Tribological analysis of hydrocarbon refrigerants applied to the hermetic compressor

This paper describes the tribological impacts of hydrocarbon refrigerants deployed in the domestic refrigerator hermetic compressor. In-use durability is examined from a tribological viewpoint. Experimental tribological information is presented from physical test procedures involving sliding tests to establish wear mechanisms and friction coefficients within critical components. Hydrocarbon refrigerant R600a is compared with hydroflourocarbon R134a using aluminium on steel samples within a novel pressurised micro-friction test rig. The refrigerant R600a is tested for its influence upon the tribological performance of mineral oil (MO) and poly-ol-ester (POE) lubricant, whilst an R134a/POE charge combination is used as a benchmark. Although wear rates were significantly greater for samples utilising POE lubricants than for MO, the friction coefficients were much lower.     

Evaporation heat transfer and pressure drop characteristics of r-134a in the oblong shell and plate heat exchanger

The evaporation heat transfer coefficienthr and frictional pressure drop δp_f of refrigerant R-134a flowing in the oblong shell and plate heat exchanger were investigated experimentally in this study. Four vertical counterflow channels were formed in the oblong shell and plate heat exchanger by four plates of geometry with a corrugated sinusoid shape of a 45° chevron angle. Upflow of refrigerant R-134a boils in two channels receiving heat from downflow of hot water in other channels. The effects of the refrigerant mass flux, average heat flux, refrigerant saturation temperature and vapor quality of R- 134a were explored in detail. Similar to the case of a plate heat exchanger, even at a very low Reynolds number, the flow in the oblong shell and plate heat exchanger remains turbulent. The results indicate that the evaporation heat transfer coefficienthr and pressure drop Δp_f increase with the vapor quality. A rise in the refrigerant mass flux causes an increase in theh _r and Δp_f. But the effect of the average heat flux does not show significant effect on the h_r and Δp_f. Finally, at a higher saturation temperature, both theh _r and Δp_f are found to be lower. The empirical correlations are also provided for the measured heat transfer coefficient and pressure drop in terms of the Nusselt number and friction factor.     

An experimental study on convective boiling of R-22 and R-410A in Horizontal smooth and micro-fin tubes

Evaporation heat transfer coefficients and pressure drops were measured for smooth and micro-fin tubes with R-22 and R-410A. Heat transfer measurements were performed for 3.0 m long horizontal tubes with nominal outside diameters of 9.52 and 7.0 mm over an evaporating temperature range of ?15 to 5°C, a mass flux range of 68 to 211 kg/m²s, and a heat flux range of 5 to 15 kW/m². It was observed that the heat transfer coefficient increased with mass flux. Evaporation heat transfer coefficients of R-22 and R-410A increased as the evaporating temperature dropped at a lower heat flux. Generally, R-410A showed the higher heat transfer coefficients than R-22 in the range of low mass flux, high heat flux and high evaporating temperature. Pressure drop increased with a decrease of evaporating temperature and a rise of mass flux. Pressure drop of R-22 was higher than that of R-410A at the same mass flux.     

Electrohydrodynamic(ehd) enhancement of boiling heat transfer with a lo-fin tube

The effect of D. C electric field on nucleate boiling heat transfer for refrigerants of R-11, HCFC-123 and FC-72 was investigated experimentally by using a single lo-fin tube shell-and-tube heat exchanger. The lo-fin tube which brought two times increase in the heat transfer area provided about 150% of boiling heat transfer enhancement compared to that of smooth surface. This experimental study has revealed that the electrical charge relaxation time was an important parameter for the boiling heat transfer enhancement under electric field. Boiling heat transfer enhancement was obtained up to 40% for R-11 which had moderate relaxation time of 1.3s. However remarkable boiling heat transfer enhancement has been obtained up to three fold increase(300%) for HCFC-123 which has the electrical charge relaxation time of 0.89 x 10^-3s. For FC-72 having longer relaxation time than the bubble detachment one, no appreciable effect on the nucleate boiling heat transfer was observed.     

