pinpoint the critical refrigerant flow rate for preventing oil film flow reversal in a vertical pipe with an 8 mm inside diameter for vapor refrigerant with miscible and immiscible lubricants and, in the first such study, for two-phase refrigerant with immiscible lubricant. In general, the data showed a higher critical refrigerant flow rate than the correlation of Jacobs et al. (1976). Liquid refrigerant flow with immiscible oil did not pose a flow reversal problem under the test conditions. An annular flow model with a vapor core, utilizing an empirical correlation for the interfacial friction factor was also developed to predict the onset of lubricant film flow reversal. The model, which was successfully extended to include a homogeneous two-phase refrigerant core, considers lubricant concentration and viscosity variations, unlike the Jacobs et al. (1976) correlation, and yields predictions within +9% and −6% of the data.