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Energies 2019, 12, 457 10 of 35 3.1. Gas Cooler In a transcritical CO2 HP, the heat rejection takes place at the supercritical temperature and pressure, and the optimum COP of the system is contingent upon the supercritical properties of CO2 at the gas cooler [34]. A significant number of experimental and numerical studies can be found in the literature that has analyzed the supercritical CO2 (scCO2) heat transfer characteristics in different types of channel geometries and arrangements. Since Cabeza et al. [35] have performed a comprehensive review of scCO2 heat transfer for many applications, several selected, recent studies on the scCO2 heat transfer mechanism in different HX configurations specifically for HP applications are presented. Additionally, different types of HX as a gas cooler and their effect on HP performance are analyzed in this section. Liao et al. [36] experimentally analyzed the heat transfer performance of scCO2 in a horizontal and inclined straight tube. The results showed that the heat transfer coefficient (HTC) was at the peak near the pseudo-critical region due to the enhanced specific heat capacity of CO2. Moreover, the HTC increased when the bulk temperature was higher than the critical temperature for horizontal and vertical flow. The effect of buoyancy was more prominent in large diameter tubes than in mini-tubes, which caused a reduction in the scCO2 Nusselt number in a mini-channel, and, subsequently, a new Nusselt number correlation was proposed based on the experimental results. In a similar study, Dang et al. [37] suggested a modification of the Gnielinski correlation to predict the HTC of scCO2 in a circular, straight tube considering the effects of mass flux, heat flux, and tube diameter on HTC. They found that an increase in mass flux enhances the scCO2 HTC, but the effects of heat flux and tube diameter depend on the scCO2 property variation in a radial direction. Studies on horizontally circular tube-in-tube HXs showed that the HTC of scCO2 is a combination of free and forced convection because of the buoyancy effect close to the pseudo-critical vicinity [38–40]. Helically coiled tubes HX were found to enhance scCO2 HTC compared to straight tubes HX [41,42]. In helical structures, the centrifugal and buoyancy forces affect the flow field and heat transfer. The effect of buoyancy force can be ignored if the buoyancy number (Bo) is below 5.6 × 10−7 [43]. Liu et al. [44] numerically analyzed the effects of buoyancy and the centrifugal force on the HTC of scCO2 in horizontal and inclined helically coiled tubes for Bo > 5.6 × 10−7. The HTC vacillates significantly due to a dominating buoyancy force compared to a centrifugal force when the CO2 bulk temperature is lower than the pseudo-critical temperature. Forooghi et al. [45] further explained the buoyancy induced supercritical heat transfer in vertical and inclined tubes. At a significantly low buoyancy number (Bo < 2.26 × 10−6), the heat transfer deteriorates due to low near-wall turbulence, and the heat transfer starts to increase with the increase of the buoyancy effect due to a velocity gradient rise between the outer tube wall and the centerline. In addition, the scCO2 HTC decreases with the increase in heat flux due to the reduced heat transfer capacity and thermal conductivity of CO2 [46]. However, increasing the CO2 mass flux in the supercritical region enhances the turbulence diffusivity and, as a result, improves the heat transfer performance and the effect of the turbulence Prandtl number on heat transfer is insignificant. Furthermore, a comparison demonstrated that the HTC in the inclined helical tube was higher than horizontally helical tubes due to the reduced centrifugal force and the dominating buoyancy force [47]. A simulation of a finned tube gas cooler indicated that the flow field characteristics and heat transfer depend on the fin configuration, and a slit fin has higher HTC than a continuous fin for both the air-side and refrigerant-side, which improves heat rejection [48]. Li et al. [49] developed a low-cost fin and micro-channel integrated gas cooler, where the fin and flat tube were integrated in a single aluminum plate to eliminate the contact resistance due to welding, which enhanced the HTC. They reported that the fin and micro-channel configuration could avoid air-side mal-distribution and had better performance at higher air velocities. Additionally, Garimella [50] and Chang et al. [34] proposed the use of near-counter-flow fin and tube type gas cooler. Yang et al. [18] analyzed a multi-twisted-tube gas cooler where a counter-current double pipe copper HX was used as the gas cooler. In their HX configuration, the inner tubes were twisted togetherPDF Image | Recent Advances in Transcritical CO2 (R744) Heat Pump System
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