Nucleate boiling heat transfer coefficients of HFO1234yf on various enhanced surfaces

In this study, nucleate boiling heat transfer coefficients (HTCs) of HFO1234yf HFC134a are measured on a flat plain, Turbo-B, Turbo-C, and Thermoexcel-E surfaces. All data are taken at the liquid pool temperature of 7 °C on small flat horizontal square copper plates (9.53 mm × 9.53 mm) at heat fluxes from 10 kW m⁻² to 200 kW m⁻² with an interval of 10 kW m⁻². Test results show that nucleate boiling HTCs of HFO1234yf on all four surfaces are similar to those of HFC134a at all heat fluxes tested in this study. At heat fluxes below 150 kW m⁻², Thermoexcel-E surface shows the highest heat transfer performance and hence is the best surface for the manufacture of the evaporators in refrigeration and air-conditioning equipment. On the other hand, at high heat fluxes above 150 kW m⁻², Turbo-B and Turbo-C show better heat transfer performance than Thermoexcel-E and hence are good for electronic cooling applications. Overall, HFO1234yf is a good long term candidate with excellent environmental properties to replace successfully HFC134a from the view point of pool boiling heat transfer. Hence HFO1234yf can be readily applied to the conventional evaporators designed for HFC134a.     

R1234yf热泵技术综述与潜力分析

为了满足逐步严苛的环保法规要求,R1234yf成为车用热泵制冷剂R134a的热门替代制冷剂之一。本文对R1234yf热泵技术的研究进行了综述与分析,其GWP<1,各方面性质均符合车用热泵系统的工作需求。在传热效果上,R1234yf的沸腾传热性能略优于R134a,且冷凝过程压降比R134a低5%～10%,优于R134a系统。在诸多R1234yf和R134a系统的仿真和实验研究中,R1234yf热泵性能略低于R134a,但可以通过优化零部件、强化补气、改善工况等方式使其与R134a十分接近甚至超越。R1234yf低压饱和压力比R134a高约15%,可以适配更高的压缩机转速,低温下制热性能比R134a更好,且较低的压缩机排气温度使系统工作更为稳定,强化补气的效果也优于R134a。因此,R1234yf在车用热泵中具有较好的工作性能和发展前景,可以作为R134a的替代制冷剂。     

Condensation heat transfer coefficients of binary HFC mixtures on low fin and Turbo-C tubes

In this study, external condensation heat transfer coefficients (HTCs) are measured for nonazeotropic refrigerant mixtures (NARMs) of HFC32/HFC134a and HFC134a/HCFC123 on a low fin and Turbo-C tubes. All measurements are taken at the vapor temperature of 39 °C with the wall subcooling of 3–8 °C. Test results showed that condensation HTCs of NARMs on enhanced tubes were severely degraded from the ideal values showing up to 96% decrease. HTCs of the mixtures on Turbo-C tube were degraded more than those on low fin tube such that HTCs of the mixtures at the same composition were similar regardless of the tube. The mixture with larger gliding temperature differences (GTDs), HFC134a/HCFC123, showed a larger heat transfer reduction from the ideal values than the mixture with smaller GTDs, HFC32/HFC134a. Heat transfer enhancement ratios of the enhanced tubes with NARMs were almost 2 times lower than those with pure refrigerants and they decreased more as the GTDs of the mixtures increased.     

Condensation heat transfer coefficients of enhanced tubes with alternative refrigerants for CFC11 and CFC12Coefficients de transfert de chaleur de condensation de tubes à surface augmentée fonctionnant avec des frigorigènes de remplacement de CFC11 et de CFC12

In this study, condensation heat transfer coefficients (HTCs) of a plain tube, low fin tube, and Turbo-C tube were measured for the low pressure refrigerants CFC11 and HCFC123 and for the medium pressure refrigerants CFC12 and HFC134a. All data were taken at the vapor temperature of 39°C with a wall subcooling of 3–8°C. Test results showed that the HTCs of HFC123, an alternative for CFC11, were 8.2–19.2% lower than those of CFC11 for all the tubes tested. On the other hand, the HTCs of HFC134a, an alternative for CFC12, were 0.0–31.8% higher than those of CFC12 for all the tubes tested. For all refrigerants tested, the Turbo-C tube showed the highest HTCs among the tubes tested showing almost an 8 times increase in HTCs as compared to the plain tube. Nusselt's prediction equation yielded a 12% deviation for the plain tube data while Beatty and Katz's prediction equation yielded a 20.0% deviation for the low fin tube data.     

