Abstract
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.
Highlights
According to the Kyoto Protocol, the HFC refrigerants that are extensively used have to be replaced due to their negative impact on the greenhouse effect [1]
The heat transfer performance and pressure drop of three brazed plate heat exchangers serving as the gas coolers in a CO2 system are investigated
The increase of CO2 inlet pressure can reduce the heat transfer coefficients except at the high temperature region, i.e., the GC1 in this paper, and this reduction can reach up to 39.7% for CO2 side heat transfer coefficient
Summary
According to the Kyoto Protocol, the HFC refrigerants that are extensively used have to be replaced due to their negative impact on the greenhouse effect [1]. CO2 is an excellent candidate due to its nontoxicity, incombustibility, safety, low cost and environmentally benign (ODP = 0, GWP = 1) [3]. It is widely implemented in refrigeration and heat pump systems, air conditioning and various industrial uses [4,5]. In the transcritical CO2 cycle, the heat absorption process occurs at the subcritical pressure whereas the heat rejection process happens at the supercritical pressure [5]. For the water heating application that requires a large temperature lift, the transcritical CO2 cycle shows a special advantage compared with the traditional refrigerants [7]. The temperature glide of supercritical CO2 can reduce the heat transfer temperature difference, and decrease the energy loss and entropy generation [8,9]
Sign-in/Register to view highlights & summary Sign-in/Register to access full text options 
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a callDisclaimer: All third-party content on this website/platform is and will remain the property of their respective owners and is provided on "as is" basis without any warranties, express or implied. Use of third-party content does not indicate any affiliation, sponsorship with or endorsement by them. Any references to third-party content is to identify the corresponding services and shall be considered fair use under The CopyrightLaw.
