Condensation heat transfer of a pure fluid and binary mixture outside a bundle of smooth horizontal tubes. Comparison of experimental results and a classical model

The condensation of pure HFC134a and different zeotropic mixtures with pure HFC134a and HFC23 on the outside of a bundle of smooth tubes was studied. The local heat transfer coefficient for each row was experimentally determined using a test section composed by a 13×3 staggered bundle of smooth copper tubes, measuring cooling water temperature in the inlet and the outlet of each tube, and measuring the vapour temperature along the bundle. All data were taken at the inlet vapour temperature of 40°C with a wall subcooling ranging from 4 to 26 K. The heat flux was varied from 5 to 30 kW/m² and the cooling water flow rate from 120 to 300 l/h for each tube. The visualisation of the HFC134a condensate flow by means of transparent glass tubes reveals specific flow patterns and explains the difference between the measured values of the heat transfer coefficient and the calculated values from Nusselt's theory. On the other hand, the experimental heat transfer data with the binary mixtures HFC23-HFC134a show the important effects of temperature glide and the strong decrease of the heat transfer coefficient in comparison with the pure HFC134a data. The measured values with the different zeotropic mixtures were compared with the data calculated with the classical condensation model based on the equilibrium model. An improvement of this model is proposed.     

Condensation heat transfer coefficients of enhanced tubes with alternative refrigerants for CFC11 and CFC12Coefficients de transfert de chaleur de condensation de tubes à surface augmentée fonctionnant avec des frigorigènes de remplacement de CFC11 et de CFC12

In this study, condensation heat transfer coefficients (HTCs) of a plain tube, low fin tube, and Turbo-C tube were measured for the low pressure refrigerants CFC11 and HCFC123 and for the medium pressure refrigerants CFC12 and HFC134a. All data were taken at the vapor temperature of 39°C with a wall subcooling of 3–8°C. Test results showed that the HTCs of HFC123, an alternative for CFC11, were 8.2–19.2% lower than those of CFC11 for all the tubes tested. On the other hand, the HTCs of HFC134a, an alternative for CFC12, were 0.0–31.8% higher than those of CFC12 for all the tubes tested. For all refrigerants tested, the Turbo-C tube showed the highest HTCs among the tubes tested showing almost an 8 times increase in HTCs as compared to the plain tube. Nusselt's prediction equation yielded a 12% deviation for the plain tube data while Beatty and Katz's prediction equation yielded a 20.0% deviation for the low fin tube data.     

Flow condensation heat transfer coefficients of R22, R134a, R407C, and R410A inside plain and microfin tubes

Flow condensation heat transfer coefficients (HTCs) of R22, R134a, R407C, and R410A inside horizontal plain and microfin tubes of 9.52 mm outside diameter and 1 m length were measured at the condensation temperature of 40 °C with mass fluxes of 100, 200, and 300 kg m⁻² s⁻¹ and a heat flux of 7.7–7.9 kW m⁻². For a plain tube, HTCs of R134a and R410A were similar to those of R22 while HTCs of R407C are 11–15% lower than those of R22. For a microfin tube, HTCs of R134a were similar to those of R22 while HTCs of R407C and R410A were 23–53% and 10–21% lower than those of R22. For a plain tube, our correlation agreed well with the present data for all refrigerants exhibiting a mean deviation of 11.6%. Finally, HTCs of a microfin tube were 2–3 times higher than those of a plain tube and the heat transfer enhancement factor decreased as the mass flux increased for all refrigerants tested.     

Experimental and theoretical study on flow condensation with non-azeotropic refrigerant mixtures of R32/R134a

Experiments on flow condensation have been conducted with both pure R32, R134a and their mixtures inside a tube (10 m long, 6 mm ID), with a mass flux of 131–369 kg m⁻²s⁻¹ and average condensation temperature of 23–40°C. The experimental heat transfer coefficients are compared with those predicted from correlations. The maximum mean heat transfer coefficient reduction (from a linear interpolation of the single component values) occurs at a concentration of roughly 30% R32 for the same mass flux basis, and is approximately 20% at Gr = 190 kg m⁻²s⁻¹, 16% at Gr = 300 kg m⁻²s⁻¹. Non-ideal properties of the mixture have a certain, but relatively small, influence on the degradation. Among others, temperature and concentration gradients, slip, etc. are also causes of heat transfer degradation.     

