Comparison and validation of heat transfer correlations for in-tube cooling of supercritical CO2

The great variation in thermo-physical properties (specially in the pseudo-critical region) cause the heat transfer coefficient of carbon dioxide to be greatly dependent on both the local temperature and the heat flux in gas cooler tubes. This study presents a detailed comparison of forced convective heat transfer correlations available for supercritical carbon dioxide cooling in mini and micro tubes. For the comparison, a general numerical model has been established for the gas cooler. The configuration considered here is a counter-flow, tube-in-tube type with CO₂ gas flowing in inner tube and the secondary fluid (coolant) water flowing in the annulus. The recently developed heat transfer correlations for CO₂ cooling are based on the limited set of diameters and flow rates (about 3 to 4 sets). So the correlations are required to be validated for other dimensions and flow rates, because we need a more general correlation for the design of an entire heat exchanger. A test facility has been developed to investigate the supercritical carbon dioxide cooling in tubes with water as coolant. Subsequently the correlations have been validated for convective heat transfer coefficient with the experimental heat transfer data generated. This study is expected to help the design engineer by recommending an appropriate heat transfer correlation for actual gas cooler design to be employed in transcritical CO₂ cycle based heating-cooling systems.