Heat Transfer and Pressure Drop of R1123/R32 Flow in Horizontal Microfin Tubes During Condensation and Evaporation

R1123 (tri fluoro ethylene; CF2 = CHF) has a GWP of approximately 1, a normal boiling point of -59 °C and a critical temperature of 59 °C, those are considered to be desirable for air condition systems. However, the concentration is limited below 60% because of the instability. Therefore, using this substance as a mixture coupled with R32 is attempted. In this study, heat transfer characteristics in a horizontal microfin tube are assessed for the new candidate of low GWP refrigerant mixture R1123 / R32 (40 / 60 mass%). Of the tested microfin tube, an outer diameter, an equivalent inner diameter, a number of fins, and a helix angle are respectively 6 mm, 5.21 mm, 60, and 18 deg. The heat transfer coefficient and pressure drop during condensation and evaporation process are experimentally quantified at saturation temperatures of 40 °C and 10 °C, respectively, and compared to those of R32 alone. The condensation heat transfer coefficient of R1123 / R32 (40 / 60 mass%)　was somewhat lower than that of R32; while, the evaporation heat transfer coefficient was almost comparable to R32. The temperature glide of R1123 / R32　(40 / 60 mass%) is approximately 1 K. Consequently, the mass transfer resistance caused by volatility difference was not obvious in heat transfer coefficient. On the other hand, the pressure drop during both process of condensation and evaporation was obviously lower than that of R32 because vapor density of R1123 / R32 (40 / 60 mass%) is higher than that of R32 alone. From the aspect of heat exchanger designing, the tested mixture offers good solution to shifting lower GWP

Thermodynamic Assessment of High-Temperature Heat Pumps for Heat Recovery

To reduce energy consumption utilizing heat recovery systems is increasingly important in industry. The screening of refrigerants and primary energy efficiency are mainly discussed with a case study for high-temperature heat pump cycles in this study. The overall coefficients of performance of four different cycle configurations to increase temperature of compressed water up to 160 oC using waste heat of 80 oC are calculated and thermodynamically compared for selected refrigerants, R717, R365mfc, R1234ze(E), and R1234ze(Z). The calculation results suggest that a multiple stage cycle drastically reduces throttling loss in expansion valve and exergy loss in condensers, and consequently achieves the highest overall coefficient of performance (COP) among the calculated cases with a refrigerant R1234ze(Z) having the critical temperature about the target outlet water temperature. A cascade cycle using R1234ze(Z) and R365mfc performs relatively high COP and also brings many practical benefits, such as varied combination of refrigerants and lubricant oils and prevention of the liquid-back caused by cold start. At a compressor efficiency of 0.7 and an approach temperature difference in heat exchangers of 2 K, the calculated overall COP is ranging from 4.3 to 4.94. This is corresponding to the primary energy efficiency 1.62 to 1.83, when the transmission-end-efficiency of electric power generation is 0.37. Even where the compressor efficiency of 0.61 and the approach temperature difference of 8 K, the efficiency is above 1.3. As remarked above, the thermodynamic assessment demonstrated that the high-temperature heat pumps to recover waste heat are promising system to reduced primary energy consumption for industrial applications

Surface Tension Measurement of Low GWP Refrigerant Mixture HFO-1123/HFC-32 and HFO-1234ze(E)/HFC-32

