Research paperCondensation heat transfer and pressure drop of R32/R1123 inside horizontal multiport extruded tubesTransfert de chaleur par condensation et chute de pression du R32/R1123 dans des tubes horizontaux extrudés multi-ports
Introduction
Global warming is a major problem that is severely damaging the environment and critically endangering the livelihood of all living beings on earth. Hydrofluorocarbons (HFCs) have a high global warming potential (GWP) and are thus one of the chief pollutants causing global warming. Therefore, they must be replaced with low GWP refrigerants, such as hydrofluoroolefins (HFOs) and natural refrigerants. In recent years, low GWP refrigerants and their mixtures composed of 2–5 compounds of HFC, HFO, and natural refrigerants have been developed as alternatives to the existing HFCs. However, the candidature of the alternative refrigerants R410A and R32 is unpromising from the viewpoints of safety and system performance. Further, HFO-1123 (R1123, CF2=CHF), which has no ozone depletion potential (ODP), significantly low GWP of less than 1, and low normal boiling temperature, is expected to be an alternative refrigerant to R410A and R32 for refrigeration and air-conditioning systems. Studies on the thermodynamic properties of R1123 have been reported by Higashi and Akasaka (2016), Higashi et al. (2018), and Fukushima et al., 2015, and the Helmholtz energy equation of state was developed by Akasaka et al. (2016). However, R1123 has a risk of disproportionation reaction, which is a self-decomposition reaction accompanied by substantial temperature and pressure rise, under certain high-temperature and high-pressure conditions; therefore, it is expected to be used along with R32 as a mixture to suppress the disproportionation reaction. Although the binary mixture R32/R1123 is a zeotropic mixture, it is expected to be an alternative to R410A and R32 because of its small temperature glide and lower GWP.
Furthermore, the condensation and evaporation heat transfer characteristics of a heat exchanger must be quantified for designing a heat exchanger. Kondou (2019) experimentally investigated the heat transfer and pressure drop characteristics of the refrigerant mixture R32/R1123 (60/40 mass%) during condensation and evaporation inside a horizontal microfin tube with an outer diameter of 6.0 mm. The pressure drop and condensation heat transfer of R32/R1123 are smaller than those of R32. However, the evaporation heat transfer coefficient of R32/R1123 is comparable to that of R32 because R1123/R32—with lower surface tension— enhances nucleate boiling and compensates for the heat transfer deterioration due to a difference in volatility. Kariya and Miyara (2019) investigated the condensation and evaporation heat transfer of R32/R1123 (60/40 mass%) inside a plate heat exchanger, and discussed the local heat transfer characteristics. The condensation heat transfer coefficient of R1123/R32 is similar to that of R32.
To mitigate global warming, reducing the refrigerant charge by downsizing the heat exchanger can be effective in addition to developing low GWP refrigerants; furthermore, compact and high-performance heat exchangers using multiport extruded tubes have been developed. Multiport extruded tubes with many minichannels having hydraulic diameters of less than 1 mm contribute to improving the performance and downsizing of heat exchangers. Numerous studies have been reported on the condensation heat transfer inside multiport extruded tubes.
Yang and Webb (1996) investigated the condensation heat transfer of R22 inside horizontal multiport extruded tubes with the hydraulic diameters of 1.56 mm and 2.64 mm, and discussed the effects of vapor shear stress and surface tension on a condensate film. Koyama et al. (2003) conducted experiments on the condensation flow of R134a inside multiport extruded tubes with hydraulic diameters of 0.81–1.06 mm, and proposed the correlations for frictional pressure drop and condensation heat transfer coefficient. Cavallini et al. (2005) investigated the pressure drop and condensation heat transfer of R134a and R410A inside a multiport extruded tube with a hydraulic diameter of 1.4 mm. Park and Hrnjak (2009) investigated the condensation heat transfer and pressure drop of CO2 inside a multiport extruded tube with circular minichannels of diameter 0.89 mm; they reported that the heat transfer coefficient increases with the increase in mass flux and quality, whereas it is almost independent of wall subcooling.