Flow condensation heat transfer coefficients of R22, R134a, R407C, and R410A inside plain and microfin tubes

Flow condensation heat transfer coefficients (HTCs) of R22, R134a, R407C, and R410A inside horizontal plain and microfin tubes of 9.52 mm outside diameter and 1 m length were measured at the condensation temperature of 40 °C with mass fluxes of 100, 200, and 300 kg m⁻² s⁻¹ and a heat flux of 7.7–7.9 kW m⁻². For a plain tube, HTCs of R134a and R410A were similar to those of R22 while HTCs of R407C are 11–15% lower than those of R22. For a microfin tube, HTCs of R134a were similar to those of R22 while HTCs of R407C and R410A were 23–53% and 10–21% lower than those of R22. For a plain tube, our correlation agreed well with the present data for all refrigerants exhibiting a mean deviation of 11.6%. Finally, HTCs of a microfin tube were 2–3 times higher than those of a plain tube and the heat transfer enhancement factor decreased as the mass flux increased for all refrigerants tested.     

Condensation heat transfer and two-phase frictional pressure drop in a single minichannel with R1234ze(E) and other refrigerants

R1234ze(E), trans-1, 3, 3, 3-tetrafluoropropene, is a fluorinated propene isomer which may be a substitute of R134a for refrigeration applications. R1234ze(E) has a much lower GWP_100-years than that of R134a. In this paper, the local heat transfer coefficient during condensation of R1234ze(E) is investigated in a single minichannel, horizontally arranged, with hydraulic diameter equal to 0.96 mm. Since the saturation temperature drop directly affects the heat transfer rate, the pressure drop during adiabatic two phase flow of R1234ze(E) is also measured. Predictive models are assessed both for condensation heat transfer and pressure drop. A comparative analysis is carried out among several fluids (R1234ze(E), R32, R134a and R1234yf) starting from experimental data collected at the same conditions and using the Performance Evaluation Criteria (PEC) named Penalty Factor (PF) and Total Temperature Penalization (TTP) to rank the tested refrigerants in forced convective condensation.     

HFO1234ze(E) vaporisation inside a Brazed Plate Heat Exchanger (BPHE): Comparison with HFC134a and HFO1234yf

This paper investigates the effects of heat flux, saturation temperature, and outlet conditions on HFO1234ze(E) boiling inside a Brazed Plate Heat Exchanger (BPHE). The effect of the heat flux on the heat transfer coefficients was remarkable. Similar consideration applies for outlet condition effects whereas the impact of saturation temperature was found to be lower. The frictional pressure drop shows a linear dependence on the refrigerant kinetic energy per unit volume. The two-phase flow boiling heat transfer coefficients were compared with a new model for refrigerant boiling inside BPHE (Longo et al., 2015): the mean absolute percentage deviation between calculated and experimental data is 7.2%. The present data points were compared with those of HFC134a and HFO1234yf previously measured inside the same BPHE under the same operating conditions: HFO1234ze(E) exhibits heat transfer coefficients very similar to HFC134a and HFO1234yf and frictional pressure drops slightly higher than HFC134a and HFO1234yf.     

Nucleate boiling heat transfer coefficients of HCFC22, HFC134a, HFC125, and HFC32 on various enhanced tubes

In this study, nucleate boiling heat transfer coefficients (HTCs) of HCFC22, HFC134a, HFC125, HFC32 were measured on a low fin, Turbo-B, and Thermoexcel-E tubes. All data were taken at the liquid pool temperature of 7 °C on horizontal tubes of 152 mm length and 18.6–18.8 mm outside diameter at heat fluxes of 10–80 kW m⁻² with an interval of 10 kW m⁻² in the decreasing order of heat flux. For a plain and low fin tubes, refrigerants with higher vapor pressures showed higher nucleate boiling HTCs consistently. This was due to the fact that the wall superheat required to activate given size cavities became smaller as pressure increased. For Turbo-B and Thermoexcel-E tubes, HFC125 showed a peculiar behavior exhibiting much reduced HTCs due to its high reduced pressure. The heat transfer enhancement ratios of the low fin, Turbo-B, and Thermoexcel-E tubes were 1.09–1.68, 1.77–5.41, 1.64–8.77 respectively in the range of heat fluxes tested.     

