Review of flow condensation of CO2 as a refrigerant

Carbon dioxide (CO2) has emerged as an excellent substitute natural refrigerant for low temperature refrigeration applications, but a better understanding of its in-tube flow condensation is needed in order to achieve its full potential. From experimental studies in the open literature we review the effects of mass flux, vapour quality and saturation pressure on CO2 flow condensation heat transfer, frictional pressure drop and flow regime transition inside smooth, micro-fin and microchannel tubes. Successful condensation models which were developed from experiments with other refrigerants are evaluated against the CO2 flow condensation experimental data. Comparison between the predicted and experimental data shows that the unique thermophysical properties of CO2 at high reduced pressure conditions lead to these correlations having high prediction errors on the flow condensation heat transfer inside smooth tubes and microchannels, but have less significant effects on the flow condensation heat transfer and two-phase frictional pressure drop under high mass flux conditions inside micro-fin tubes. Recommendations for condensation and pressure drop models to apply to CO2 flow condensation in different tubes are made. As there is inconsistency between the experimental data in smooth tubes from different sources, and the effects of microchannel and micro-fin tube geometries, on the flow regime transition and condensation heat transfer of CO2, are unclear, a more extensive range of the experimental data in different tubes is needed for a fully understanding of in-tube CO2 flow condensation.