HFOs (hydro-fluoro-olefins), HFO-1123 and HFO-1234ze(E) with GWP (global warming potential) of less than 1 attracted attention in a field of air conditioning and refrigeration engineering. However, the use of those HFO alone in such heat pump systems lucks adequate capacity or chemical stability. To compensate the shortcomings, mixtures coupled with R32 is considered to be one solution. This paper presents the surface tension measurement data of HFC-32, HFO-1123, and HFO-1234ze(E), and also their binary mixtures HFO-1123/HFC-32 and HFO-1234ze(E)/HFC-32 at various mass fractions. The capillary constant and surface tension of those are measured across temperature range from 267 K to 307 K using a measuring apparatus based on the differential capillary rise method to discuss the effects of mass fraction to the surface tension of HFO-1123/HFC-32 and HFO-1234ze(E)/HFC-32. The propagated uncertainty in the surface tension is typically 0.3 mN m-1. The experimentally quantified surface tension of HFC-32 alone well agrees with the calculated value of REFPROP ver. 9.1, and also data in literatures. However, data of HFO-1234ze(E) showed slightly negative deviation to REFPROP. To the authors’ best knowledge, available data for HFO-1123 was not found and presented data are new, so far. Among the selected predicting methods, the correlations of Di Nicola et al. (2011) and Gharagheizi et al. (2012) show the best agreement with data of two HFOs. The measured surface tension data of HFO-1123/HFC-32 at a HFC-32 mass fraction of 0.48 well agree with the correlation of Di Nicola et al. (2017). However, the correlation deviates from the present data as mass fraction approaches to 0 and 1. Similarly, data of HFO-1234ze(E)/HFC-32 shows considerable deviations

Heat rejection in condensers close to critical point - desuperheating, condensation in superheated region and condensation of two phase fluid

Paper presented at the 8th International Conference on Heat Transfer, Fluid Mechanics and Thermodynamics, Mauritius, 11-13 July, 2011.mp201

Condensation and Evaporation of R134a, R1234ze(E) and R1234ze(Z) Flow in Horizontal Microfin Tubes at Higher Temperature

Hydrofluoro-olefin R1234ze(E) and the isomer R1234ze(Z) are anticipated to be environment-friendly alternatives of R134a. Especially, R1234ze(Z) is most likely to be suitable for high-temperature heat pumps in industries. The heat transfer characteristics of those refrigerants R134a, R1234ze(E) and R1234ze(Z) are experimentally compared in this study. Their heat transfer coefficient and pressure gradient in a horizontal microfin tube are measured at temperatures of 40 and 50 oC for condensation, and temperature of 30 oC for evaporation. The equivalent inner diameter, fin height, and surface enlargement to the equivalent smooth tube of the microfin tube are 5.34 mm, 0.255 mm, and 2.24, respectively. The thermodynamic and transport properties of R1234ze(Z) are calculated by Reprop (Lemmon et al., 2013) associated the fluid file provided by Akasaka (2013) that is proposed to fit the experimental data by Higashi et al. (2013) and Miyara et al. (2013). For condensation, the heat transfer coefficient and pressure gradient of R1234ze(Z) are higher than those of R1234ze(E) and R134a at mass flux of 200 kg m-2s-1 and heat flux of 10 kW m-2. This is mainly because of the higher vapor velocity due to lower vapor density, larger liquid thermal conductivity and latent heat comparing to R134a and R1234ze(E). The experimentally determined heat transfer coefficient and pressure gradient are reasonably agree with the predicted values by Cavallini et al. (2009), Yonemoto et al. (2006), and Kadzierski et al. (1998). For evaporation, at mass flux of 200 kg m-2s-1 and heat flux of 10 kW m-2, the heat transfer coefficient of R1234ze(Z) is somewhat higher than that of R134a and R1234ze(E) at only higher vapor qualities. The pressure gradient of R1234ze(Z) is notably higher than that of R134a and R1234ze(E). The experimental heat transfer coefficient and pressure gradient are agree well with predicted values by Chamra et al. (2007), Mori et al. (2002), Thome et al. (1997)

Condensation and Evaporation of R744/R32/R1234ze(E) Flow in Horizontal Microfin Tubes

