The influence of a low viscosity oil on the pool boiling heat transfer of the refrigerant R507Influence d'une huile à faible viscosité sur le transfert de chaleur lors de l'ébullition libre du frigorigène R507

This paper describes the influence of a low viscosity polyolester based lubricating oil on the pool boiling heat transfer of the refrigerant R507. The pool boiling heat transfer coefficients for this refrigerant–oil mixture are measured on a smooth tube and on an enhanced tube. The investigation is made for oil mass fractions up to 10% and for saturation temperatures between −28.6°C and +20.1°C. For the smooth tube the heat transfer increases for increasing oil mass fractions up to 3% at lower saturation temperatures. At higher saturation temperatures the heat transfer decreases for increasing oil mass fractions for both tubes. For oil mass fractions greater than 1% at the higher saturation temperatures a range of decreasing heat transfer coefficient is found for increasing heat flux. The effect is caused by the different miscibility of the oil and the components of the refrigerant mixture.     

Coefficients d'échange locaux au cours de l'ébullition du R22 et du R407C dans des tubes horizontaux, lisse ou micro-ailetéLocal heat transfer coefficients during boiling of R22 and R407C in horizontal smooth and microfin tubes

The purpose of this study is to experimentally investigate forced convective boiling. The heat transfer coefficients of pure refrigerant R22 and non azeotropic refrigerant mixture R407C were measured in both a smooth tube and a microfin tube. The tests have been carried out with a uniform heat flux all along the tube length. The refrigerant mass flux was varied from 100 to 300 kg m⁻² s⁻¹ and heat fluxes from 10 to 30 kW m⁻². Local heat transfer coefficients depend strongly on heat flux at a low quality and on mass fluxes at a high quality. When compared to smooth tube, the microfin tubes exhibit a significant heat transfer enhancement, up to 180%. In comparison to R22, the R407C heat transfer coefficients of smooth and microfin tubes are 15 to 35% lower, respectively. The best heat transfer enhancement is obtained at low heat flux and mass flow rate.     

Numerical analysis of evaporation performance in a finned-tube heat exchanger

This study discusses the effects of the heat exchanger type, refrigerant, inner tube configuration, and fin geometry on evaporator performance by adopting updated correlations of EVSIM, a numerical analysis model based on the tube-by-tube method developed by Domanski. The heat exchanger types considered are the cross-counter flow type and cross-parallel flow type. The refrigerants considered for the numerical test as a working fluid are R-134a, R-410A and R-22. For inner tube configuration, enhanced tube and smooth tube cases are considered. For the air side evaporation performance, heat exchangers using plate fins, wavy fins and slit fins are analyzed. Results show that the heat transfer rate of the cross-counter flow type heat exchanger is 3% higher than that of the cross-parallel flow type with R-22. The total heat transfer rate of the evaporator using R-410A is higher than those using R-22 and R-134a, while the total pressure drop of R-410A is lower than those of R-22 and R-134a. The heat transfer rate of the evaporator using enhanced tubes is two times higher than that using smooth tubes, but the pressure drop of the enhanced tube is 45–50% higher than that of the smooth tubes. The evaporation performance of slit fins is superior to that of plate fins by 54%.     

Numerical study on heat transfer and entropy generation of developing laminar nanofluid flow in helical tube using two-phase mixture model

Nanofluids and helical tubes are among the best methods for heat transfer enhancement. In the present study, laminar, developing nanofluid flow in helical tube at constant wall temperature is investigated. The numerical simulation of Al₂O₃-water nanofluid with temperature dependent properties is performed using the two-phase mixture model by control volume method in order to study convective heat transfer and entropy generation. The numerical results is compared with three test cases including nanofluid forced convection in straight tube, velocity profile in curved tube and Nusselt number in helical tubes that good agreement for all cases is observed. Heat transfer coefficient in developing region inside a straight tube using mixture model shows a better prediction compared to the homogenous model. The effect of Reynolds number and nanoparticle volume fraction on flow and temperature fields, local and overall heat transfer coefficient, local entropy generation due to viscous dissipation and heat transfer, and the Bejan number is discussed in detail and compared with the base fluid. The results show that the nanofluid and the base fluid have almost the same axial velocity profile, but their temperature profile has significant difference in developing and fully developed region. Entropy generation ratio by nanofluid to the base fluid in each axial location along the coil length showed that the entropy generation is reduced by using nanofluid in at most length of the helical tube. Also, better heat transfer enhancement and entropy generation reduction can be achieved at low Reynolds number.     

