Effects of saturation temperature on the boiling properties of carbon dioxide in small diameter pipes at low vapour quality: Heat transfer coefficient

The renewed interest in carbon dioxide as a refrigerant stems from both the environmental friendly properties of this fluid and from its extremely favourable performance as compared to most standard refrigerants at the same temperature, in particular for all applications where small size evaporators are required. Since a dependable forecast of heat transfer is crucial for these applications and reliable predictive methods spanning a large temperature range are so far missing, a long-term study has been launched to create a consistent and reliable experimental database studying the peculiarities of boiling carbon dioxide in mini- and micro-channels, aiming at a future improvement of predictive models. This study presents results on the local heat transfer coefficient of boiling carbon dioxide at low vapour quality (0 < x < 0.4) in 200 mm-long stainless steel tubes with inner diameters of 2.15 mm, 1 mm and 0.5 mm. The dedicated test setup equipped with high precision sensors allows to probe a wide range of saturation temperatures (+15◦C to -25◦C) and mass fluxes between 100 and 1800 kg/m2s have been studied under diabatic test conditions for heat fluxes from 5 to 35 kW/m2. The results discussed focus on the influence of the saturation temperature on the two-phase heat transfer coefficient. It is suggested that the combination of shifting physical properties of carbon dioxide and different flow confinement conditions cause a change in the phenomenological behaviour of the flow and that a transition between macro- and micro-scale most likely occurs within the range of tested parameters. Furthermore it is found that a shift in the applicability of existing prediction methods is caused by those effects and no correlation is able to predict the experimental data and trends in the whole temperature range observed. However, a critical performance analysis of selected correlations provides useful hints on the variation of the phenomenological flow boiling features induced by the combined effects of changing saturation temperature and evaporator diameter.