Performance prediction for oil contaminated carbon dioxide flow boiling heat transfer in a smooth horizontal tube

Abstract

In recent years research into the identification and utilization of natural refrigerants with a low global warming potential, as a working fluid in heat pump cycles, has received much attention. One of these natural refrigerants is carbon dioxide (CO2). This study investigates correlations used in predicting the convection heat transfer coefficient of CO2 during flow boiling for both oil contaminated and uncontaminated CO2 in horizontal smooth tubes and a new correlation is also developed and evaluated. Various CO2 specific convection heat transfer correlations were identified from literature. Evaluations performed by Li et al. (2014) indicated that the correlation of Aiyoshizawa et al. (2006) is the most accurate correlation among the six correlations considered in their study. A limitation of the study by Li et al. (2014) is that the correlations were only evaluated for a vapour quality range of 0 to 0.7. In this study the correlation of Aiyoshizawa et al. (2006) is evaluated over the entire vapour quality range from 0 to 1 using experimental data obtained from the study by Dang et al. (2013). The evaluation showed that 49.27% of the predicted data points fall within a 20% deviation of their corresponding experimental values. It was found that the correlation is inadequate for accurately predicting convection heat transfer coefficients over the entire range of the two-phase flow boiling process, specifically under post-dryout conditions. As a result it was decided to develop a new correlation in this study to improve the flow boiling convection heat transfer coefficient predictions for both oil contaminated and uncontaminated CO2 in horizontal smooth tubes. Utilizing the experimental data from Dang et al. (2013), the new correlation was developed to take into consideration the effect of the oil concentration ratio over the entire vapour quality range from 0 to 1. The correlation proposes a new boiling suppression factor, S , and an oil contamination boiling suppression factor, oilS . A new correlation for the dryout vapour quality was also developed in order to improve the prediction accuracy of the post-dryout convection heat transfer coefficients. When evaluated over the entire vapour quality range using the experimental data of Dang et al. (2013), the new correlation predicted 72.20% of the data points within a 20% deviation of their corresponding experimental values. This includes predictions for all calculation conditions, both pre- and post-dryout, with and without oil contamination and for oil concentration ratios ranging from 0% to 5%