Refrigerant charge, pressure drop, and condensation heat transfer in flattened tubes

Horizontal smooth and microfinned copper tubes with an approximate diameter of 9 mm were successively flattened in order to determine changes in flow field characteristics as a round tube is altered into a flattened tube profile. Refrigerants R134a and R410A were investigated over a mass flux range from 75 to 400 kg m⁻² s⁻¹ and a quality range from approximately 10–80%. For a given refrigerant mass flow rate, the results show that a significant reduction in refrigerant charge is possible. Pressure drop results show increases of pressure drop at a given mass flux and quality as a tube profile is flattened. Heat transfer results indicate enhancement of the condensation heat transfer coefficient as a tube is flattened. Flattened tubes with an 18° helix angle displayed the highest heat transfer coefficients. Smooth tubes and axial microfin tubes displayed similar levels of heat transfer enhancement. Heat transfer enhancement is dependent on the mass flux, quality and tube profile.     

Convective boiling performance of refrigerant R-134a in herringbone and microfin copper tubes

This paper reports an experimental investigation of convective boiling heat transfer and pressure drop of refrigerant R-134a in smooth, standard microfin and herringbone copper tubes of 9.52 mm external diameter. Tests have been conducted under the following conditions: inlet saturation temperature of 5 °C, qualities from 5 to 90%, mass velocity from 100 to 500 kg s⁻¹ m⁻², and a heat flux of 5 kW m⁻². Experimental results indicate that the herringbone tube has a distinct heat transfer performance over the mass velocity range considered in the present study. Thermal performance of the herringbone tube has been found better than that of the standard microfin in the high range of mass velocities, and worst for the smallest mass velocity (G=100 kg s⁻¹ m⁻²) at qualities higher than 50%. The herringbone tube pressure drop is higher than that of the standard microfin tube over the whole range of mass velocities and qualities. The enhancement parameter is higher than one for both tubes for mass velocities lower than 200 kg s⁻¹ m⁻². Values lower than one have been obtained for both tubes in the mass velocity upper range as a result of a significant pressure drop increment not followed by a correspondent increment in the heat transfer coefficient.     

新齿型内螺纹传热管蒸发性能研究

针对开发的瘦齿、交叉齿、光管及普通齿型内螺纹传热管单管蒸发传热实验数据进行了对比,相同工况下,各传热管管内换热系数均随着冷媒流量的增大而增大,交叉齿管换热系数比瘦齿管高出10％,瘦齿管换热系数高出普通齿管10％左右。四种齿型中,交叉齿管管内压降最大,光管其次,瘦齿管比普通齿管略有增加。相同结构尺寸,相同工况下,利用稳态分布参数模型,对以上四种内螺纹传热管蒸发器传热和流动性能进行了数值模拟分析比较。结果表明,交叉齿管蒸发器换热量最大,瘦齿管稍差,普通齿管其次,但均好于光管。整个流量变化范围内,4种蒸发器压降变化都相差不大,光管蒸发器压降变化较其它管型略有增加。研究结果对内螺纹传热管的开发和研究具有指导意义。     

A visual study of R-404A/oil flow through adiabatic capillary tubes

The present study explores the potential of using visualization techniques to investigate refrigerant/oil flow through adiabatic capillary tubes. A literature review shows that these techniques have been used before for capillary tube investigations, but none of these studies focused on the refrigerant/oil phenomena. Therefore, the main objective is to investigate the flow of a refrigerant/oil mixture through a glass capillary tube, with special emphasis on the behavior of the vaporization point. The test fluids are R-404A (a near azeotropic blend) and a polyolester-type oil. Experimental data cover oil concentrations ranging from 5.6 to 6.9% (by mass), degrees of subcooling ranging from 6.2 to 21.5 °C (11.2 F to 38.7 F), and a condensing pressure of 1825 kPa (250 psig). The results show trends of mass flow rate, and give some useful insights about the location of the vaporization point for various oil concentrations and operating conditions.     

