Two-Phase Heat Transfer and Pressure Drop of Carbon Dioxide in Pipes

Applying two-phase flows improves heat transfer rate which is an important objective in most of applications. Due to environmental concerns and corresponding regulations, carbon dioxide ( ), naturally exists in the atmosphere, has been reintroduced as a possible replacement for the most of commonly used refrigerants in air conditioning applications. High pressure working conditions of carbon dioxides had been the only difficulty in applying it in industrial applications for many years. Nowadays, by progressing in manufacturing technologies, the mentioned difficulty can be simply solved and attempts are resumed for replacing HFCs by carbon dioxide.  There are limited numbers of experiments applying carbon dioxide as working fluid and due to common laboratories restrictions, that decrease the possibilities of repetition of experiments, there are dominant differences in results of present experiments in the literature. Having specific thermo-physical characteristics, carbon dioxide behaves different from other refrigerants in two-phase conditions. Therefore applying models based on other refrigerants for simulating carbon dioxide behavior results in dominant deviations from reality. In the present article the most recent experiments on carbon dioxide is gathered and investigated from aspect of heat transfer and pressure drop. Most of internal two-phase flows reach the annular flow pattern which has further heat transfer rate in compare with other flow patterns. This issue leads us to select this flow pattern in this article. The most important objective of the present study, apart from having a review on the performed studies, is to compare carbon dioxide efficiency with that of other refrigerants. At last it will appear that applying carbon dioxide as working fluid results in much more heat transfer rate and much less pressure drop level in compare with similar conditions of other refrigerants.