冷媒R-134a與HFO-1234yf在平滑管內之流動沸騰熱流分析與比較

An experimental investigation of flow boiling heat transfer coefficient and pressure drop of R410A in various minichannel multiport tubes

An experimental investigation of refrigerant R-134a two-phase flow condensation heat transfer coefficient and pressure drop in condenser tube section of refrigeration system under different operating conditions is presented. The experimental and theoretical investigations are based on test conditions in range of 10 -17 kW/m2 for heat flux, 42-63 kg/m2s for mass flux, vapor quality 1-0.03 and saturation temperature 44 to 49˚C. The experimental tests are conducted on test rig supplied with a test section to simulate the water cooled double pipe heat exchanger, which is designed and constructed in the present work. “The experimental results have revealed that, the heat flux and mass flux have significant impacts on the heat transfer coefficient. “The heat transfer coefficient was increased with increase in heat flux and mass flux at prescribed test conditions, where the enhancement in heat transfer coefficient was about 47% and 14% for relatively higher heat flux and mass flux, respectively. “The enhancement in the heat transfer coefficient was about 51% for relatively lower saturation temperature 45.97˚C and 43% for higher vapor quality 0.88 compared to other values at constant test conditions. “The pressure drop was higher in the range of 12% and 49% for relatively higher mass flux and heat flux respectively. “The present work results have validated by comparison with predictive models and with similar research work results and the comparison has revealed an acceptable agreement.

• Flow boiling heat transfer of R32 in herringbone and dimple enhanced tubes are investigated. • Effects of mass flux, vapor quality and saturation temperature on heat transfer are investigated. • Heat transfer coefficient correlations for flow boiling in the enhanced tubes are developed. This paper experimentally investigates the flow boiling heat transfer characteristics of smooth tubes (ST) and advanced new enhanced tubes (HT and EHT) made of aluminum, using R32 refrigerant. The effects of the tubes on heat transfer coefficients (HTC) and frictional pressure drop during flow boiling were explored. The study examined the flow boiling heat transfer performance of the enhanced tubes and ST tubes under varying conditions of mass flux (100∼350 kg/m 2 s), saturation temperatures (279∼288 K), and refrigerant inlet vapor quality (0.1∼0.9), and analyzed the impact of the enhancement structures on flow boiling heat transfer. The results are that the HTC increases with rising saturation temperature, mass flux, and average vapor quality, with the HT tube exhibiting the best HTC, followed by the EHT tube. The result is that the frictional pressure drop increases with mass flux and average vapor quality and decreases with saturation temperature. Among them, the EHT tube exhibits the highest frictional pressure drop, while the pressure drop of the HT tube is closer to that of the ST tube. Additionally, four flow boiling heat transfer correlations were predicted the HTC of the ST tube, and the correlation with the best prediction results was selected to predict the HTC for the HT and EHT tubes. The revised correlation can verify over 95 % of the data within a ± 15 % range, establishing a new HTC correlation for enhanced tubes.

The characteristics of evaporation heat transfer and pressure drop for refrigerant R134a flowing in a plate heat exchanger were investigated experimentally in this study. Two vertical counter flow channels were formed in the exchanger by three plates of commercialized geometry with a corrugated sine shape of a chevron angle of 60°. Upflow boiling of refrigerant R134a in one channel receives heat from the hot downflow of water in the other channel. The effects of the heat flux, mass flux, quality and pressure of R134a on the evaporation heat transfer and pressure drop were explored. The preliminary measured data for the water to water single phase convection showed that the heat transfer coefficient in the plate heat exchanger is about 9 times of that in a circular pipe at the same Reynolds number. Even at a very low Reynolds number, the present flow visualization in a plate heat exchanger with the transparent outer plate showed that the flow in the plate heat exchanger remains turbulent. Data for the pressure drop were also examined in detail. It is found that the evaporation heat transfer coefficient of R134a in the plates is quite different from that in circular pipe, particularly in the convective evaporation dominated regime at high vapor quality. Relatively intense boiling on the corrugated surface was seen from the flow visualization. More specifically, the present data showed that both the evaporation heat transfer coefficient and pressure drop increase with the vapor quality. At a higher mass flux the pressure drop is higher for the entire range of the vapor quality but the heat transfer is only better at high quality. Raising the imposed wall heat flux was found to slightly improve the heat transfer. While at a higher system pressure the heat transfer and pressure drop are both slightly lower.

