Condensation heat and mass transfer characteristics of low GWP zeotropic refrigerant mixture R1234yf/R32 inside a horizontal smooth tube: An experimental study and non-equilibrium film model development

https://doi.org/10.1016/j.ijthermalsci.2021.107090Get rights and content

Highlights

  • Condensation heat and mass transfer of R1234yf/R32 was experimentally studied.

  • A non-equilibrium film model was developed considering the mass transfer resistance.

  • Heat transfer deterioration caused by the mass transfer difference was evaluated.

  • Heat transfer degradation coefficient and diffusion flux were reasonably determined.

Abstract

Condensation heat and mass transfer characteristics of low GWP zeotropic refrigerant mixtures R1234yf/R32 (mass fractions of 0.52:0.48 and 0.77:0.23, respectively) inside a horizontal smooth tube (inner diameter 4 mm) were studied experimentally. Moreover, a non-equilibrium film heat and mass transfer model was also developed by considering the mass transfer resistance on the vapor and the liquid side. Effects of mass flux, vapor quality, thermophysical properties and mass fraction on the heat transfer coefficients (HTCs) were analyzed. The heat transfer characteristics, especially the heat transfer deterioration caused by the mass transfer difference of the zeotropic refrigerant mixture, were evaluated. The experiment data show the HTCs of R1234yf/R32 (0.48:0.52) are lower than that of R1234yf at the inlet nearby of the condenser, then that becomes higher gradually than R1234yf due to mass transfer resistance decreasing. The non-equilibrium film model shows good agreement with the experimental results with the mean deviation in HTC of 22.9% at a mass fraction of 0.77:0.23 and 17.8% at a mass fraction of 0.48:0.52. Furthermore, the heat transfer degradation coefficient, interface temperature, diffusion flux of R32 and mass transfer Nusselt number were also reasonably determined with the non-equilibrium film model.

Introduction

The refrigerants from chlorofluorocarbons (CFCs) and hydrochlorofluorocarbons (HCFCs) to hydrofluorocarbons (HFCs) have been evolved all over the world because of the ozone layer destruction. However, most HFC refrigerants have great global warming potential (GWP), for example R134a with a GWP of 1430, which are widely used in automobile air conditioning. And, R410a used in room air-conditioner widely with a GWP of 2088. The refrigerants with GWP higher than 150 are being phased out. With Europe's mobile air-conditioning (MAC) Directive went into force on Jan. 1, 2013, which requires new type-approved cars sold in EU market to use a refrigerant with GWP less than 150, R1234yf has been proposed as a drop-in solution for current automotive air conditioners due to its low GWP of 4 [1], and similar thermophysical properties to those of R134a [2]. Meanwhile, R1234yf is not suitable as a substitute for R410a because of the limitation of thermalphysical properties. Therefore, one of the resolution is the refrigerant mixtures, such as R1234yf/R32, that has a low GWP and higher performance.

Several drop-in experiments on system performance and heat transfer performance measurements on both the flow boiling and condensation processes have been carried out by R1234yf or R1234yf/R32 mixtures for the applications of HVAC&R and waste heat recovery. The flow boiling heat transfer characteristics of R1234yf was evaluated by Saitoh et al. [3], and the heat transfer correlation proposed for R134a was found agreed well with that of R1234yf. An experimental study on the condensation heat transfer of R1234yf by Wang et al. [4] showed the largest deviation of condensation HTC between R1234yf and R134a of 24%, and found that the HTCs could be reasonably predicted using Haraguchi correlation [5]. Xu et al. [6] experimentally studied the heating performance of a heat pump system by using R1234yf/R32 mixture as a working fluid, and they concluded that R1234yf/R32 could operate at the evaporating temperature of −20 °C and it has the highest heating energy efficiency over R1234yf and R32. Yang et al. [7] evaluated the performance of R1234yf/R32 mixture in organic Rankine system for low-grade heat recovery. They found R1234yf/R32 is superior to pure R1234yf and R32 by up to 1.46% and 4.88% from the perspective of thermo-economic performance. The experimental study on flow boiling heat transfer of R1234yf/R32 by Li et al. [8,9]showed the great heat transfer degradation due to the formation of concentration layer around the boiling bubbles and near the evaporation vapor-liquid interface. Jige et al. [10] investigated the flow boiling heat transfer and pressure drop of R1234yf/R32 experimentally inside a horizontal multiport tube with rectangular minichannels. And, the proposed correlation showed good agreement with the HTCs of the R1234yf/R32. Also, thermodynamic properties of R1234yf/R32 mixtures were experimentally measured including vapor-liquid equilibrium and dynamic viscosity at various R32 mass fractions by Kamiaka et al. [11]. The maximum temperature glide at a saturation temperature of 15 °C was found to be 7.7 °C at R32 mass fraction of 22%.