Effect on boiling heat transfer of horizontal smooth minichannel for R-410A and R-407C

An experimental study of boiling heat transfer with refrigerants R-410A and R-407C is presented. The present paper is focused on pressure drop and boiling heat transfer coefficient of the refrigerants inside a horizontal smooth minichannel. To evaluate the diameter size effect on pressure and heat transfer characteristics, minichannels with inner diameters of 1.5 mm and 3.0 mm and with lengths of 1500 mm and 3000 mm respectively are used. The pressure drop increases with mass flux and heat flux for both inner tube diameters and for both the refrigerants. The pressure drop of R-407C is higher than that of R-410A, but the heat transfer coefficient of R-410A is higher than of R-407C at the low quality region. The heat transfer coefficient in the tube with an inner diameter of 1.5 mm is higher than that of 3.0 mm diameter tube at the low quality region. The comparison of present heat transfer coefficient with the predictions of some previous correlations shows a large deviation. Therefore, there is a necessity to develop a new correlation.     

Two-phase flow heat transfer of R-134a in microtubes

The characteristics of the two-phase flow heat transfer of R-134a in microtubes with inner diameters of 430 μm and 792 μm were experimentally investigated. The effect of the heat flux on the heat transfer coefficient for microtubes was significant before the transition quality. The boiling number expressed the interrelation between the heat flux and the mass about the heat transfer coefficients. The smaller microtube had greater heat transfer coefficients; the average heat transfer coefficient for the tube A (D _i = 430 μm) was 47.0% greater than that for the tube B (D _i = 792 μm) at G = 370 kg/m²·s and q″ = 20 kW·m². A new correlation for the evaporative heat transfer coefficients in microtubes was developed by considering the following factors: the laminar flow heat transfer coefficient of liquid-phase flow, the enhancement factor of the convective heat transfer, and the nucleate boiling correction factor. The correlation developed in present study predicted the experimental heat transfer coefficients within an absolute average deviation of 8.4%.     

Enhancement of pool boiling heat transfer coefficients using carbon nanotubes

In this study, the effect of carbon nanotubes (CNTs) on nucleate boiling heat transfer is investigated. Three refrigerants of R22, R123, R134a, and water were used as working fluids and 1.0 vol.% of CNTs was added to the working fluids to examine the effect of CNTs. Experimental apparatus was composed of a stainless steel vessel and a plain horizontal tube heated by a cartridge heater. All data were obtained at the pool temperature of 7°C for all refrigerants and 100°C for water in the heat flux range of 10–80 kW/m². Test results showed that CNTs increase nucleate boiling heat transfer coefficients for all fluids. Especially, large enhancement was observed at low heat fluxes of less than 30 kW/m². With increasing heat flux, however, the enhancement was suppressed due to vigorous bubble generation. Fouling on the heat transfer surface was not observed during the course of this study. Optimum quantity and type of CNTs and their dispersion should be examined for their commercial application to enhance nucleate boiling heat transfer in many applications.     

Condensation heat transfer coefficients of HFC245fa on a horizontal plain tube

Condensation heat transfer coefficients (HTCs) of HCFC22, HCFC123, HFC134a and HFC245fa are measured on a horizontal plain tube 19.0 mm outside diameter. All data are taken at the vapor temperature of 39°C with a wall subcooling temperature of 3–8°C. Test results show the HTCs of newly developed alternative low vapor pressure refrigerant, HFC245fa, on a smooth tube are 9.5% higher than those of HCFC123, while they are 3.3% and 5.6% lower than those of HFC134a and HCFC22, respectively. Nusselt’s prediction equation for a smooth tube underpredicts the measured data by 13.7% for all refrigerants, while a modified equation yielded 5.9% deviation against all measured data. From the view point of environmental safety and condensation heat transfer, HFC245fa is a long-term good candidate to replace HCFC123 used in centrifugal chillers.     

Measurement and correlation of boiling heat transfer coefficient of R-1234yf in horizontal small tubes

We report experimental data of boiling heat transfer of R-1234yf in horizontal small tubes. The experimental data obtained in the horizontal circular small tubes of 1.5 and 3.0 mm inner diameter, the lengths of 1000 and 2000 mm, the mass flux range from 200–650 kg/m²s, the heat flux range from 5–40 kW/m² and saturation temperature of 10 and 15°C, was used to develop a modified correlation for the heat transfer coefficient. The flow pattern of the experimental data was mapped and analyzed with existing flow pattern maps. The heat transfer coefficient was also compared with some well-known correlations.     

