Experimental investigation on the heat transfer and pressure drop characteristics of R600a in a minichannel condenser with different inclined angles
Introduction
With the market launch of a new generation of chlorine free refrigerants, the refrigeration and air-conditioning industries have promoted the conversion from CFC and HCFC to HFC and natural refrigerants since 2004 [1]. As a natural refrigerant, R600a has zero ozone depletion potential and quite low global warming potential, which is considered as one of the most appropriate alternative refrigerants in the refrigeration and air conditioning devices [2]. In addition, thermodynamic characteristics of refrigerant in the heat exchanger strongly influence the overall efficiency of a refrigeration system. Recent years, the high-efficiency and compact condenser had been widely used in refrigeration systems [3]. The heat transfer and pressure drop characteristics of R600a in a condenser are important prerequisites for evaluating and optimizing the condenser. Therefore, comprehensive experimental investigation on flow condensation heat transfer and pressure drop of R600a in a compact condenser is very important.
Over the past few decades, several studies have been conducted on R600a single tube condensation. Darzi et al. [4] studied heat transfer and pressure drop characteristics during condensation of R600a in a horizontal plain tube and different flattened channels. Round copper tubes of 8.7 mm were deformed into flattened channels with different interior heights of 6.7 mm, 5.2 mm and 3.1 mm as test sections. Results indicate that flattening the tubes causes significant enhancement of heat transfer coefficient which is also accompanied by simultaneous augmentation in flow pressure drop. Lee et al. [5] examined the condensing pressure drop of R600a in tubes with inner diameters of 9.52 mm and 12.7 mm, and it was compared with that of R22. Results showed that pressure drop of the R600a are higher than that of R22 under similar test conditions. Wen et al. [6] performed an experiment to research the characteristics of sub-cooled flow boiling heat transfer and pressure drop of R600a. Experimental results showed that mass flux and heat flux have positive effects on heat transfer coefficient, while vapor quality has a negative effect on heat transfer coefficient. The heat transfer coefficient in tube with porous inserts is larger than that in the open tube. With respect to the open tube, the pressure drop greatly increases in the porous inserts. Yang et al. [7] carried out an experimental study to investigate the flow boiling heat transfer and pressure drop characteristics of R600a in a smooth horizontal tube with an inner diameter of 6 mm. They showed that the heat transfer coefficient is nearly independent of mass flux in low vapor quality region, while it increases with heat flux in low vapor quality region. But this increasing tendency tends to be suppressed in high vapor quality region. As for two-phase frictional pressure drop, it rises with mass flux, while reduces with saturation pressure. Ağra and Teke [8] conducted condensation studies on R600a in a 4 mm tube at low mass fluxes of 47 and 116 kg∙(m2∙s)−1, saturation temperatures between 30 and 43℃.Their results showed that a change in the saturation temperature of the working fluid has almost no effect on the heat transfer coefficient. An inverse relationship between temperature difference and heat transfer coefficient was reported.
Previous researches on inclined tubes also found that the heat transfer coefficient and pressure drop affected by the inclined angle. Nitheanandan and Soliman [9] published an experimental study on flow condensation heat transfer in an inclined tube. Condensation experiments were carried out in an inclined pipe with inner diameter of 18 mm with benzene as the working fluid, and the inclined angle is 15°. The experimental results found that the heat transfer coefficient is greatly affected by inclination. Mohseni et al. [10] carried out condensation experiments of different heat exchanges with inclined angles between −90° and +90°, the mass flux ranged from 53 to 212 kg∙(m2∙s)−1. Results indicated that the heat transfer coefficient changes significantly with inclined angle, and the best heat transfer performance was obtained at the inclined angle of + 30°. The condensation heat transfer of the R245fa in a slight angle shell-and-tube heat exchanger was investigated by Cao et al. [11]. It was found that a slight inclination in the upward and downward flow is better for the condensation heat transfer. However, the downward flow is more stable than the upward flow. Yildiz et al. [12] also reported that inclined angle had a great influence on the condensation heat transfer. The smaller inclination of the vertical direction causes a better heat transfer performance with the best inclination angle of 30°.
As mentioned above, although considerable researches have been conducted on in-tube condensation heat transfer and pressure drop of R600a, most of them only focus on the single tube. The overall heat transfer performance of a condenser especially for an air-cooling condenser was seldom involved. In the present work, a heat exchanger performance test system was set up. The heat transfer performance and pressure drop of a minichannel condenser with multi-louvered fins was experimentally tested by this system. The experimental data were compared with several well-known condensation heat transfer models. Based on analysis results, a modified calculation method of the average temperature of the refrigerant was proposed to calculate the logarithmic average temperature difference. In addition, the improved correlations for heat transfer and friction factor were proposed. Moreover, there is still a lack of in-depth study on the effects of the installation angle (inclined angle) on the performance of a whole heat exchanger. Experiments with different inclined angles of heat exchanger were carried out in this paper. The inclined angle of the condenser changed from 0° to 180° in horizontal and vertical directions, respectively.
Section snippets
Test facility and method
Fig. 1 shows the schematic view of the heat exchanger performance test system. It consists of three main subsystems: the test circuit, pressure control unit for the reservoir (shown by the red line), and temperature control unit for the preheater 2 (shown by the blue line). The test circuit mainly includes compressor, preheater 2, tested condenser, auxiliary condenser, reservoir, sub-cooler, dry-filter, mass flow meter, preheater 1, expansion valve, tested evaporator and auxiliary evaporator.
Heat transfer
The heat transfer capacity of the condenser can be calculated by Eq. (1):Where Qc is the heat transfer capacity of the condenser, W; ṁ is the mass flow rate of the refrigerant flowing through the condenser, g∙s−1; and are the specified enthalpy of the test fluid in the inlet and outlet of the condenser, respectively, which are determined by the corresponding pressure and temperature, kJ∙kg−1.
The overall heat transfer coefficient obtained in the experimental test is
Verification of the reliability of the experimental system
The variation of the refrigerant mass flux with time at different opening degrees of the electronic expansion valve is shown in Fig. 3. It is obvious that the mass fluxes of the refrigerant are stable at 22.5 kg∙(m2∙s)−1, 25 kg∙(m2∙s)−1 and 36.25 kg∙(m2∙s)−1, when the opening of electronic expansion valve are 70%, 72% and 79%, respectively. The fluctuation range does not exceed 5.6%. In conclusion, the mass flux of the refrigerant in the condenser is precisely controlled by the electronic
Conclusions
In the present work, a heat exchanger performance test system was set up. Heat transfer and pressure drop characteristics of R600a in a compact minichannel heat exchanger with multi-louvered fins were experimentally investigated. Experiments were performed with the mass fluxes from 25 to 41.25 kg∙(m2∙s)−1, saturation pressures from 530 to 620 kPa. The reliability of the test system was confirmed by repeated experiments and comparisons with public data. The effects of mass flux, saturation
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.
Acknowledgements
This work is supported by the National Natural Science Foundation of China under contract No. 51876094.
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