Due to the different boiling points of each component in the refrigerant mixtures, there is a mass transfer resistance in the process of condensation or evaporation. A number of experimental and theoretical studies have been conducted to clarify the heat and mass transfer characteristics of different refrigerant mixtures in condensation. Shao and Granryd [12] conducted experiments on flow condensation with R32, R134a and their mixtures inside a horizontal tube with an inner diameter of 6 mm. They pointed out that the temperature and concentration gradients are the causes of heat transfer degradation. Koyama et al. [13] established a prediction model of the binary refrigerant mixture condensation for R134a/R123 mixtures, which the radial distribution of the mass fraction on the vapor side is considered, while the mass fraction distribution in the liquid film is neglected. Jin et al. [14] developed a condensation heat transfer model of binary zeotropic refrigerant mixtures considering both the liquid and vapor side mass transfers. They validated their model using the experimental data of R134a/R123 mixtures by Koyama et al. [13]. Afroz et al. [15] experimentally studied the local HTCs and pressure drops of pure Dimethyl Ether (DME) and non-azeotropic mixtures of CO2 and DME during condensation inside a horizontal smooth tube. Their research shows that the heat transfer degradation of the mixture at the beginning of the condensation is more severe than that downstream due to mass transfer resistance. Shagawa et al. [16] investigated the effect of mass concentration of R32 on R1234ze(E)/R32 mixtures experimentally and compared with near azeotropic mixture R410A for condensation and evaporation HTCs and pressure drop. They compared experimental results with some correlations. They found that the heat transfer degradation due to the mass transfer resistance results in lower HTCs of refrigerant mixtures compared with that of the pure refrigerant at the same conditions. Garimella et al. [17] measured the condensation HTCs and frictional pressure drop of a zeotropic mixture of R245fa and n-pentane in 7.75 mm smooth horizontal tubes experimentally, and compared with the zeotropic modeling approaches using various correlations. They concluded that the predictions of the zeotropic mixture HTC are heavily dependent on the underlying pure fluid model. Deng et al. [18] presented experimental data of R32/R1234ze(E) in condensation process inside 8 mm horizontal tubes, and proposed a new method to predict condensation HTC of zeotropic mixtures. That showed a good agreement with experimental data covering various mixtures and a wide range of working conditions. Based on the non-equilibrium film theory, Zhang et al. [19] developed an analytical model for condensation heat and mass transfer characteristics of binary zeotropic mixtures inside a horizontal tube, in which the variation of liquid surface tension with concentration was considered. The results showed that the vapor mass transfer resistance is important in the high vapor quality region, and the liquid heat transfer resistance is also more significant in some cases.

In addition, there are also some research on experiments and predictive models of refrigerant mixtures condensation in small-scale tubes. The condensation heat transfer characteristics including heat transfer coefficient and pressure drop of refrigerant mixture R744/R32/R1234ze(E) in a horizontal microfin tube were investigated experimentally by Kondou et al. [20]. Azzolin et al. [21] measured the condensation HTC and two-phase frictional pressure drop of R455A and R452B inside a 0.96 mm minichannel and a 8.0 mm tube, and the results were compared to predictive correlations. They summarized that the effect of the mass transfer resistance is more penalizing at high vapor quality, and in the case of ID 8.0 mm at low mass velocities due to the high temperature glide. Zhang et al. [22] conducted flow condensation experiments of R134a/R245fa in a rectangular micro-channel, and prediction correlations were also developed to evaluate the heat transfer coefficients and frictional pressure gradients, respectively. They found that the impact of temperature glide on the flow pattern transition mainly reflected in the vapor-liquid interface. The condensation heat transfer of R32/R1234ze(E) were investigated experimentally with ID 4.35 mm by Mazumder et al. [23]. And, a condensation HTC correlation was developed considering the mass transfer resistance. An experimental investigation on condensation flow pattern and heat transfer coefficient of methane/ethane mixtures in a horizontal smooth tube with ID 4 mm was carried out by Zhuang et al. [24]. And they proposed an improved heat transfer correlation for zeotropic mixtures based on flow patterns. Qiu et al. [[25], [26]] [[,26] [][25], [26][] studied the condensation heat transfer, frictional pressure drop and refrigerant charge characteristics of R290 in minichannels with different diameters and different inclined angles by numerical simulations. They found that continuing to reduce the diameter will have little effect on the refrigerant charge when the diameter is reduced to a certain value, and changing the inclined angle is an effective approach to reduce the refrigerant charge of R290 in minichannel heat exchangers.

