variavel0=<b>J.R. BARBOSA JR.</b> - jrb@nrva.ufsc.br UFSC
<b>V.T. LACERDA</b> - vitor@nrva.ufsc.br UFSC
<b>A.T. PRATA</b> - prata@nrva.ufsc.br UFSC
<b>Abstract.</b> This paper presents an analysis of the available prediction methodologies for frictional pressure drop in two-phase gas-liquid flows of refrigerant-lubricant oil mixtures in a small diameter pipe. In this particular application, the liquid-vapour phase change 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-the-art methods developed based on data for small diameter channels (Mishima and Hibiki, 1996; Tran et al., 2000; Wang et al., 2000; Chen et al., 2001). As will be seen, none of the above methodologies perform satisfactorily over the wide range of conditions of the refrigerant-lubricant oil mixtures flows tested by Lacerda et al. (2000) in a 2.86 mm tube. Reasons for such discrepancies are explored in the manuscript and the best approach to predict the frictional pressure gradient for such flows is outlined.
<b>Keywords.</b> Phase change, refrigerant-oil mixtures, small diameter pipes, refrigerant outgassing.