Effect of refrigerant oil additive on R134a and R123 boiling heat transfer performance

This paper investigates the effect that an additive had on the boiling performance of an R134a/polyolester lubricant (POE) mixture and an R123/naphthenic mineral oil mixture on a roughened, horizontal flat surface. Both pool boiling heat transfer data and lubricant excess surface density data are given for the R134a/POE (98% mass fraction/2% mass fraction) mixture before and after use of the additive. A spectrofluorometer was used to measure the lubricant excess density that was established by the boiling of the R134a/POE lubricant mixture before and after use of the additive. The measurements obtained from the spectrofluorometer suggest that the additive increases the total mass of lubricant on the boiling surface. The heat transfer data show that the additive caused an average and a maximum enhancement of the R134a/POE heat flux between 5 kW m⁻² and 22 kW m⁻² of approximately 73% and 95%, respectively. Conversely, for nearly the same heat flux range, the additive caused essentially no change in the pool boiling heat flux of an R123/mineral oil mixture. The lubricant excess surface density and interfacial surface tension measurements of this study were used to form the basis of a hypothesis for predicting when additives will enhance or degrade refrigerant/lubricant pool boiling.     

Condensation of downward-flowing HFC134a in a staggered bundle of horizontal finned tubes: effect of fin geometry

Experimental results are presented that show the effect of fin geometry on condensation of refrigerant HFC134a in a staggered bundle of horizontal finned tubes. Two types of conventional low-fin tubes and three types of three-dimensional fin tubes were tested. The refrigerant mass velocity ranged from 8 to 23 kg/m²s and the condensation temperature difference from 1.5 to 12 K. The effect of condensate inundation was more significant for the three-dimensional fin tubes than for the low-fin tubes. In most cases, the highest performance was obtained by the tube with a three-dimensional structure at the tip of low fins. In the case of high mass velocity and high condensate inundation rate, however, the highest performance was obtained by one of the low-fin tubes. The results were compared with previous results for bundles of smooth tubes and low-fin tubes.     

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”.     

An experimental study on condensation of refrigerant R134a in a multi-port extruded tube

In the present study, the local characteristics of pressure drop and heat transfer are investigated experimentally for the condensation of pure refrigerant R134a in two kinds of 865 mm long multi-port extruded tubes having eight channels in 1.11 mm hydraulic diameter and 19 channels in 0.80 mm hydraulic diameter. The pressure drop is measured at an interval of 191 mm through small pressure measuring ports. The local heat transfer rate is measured in every subsection of 75 mm in effective cooling length using heat flux sensors. It is found that the experimental data of frictional pressure drop agree with the correlation of Mishima and Hibiki [Trans. JMSE (B) 61 (1995) 99], while the correlations of Chisholm and Laird [Trans. ASME 80 (1958) 227], Soliman et al. [Trans. ASME, Ser. C 90 (1998) 267], and Haraguchi et al. [Trans. JSME (B) 60 (1994) 239], overpredict. As a trial, the data of local heat transfer coefficient are also compared with correlations of Moser et al. [J. Heat Transfer 120 (1998) 410] and Haraguchi et al. [Trans. JSME (B) 60 (1994) 245]. The data of high mass velocity agree with the correlation of Moser et al., while those of low mass velocity show different trends. The correlation of Haraguchi et al. shows the trend similar to the data when the shear stress in their correlation is estimated using the correlation of Mishima and Hibiki.     

Experimental study and modelling of heat transfer during condensation of pure fluid and binary mixture on a bundle of horizontal finned tubes

An experimental investigation was conducted to measure the local heat transfer coefficient for each row in a trapezoidal finned horizontal tube bundle during condensation of both pure fluid (HFC 134a) and several compositions of the non-azeotropic binary mixture HFC 23/HFC 134a. The test section is a 13×3 (rows × columns) tube bundle and the heat transfer coefficient is measured using the modified Wilson plot method. The inlet vapour temperature is fixed at 40 °C and the water flow rate in each active row ranges from 170 to 600 l/h. The test series cover five different finned tubes all commercially available, K11 (11 fins/inch), K19 (19 fins/inch), K26 (26 fins/inch), K32 (32 fins/inch), K40 (40 fins/inch) and their performances were compared. The experimental results were checked against available models predicting the heat transfer coefficient during condensation of pure fluids on banks of finned tubes. Modelling of heat exchange during condensation of binary mixtures on bundles of finned tubes based on the curve condensation model is presented.     