微尺度通道内混合物流动沸腾特性研究

对非共沸混合工质R32／R134a(25％／75％)在微尺度管内的流动沸腾换热特性进行了试验研究。试验结果表明,在较高热流密度下,微尺度管内流动沸腾换热与质量干度和质量流量基本无关,热流密度对换热有着很大的影响,在较宽的热流密度范围内,核态沸腾在换热过程中占据主导地位。和细小管道相比,在相同条件下,微尺度管道内的流动沸腾表面传热系数高于细小管道。     

替代制冷剂流动沸腾传热实验台设计建造与探讨

以R134a为流动工质,设计并建造了流动沸腾传热实验台,从原理设计到调试运行作了详细介绍,并探讨了用材与焊接、密封与保温、特殊加工与制作以及附件布置等环节的流体与机械相关技术问题.该实验台可作为两相流动和沸腾传热特性以及流体模化技术实验研究平台.     

Prediction of forced convective boiling heat transfer coefficient of pure refrigerants and binary refrigerant mixtures inside a horizontal tube

Forced convective boiling heat transfer coefficients were predicted for an annular flow inside a horizontal tube for pure refrigerants and nonazeotropic binary refrigerant mixtures. The heat transfer coefficients were calculated based on the turbulent temperature profile in liquid film and vapor core considering the composition difference in vapor and liquid phases, and the nonlinearity in mixing rules for the calculation of mixture properties. The heat transfer coefficients of pure refrigerants were estimated within a standard deviation of 14% compared with available experimental data. For nonazeotropic binary refrigerant mixtures, prediction of the heat transfer coefficients was made with a standard deviation of 18%. The heat transfer coefficients of refrigerant mixtures were lower than linearly interpolated values calculated from the heat transfer coefficients of pure refrigerants. This degradation was represented by several factors such as the difference between the liquid and the overall compositions, the conductivity ratio and the viscosity ratio of both components in refrigerant mixtures. The temperature change due to the concentration gradient was a major factor for the heat transfer degradation and the mass flux itself at the interface had a minor effect.     

Swirl flow post-CHF heat transfer with refrigerant 113

Heat transfer rate was experimentally determined in the post-CHF region of a steady-state two-phase flow of a refrigerant in a vertical tube with swirl induced by twisted-tape inserts. Experiments were performed with the vertical flow of refrigerant-113 in a tube with inside diameter of 7.75 mm, a heated length of 3.66 m and mass flux of 375–535 kg/m²s for swirl flow at a pressure of 0.184 MPa. Four tapes were used with twist-ratio of 2.5 to 9.2 for swirl flow. Liquid heating produced the low wall-superheat in the post-CHF region at steady-state, which is typical of heat exchanger operation. Superheated vapor measured at the test section exit in most tests ensured that entire post-CHF region was included. All refrigerant-113 data were compared with the data of water and refrigerant-12. The existing post-CHF heat transfer correlation of swirl flow was modified to predict the magnitude and trends of the data of the three fluids such as water, R-12 and R-113.     

The performance analysis of a hydrocarbon refrigerant R-600a in a household refrigerator/freezer

The performance of a hydrocarbon refrigerant, R-600a, as an alternative for R-12 has been evaluated in a 215 ℓ household auto-defrost refrigerator/freezer. A theoretical analysis was performed with NIST REFPROP, based on the ASHRAE refrigeration cycle and a series of tests with R-600a was conducted according to the Korea Standard (KS C 9305). All the tests were performed in the climate chamber of which the temperatures and relative humidities were maintained at 30±1°C and 75±5%, respectively. The test results showed that the energy efficiencies and the cooling speeds with R-600a were improved by 1–11% and 3–10%, respectively, compared to R-12.     

Lubricity of environmentally friendly HFO-1234yf refrigerant

Global warming concerns have triggered the interest in alternative refrigerants for air-conditioning and refrigeration compressors. Newly developed refrigerant HFO-1234yf is an attractive and environmentally friendly candidate to replace existing hydrofluorocarbon refrigerants, such as R-134a. Herein, the tribological compatibility of gray cast iron material with this new refrigerant was investigated under aggressive unlubricated conditions. HFO-1234yf exhibits excellent performance compared to R-134a due to the formation of a fluorine-containing protective tribolayer on the topmost surface as X-ray photoelectron spectroscopy (XPS) analysis, inside and outside the wear tracks, revealed. The morphology of the layer was also studied using scanning electron microscopy (SEM) studies.     