Condensation heat transfer coefficients of R22, R407C, and R410A on a horizontal plain, low fin, and turbo-C tubes

In this study, condensation heat transfer coefficients (HTCs) were measured on a horizontal plain tube, low fin tube, and Turbo-C tube at the saturated vapor temperature of 39 °C for R22, R407C, and R410A with the wall subcooling of 3–8 °C. R407C, a non-azeotropic refrigerant mixture, exhibited a quite different condensation phenomenon from those of R22 and R410A and its condensation HTCs were up to 50% lower than those of R22. For R407C, as the wall subcooling increased, condensation HTCs decreased on a plain tube while they increased on both low fin and turbo-C tubes. This was due to the lessening effect of the vapor diffusion film with a rapid increase in condensation rate on enhanced tubes. On the other hand, condensation HTCs of R410A, almost an azeotrope, were similar to those of R22. For all refrigerants tested, condensation HTCs of turbo-C tube were the highest among the tubes tested showing a 3–8 times increase as compared to those of a plain tube.     

Development and validation of a micro-fin tubes evaporator model using R134a and R1234yf as working fluids

This paper presents a model of shell and tube evaporator with micro-fin tubes using R1234yf and R134a. The model developed for this evaporator uses the ε-NTU method to predict the evaporating pressure, the refrigerant outlet enthalpy and the outlet temperature of the secondary fluid. The model accuracy is evaluated using different two-phase flow boiling correlations for micro-fin tubes and comparing predicted and experimental data. The experimental tests were carried out for a wide range of operating conditions using R134a and R1234yf as working fluids. The predicted parameter with maximum deviations, between the predicted and experimental data, is the evaporating pressure. The correlation of Akhavan– Behabadi et al. was used to predict flow boiling heat transfer, with an error on cooling capacity prediction below 5%. Simulations, carried out with this validated model, show that the overall heat transfer coefficient of R1234yf has a maximum decrease of 10% compared with R134a.     

Comparative performance analysis of the low GWP refrigerants HFO1234yf,HFO1234ze(E) and HC600a inside a roll-bond evaporator

This paper presents the comparative performance analysis of the low GWP refrigerants HFO1234yf, HFO1234ze(E) and HC600a inside a commercial roll-bond evaporator for household refrigerators. The vaporisation performances were evaluated at two evaporation temperatures, −15 and −20 °C, and different refrigerant mass flow rates and compared with those of the traditional refrigerant for domestic refrigeration HFC134a. The performance analysis was carried out using both thermocouples installed on the rear side of the roll-bond evaporator and an IR thermo-camera. Each of the low GWP refrigerants tested can be considered a good substitute for HFC134a, provided that the compressor displacement is adjusted to deliver the proper refrigerant mass flow rate. Only HFO1234yf exhibits performance similar to HFC134a at the same mass flow rate, therefore it can be considered a direct drop-in substitute for HFC134a.     

Condensation heat transfer coefficients of halogenated binary refrigerant mixtures on a smooth tube

In this study, external condensation heat transfer coefficients (HTCs) of nonazeotropic refrigerant mixtures of HFC32/HFC134a and HFC134a/HCFC123 at various compositions were measured on a horizontal smooth tube of a 19 mm outside diameter. All data were taken at the vapor temperature of 39 °C with a wall subcooling of 3–8 °C. Test results showed that HTCs of the tested mixtures were 19.4–85.1% lower than the ideal values calculated by the mole fraction weighting of the HTCs of the pure components. A thermal resistance due to the diffusion vapor film seemed to be partly responsible for the significant reduction of HTCs with these nonazeotropic mixtures.     