R134a单元式风冷冷风机组冷凝器设计

单元式风冷冷风空调机组普遍采用波纹翅片管冷凝器。对冷凝器进行设计的关键是确定制冷工质在铜管内的冷凝换热系数及空气在翅片侧的表面换热系数，同时也需要考虑空气流过冷凝器的压降，以便选择风机。采用数学模型及换热关联式计算相关参数，在此基础上对R134a单元式风冷冷风空调机组的冷凝器进行设计。     

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.     

Prediction of heat transfer coefficient during condensation of water and R-134a on single horizontal integral-fin tubes

This paper presents a few salient features of an investigation carried out to study the heat transfer augmentation during condensation of water and R-134a vapor on horizontal integral-fin tubes. The experimental investigation was performed on two different experimental set-ups for water and R-134a. The test-sections were manufactured by machining fins over plain copper tubes of 24.4 ± 0.6 mm outside diameter. The performance of two types of finned tubes viz. circular integral-fin tubes (CIFTs) and spine integral-fin tubes (SIFTs) was studied for the condensation of water and R-134a. These tubes were positioned one by one inside the test-condenser to perform the experiments. All together the experiments were conducted for the condensation on 10 different test-section tubes. With the help of the experimental results, authors have developed an empirical equation. This equation predicts the condensing heat transfer coefficient from their own experimental data for the condensation over CIFTs and SIFTs within a range of ± 15% and experimental data of other thirteen investigators in a range of ± 35% for condensation of water and different refrigerants.     

Experimental correlation of falling film condensation on enhanced tubes with HFC134a; low-fin and Turbo-C tubes

The objectives of this paper are to develop experimental correlations of heat transfer for enhanced tubes used in a falling film condenser, and to provide a guideline for optimum design of the falling film condenser with a high condensing temperature of 59.8 °C. Tests are performed for four different enhanced tubes; a low-fin and three Turbo-C tubes. The working fluid is HFC134a, and the system pressure is 16.0 bar. The results show that the heat transfer enhancement of low-fin tube, Turbo-C (1), Turbo-C (2) and Turbo-C (3) ranges 2.8–3.4 times, 3.5–3.8 times, 3.8–4.0 times and 3.6–3.9 times, respectively, compared with the theoretical Nusselt correlation. It was found that the condensation heat transfer coefficient decreased with increasing the falling film Reynolds number and the wall subcooling temperature. It was also found that the enhanced tubes became more effective in the high wall subcooling temperature region than in the low wall subcooling temperature region. This study developed an experimental correlation of the falling film condensation with an error band of ±5%.     

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.     

Flow condensation heat transfer coefficients of pure refrigerants

Flow condensation heat transfer coefficients (HTCs) of R12, R22, R32, R123, R125, R134a, and R142b were measured experimentally on a horizontal plain tube. The experimental apparatus was composed of three main parts; a refrigerant loop, a water loop and a water-glycol loop. The test section in the refrigerant loop was made of a copper tube with an outside diameter of 9.52 mm and 1 m length. The refrigerant was cooled by cold water passing through an annulus surrounding the test section. All tests were performed at a fixed refrigerant saturation temperature of 40 °C with mass fluxes of 100, 200, 300 kg m⁻² s⁻¹ and heat flux of 7.3–7.7 kW m⁻². Experimental results showed that flow condensation HTCs increase as the quality and mass flux increase. At the same mass flux, the HTCs of R142b and R32 are higher than those of R22 by 8–34% while HTCs of R134a and R123 are similar to those of R22. On the other hand, HTCs of R12 and R125 are lower than those of R22 by 24–30%. Previous correlations predicted the present data satisfactorily with mean deviations of less than 20% substantiating indirectly the reliability of the present data. Finally, a new correlation was developed by modifying Dobson and Chato's correlation with an introduction of a heat and mass flux ratio combined with latent heat of condensation. The correlation showed a mean deviation of 10.7% for all pure halogenated refrigerants' data obtained in this study.     