However, the research on the condensation prediction model of R1234yf/R32, especially in small-scale tube, is still limited. And, there are seldom literature about the analysis of the heat transfer degradation related parameters of refrigerant mixtures by modeling, for example, the heat transfer degradation coefficient, interface temperature, the diffusion flux, mass transfer Nusselt number.

In this study, the condensation heat transfer characteristics of R1234yf/R32 with mass fractions of 0.52:0.48 and 0.77:0.23 inside a horizontal smooth tube (ID 4 mm) were studied experimentally. The mass fluxes ranged from 100 to 400 kg m−2 s−1 and saturation temperature was 40 °C. Effects of mass flux, vapor quality and mass fraction on the HTC were analyzed. Also, a non-equilibrium film condensation heat and mass transfer model considering the mass transfer resistances of the vapor and liquid side was developed to predict the measured local HTCs. Moreover, the heat transfer degradation coefficient, interface temperature, diffusion flux and mass transfer Nusselt number which have significant effects on mass transfer were also obtained and analyzed.

Section snippets

Experimental facility

A schematic of the experimental facility is shown in Fig. 1, which includes a test section, sight glasses, expansion valve, evaporator, post-condenser, accumulator, liquid pump, mass flow meter, and water baths. The saturation temperature and pressure of the test section are determined by controlling the expansion valve. The inlet superheating of the test section is adjusted by the water baths, which also providing the coolant for the test section.

Fig. 2 is a detailed diagram of a test

Data reduction

The thermodynamic and transport properties of all the refrigerant mixtures including application in the non-equilibrium film model were obtained by REFPROP 9.0 (Lemmon et al. [27]).

The average HTC hexp of each subsection was calculated by Eq. (1).hexp=qTsTwallwhere q is the average heat flux, Ts is the saturation temperature, Twall is the inner wall temperature. The heat exchange rate Q of coolant was calculated by Eq. (2).Q=cpmw(ToutTin)where Tin is the inlet temperature, Tout is the outlet

Non-equilibrium film model of refrigerant mixtures

Fig. 3 shows the temperature glide of R1234yf/R32 and R134a/R32 with a different mass fractions of R32 at the pressure of 2.0 MPa. The maximum temperature glide of R1234yf/R32 is 6.5 °C (at R32 mass fraction of 0.24) is much larger than that of R134a/R32 (4.97 °C at R32 mass fraction of 0.35). The temperature glide at R32 mass fraction of 0.5 is 4.11 °C and 4.52 °C for the R1234yf/R32 and R134a/R32, respectively.

Fig. 4 shows the non-equilibrium film model of the condensation process of the

Measurement uncertainty

The measurement uncertainty of the HTC is illustrated using a error bar for each experimental data in Fig. 6. The first measurement point near the condenser inlet has the highest measurement uncertainty because of the small temperature difference between the saturation temperature and the wall temperature. When the mass flux is 100 kg m−2 s−1, the maximum uncertainty of the first point can reach 15.2%. The average measurement uncertainty of the HTC for the two refrigerant mixtures at this mass

Conclusions

The condensation experiments were carried out and the non-equilibrium film model was developed for zeotropic mixture R1234yf/R32 (0.77:0.23 and 0.48:0.52) at the mass flux ranging from 100 to 400 kg m−2 s−1 in a horizontal tube within ID 4 mm. The main conclusions are as follows:

  • (1)

    The heat transfer coefficients (HTCs) of R1234yf/R32 could be lower than that of R1234yf at low mass flux and early stage of condensation due to the large mass transfer resistance.

  • (2)

    The heat transfer degradation could be

Declaration of competing interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgments

This study was sponsored by the “Development of Non-fluorinated Energy-Saving Refrigeration and Air Conditioning Systems” project of the New Energy and Industrial Technology Development Organization of Japan (No. 106716), and supported by the National Natural Science Foundation of China (No. 51806151) and the Natural Science Foundation of Tianjin City (No. 20JCQNJC00600).

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