具有蒸气过热的R-134a在螺旋管内凝结换热的实验研究

对具有蒸气过热的替代制冷剂R-134a在螺旋管内的对流凝结换热特性进行了实验研究.在制冷剂R-134a的质量流量变化范围为100(400kg/(m2s)和蒸气过热度为3.8℃和8.4℃的条件下,得到了螺旋管内具有蒸气过热的R-134a对流凝结换热特性的实验数据.实验结果表明,蒸气过热对R-134a在螺旋管内的凝结换热特性具有明显的影响.通过与已有研究结果的比较,简要分析了蒸气过热对R-134a在螺旋管内凝结换热特性的影响机理.     

R-134a在三种不同放置方式螺旋管内凝结换热的实验研究

对替代制冷剂R-134a在螺旋管内的对流凝结换热特性进行了实验研究.在制冷剂R-134a的质量流量变化范围为100～400kg/(m2s) 和冷却水的平均温度分别为12℃和22℃的条件下,实验得到了在三种不同放置方式螺旋管内(水平,垂直和倾斜)R-134a的对流凝结换热特性数据.实验结果表明,螺旋管的不同放置方式对R-134a在螺旋管内的凝结换热特性具有重要的影响.通过与已有研究结果的比较,简要分析了螺旋管不同放置方式对R-134a对流凝结换热特性的影响机理.     

R134a在水平强化管外凝结换热的实验研究

对氟利昂R134a在水平单管外的凝结换热性能进行了试验研究,试验管为光管和三根强化管,采用热阻分离法得到蒸气侧凝结换热系数。试验结果表明:光管管外Nusselt理论值与实验数据偏差小于10%。强化管No.1-3的传热性能均好于光管,当Re=40000时,No.1-4管的总传热系数分别为:5295,5818,5904,1502W/m2.K。在相同热流密度条件下,No.1-3管的管外换热系数分别是光管的7.0-8.8倍,9.0-10.8倍,9.9-12.0倍。管外强化后,管内外的换热系数已比较接近。     

R134a臣卧式螺旋管内流动沸腾换热特性实验研究

对R134a在水平直管和螺旋管内的沸腾换热特性进行了实验研究.在三个不同的蒸发温度(5℃、10℃和20℃),工质R134a的质量流量范围为100～400kg/(m~2·s)和干度范围为0.1～0.8的条件下,实验得到了R134a在水平直管和螺旋管内的沸腾换热系数随其质量流量和干度的变化关系,将水平直管和螺旋管内的沸腾换热特性数据进行了比较,结果显示,在实验条件下,卧式螺旋管的传热系数比直管的平均增加13.7%.     

R22和R417A在水平强化管外的凝结换热实验研究

用实验的方法研究了非共沸工质R417A在水平强化换热管管外的凝结换热性能,并与R22做了对比.试验管为两种强化换热管-斜翅管和矩翅管.结果表明:对于斜翅管,同等的壁面过冷度下,R417A的凝结换热系数大于R22的管外换热系数;对于矩翅管,同等的壁面过冷度下,R22的凝结换热系数大于R417A的凝结换热系数;在工质R417A下,两种强化管的凝结管外换热系数随壁面过冷度的变化率都比R22大,其原因应该与R417A作为一种非共沸制冷剂的温度滑移特性有关.从强化换热的角度考虑,对于表面张力较小的工质,选用斜翅管更有利.     

纯工质水平管内凝结换热研究进展

相对于混合工质,纯工质(单一成分制冷剂)不存在温度滑移、系统泄漏时的成分变化等问题,在制冷设备中得到了广泛应用.研究纯工质水平管内凝结换热特性对其及其混合物在制冷、空调、热泵方面的应用具有重要意义.文章从理论和实验两个方面,综述了国外对纯工质在水平管内流动凝结换热的研究,分析了凝结换热关联式的适用性和局限性,并提出了进一步研究的建议.     

