A model for the condensation heat transfer of binary refrigerant mixtures

In the present work turbulent film condensation of nonazeotropic binary mixtures inside a horizontal tube is studied theoretically. The combined heat and mass transfer involved is analyzed through an integral formulation of the continuity, momentum, energy and diffusion equations. As the mass velocity of refrigerant mixtures increases, the condensation heat transfer coefficient increases. The heat transfer coefficient becomes smaller at higher mass quality. As the mole fraction of the more volatile component in binary mixtures increases, the back-diffusion mass flux of the more volatile component reduces in the vapor. As a result the condensation heat transfer coefficient improves with the increase of the inlet mole fraction of the more volatile component especially in the upstream of condenser. The results of the present study show good agreement with the experimental data available.     

Nucleate Boiling Heat Transfer Coefficients of Mixtures Containing Propane, Isobutane and HFC134a

Nucleate pool boiling heat transfer coefficient (HTCs) were measured with one nonazeotropic mixture of propane/isobutane and two azeotropic mixtures of HFC134a/isobutane and propane/HFC 134a. All data were taken at the liquid pool temperature of 7°C on a horizontal plain tube of 19.0 mm outside diameter with heat fluxes of 10kW/m² to 80 kW/m² with an interval of 10 kW/m² in the decreasing order of heat flux. The measurements were made through electrical heating by a cartridge heater. The nonazeotropic mixture of propane/isobutane showed a reduction of HTCs as much as 41% from the ideal values. The azeotropic mixtures of HFC134a/isobutane and propane/HFC 134a showed a reduction of HTCs as much as 44% from the ideal values at compositions other than azeotropic compositions. At azeotropic compositions, however, the HTCs were even higher than the ideal values due to the increase in the vapor pressure. For all mixtures, the reduction in heat transfer was greater with larger gliding temperature difference. Stephan and Körner’s and Jung et al’s correlations predicted the HTCs of mixtures with a mean deviation of 11 %. The largest mean deviation occurred at the azeotropic compositions of HFC 134a/isobutane and propane/HFC 134a.     

An experimental investigation of heat transfer in forced convective boiling of R134a,R123 and R134a/R123 in a horizontal tube

This paper reports an experimental study on flow boiling of pure refrigerants R134a and R123 and their mixtures in a uniformly heated horizontal tube. The flow pattern was observed through tubular sight glasses with an internal diameter of 10 mm located at the inlet and outlet of the test section. Tests were run at a pressure of 0.6 MPa in the heat flux ranges of 5–50 kW/m², vapor quality 0–100 percent and mass velocity of 150–600 kg/m²s. Both in the nucleate boiling-dominant region at low quality and in the two-phase convective evaporation region at higher quality where nucleation is supposed to be fully suppressed, the heat transfer coefficient for the mixture was lower than that for an equivalent pure component with the same physical properties as the mixture. The reduction of the heat transfer coefficient in mixture is explained by such mechanisms as mass transfer resistance and non-linear variation in physical properties etc. In this study, the contribution of convective evaporation, which is obtained for pure refrigerants under the suppression of nucleate boiling, is multiplied by the composition factor by Singal et al. (1984). On the basis of Chen’s superposition model, a new correlation is presented for heat transfer coefficients of mixture.     

组分迁移对R407C相变换热影响的理论分析

为了研究组分迁移对非共沸工质在换热器中相变换热特性的影响,探明制冷剂和换热流体间的温差在换热器中的变化规律,进行了相应的理论分析。基于窄点理论和空泡模型,将换热器按等焓降划分为若干控制体,建立相应的数学模型,然后在给定工况下,计算出R407C在换热器内冷凝和蒸发的组分迁移情况,进而得到考虑组分迁移的非共沸工质温焓非线性关系,发现制冷剂和换热流体间的温差在换热器中的变化规律,再与未考虑组分迁移时作对比。结果发现,考虑组分迁移对窄点或最大传热温差及其在换热器中出现的位置、可用能损失的影响很大,在合理设计换热器时应给予考虑。     

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.     

