gas cooler, CO2, R-744, trans-critical, heat pump
With on-going developments in the field of trans-critical carbon dioxide (R-744) vapour compression cycles, a need to effectively describe the heat transfer of supercritical carbon dioxide for application in larger diameter tube-in-tube heat exchangers was identified. This study focuses on the in-tube cooling of supercritical carbon dioxide for application in the gas cooler of a trans-critical heat pump. A literature study has revealed Nusselt number correlations specifically developed for the cooling of supercritical carbon dioxide. These correlations were proven to be accurate only for certain operating conditions and tube geometries. A shortcoming identified in the literature reviewed was a generic heat transfer correlation that can be applied over a wide range of fluid conditions for supercritical carbon dioxide cooling. The objective of this study was to compare experimental data obtained from a trans-critical heat pump test bench with different Nusselt number correlations available in literature. The experimental tube diameter used for this study (16mm), was considerably larger than the validated tube diameters used by the researchers who developed Nusselt number correlations specifically for the supercritical cooling of carbon dioxide. The experimental Reynolds number ranges were very high (350’000 < Re < 680’000) compared to those found in literature (Re < 300’000). Experimental results showed that correlations specifically developed for supercritical carbon dioxide cooling generally over-predicts the Nusselt numbers with an average relative error of between 62% and 458% and subsequently also over-predicts the convection heat transfer coefficients. Furthermore, generic heat transfer correlations were compared to the experimental results which over-predicted the Nusselt number with an average relative error of between 20% and 45% over the entire Reynolds number range. More specifically, the correlation by Dittus & Boelter (1985) correlated with an average relative error of 9% for 350’000 < Re < 550’000. From the results of this study it was concluded that cooling heat transfer of supercritical carbon dioxide in larger tube diameters and at higher Reynolds numbers is more accurately predicted by the generic Dittus & Boelter (1985) and Gnielinski (1975) correlations mainly due to the absence of thermo-physical property ratios as seen in the CO2-specific correlations.