R1234ze(E) has been anticipated to become an alternative of conventional refrigerant R410A for air conditioning systems. Latest studies revealed that the COP of R1234ze(E) alone is unexpectedly lower than that of R410A, mainly caused by irreversible loss in consequent of the small volumetric capacity. To increase volumetric capacity as maintaining the global warming potential (GWP) less than 300, adding R744 and R32 into R1234ze(E) has been attempted recently. For understanding the transport phenomenon of this ternary mixture, the heat transfer coefficient and pressure gradient of in a horizontal microfin tube is experimentally investigated in this study. Experimental data of R744/R32/R1234ze(E) (4/43/53 mass%) are compared to R32/R1234ze(E) (40/60 mass%) as the combination of GWP 300; data on R744/R32/R1234ze(E) (9/29/62 mass%) are compared to R32/R1234ze(E) (30/70 mass %) as the combination of GWP 200. At average saturation temperature of 40 oC, mass flux of 200 kg m-2s-1, and heat flux of 10 kWm-2, condensation heat transfer coefficient of R744/R32/R1234ze(E) (9/29/62 mass%) is somewhat lower than that of other mixtures R744/R32/R1234ze(E) (4/43/53 mass%), R32/R1234ze(E) (40/60 mass%) and (30/70 mass%). The temperature glide of R744/R32/R1234ze(E) (9/29/62 mass%), (4/43/53 mass%), R32/R1234ze(E) (30/70 mass%), and (40/60 mass%) is approximately 18, 11, 10, and 8 K, respectively, at 40 oC. Likewise, the magnitude of decrease in heat transfer coefficient is much related to the temperature glide during condensation. Similar effects of temperature glide is seen in data of evaporation heat transfer coefficient. At average saturation temperature of 10 oC, the evaporation heat transfer coefficient of R744/R32/R1234ze(E) (9/29/62 mass%) is slightly lower than that of other mixtures. The temperature glide of R744/R32/R1234ze(E) (9/29/62 mass%), (4/43/53 mass%), R32/R1234ze(E) (30/70 mass%), and (40/60 mass%) is approximately 22, 13, 11, and 9 K, respectively. The pressure gradient of those refrigerants are almost equal, and the difference is within measurement uncertainty. The experimental pressure gradient agrees well with prediction of Kubota et al. (2001), Filho et al. (2004), Newell and Shah (2001)

Enhancement of R1234ze(Z) Pool Boiling Heat Transfer on Horizontal Titanium Tubes for High Temperature Heat Pumps

Low GWP refrigerant R1234ze(Z) is promising candidate of refrigerant used in industrial high temperature heat pumps. The heat exchangers of such heat pumps designed for waste heat recovery systems is exposed to the exhaust containing acid substances. Using titanium as the material can be one solution to prevent oxidation. In this study, pool boiling heat transfer characteristics outside of horizontal titanium tubes were experimentally investigated for R1234ze(Z) at relatively higher temperatures. A plain tube and three enhanced tubes having different fin geometry were tested in a pressure vessel and the bubble behavior was observed. The experiment covers saturation temperatures from 10 to 60 °C and heat fluxes from 0.55 to 79.8 kWm-2. With the plain tube, it was confirmed that the measured heat transfer coefficients on the plain tube agree with the calculated heat transfer coefficients by Jung et al. correlation proposed for other HFC refrigerants within ±15%. The notable difference to the typical material copper is thermal resistance over the tube wall. Because the thermal conductivity of titanium is only 19 W m-1K-1 (that of copper is approximately 360 W m-1K-1), the thermal resistance is onsiderable. Comparing to the plain tube, the tested enhanced tube exhibited 2.8 to 5.1 times higher heat transfer coefficient, on average, in the test range. This could compensate the disadvantage in thermal conductivity for titanium. The enhancement ratio predominantly depends on the saturation temperature and wall heat flux. At conditions of the higher saturation temperatures and lower heat fluxes, where smaller bubbles were observed, the test tube having smaller fin spaces exhibits higher heat transfer coefficient. The experimental results remark the importance of fin geometry optimization to the operation conditions