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.     

Performance of heat transfer and pressure drop in a spirally indented tube

In an effort to develop a heat transfer enhancement technique for low temperature applications such as utilization of LNG cold energy, an experiment was carried out to evaluate the heat transfer and the pressure drop performance for a spirally indented tube using ethylene-glycol and water solutions and pure water under horizontal single-phase conditions. The test tube diameter was 14.86 mm and the tube length was 5.38 m. Heat transfer coefficients and friction factors for both inner and outer surfaces of the test tube were calculated from measurements of temperatures, flowrates and pressure drops. Correlations of heat transfer coefficients in the spirally indented tube, which were applicable for laminar and turbulent regimes were proposed for inner, and outer surfaces. The correlations showed that heat transfer coefficients for the spirally indented tube were much higher than those for smooth tubes, increased by more than 8 times depending upon the Reynolds number. The correlations were compared with other correlations for various types of surface roughness. The effect of the Prandtl number on the heat transfer characteristics was discussed. The critical Reynolds number from the laminar flow to the turbulent flow inside the spirally indented tube was found to be around Re=1,000.     

Condensation heat transfer correlation for smooth tubes in annular flow regime

Condensation heat transfer coefficients in a 7.92 mm inside diameter copper smooth tube were obtained experimentally for R22, R134a, and R410A. Working conditions were in the range of 30–40°C condensation temperature, 95–410 kg/m²s mass flux, and 0.15–0.85 vapor quality. The experimental data were compared with the eight existing correlations for an annular flow regime. Based on the heat-momentum analogy, a condensation heat transfer coefficients correlation for the annular flow regime was developed. The Breber et al. flow regime map was used to discern flow pattern and the Muller-Steinhagen & Heck pressure drop correlation was used for the term of the proposed correlation. The proposed correlation provided the best predicted performance compared to the eight existing correlations and its root mean square deviation was less than 8.7%.     

套管式冷凝器冷凝混合工质的分析研究

套管式冷凝器广泛应用于各种小型制冷装置系统。本文针对混合制冷工质在套管式冷凝器中的冷凝过程,依据混合工质在冷凝过程中存在温度滑移的特性,分析了气液相各组分质量百分数随温度变化的特性。应用混合制冷工质物性计算软件Refprop8.0分析了其初始过热状态质量成分比例一定时,在温度不断降低情况下,气相和液相中质量成分的变化规律。通过传热学理论及换热器设计理论,较详细地分析计算了混合制冷工质在套管式冷凝器中冷凝时的热力参数,并得到了这种工况条件下套管式冷凝器的几何尺寸。     

Influence of the inclination angle and liquid charge ratio on the condensation in closed two-phase thermosyphons with axial internal low-fins

This study concerns the performance of the heat transfer of the thermosyphons having 60, 70, 80, 90 axial internal low-fins in which boiling and condensation occurr. Water, HCFC-141b and CFC-11 have been used as the working fluids. The operating temperature, the liquid charge ratio and the inclination angle of thermosyphons have been used as the experimental parameters. The heat flux and heat transfer coefficient at the condenser are estimated from experimental results. The experimental results have been assessed and compared with existing theories. As a result of the experimental investigation, it was found that the maximum heat flow rate in the thermosyphons is dependent upon the liquid charge ratio and inclination angle. A relatively high rate of heat transfer has been achieved by the thermosyphon with axial internal low-fins. The inclination of a thermosyphon has a notable influence on the condensation. In addition, the overall heat transfer coefficients and the characteristics at the operating temperature are obtained for the practical applications.     

