Heat transfer and pressure drop of supercritical CO₂ in brazed plate heat exchangers of the tri-partite gas cooler

The heat transfer characteristic of supercritical CO₂ is an essential research topic due to its significant influence on the performance of heat exchangers and systems. In this paper, the heat transfer and pressure drop of supercritical CO₂ in the brazed plate heat exchangers are experimentally researched. The heat exchangers belong to a tri-partite gas cooler which can simultaneously fulfill the demands of domestic hot water and space heating. The results demonstrates that the thermal resistance in the CO₂ side is the main factor that influences the total heat transfer. The increase of CO₂ inlet pressure can reduce the heat transfer coefficients except at the high temperature region. The improvement of heat transfer coefficient by increasing the CO₂ mass flow rate is more significant in the space heating (SH) and domestic hot water (DHW) preheating gas coolers, and is lowest in the DHW reheating gas cooler. The influence of DHW inlet temperature is more obvious in the DHW preheating gas cooler that connected to the water inlet. The influence of water mass flow rate is different in the DHW and SH operation modes. Moreover, the effects of CO₂ pressure and mass flow rate on the buoyancy force are discussed and the influence of buoyancy force on heat transfer is verified. The inaccuracy of the correlations from the literature is proved and then new correlations are established. The mean absolute relative errors of the new correlations are 11.61% and 12.82% for the one-pass and two-pass configurations, respectively. Furthermore, the frictional pressure drop in the heat exchangers is low (up to 36.51 kPa) and basically increases as the Reynolds number increases.