含油制冷剂在泡沫金属圆管内流动沸腾的换热特性

实验研究了填充泡沫金属的圆管内制冷剂与润滑油混合物流动沸腾换热特性。实验对象为两根分别填充5PPI、90%孔隙率与10PPI、90%孔隙率泡沫铜的圆管,以及相同管径的光管。实验工况为蒸发压力995kPa,质流密度为10~30 kg/(m2.s),热流密度为3.1~9.3kW/m2,入口干度0.175~0.775,油浓度为0~5%。实验结果表明:纯制冷剂工况下,泡沫金属的存在强化流动沸腾换热,换热系数最多提高185%;含油工况下,泡沫金属强化换热的效果弱化;相同工况下,更小的孔径可以提高流动沸腾换热系数,相比5PPI泡沫金属的实验数据,10PPI的泡沫金属可以使换热系数最多提高0.6倍。基于流型建立了填充泡沫金属的圆管内制冷剂与润滑油流动沸腾换热系数的预测模型,预测模型与98%的实验数据误差在±30%以内。     

Investigation of entropy generation in a helically coiled tube in flow boiling condition under a constant heat flux

There are many methods to augment the heat transfer rate in flow boiling in industrial applications. The helically coiled tubes are one of the best geometries to enhance the heat transfer rate. The entropy generation analysis is an appropriate tool to evaluate the contribution of heat transfer and pressure drop mechanisms. In the present paper, the entropy generation in the helically coiled tube under flow boiling is studied. The optimum tube and coil diameters under specified conditions are found. The effect of different flow conditions such as mass velocity, inlet vapor quality, saturation temperature, and heat flux on contributions of pressure drop and heat transfer in entropy generation is discussed. The Bejan number (Be) and irreversibility distribution ratio (IDR) at different saturation temperatures versus mass velocity are plotted. The comparison between entropy generation and contributions of pressure drop and heat transfer for the helically coiled tube and the straight one is presented. The entropy generation number (N_s) for different flow conditions is plotted. The entropy generation analysis shows that there is a favorable region to use the helically coiled tube with respect to the straight one.     

R404A在水平强化管外的冷凝实验及数据处理方法

针对一种双侧强化换热管,实验测试和分析了制冷工质R404A在管外凝结与水在管内对流的传热规律,采用"Wilson图解法"和"Gnielinski法"两种不同的方法对实验数据进行了处理。经理论分析和实验研究表明,Wilson图解法对于双侧强化换热管管内、管外表面传热系数实验容易产生较大误差,"Gnielinski法"是更合适的方法。实验得出了管内对流传热和管外凝结传热的计算关联式及传热的强化倍率。对于制冷剂R404A,在强化管外凝结的表面传热系数随着壁面过冷度的增加而增大,呈现出与纯工质光滑管外冷凝时不同的变化趋势。     

Measurement of spray boiling refrigerant coefficients in an integral-fin tube bundle segment simulating a full bundle

Outside (refrigerant) boiling coefficients for a combination of spray and drip boiling for a low pressure refrigerant have been obtained from overall heat transfer coefficients in a 1024 fins per meter tube bundle segment. The tubes were heated by water on the inside; liquid refrigerant was sprayed and/or dripped on the outside. Also, refrigerant vapor was supplied at the bottom of the bundle segment. This configuration simulates an actual flooded evaporator under spray boiling conditions. The dripping corresponds to liquid film falling from upper rows while the inlet vapor is equivalent to the vaporized refrigerant rising from lower tubes; the refrigerant vapor can influence heat transfer performance by the combined effects of gas convection and liquid shear on the tubes. For a nominal heat flux of 23,975 W/m², a bundle average outside heat transfer coefficient of 8522 W/m² °C, based on nominal tube outer diameter, was found at an average bundle vapor mass flux equal to 12.4 kg/s m². The distributor plate below the bundle enhanced the heat transfer, especially at lower vapor mass fluxes, by providing a level of liquid hold-up just below the bottom tube row.     

