Development of a general heat transfer correlation for horizontal flow of supercritical low GWP refrigerants under heating conditions

Over the years, numerous supercritical heat transfer correlations have been developed for the design of the vapor generator of supercritical refrigeration systems. However, depending on the refrigerant and the applied operating conditions, the prediction capability of existing correlations is limited. This is because of the strong thermophyscial property variations that occur during heat transfer under supercritical conditions, resulting in complex secondary flow phenomena influencing the heat transfer. In addition, for horizontal flow, there is a different heat transfer performance between the top and bottom of the tube complicating the prediction even further. In this work, the prediction capability of existing correlations and several correction factors are evaluated based on experimental heat transfer data on supercritical refrigerants flowing in heated horizontal tubes. Based on this analysis, new more generally applicable heat transfer correlations for the top and bottom of a horizontal tube are proposed. Both correlations include a density-based correction factor to take into account the radial property variations and a correction term accounting for pseudo-boiling effects. The prediction capability of the correlations for the bottom and the top outperform previously developed correlations, predicting 88% and 86% of the data within a relative error of 20%, respectively.