Study On Heat Transfer And Pressure Drop Characteristics Of Supercritical CO₂ Cooled In Horizontal Helically Coiled Tube

As a natural substitution working fluid, CO2 has been widely concerned. The transcritical CO2 cycle has been successfully applied in air conditioning and heat pump system. The gas cooler, one of the main components of transcritical CO2 heat pump system, will directly influence the performance of the system. Now, the types of compact CO2 gas coolers include micro-channel, double-pipe, helical-coil-in-fluted-tube gas cooler and so on. Thehelical-coil-in-fluted-tube gas cooler was very suitable for the CO2 heat pump water heaters for its structural advantage, easy processing and spread. But the cooling heat transfer and flow characteristics of CO2 in helically coiled tube were still a blank field, and the mechanism of them was unknown. Based on this situation, this paper investigated the cooling heat transfer and flow characteristics of supercritical CO2 in horizontal circular tube and helically coiled tube experimentally and numerically.An experiment platform was designed and set up for the cooling heat transfer and flow of supercritical CO2 in helically coiled tube. In order to verify the reliability of the experiment platform, the cooling heat transfer and flow of supercritical CO2 in horizontal circular tube was studied, and analyzed the effects of mass flux and pressure on heat transfer. The results are compared with the calculated values from some existing correlations, and studied the influence of Ri on the cooling heat transfer in horizontal tube. The results showed that the experimental data and system had a high reliability.The cooling heat transfer of supercritical CO2 in a helically coiled tube was experimentally studied. The effects of mass flux and pressure on heat transfer and pressure drop were analyzed, and the heat transfer and flow in helically coiled tube were compared with that in circular tube. The results showed that heat transfer coefficient and pressure drop of the helically coiled tube are higher than that of circular tube, and the heat transfer coefficient and the pressure drop increased with the increasing of the mass flux. The heat transfer coefficient reached the peak value near the pseudo-critical temperature, and the peak value decreased and shifted to a higher temperature region as the pressure increased. Pressure drop increased as the fluid temperature increased. Below the pseudo-critical temperature, the pressure drop was slightly influenced by the pressure. Above the pseudo-critical temperature, the pressure drop decreased as the pressure increased. As the properties of CO2 change dramatically, introducing the specific heat and density to Dittus-Boelter correlation, and proposed a new correlation. The result of the calculation had an error of 20% compared with the test value.The cooling heat transfer of supercritical CO2 in a horizontal circular tube and a horizontal helically coiled tube was numerically studied. The distribution of section local temperature, velocity, second flow and turbulent kinetic energy was analyzed, that could better understanding the difference of the mechanism of heat transfer and flow in circular tube and helically coiled tube. The effect of heat flux was also analyzed. The results showed that the heat transfer coefficient and pressure drop of helically coiled tube are both higher than that in circular tube, because the flow of the supercritical CO2 in helically tube dramatically disturbed for the secondary flow caused by the centrifugal and buoyancy forces. The heat transfer coefficient of CO2 increased as the heat flux increased in the gas-like region, while the heat transfer coefficient was unaffected by the heat flux in the liquid-like region.