R134a单元式风冷冷风空调机蒸发器设计

以理论模型为基础，对R134a单元式风冷冷风机组翅片管式蒸发器进行设计。应用管内流动沸腾换热模型仿真分析R134a的质量流量对沸腾换热的影响，利用外掠翅片管束换热关联式计算管外翅片侧表面换热系数，进而得出翅片管蒸发器总传热系数，利用计算结果进行设计。     

Prediction of in-tube condensation heat transfer characteristics of binary refrigerant mixtures

This study presents a prediction model for the condensation heat transfer characteristics of binary zeotropic refrigerant mixtures inside horizontal smooth tubes. In this model, both the vapor-side and liquid-side mass transfers are considered, and the high flux mass transfer correction factor is used to evaluate mass transfer coefficients. The model was applied to the binary zeotropic refrigerant mixture R134a/R123, which has a large temperature glide. Calculation results showed that the heat transfer degradation of R134a/R123 due to gradients in the mass fraction and temperature is considerable, and depends on the mass fraction of the more volatile component and the vapor mass quality of the refrigerant mixture. By comparison with experimental data, incorporating the present finite mass transfer model for the liquid film side into the calculation algorithm was shown to reasonably well predict the condensation heat transfer coefficients of binary refrigerant mixtures with the mean deviation of about 10.3%. In the present calculations, however, it was also found that the high flux mass transfer correction factor had only a slight effect on the condensation heat transfer.     

Enhancement of R123 pool boiling by the addition of hydrocarbonsAmélioration de l'ébullition libre du R123 par l'ajout d'hydrocarbures

This paper presents pool boiling heat transfer data for 10 different R123/hydrocarbon mixtures. The data consisted of pool boiling performance of a GEWA-T^™ surface for pure R123 and for 10 dilute solutions of five different hydrocarbons: (1) pentane, (2) isopentane, (3) hexane, (4) cyclohexane, and (5) heptane with R123. The heat flux and the wall superheat were measured for each fluid at 277.6 K. A maximum (19±3.5)% increase over the pure R123 heat flux was achieved with the addition of 0.5% mass isopentane to R123. Other mixtures of isopentane, pentane, hexane, and cyclohexane with R123 exhibited smaller maximums than that of the R123/isopentane (99.5/0.5) mixture. Presumably, a layer enriched in hydrocarbon at the heat transfer surface caused the heat transfer enhancement. Conversely, an R123/heptane (99.5/0.5) mixture and an R123/cylcohexane (99.5/0.5) mixture exhibited only degradations with respect to the pure component performance for all test conditions. Several characteristics of the hydrocarbons were examined to determine their influence on the boiling heat transfer performance: molecular weight, molecular structure, composition, surface tension, and vapor pressure.     

Boiling heat transfer of R-22, R-134a, and CO2 in horizontal smooth minichannels

This study examined convective boiling heat transfer in horizontal minichannels using R-22, R-134a, and CO₂. The local heat transfer coefficients were obtained for heat fluxes ranging from 10 to 40 kW m⁻², mass fluxes ranging from 200 to 600 kg m⁻² s⁻¹, a saturation temperature of 10 °C, and quality up to 1.0. The test section was made of stainless steel tubes with inner diameters of 1.5 mm and 3.0 mm, and a length of 2000 mm. The section was heated uniformly by applying an electric current to the tubes directly. Nucleate boiling heat transfer was the main contribution, particularly at the low quality region. An increasing and decreasing heat transfer coefficient occurred at the lower vapor quality with increasing heat flux and mass flux. The mean heat transfer coefficient ratio of R-22:R-134a:CO₂ was approximately 1.0:0.8:2.0. Laminar flow was observed in the minichannels. A new boiling heat transfer coefficient correlation based on the superposition model for refrigerants in minichannels was developed with a mean deviation of 11.21%.     

R134a制冷剂在微通道中的相变传热特性实验

设计了一个可控制制冷剂流量、压力和温度等实验工况的微通道换热器相变流动与换热的可视化实验平台,对R134a制冷剂流经微通道换热器进行了冷凝换热实验研究.试验测量了小质量流率下的R134a制冷剂在多个饱和状态工况下的冷凝换热性能,涉及质量流量、进出口压力和温度等参数.实验分析了传热系数与雷诺数的关系,与Koyama的关联式预测比较接近.分析了摩擦系数随雷诺数的变化,与H L MO和Wu＆Little方程计算得到的数值相近.