Experimentation and modeling of refrigerant flow through coiled capillary tubes

Air-conditioners use spirally coiled capillary tubes as an expansion device to enhance compactness of the unit. However, most empirical correlations for predicting refrigerant flow rate through capillary tubes were developed for straight capillary tubes without consideration of coiled effects. The objectives of this study are to investigate the flow characteristics of the coiled capillary tubes and to develop a generalized correlation for the mass flow rate through the coiled capillary tubes. The mass flow rate of R22 through the coiled capillary tubes and straight capillary tubes was measured for various operating conditions and tube geometries. The mass flow rates of the coiled capillary tubes decreased by 5–16% more than those of the straight capillary tubes at the same operating conditions. A generalized correlation for predicting refrigerant mass flow rate through coiled capillary tubes was developed by introducing the parameter of capillary equivalent length. The present correlation showed good predictions with the present database for R22, R407C and R410A in the straight and coiled capillary tubes, yielding average and standard deviations of 0.24% and 4.4%, respectively.     

In-tube condensation of mixture of R134a and ester oil: empirical correlations

This paper presents a comparative study of the condensation heat transfer coefficients in a smooth tube when operating with pure refrigerant R134a and its mixture with lubricant Castrol “icematic sw”. The lubricant is synthetic polyol ester based oil commonly used in lubricating the compressors. Two concentrations of R134a-oil mixtures of 2% and 5% oil (by mass) were analysed for a range of saturation temperatures of refrigerant R134a between 35 °C and 45 °C. The mass flow rate of the refrigerant and the mixtures was carefully maintained at 1 g/s, with a vapour quality varying between 1.0 and 0. The effects of vapour quality, flow rate, saturation temperature and temperature difference between saturation and tube wall on the heat transfer coefficient are investigated by analysing the experimental data. The experimental results were then compared with predictions from earlier models [Int J Heat Mass Transfer (1979), 185; 6th Int Heat Transfer Congress 3 (1974) 309; Int J Refrig 18 (1995) 524; Trans ASME 120 (1998) 193]. Finally two new empirical models were developed to predict the two-phase condensation heat transfer coefficient for pure refrigerant R134a and a mixture of refrigerant R134a with Castrol “icematic sw”.     

Review of flow condensation of CO2 as a refrigerant

Carbon dioxide (CO₂) has emerged as an excellent substitute natural refrigerant for low temperature refrigeration applications, but a better understanding of its in-tube flow condensation is needed in order to achieve its full potential. From experimental studies in the open literature we review the effects of mass flux, vapour quality and saturation pressure on CO₂ flow condensation heat transfer, frictional pressure drop and flow regime transition inside smooth, micro-fin and microchannel tubes. Successful condensation models which were developed from experiments with other refrigerants are evaluated against the CO₂ flow condensation experimental data. Comparison between the predicted and experimental data shows that the unique thermophysical properties of CO₂ at high reduced pressure conditions lead to these correlations having high prediction errors on the flow condensation heat transfer inside smooth tubes and microchannels, but have less significant effects on the flow condensation heat transfer and two-phase frictional pressure drop under high mass flux conditions inside micro-fin tubes. Recommendations for condensation and pressure drop models to apply to CO₂ flow condensation in different tubes are made. As there is inconsistency between the experimental data in smooth tubes from different sources, and the effects of microchannel and micro-fin tube geometries, on the flow regime transition and condensation heat transfer of CO₂, are unclear, a more extensive range of the experimental data in different tubes is needed for a fully understanding of in-tube CO₂ flow condensation.     

R404A和R407C在水平强化管外的凝结换热实验研究

实验研究了近共沸制冷工质R404A与非共沸制冷工质R407C在水平强化换热管管外的凝结换热性能。采用"Wilson图解法"对实验数据进行处理。结果表明:对于R404A和R407C,强化管外的凝结换热系数随着壁面过冷度的增加而增大,呈现出与纯工质冷凝时不同的变化趋势,这主要是近共沸或非共沸工质凝结过程中,某些组分的凝结会遇到其它组分的凝结气膜热阻所造成的;随着过冷度增加,易挥发组分开始凝结,气膜变薄,冷凝传热系数增大。R407C在强化换热管管外的凝结换热系数比R404A要小70%左右,这是由于R407C的温度滑移较R404A要大,管外形成的凝结扩散气膜造成的影响更大。R407C在高热流密度工况下的换热效果提升明显,故应尽量工作在高热流密度区域。     

Heat transfer, pressure drop, and flow pattern recognition during condensation inside smooth, helical micro-fin, and herringbone tubes