小通道钎焊板式冷凝器性能仿真与实验研究

本文针对当量直径为1.5 mm的小通道钎焊板式冷凝器的换热和压降特性进行了仿真和实验研究。采用有限体积法建立了一维稳态分布参数模型,对R134a和R1234yf两种制冷剂在板间冷凝换热的性能进行仿真模拟,并对模型进行了实验验证。实验结果表明:本文所建立的仿真模型精度较高,换热性能平均误差为4%,压降平均误差为16%,可用于分析换热器的整体性能。最后用此模型仿真对比了R134a和R1234yf在小通道钎焊板式换热器内的冷凝换热特性,结果显示,在相同工况下,用R1234yf替代R134a,传热系数平均下降9%,压降平均下降8%。     

Condensation heat transfer coefficients of flammable refrigerants

In this study, external condensation heat transfer coefficients (HTCs) of six flammable refrigerants of propylene (R1270), propane (R290), isobutane (R600a), butane (R600), dimethylether (RE170), and HFC32 were measured at the vapor temperature of 39 °C on a plain tube of 19.0 mm outside diameter with a wall subcooling of 3–8 °C under a heat flux of 7–23 kW m⁻². Test results showed a typical trend that external condensation HTCs decrease with the wall subcooling. No unusual behavior or phenomenon was observed for these flammable refrigerants during experiments. HFC32 and DME showed 28–44% higher HTCs than those of HCFC22 due to their excellent thermophysical properties. Propylene and butane showed the similar HTCs as those of HCFC22 while propane and isobutane showed 9% lower HTCs than those of HCFC22. Finally, a general correlation was made by modifying Nusselt's equation based upon the measured data of eleven fluids of various vapor pressures including halogenated refrigerants. The general equation showed an excellent agreement with all data exhibiting a deviation of less than 3%.     

Flow condensation heat transfer coefficients of pure refrigerants

Flow condensation heat transfer coefficients (HTCs) of R12, R22, R32, R123, R125, R134a, and R142b were measured experimentally on a horizontal plain tube. The experimental apparatus was composed of three main parts; a refrigerant loop, a water loop and a water-glycol loop. The test section in the refrigerant loop was made of a copper tube with an outside diameter of 9.52 mm and 1 m length. The refrigerant was cooled by cold water passing through an annulus surrounding the test section. All tests were performed at a fixed refrigerant saturation temperature of 40 °C with mass fluxes of 100, 200, 300 kg m⁻² s⁻¹ and heat flux of 7.3–7.7 kW m⁻². Experimental results showed that flow condensation HTCs increase as the quality and mass flux increase. At the same mass flux, the HTCs of R142b and R32 are higher than those of R22 by 8–34% while HTCs of R134a and R123 are similar to those of R22. On the other hand, HTCs of R12 and R125 are lower than those of R22 by 24–30%. Previous correlations predicted the present data satisfactorily with mean deviations of less than 20% substantiating indirectly the reliability of the present data. Finally, a new correlation was developed by modifying Dobson and Chato's correlation with an introduction of a heat and mass flux ratio combined with latent heat of condensation. The correlation showed a mean deviation of 10.7% for all pure halogenated refrigerants' data obtained in this study.     

R1234yf as a substitute of R134a in automotive air conditioning. Solubility measurements in two commercial PAG oils

Starting from January 1st 2011, as stated by the Directive 2006/40/EC, fluorinated greenhouse gases with a global warming potential (GWP) higher than 150 can not be used in automotive applications any more. For this reason, 1,1,1,2-tetrafluoroethane (R134a), commonly used for these applications, will be abandoned and substituted by refrigerants with lower GWP. In recent times, a new fluid, 2,3,3,3-tetrafluoroprop-1-ene (R1234yf) has been proposed as an interesting alternative, since it has a very low GWP and thermodynamic properties very similar to R134a. At the moment, only few data can be found on the thermodynamic properties of this new refrigerant and, in particular, its behaviour in solution with commonly used compressor lubricants is still to be evaluated. Here, solubility experimental data of R1234yf in a Polyalkylene Glycol (PAG) and in a specifically modified Double-Capped PAG (DC-PAG) commercial lubricants are measured with a static synthetic method at isothermal conditions, in the temperature range between 258 K and 338 K.     