A study on the improvement of heat transfer performance in low temperature closed Thermosyphon

The study focuses on the heat transfer performance of two-phase closed thermosyphons with plain copper tube and tubes having 50, 60, 70, 80, 90 internal grooves. Three different working fluids (distilled water, methanol, ethanol) are used with various volumetric liquid fill charge ratio from 10 to 40%. Additional experimental parameters such as operating temperature and inclination angle of zero to 90 degrees are used for the comparison of heat transfer performance of the thermosyphon. Condensation and boiling heat transfer coefficients, heat flux are obtained using experimental data for each case of specific parameter. The experimental results are assessed and compared with existing correlations. The results show that working fluids, liquid fill charge ratio, number of grooves and inclination angle are very important factors for the operation of thermosyphons. The relatively high rate of heat transfer is achieved when the thermosyphon with internal grooves is used compared to that with plain tube. The optimum liquid fill charge ratio for the best heat transfer performance lies between 25% and 30%. The range of the optimum inclination angle for this study is 20°-30° from the horizontal position.     

Pool boiling of R-123/MO and R-134a/POE lubricant mixtures on pored surfaces

In a flooded refrigerant evaporator, where enhanced tubes are frequently used, lubrication oil inevitably circulates with the refrigerant. However, the literature shows that systematic studies on this subject are lacking. In this study, the effects of oil on the pool boiling of pored surfaces having a range of pore diameter (0.1 to 0.3 mm) and pitch (0.75 to 3.0 mm) were investigated using R-134a/polyester oil (POE) and R-123/mineral oil (MO) mixtures. The saturation temperature was 26.7 °C, and the oil concentration was varied up to 10 %. The results showed that the oil reduced the heat transfer coefficient. This was true for all the pored surface as well as the smooth surface. Overall, the samples having a ‘small open area’ yielded a significant degradation at a low heat flux, whereas the samples having a ‘large open area’ yielded a noticeable degradation at a high heat flux. Meanwhile, the heat transfer degradation was larger for R-134a/POE than R-123/MO, and the reason was attributed to the oil effect on the surface tension, which was stronger for R-134a/POE. The flow visualization results showed that, with the increase of oil concentration, the bubble departure diameter decreased. Similarly, the bubble generation frequency and nucleation site density decreased. These changes of the bubble dynamic parameters certainly were responsible for the heat transfer degradation. A model was developed extending that of Pastuszko et al. [27] to predict the heat transfer coefficients as well as bubble dynamic parameters. The model predicted 92 % of the heat transfer coefficients within ± 40 %.

     

Two-phase flow heat transfer of propane vaporization in horizontal minichannels

Experiments were performed on the convective boiling heat transfer in horizontal minichannels using propane. The test section was made of stainless steel tubes with inner diameters of 1.5 mm and 3.0 mm and lengths of 1000 mm and 2000 mm, respectively, and it was uniformly heated by applying an electric current directly to the tubes. Local heat transfer coefficients were obtained for a heat flux range of 5–20 kW m⁻², a mass flux range of 50–400 kg m⁻² s⁻¹, saturation temperatures of 10, 5, and 0°C and quality ranges of up to 1.0. The nucleate boiling heat transfer contribution was predominant, particularly at the low quality region. Decreases in the heat transfer coefficient occurred at a lower vapor quality with a rise of heat flux and mass flux, and with a lower saturation temperature and inner tube diameter. Laminar flow appeared in the minichannel flows. A new boiling heat transfer coefficient correlation that is based on the superposition model for propane was developed with 8.27% mean deviation. This paper was recommended for publication in revised form by Associate Editor Jae Young Lee Jong-Taek Oh received his B.S., M.S. and Ph.D. degrees in Refrigeration Engineering from Pukyong National University, Korea. Dr. Oh is currently a Professor at Department of Refrigeration and Air Conditioning Engineering, Chonnam National University at Yeosu, Korea. Dr. Oh’s research interests are in the area of boiling and condensation heat transfer and pressure drop of refrigerants with small tubes, heat pump and transportation refrigeration.     