This paper presents a study of flow regimes, pressure drops, and heat transfer coefficients during refrigerant condensation inside a smooth, an 18° helical micro-fin, and a herringbone tubes. Experimental work was conducted for condensing refrigerants R-22, R-407C, and R-134a at an average saturation temperature of 40 °C with mass fluxes ranging from 400 to 800 kg m⁻² s⁻¹, and with vapour qualities ranging from 0.85 to 0.95 at condenser inlet and from 0.05 to 0.15 at condenser outlet. These test conditions represent annular and intermittent (slug and plug) flow conditions. Results showed that transition from annular flow to intermittent flow, on average for the three refrigerants, occurred at a vapour quality of 0.49 for the smooth tube, 0.29 for the helical micro-fin tube, and 0.26 for the herringbone tube. These transition vapour qualities were also reflected in the pressure gradients, with the herringbone tube having the highest pressure gradient. The pressure gradients encountered in the herringbone tube were about 79% higher than that of the smooth tube and about 27% higher than that of the helical micro-fin tube. A widely used pressure drop correlation for condensation in helical micro-fin tubes was modified for the case of the herringbone tube. The modified correlation predicted the data within a 1% error with an absolute deviation of 7%. Heat transfer enhancement factors for the herringbone tube against the smooth tube were on average 70% higher while against the helical micro-fin tube it was 40% higher. A correlation for predicting heat transfer coefficients inside a helical micro-fin tube was modified for the herringbone tube. On average the correlation predicted the data to within 4% with an average standard deviation of 8%.     

Condensation of hydrocarbons – A review

Hydrocarbons are one of the candidates for refrigerants of next generation heat pump and refrigeration systems. Although the hydrocarbons have superior thermophysical properties as a refrigerant to fluorocarbons and are widely used in domestic refrigerators, their flammability prevents their wide application to larger systems, such as residential and packaged air conditioners, car air conditioners, heat pumps, etc. In this paper, recent studies on condensation of hydrocarbons are reviewed since it is one of the key technologies. For in-tube condensation, heat transfer coefficients of smooth tubes are correlated well with previously proposed equation developed with data of the fluorocarbons. On the other hand, few data are available for enhanced tubes. In the case of condensation on a horizontal tube, heat transfer of smooth tube is explained well by the Nusselt theory. However, different heat transfer characteristics appear for enhanced tubes. Since blended hydrocarbons would be used to obtain suitable properties, models of mixed vapor condensation are also reviewed.     

Influences of miscible and immiscible oils on flow characteristics through capillary tube—part I: experimental study

A capillary tube is widely used as an expansion device for small refrigeration cycles. In a practical refrigeration cycle, some amount of refrigeration oil is discharged from a compressor and refrigerant/oil mixture flows through the capillary tube. This study investigated experimentally the influence of mixing of the refrigeration oil with the refrigerant on the flow through the capillary tube. The experiments are carried out with not only a miscible combination of refrigerant and oil but also an immiscible combination. In both cases, the mass flow rate through the capillary tube and temperature and pressure distributions along the tube are measured under several conditions of subcooled degree and oil concentration. In the case of miscible combination, the mass flow rate of refrigerant decreases with increasing the oil concentration because the viscosity of liquid phase increases by the mixing of viscous oil. Even in the case of the immiscible combination, the oil droplet is so small that it mixes homogeneously in the liquid phase in the capillary tube and the refrigerant mass flow rate decreases by the mixing of immiscible oil. There is no significant influence of the oil concentration on the underpressure, which means pressure difference between saturation pressure and flash inception pressure, in both miscible and immiscible combinations.     

R32/R134a水平内螺纹管内流动沸腾强化换热实验研究

对非共沸混合制冷剂R32／R134a(25％Wt／75％Wt)在水平内螺纹强化管中的流动沸腾换热特性进行了研究。实验结果表明：在内螺纹强化管中的流动沸腾换热性能比在光管中有较明显的提高,强化管强化系数变化的大致范围为1．5～2．2。根据活化穴、二次流和毛细提升三者之间的相互作用,探讨了内螺纹强化管的强化机理;并从重力影响、非共沸混合工质组分差与二次流影响的角度上,对在环状流下混合工质剂2／R134a在内螺纹强化管和光管中流动沸腾换热时管子周向壁温的变化特征作出了分析。