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Energies 2019, 12, 457 12 of 35 evaporator works below 0.10 at a saturation temperature of 10 ◦C [62]. Consequently, the transport properties of CO2 in the subcritical region (e.g., high vapor density and low vapor viscosity) are substantially different from the other refrigerants. The CO2 flow in the evaporator is characterized by two-phase flow, and the transport properties drastically influence the nucleate boiling, the convective heat transfer, and the CO2 pressure drop in the flow. Recent numerical and experimental studies focus on improving the efficiency of CO2 evaporator considering the flow characteristics. To ensure the feasibility of a CO2 HP, it is essential to design the evaporator as a compact, lightweight, and reliable system [63]. The CO2 two-phase heat transfer in macro-channels and micro-channels depends on the refrigerant mass flux, heat flux, channel geometry, and saturation temperature [62]. In an investigation of CO2 two-phase heat transfer and pressure characteristics in a conventional macro-channel, Yoon et al. [64] reported that CO2 boiling HTC increased with the increase in heat flux at the low vapor quality but decreased with the increase in heat flux when the vapor quality was above a specific value. This can be explained by the inception of vapor dryout at a high vapor quality due to an increase in heat flux, low surface tension, and reduced viscosity. The pressure drop was found to increase with an increase in mass flux, while there was an opposite trend for increasing the saturation temperature. Bredesen et al. [65] and Knudsen et al. [66] found in their experimental studies that the two-phase heat transfer in large tubes could be enhanced significantly by increasing the heat flux without a considerable pressure drop. From the perspective of the physical aspect, a microchannel and minichannel HXs as the CO2 evaporators rather than macro HXs have been the recent research focus due to enhanced HTC, minimized leakage, and reduced refrigerant charge. Choi et al. [67] investigated CO2 two-phase heat transfer in horizontal mini-channels and reported that CO2 HTC increases with an increase in vapor quality. Moreover, CO2 HTC was found to be three times higher than R143a. Based on their experimental study, they proposed a separate two-phase flow heat transfer model for CO2 flowing in smooth mini-channles. Oh et al. [68] experimentally studied five different refrigerants in mini-channels and found that CO2 has the highest boiling HTC. Wu et al. [69] and Yun et al. [70] further reported that the nucleate boiling is predominant at a lower vapor quality because the heat flux and saturation temperature dictate the local HTC in this region, while the convective HTC dominates due to a high vapor velocity at a higher vapor quality. The pressure drop across the mini-channel was evaluated in Reference [69] by modifying a frictional factor developed by Cheng et al. [71] to address the overprediction of a frictional pressure drop in the mist flow region. Concerning the evaporator configuration, several types of CO2 HXs have been studied. In a study of an electric vehicle HP system in a cold climate, Wang et al. [72] found that the micro-channel evaporator had less than 4% exergy loss in all operating conditions. Yun et al. [22] simulated a three-slabbed micro-channel evaporator using R134a and CO2 as the working fluid and compared their performance. The numerical results showed that the overall system performance could be enhanced by increasing the two-phase flow region in the micro-channel. There was a 70% increase in the two-phase flow region when the fin spacing was manipulated from 1.5 to 2.0 mm, and such a marginal increase in the spacing not only reduced material cost but also helped to eradicate the defrosting and condensation drainage problem. Furthermore, the selection of an appropriate circuiting arrangement is found to be imperative to ensure a higher heat transfer in a CO2 evaporator. Bendaoud et al. [73] developed a new model accounting for the thermal and hydrodynamic behavior of a fin and tube HX. The developed tool was used to study a typical CO2 evaporator coil consisting of two circuits in a parallel counter-current configuration. The study showed that the pressure drop was very low for CO2 compared to other refrigerants, and, as a result, the temperature glide was limited considerably. In another study, Yamaguchi et al. [21] experimentally and numerically studied a cross-fin tube HX with smooth plate fins as a CO2 evaporator for the water heating application, and the results confirmed that the heat exchange rate in the evaporator decreased (20 to 12 kW) with a rise in inlet water temperature (8 to 44 ◦C). Yun et al. [22] compared a two-slabbed micro-channel evaporator and a conventionalPDF Image | Recent Advances in Transcritical CO2 (R744) Heat Pump System
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