Prediction of pressure drop in refrigerant–lubricant oil flows with high contents of oil and refrigerant outgassing in small diameter tubes

This paper presents an analysis of the available prediction methodologies for frictional pressure drop in two-phase gas–liquid flows of oil-rich refrigerant–lubricant oil mixtures in a small diameter tube. In this particular application, the liquid–vapour phase change (flashing) is caused by a reduction of the solubility of the refrigerant in the mixture. The very low vapour pressure of the oil causes it to remain in the liquid state throughout the pipe length, whilst the refrigerant progressively evaporates from the liquid mixture (outgassing). Several correlations and methods for the calculation of the frictional two-phase pressure drop were investigated. Some of these correlations are state-of-art methods developed based on data for small diameter channels. As will be seen, none of the above methodologies perform satisfactorily over the range of conditions covered during refrigerant–lubricant oil mixtures flow tests in a 5.3 m long, 2.86 mm ID tube. Reasons for the observed discrepancies are explored in the manuscript and the best approach to predict the frictional pressure gradient for such flows is outlined.