国产制冷剂HFC－134a的凝结换热特性

本文报导了国产新制冷剂工质ＨＦＣ－１３４ａ及其所替代的工质ＣＦＣ－１２在水平光管内凝结换热的实验结果。实验管内径为６ｍｍ,管长为５ｍ。实验结果表明,在相同的凝结温度和工质质量流量的条件下,国产ＨＦＣ－１３４ａ的凝结换热系数比ＣＦＣ－１２大３０％左右,而两者压力降数值基本相同。文中同时给出了计算这两种工质凝结换热过程中局部换热系数、平均换热系数和压力降的计算公式。     

水平微圆管内R22和R410a凝结压降的实验研究

R410a是一种被广泛看好的R22替代物,研究R410a的凝结压降特性对开发适用此种制冷工质的凝结换热设备有重要意义。搭建了微细尺度凝结压降实验台,实验研究了R22和R410a在内径为0.941mm水平不锈钢圆管内饱和温度为40℃、质量流速为200-1000 kg/m^2.s、干度为0.2-0.8时的凝结压降特性。实验结果表明：凝结压降随着质量流速的增大而增大,在较高干度时更加明显。与R22相比,R410a的凝结压降在较低干度和质量流速时与R22相当,在较高干度和较高质量流速时明显低于R22。     

R290和R22在水平细圆管内流动凝结换热的实验研究

实验研究了R290、R22在细圆管中的流动凝结换热特性。实验管内径为1.085 mm,R22的质量流率为200~1200 kg/(m2·s),R290的质量流率为200~650 kg/(m2·s),饱和温度分别为40℃与50℃。实验结果表明,高质量流率时R22在较高干度下换热系数随干度增加缓慢或略有下降,低质量流率时,R290在较小干度下出现换热系数下降。两种制冷剂蒸气相比,相同条件下R290的凝结换热系数高于R22的。本文的实验结果还与现有典型关联式的计算结果作了对比,其中,Wang et al.(2002)关联式对R290的实验数据预测偏差在17.5%之内,Kim et al.(2013)关联式对R22的实验数据预测偏差在18.4%之内。     

Thermodynamic Properties of Mixtures of R-32, R-125, R-134a, and R-152a

A mixture model explicit in Helmholtz energy has been developed that is capable of predicting thermodynamic properties of refrigerant mixtures containing R-32, R-125, R-134a, and R-152a. The Helmholtz energy of the mixture is the sum of the ideal gas contribution, the compressibility (or real gas) contribution, and the contribution from mixing. The contribution from mixing is given by a single equation that is applied to all mixtures used in this work. The independent variables are the density, temperature, and composition. The model may be used to calculate thermodynamic properties of mixtures, including dew and bubble point properties and critical points, generally within the experimental uncertainties of the available measured properties. It incorporates the most accurate published equation of state for each pure fluid. The estimated uncertainties of calculated properties are ±0.25% in density, ±0.5% in the speed of sound, and ±1% in heat capacities. Calculated bubble point pressures are generally accurate to within ±1%.     

Equations for the Thermal Conductivity of R-32, R-125, R-134a,and R-143a

At present hydrofluorocarbons (HFCs) such as R32, R-125, R-134a, and R-143a are widely used, and it is required to obtain accurate information of thermophysical properties, especially of the thermal conductivity of HFCs. In this paper new thermal conductivity equations for R-32, R-125, R134a, and R143a are proposed, applicable over a wide range of temperature and pressure including the critical region based on existing experimental data, and the reliability of the present equations is summarized. The problem that the thermal conductivity calculated from the thermal diffusivity in the critical region differs depending on the equation of state is also discussed. Paper presented at the Sixteenth European Conference for Thermophysical Properties, September 1–4, 2002, London, United Kingdom.     

Thermal stability of R-134a, R-141b, R-13I1, R-7146, R-125 associated with stainless steel as a containing material

An experimental apparatus for assessing the thermal stability threshold of refrigerant working fluids is described and results for R-134a (1,1,1,2-tetrafluoroethane), R141b (1,1-dichloro-1-fluoroethane), R-13I1 (trifluoromethyl iodide), R-7146 (sulphur hexafluoride), R-125 (pentafluoroethane) are presented. The information is a concern for the design of refrigeration systems, high temperature heat pumps and Organic Rankine Cycles (ORC), for which the above refrigerants are proposed. The method aims to identify a maximum temperature for plant operation in contact with stainless steel and involves the evaluation of four indicators: (1) pressure variation while the fluid is maintained at set temperature; (2) saturation pressure comparison after heat treatment; (3) chemical analysis; and (4) vessel visual inspection after the test session. The highest temperatures at which no evident degradation occured are: 368°C for R-134a; 102°C for R-13I1; 90°C for R-141b; 204°C for R-7146; and 396°C for R-125.     