For the past few years, research in the field of flow boiling heat transfer has gained immense popularity for unravelling the dominant mechanism responsible for controlling heat transfer and identifying a parametric trend for understanding the characteristics of flow boiling heat transfer. This has led to several assumptions and models for predicting heat transfer during flow boiling without any known generalized mechanism. This study therefore seeks to experimentally study the characteristics of heat transfer during flow boiling over a wide range but small increase in vapor quality from a single-phase subcooled region through to a two-phase superheated vapor region. The study was performed with an R134a refrigerant in a single horizontal circular stainless-steel smooth tube that had an internal diameter of 5 mm. In this experimental study, local heat transfer coefficients and frictional pressure drop were measured for low heat fluxes of 4.6–8.5 kW/m2, mass fluxes of 200–300 kg/(m2s), vapor quality from −0.1 to 1.2 and a low constant saturation pressure of 460 kPa. Flow patterns observed during the study were recorded with a high-speed camera at 2000 fps. In covering a wide range of vapor quality, a peak of heat transfer coefficient near a vapor quality of zero and a local minimum observed in the low vapor quality region were observed, and both were sensitive to heat flux and mildly sensitive to mass flux. Generally, at low vapor quality, the heat transfer coefficient deteriorated with vapor quality and this was sensitive to heat flux but insensitive to mass flux and vapor quality, indicating nucleate boiling dominance in low vapor quality regions. In high vapor quality regions, the heat transfer coefficient was sensitive to mass flux and insensitive to heat flux. This indicates the dominance of convective boiling. In the low vapor quality regions, the flow patterns observed were slug and intermittent, while in the high vapor quality region, annular and mist flow patterns were observed. Generally, frictional pressure drop increased with increasing mass flux and vapor quality in the two-phase region.

The evaporation heat transfer coefficients and pressure drops of R-410A and R-134a flowing through a horizontal-aluminium rectangular multiport mini-channel having a hydraulic diameter of 3.48 mm are experimentally investigated. The test runs are done at refrigerant mass fluxes ranging between 200 and 400 kg/m2s. The heat fluxes are between 5 and 14.25 kW/m2, and refrigerant saturation temperatures are between 10 and 30 °C. The effects of the refrigerant vapour quality, mass flux, saturation temperature and imposed heat flux on the measured heat transfer coefficient and pressure drop are investigated. The experimental data show that in the same conditions, the heat transfer coefficients of R-410A are about 20–50% higher than those of R-134a, whereas the pressure drops of R-410A are around 50–100% lower than those of R-134a. The new correlations for the evaporation heat transfer coefficient and pressure drop of R-410A and R-134a in a multiport mini-channel are proposed for practical applications.

Experimental study of boiling heat transfer in smooth/micro-fin tubes of four refrigerants

Condensation heat transfer of R410A on outside of horizontal smooth and three-dimensional enhanced tubes

This work evaluates the evaporation heat transfer of R455A, a low Global Warming Potential (GWP) refrigerant, in a horizontal tube. As high-GWP refrigerants like R410A and R404A are being phased down under stricter environmental regulations, R455A has gained attention as a potential alternative. With a GWP of around 146, it not only meets regulatory requirements but also offers thermodynamic properties comparable to R404A. However, experimental data for small-diameter tubes under low saturation temperature (pressure) and low heat flux conditions remain limited. Therefore, this work investigates the evaporation heat transfer coefficient (HTC) of R455A under conditions representative of compact systems and low-temperature operation. Experiments were carried out in a 1 m long smooth horizontal tube with an inner diameter of 4.6 mm. In the experiments, the parameters covered were mass flux (200–400 kg/m2·s), heat flux (10–20 kW/m2), and saturation temperature (–10 °C to 10 °C), while the vapor quality (x) varied from 0.1 to 0.9 for HTC measurement. It was observed that HTC decreased with increasing saturation temperature but increased with both mass flux and vapor quality. The pressure drop decreased with higher saturation temperature and increased with higher mass flux. Relative to other correlations, the Kim and Mudawar (2013) model showed superior prediction accuracy. These findings provide experimental evidence supporting the practical applicability of R455A as an alternative refrigerant.

An empirical investigation on flow pattern, heat transfer, and pressure drop during flow boiling of R1234yf in an inclined plain tube

Experimental investigation on the flow condensation of R450A, R515B, and R1234ze(E) in a 7.0 mm OD micro-fin tube

A comprehensive criterion for slug-annular flow transition based on flow boiling of R134a in microchannels

Investigation of two phase heat transfer and pressure drop of propane in a vertical circular minichannel

The current study presents the experimental results for the boiling heat transfer coefficient of R134a flowing inside a 5 mm inner diameter, smooth horizontal tube. The study cases are set for pressures ranging from 5 to 10 bar, and mass fluxes from 300 to 400 kg/m2s. The heat transfer coefficients are measured at vapor qualities from 0.01 to 1.3, particularly in small quality increments over the high quality region. The experimental heat transfer coefficients are compared with the predicted results by the method of Wojtan et al. [1,2]. The method gives a good prediction in the quality range 0.1–0.7. There exist large errors in the high quality region, and the maximum value can reach −90%. The method is modified using the correlation by Del Col et al. [2] for dryout inception and a new proposed approach for the dry angle. The modified method improves the predicted results with good estimation in the quality range 0.1–0.8, and the maximum error in the high quality region is reduced to −60%.

Experimental analysis for the determination of the convective heat transfer coefficient by measuring pressure drop directly during annular condensation flow of R134a in a vertical smooth tube