CO_2/润滑油混合物在水平管内流动沸腾换热的试验研究

为了测试润滑油对CO2流动沸腾换热特性的影响,以指导CO2换热器的设计,对CO2/润滑油混合物在水平管内的流动沸腾换热系数进行了试验研究,试验工况质量流量为2.74~5.61kg/h,热流密度为3.2~5k W/m2,测试段入口干度为x=0.2~0.5,蒸发温度在-4~8℃之间,选择PAG作为润滑油,浓度为0~6%。试验结果表明,润滑油浓度越大,CO2的局部换热系数越小;润滑油浓度较低时(<3%),换热系数下降较大,再增大含油量,换热系数下降的趋势减缓。增大蒸发温度可以延迟干涸的发生,相反地,大的热流密度和质量流量可以使管内提前出现干涸。CO2/润滑油混合物的换热系数随蒸发温度的升高而增大,随热流密度和质量流量的增大而减小。     

Structure effect of refrigeration polyolester oils for use in HFC-134a compressors

A study of polyolester oils (POE) for compressors using an ozone-friendly refrigerant is presented here. The effect of the chemical structure of POE was studied in relation to three lubricant properties: miscibility with HFC-134a (CF₃-CH₂F), lubricity (steel-on-steel and aluminium-on-steel contacts), and viscosity. The paper shows the strong structure effect of POE on miscibility and lubricity. Based on the data obtained, some model lubricants were blended, and the performance of these lubricants was assessed on refrigeration test rigs.     

Effects of saturation temperature variation due to pressure drop of working fluid in heat exchanger on heat transfer performance

The effect of the saturation temperature drop on the heat transfer performance of a heat exchanger was analyzed under air-conditioner condensing condition, air-conditioner evaporating condition, and refrigerator evaporating condition. The thermodynamic analysis results show that the heat transfer capacity due to the pressure drop of the saturated refrigerant was at least 2.3 % and at most 91.1 % compared to the evaluated heat transfer capacity assuming no pressure loss. The rate of change of heat transfer capacity was the largest in the order of R600a, R1234yf, R134a, R410A, and R32. Heat exchanger performance simulation under practical air-conditioner operating conditions showed that the heat transfer capacity was reduced by 0.72 % due to refrigerant pressure drop under the condensing condition. On the other hand, the heat transfer capacity was increased by 26.55 % under the evaporating condition.

     

A New High-Pressure Viscometer for Oil/Refrigerant Solutions and Preliminary Results

The reliability and efficiency of refrigeration compressors is dependent on the lubricating effectiveness of the refrigeration oil, which is necessarily a solution of oil and refrigerant. In this article, the development of a high-pressure viscometer that can operate at elastohydrodynamic lubrication (EHL) inlet pressures for oil/refrigerant solutions is explained in detail. New viscosity measurements are presented for two compressor oils to 1.2 GPa. Preliminary results are given for a polyol ester with two concentrations of R134a refrigerant to 0.35 GPa. These are the first measurements of oil/refrigerant solutions to such high pressure. Temperature–pressure–viscosity correlations are applied to all materials for use in modeling. Correlations are provided for the temperature and pressure dependencies of the viscosity of the oils, refrigerant, and mixtures. The definition of pressure–viscosity coefficient is a pressing problem for elastohydrodynamics.     

Performance comparison of microchannel evaporators with refrigerant R-22

An experimental study on the performance comparison of microchannel evaporators with refrigerant R-22 was conducted. Six microchannel evaporators were designed and manufactured for a residential air-conditioner. They were tested with psychrometric calorimeter test facilities. The experiment was performed with both vapor compression system and refrigerant circulation system. Each evaporator was made up of two parallel flow heat exchangers connected with several return pipes. The parallel flow heat exchanger had 41 microchannel tubes inserted between inlet and outlet headers. The microchannel tube had 8 rectangular ports with the hydraulic diameter of 1.3 mm. For the vapor compression system, the flow area ratio and the number of return pipes had a great effect on the cooling capacity. Type 3 with a flow area ratio of 73% and 58% showed the best cooling capacity. It had 12 return pipes and 3 circuits. There is a merging manifold in it. The effect of the number of circuits and merging manifold on the cooling capacity was relatively small. For the refrigerant circulation system, the effect of the mass flow rate on the cooling capacity was slightly superior to that of inlet quality. The effect of the number of circuits on the cooling capacity was different from the result of the vapor compression system. The effect of merging manifold was negligible, which was consistent with the result of the vapor compression system. The cooling capacity proportionally increased as the vertical inclination angle of the evaporator increased due to gravity force.