Vapor Pressure of the 1,1,1,2-Tetrafluoroethane (R-134a) + Polyalkylene Glycol System

Vapor pressures of the 1,1,1,2-tetrafluoroethane + polyalkylene glycol system were obtained at 72 points over the temperature range from 253.15 to 333.15 K at 10 K intervals and the composition range from 0 to 90 mass % polyalkylene glycol. It was found that below 273.15 K, the effect of the polyalkylene glycol on the vapor pressure was negligible up to 30 mass % polyalkylene glycol. The vapor pressure of the 1,1,1,2-tetrafluoroethane + polyalkylene glycol system decreased as the concentration of polyalkylene glycol increased. Raoults model and Flory–Huggins model were used for data reduction. Raoults model gave reasonable predictions for the vapor pressure of the system below 30 mass % polyalkylene glycol. The Flory–Huggins model gave reasonable predictions for the vapor pressure over the complete composition range. An empirical vapor pressure equation was obtained in terms of temperature and mass fraction polyalkylene glycol. The empirical equation was the most convenient way to calculate the vapor pressure.     

Condensation of R-134a vapour over single horizontal integral-fin tubes: effect of fin height

An experimental investigation has been carried out to study the condensation of R-134 vapour over five single horizontal circular integral-fin tubes of 472 fpm fin density, 417 mm length and different fin heights of 0.45, 1.14, 1.47, 1.92 and 2.40 mm. The circular fins are rectangular in shape and the fin thickness of all tubes is 0.70 mm. The tube with the fin height of 0.45 mm has given the highest enhancement in heat transfer coefficient, h, of the order of 3.18 in comparison to that predicted by the Nusselt model for a plain tube.     

R134a水平微细管内流动沸腾换热的实验研究

本文对R134a在水平微细管内的流动沸腾进行了实验研究。实验测试段选用了内径为1 mm、2 mm、3 mm共3种不同的水平光滑不锈钢管,实验的饱和温度为5~30℃,热流密度为2~70 k W/m2,流量范围为200~1500 kg/(m2·s)。实验结果表明:相同条件下,干涸前2 mm管较3 mm管换热系数平均增幅为11.6%,1 mm管较2 mm管换热增幅为26.3%,1 mm管径换热系数比3 mm管径平均增大40.8%。随着管径的减小,换热系数在更低的干度开始减小,质量流速和强制对流蒸发作用对换热系数的影响变小,热流密度的影响依然显著;塞状流和弹状流区域减小,泡状流和环状流区域增大。     

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在微肋管内的冷凝换热特性

本文在35、40和45℃三种冷凝温度下,对R134a在微肋管内的冷凝换热进行了实验研究。选用质量流量、冷凝温度、微肋管结构参数为变量,以总传热系数、水侧传热系数、制冷剂侧表面传热系数及压降为评价指标。结果表明:总传热系数、制冷剂侧表面传热系数、压降均随着质量流量的增加、冷凝温度的降低和管径的减小而增大,而水侧传热系数随质量流量的增加而稍有降低,冷凝温度对其值影响并不大。热阻分析时发现:随着质量流量的增加,水侧热阻占总热阻比值逐渐增加,而制冷剂侧热阻所占比值逐渐减小,但制冷剂侧热阻总小于水侧热阻;对换热器进行综合性能进行评价时,以表面传热系数与压降的比值(单位压降表面传热系数)为指标,发现该比值均随质量流量的增加呈先减小后增大的趋势,并随着冷凝温度的降低、管径的减小而增大。     

R134a在微肋管内流动冷凝换热计算关联式的改进

本文选用表面传热系数为评价指标,对外径为6.35 mm的微肋管内R134a两相流动冷凝换热特性进行实验研究,分析了水力工况、测试管结构参数等对管内表面传热系数的影响,还选用Cavallini et al.关联式、Miyara et al.关联式和Oliver et al.关联式对微肋管内表面传热系数进行预测,发现Cavallini et al.关联式对微肋管内换热性能的预测能力最好,关联式预测值与实验值的平均误差、标准误差分别为-21.47%和21.94%。虽然Miyara et al.关联式预测值与实验值的平均误差、标准误差分别为16.21%、30.65%,但两者之间的误差范围为-47.12%～82.32%,说明在部分工况下Miyara et al.关联式对管内换热性能的预测仍存在较大误差。三个关联式中,Oliver et al.关联式的预测能力最差,预测值与实验值之间平均误差高达-54.93%,因此,实验根据现有实验数据对Oliver et al.关联式进行了修正,修正Oliver et al.关联式对管内换热性能的预测能力大大提高,预测值与实验值的平均误差、标准误差分别为-2.37%和10.77%。