纯质制冷剂管内冷凝换热的计算方法

在分析比较了一系列的纯质制冷剂在竖管及水平管内的局部凝结换热系数计算方法的基础上,筛选出了与实验吻合较好的计算公式,并且采用分段计算的方法使其计算值与实验值更为一致。     

纯制冷剂贮罐外泄动态过程分析

建立了纯制冷剂气相、液相空间动态泄漏模型,详细描述动态泄漏的整个过程;分析了制冷剂主要内外因素对气相空间动态泄漏过程的影响。结果表明,同等条件下液相空间泄漏的质量泄漏速度远比气相空间的大,但后者比前者变化快;各内外因素对气相空间泄漏影响很大,其中当量泄漏半径对其影响最大。研究结果可用于评估、预测有毒有害、易燃易爆制冷剂在生产、储藏、运输过程中泄漏事故危险范围和强度。     

Dynamic analysis of evaporator characteristics

An analysis of the dynamic characteristics in an evaporator was numerically performed for control and design of the refrigeration and air conditioning systems. The important factors, such as refrigerant flow rate, inlet enthalpy, inlet air velocity and air temperature, are incorporated with this analysis. An evaporator is modeled for the dynamic characteristics analysis separated into three regions which are the two-phase region, the saturated vapor region and the superheated vapor region. The basic equations of each region were derived in the continuity, heat energy equilibrium and heat transfer equations. The transfer functions of the dynamic characteristics were obtained by the linearization and Laplace transformation. The dynamic response characteristics were evaluated on the Bode diagram with the frequecy response method. These results may be used for the analysis of the dynamic characteristics and design in the total system.     

Some aspects of experimental in-tube evaporation

The heat transfer characteristics of refrigerant-oil mixture for horizontal in-tube evaporator have been investigated experimentally. A smooth copper tube and a micro-fin tube with nominal 9.5 mm outer diameter and 1500 mm length were tested. For the pure refrigerant flow, the dependence of the axial heat transfer coefficient on quality was weak in the smooth tube, but in the micro-fin tube, the coefficients were 3 to 10 times greater as quality increases. Oil addition to pure refrigerant in the smooth tube altered the flow pattern dramatically at low mass fluxes, with a resultant enhancement of the wetting area by vigorous foaming. The heat transfer coefficients of the mixture for low and medium qualities were increased at low mass fluxes. In the micro-fin tube, however, the addition of oil deteriorates the local heat transfer performance for most of the quality range, except for low quality. The micro-fin tube consequently loses its advantage of high heat transfer performance for an oil fraction of 5%. Results are presented as plots of local heat transfer coefficient versus quality.     

R134a、R123、R23和R14混合工质林德循环的试验研究

提出了一种新的R134a、R123、R23和R14四元混合工质林德制冷循环并做了相关的试验,主要包括制冷剂组分混合比例的确定以及分析不同比例对循环整体性能的影响。通过对不同配比下所得数据的分析比较,总结了此四元混合工质林德循环的运行特点,并提出了整个系统循环的改进方案。     

Condensation heat transfer coefficients of flammable refrigerants on various enhanced tubes

In this study, external condensation heat transfer coefficients (HTCs) of six flammable refrigerants of propylene (R1270), propane (R290), isobutane (R600a), butane (R600), dimethylether (RE170), and HFC32 were measured at the vapor temperature of 39°C on a 1023 fpm low fin and Turbo-C tubes. All data were taken under the heat flux of 32- 116 and 42-142 kW/m² for the Iow fin and Turbo-C tubes respectively. Flammable refrigerants’ data obtained on enhanced tubes showed a typical trend that external condensation HTCs decrease with increasing wall subcooling. HFC32 and DME showed up to 30% higher HTCs than those of HCFC22 due to their excellent thermophysical properties. Propylene, propane, isobutane, and butane showed similar or lower HTCs than those of HCFC22. Beatty and Katz’ correlation predicted the HTCs of the flammable refrigerants obtained on a low fin tube within a mean deviation of 7.3%. Turbo-C tube showed the best performance due to its 3 dimensional surface geometry for fast removal of condensate.     

利用工质特性提高热泵制热温度的研究

提出了一种采用T—S图中饱和蒸气线具有右倾特性工质作为热泵工质的新思路。可使工质在压缩过程中始终接近饱和蒸气状态,压缩机排气温度约等于工质的冷凝温度,从而可较好地解决排气温度过热所引起的一系列问题。