Measurement of vapor viscosity of R1234yf and its binary mixtures with R32, R125

The vapor viscosities of the new refrigerant R1234yf and its binary mixtures, R32+R1234yf, R125+R1234yf, were measured at atmospheric pressure with a falling-ball-type viscometer. The combined expanded uncertainty of the measurement apparatus was less than 1.5%. The binary mixtures consisted of 20.0, 30.0, 40.0, and 50.0 wt% R32 for R32+R1234yf and of 20.0, 35.0, 50.0, and 70.0 wt% R125 for R125+R1234yf. The viscosities of R1234yf were correlated with the Chapman–Enskog gas kinetic theory and those of binary mixtures were correlated with the Wilke mixture rule. The average absolute deviation (AAD) is 0.189% for R32+R1234yf and 1.169% for R125+R1234yf. The deviations of experimental viscosities of the binary mixtures from data calculated using RefProp v9.1 were also obtained. The AAD is 0.555% for R32+R1234yf and 1.479% for R125+R1234yf.     

An experimental investigation of the energetic performances of HFO1234yf and its binary mixtures with HFC134a in a household refrigerator

In this paper, a comparative experimental analysis between HFC134a, HFO1234yf and a refrigerant mixture of HFC134a/HFO1234yf (10/90% weight) implemented in a domestic refrigerator is introduced. Adding 10% of HFC134a to HFO1234yf, the mixture becomes non-flammable with GWP still less than 150. The experimental tests have been conducted under sub-tropical conditions in accordance with the UNI-EN-ISO15502 standard. Two kinds of tests have been shown: pull down and 1-day energy consumption. The results show that HFC134a/HFO1234yf (10/90% weight) is the best drop-in refrigerant fluid for HFC134a in the domestic refrigerator used for the experimental tests. The refrigerant mixture has the closest behaviour to that of HFC134a in terms of temperatures and pressures. Furthermore, the cycle working with the optimal charge of the mixture shows an energy saving of 16 and 14% with respect to HFC134a and HFO1234yf, respectively.     

Nucleate boiling heat transfer coefficients of mixtures containing HFC32, HFC125, and HFC134a

Nucleate boiling heat transfer coefficients (HTCs) of binary and ternary mixtures composed of HFC32, HFC125, and HFC134a on a horizontal smooth tube of 19.0 mm outside diameter were measured. A cartridge heater was used to generate uniform heat flux on the tube. Data were taken in the order of decreasing heat flux from 80 kW m⁻² to 10 kW m⁻² with an interval of 10 kW m⁻² in the pool temperature at 7 °C. HTCs of nonazeotropic mixtures of HFC32/HFC134a, HFC125/HFC134a, and HFC32/HFC125/HFC134a showed a reduction of HTCs as much as 40% from the ideal values while the near azeotropic mixture of HFC32/HFC125 did not show the reduction. Four of the well known correlations were compared against the present data for binary mixtures. Stephan and Körner's and Schlünder's correlations yielded a good agreement with a deviation of less than 10% but they can not be easily extended to multi-component mixtures of more than three components. A new correlation was developed utilizing only the phase equilibrium data and physical properties. A regression analysis was carried out to account for the reduction of HTCs and the final correlation, which can be easily extended to multi-component mixtures of more than three components, yielded a deviation of 7% for all binary and ternary mixtures.     

Boiling heat transfer of HFO-1234yf flowing in a smooth small-diameter horizontal tube

The flow boiling heat transfer coefficient of the low-GWP (global warming potential) refrigerant HFO-1234yf inside a smooth small-diameter horizontal tube (inner diameter: 2 mm) was experimentally investigated. The local heat transfer coefficient was measured at heat fluxes of 6-24 kW m⁻², mass fluxes of 100-400 kg m⁻² s⁻¹, an evaporating temperature of 288.15 K, and an inlet vapor quality of 0-0.25. The results show that the effect of heat flux on the heat transfer was large at low vapor quality, while the effect of mass flux was large at high vapor quality. The heat transfer coefficient of HFO-1234yf was almost the same as that of R-134a. The heat transfer coefficients calculated based on correlations with Saitoh et al. agreed well with the measured values compared to other correlations. The measured pressure drop agreed well with that predicted by the Lockhart-Martinelli correlation.