The effect of the channel shape on the condensate film was discussed in previous studies (Wang and Rose, 2011; Jige et al., 2016; Toninelli et al., 2019). Wang and Rose (2011) theoretically investigated condensation heat transfer inside horizontal non-circular minichannels. They proposed a model considering the effects of interfacial shear stress, surface tension, and gravity and clarified the effect of the channel shape on the condensate film distribution and condensation heat transfer. Experimental studies also reported on the effect of surface tension on the condensation heat transfer inside rectangular minichannels; the heat transfer coefficient at low mass flux is almost constant in the low-middle vapor quality region (Jige et al., 2016). Jige et al. (2016) experimentally investigated the condensation heat transfer and pressure drop of R134a, R32, R1234ze(E), and R410A inside a multiport extruded tube with a hydraulic diameter of 0.85 mm. They reported that the heat transfer coefficients monotonically decrease with decreasing mass flux and quality at higher mass fluxes. In contrast, the heat transfer coefficients remain almost constant at low mass velocities for a wide quality range because the surface tension suppresses the increase in the liquid film thickness at the channel side. Furthermore, Jige et al. (2016) developed a condensation heat transfer model inside a rectangular minichannel by considering the flow pattern and effects of vapor shear stress and surface tension acting on a condensate film. Toninelli et al. (2019) investigated the effect of channel geometry during condensation heat transfer using a circular minichannel and a square minichannel at lower mass fluxes of 65–400 kgm-2s-1. At high mass fluxes (exceeding 400 kgm-2s-1), condensation heat transfer is dominated by shear stress, and there is no effect of the channel shape and gravity on the heat transfer coefficient. Contrarily, at a low mass flux inside square minichannel, a clear enhancement of heat transfer was indicated by experimental and numerical simulation results compared with the heat transfer inside a circular minichannel.
From these studies, it is clear that the trend of condensation heat transfer inside a multiport extruded tube is not significantly different from that inside conventional-diameter tubes, in the case of annular flow pattern observed at higher mass fluxes and quality; however, at lower mass fluxes, this trend differs for conventional-diameter tubes because the surface tension becomes dominant. For lower mass fluxes, several studies have reported that the flow pattern inside minichannels of diameter less than 1 mm is different from that inside conventional-diameter tubes: plug flow pattern, in which elongated bubbles and liquid slug flow alternately, is observed (Nino et al. (2003); Kim et al. (2012); Jige et al. (2018); Li and Hrnjak (2018)). However, no experimental study is available on the condensation and evaporation heat transfer characteristics of refrigerant mixtures containing R1123 inside a multiport extruded tube.
This study, therefore, experimentally investigates the condensation heat transfer and frictional pressure drop of a pure zeotropic binary refrigerant mixture R32/R1123 inside two horizontal multiport extruded tubes with different channel shapes. The effects of mass flux, quality, heat flux, and channel size on the condensation heat transfer and frictional pressure drop were quantified at an average saturation temperature of 40 ℃, and the obtained data were compared with those of pure R32 and the previously proposed correlations.
Section snippets
Experimental setup and methods
Fig. 1 shows a schematic of the experimental loop. The experimental loop is a forced-circulation loop, whose details are available in the previous paper (Jige et al., 2018). The liquid refrigerant discharged from a refrigerant pump flows to a water preheater and electric preheater through a Coriolis mass flow meter. The refrigerant is heated in an electric preheater to control the quality at the inlet of the test section, following which it flows to the test section. The liquid refrigerant then
Data reduction methods and experimental conditions
The specific enthalpy at the test section inlet hTS,in was calculated using the heat transfer rate in the electric preheater QE and the mass flow rate m:
The specific enthalpy of the subcool refrigerant hE,in was calculated using the measured refrigerant pressure and the bulk temperature at the inlet of the electric preheater.
The calculation method and procedures are detailed in previous papers (Jige and Koyama, 2011; Jige et al., 2016). The distribution of the refrigerant
Heat transfer of R32/R1123
The condensation heat transfer is affected by the flow pattern. Fig. 5 shows the flow pattern map for the horizontal multiport extruded tube reported by Jige et al., 2018, Jige et al., 2019 and the experimental conditions selected to measure the condensation heat transfer coefficient of R32/R1123. The heat transfer coefficients were measured by changing the quality at the test section inlet at a mass flux range of 50–400 kgm-2s-1 because the quality change in the test section was limited. This
Frictional pressure drop of R32/R1123
Fig. 10 shows the frictional pressure drop of R32/R1123 inside the horizontal multiport extruded tubes of hydraulic diameters 0.82 mm and 1.16 mm at an average saturation temperature of 40 ℃. The quality change of each measured data of R32/R1123 is 0.15–0.30 and 0.04–0.07 for mass fluxes of 100 and 400kgm-2s-1, respectively, for Rec0.82. The quality change for Rec1.16 is 0.1–0.2 and 0.03–0.05 for mass fluxes of 100 and 400kgm-2s-1, respectively. The frictional pressure drop decreased with the
Conclusions
The condensation heat transfer and frictional pressure drop characteristics of the zeotropic binary mixture R32/R1123 (60/40 mass%) were experimentally investigated inside two horizontal multiport extruded tubes having different channel shapes, and the effects of mass flux, quality, heat flux, and channel shape on these characteristics were quantified.
For the highest mass flux of 400 kgm-2s-1, the heat transfer coefficient decreased monotonically with the decreasing quality because convective
Declaration of Competing Interest
The authors have no conflicts of interest to declare.
Acknowledgments
This work was supported by the New Energy and Industrial Technology Development Organization (NEDO) of Japan. The sample refrigerant R1123 was supplied by AGC Inc., Japan. The test multiport extruded tube was provided by the UACJ Corporation. The authors express their gratitude for these supports.
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