Simulation and evaluation of flow boiling heat transfer correlations for R-744

Abstract

A need to reduce the use of conventional refrigerants in vapour-compression cycles has shifted attention towards natural refrigerants, one of which is carbon dioxide (CO2 or R-744). This study focuses on the evaluation of existing correlations to predict the heat transfer characteristics of R-744 flow boiling in an evaporator. A literature study was conducted on existing correlations developed specifically for R-744 as working fluid. Four correlations were identified, each developed for different ranges of the four main parameters believed to influence heat transfer. These factors are mass flux, evaporation temperature, heat flux and channel diameter. The objective of the study was to compare experimentally obtained data with these correlations, thereby identifying the most applicable correlation or combination of correlations. A test bench was developed consisting of a counter flow tube-in-tube heat exchanger with water as secondary fluid, whereby experimental data were obtained. From the study it was concluded that a combination of the Yoon et al.(2004) and the Pamitran et al.(2011) correlations applied to different regions are the most accurate. This combination predicted on average 55% of the data within ±10% and 88% within ±30% accuracy.