Flow and heat transfer performance of molten salt and CO2-based mixtures in printed circuit heat exchangers

Abstract

The Brayton cycle using CO2-based mixtures as the heat transfer fluid could effectively reduce the ambient temperature limitation on solar power plants. The preheater that completes the heat transfer between the molten salt and the CO2-based mixtures is vital for the system, but has yet to be reported in the literature. In this paper, the printed circuit heat exchanger (PCHE) is used as the preheater in the Brayton cycle. The CO2/butane, CO2/propane and CO2/xenon mixtures are used as the cold fluid, while the molten salt is used as the hot fluid. The thermal-hydraulic properties of the mixture-molten salt PCHEs is numerically analyzed under different operating conditions. The results show that when the mass fraction of butane, propane and xenon changes from 0 to 0.15, the cold fluid temperature changes by − 22 K, −18 K and 12 K, respectively. All three added gases could reduce the pressure loss in the cold channel, but the effect of butane is the most obvious and is further expanded with the increasing mass flow rate of cold fluid. Under the favorable effect of butane and propane, the Fanning friction factor and the Nusselt number in the cold channel are reduced by 2.7 % and increased by 13.2 %, respectively. However, the changes become more minor as the inlet temperature of the cold fluid decreases. New correlations are proposed for the flow and heat transfer performance of these mixture-molten salt PCHEs. The minimum deviation of the correlations is less than ± 1 %.