Comparison on Evaporation Heat Transfer between R32/R1234yf and R32/R1234ze(E) Flowing in Horizontal Microfin Tubes

Refrigerant mixtures R32/R1234yf and R32/R1234ze(E) are considered to be the low GWP alternatives of R32 and R410A for air conditioners. However, according to the recent reports, the severe heat transfer degradation is encountered during the evaporation process. This implies that much larger heat exchangers are required to maintain the COP and cooling/heating capacity with R32/R1234yf and R32/R1234ze(E). Therefore, the effects of mixture component and composition on the heat transfer degradation is experimentally investigated in this paper. Heat transfer coefficient of those two mixtures and their components (i.e., R32, R1234yf and R1234ze(E) alone) are experimentally quantified with horizontally set copper microfin tubes of 6.00 mm in outer diameter having 48, 58, and 64 fins of 0.26 mm in height and 19 ° in helical angle. The evaporation test was conducted at an average saturation temperature of 10 °C, a heat flux of 10 kW m-2, and mass fluxes from 150 to 400 kg m-2s-1. The heat transfer is degraded most at the composition where the temperature glide and concentration difference between vapor and liquid phases are maximized. This suggest that the relevance of the heat transfer degradation and mass transfer resistance caused by the concentration boundary layer and the additional sensible heat transfer. Although the heat transfer coefficients of R1234yf and R1234ze(E) alone are comparable, the magnitude of heat transfer degradation is obviously severer for R32/R1234ze(E) than that for R R32/R1234yf. This can be explained with the larger concentration difference and the temperature glide of R32/R1234ze(E) that those of R32/R1234yf. With increasing mass flux and number of fins, the heat transfer degradation is mitigated somewhat. It appears to be that mixing of concentration boundary layer contributes to recover the evaporation heat transfer. For the real possibility to use R32/R1234yf and R32/R1234ze(E) in air conditioners, the heat transfer recovery or enhancement could be the key technology

Experimental Assessment on Performance of a Heat Pump Cycle Using R32/R1234yf and R744/R32/R1234yf

     Hydro-fluorocarbons (HFCs) are widely used as working fluids (refrigerants) in air-conditioning and refrigeration systems. However, at the 1997 Kyoto Conference (COP3), it was determined that the product and use of HFCs should be regulated due to their high global warming potential (GWP). In the above mentioned situation for the air-conditioning and refrigeration systems, recently, R1234yf having extremely low-GWP was nominated as one of the alternates of HFCs. Some literatures reported that the heating capacity of heat pump cycles using R1234yf is less than R410A because of its smaller vapor density and latent heat. To achieve the performance comparable to R410A, much larger unit is required. Therefore, in this present study, mixing with R32 of larger latent heat and relatively low-GWP into R1234yf was attempted. Additionally, R744 having higher vapor density and extremely low-GWP was added into R32/R1234yf.  The R32/R1234yf and R744/R32/R1234yf are zeotropic mixtures that cause temperature change during the phase-change, typically called temperature glide. When this temperature glide is utilized effectively to decrease the irreversible loss in heat exchanger, the cycle performance can be improved. The degree of temperature glide is determined by the composition of refrigerant mixtures. The composition of the test refrigerants are selected from the criterions of GWPs just below 300 and 200.Experiment was conducted with a vapor compression heat pump cycle using a compressor developed for R410A. The condenser and the evaporator are tube-in tube heat exchangers of counter-flow configuration.  At compositions with GWP200 and a given heating capacity, the COP of R32/R1234yf and R744/R32/R1234yf are lower than that of R410A. The main causes were that R32/R1234yf of GWP200 has lower vapor density and R744/R32/R1234yf of GWP200 has lager temperature glide. At compositions with GWP300, R32/R1234yf and R744/R32/R1234yf exhibit comparable COP and heating capacity to R410A. Using R32/R1234yf and R744/R32/R1234yf of compositions with GWP300, as the alternatives of R410A, is feasible idea