R134a在水平内微翅管管内凝结换热的实验研究

对制冷剂R134a在水平强化换热管管内的凝结换热性能进行了实验研究。实验管为两种内微翅管,分别命名为A管和B管。实验件采用套管结构,强化内管外表面和外管内表面之间(管间)走乙二醇水溶液。实验过程中管内冷凝温度为51℃,管间乙二醇水溶液的流速为3.35 m/s,乙二醇水溶液的进口温度根据制冷剂的质量流速做相应调整,以保证试件出口制冷剂有一定的过冷度。实验结果表明:两种水平强化管的管内冷凝换热系数均随着制冷剂质量流速的增加而增大,在制冷剂质量流速从300 kg/(m2.s)增加到700kg/(m2.s)时,A管的管内冷凝换热系数比B管高1.87%到6.28%,而B管的制冷剂流动阻力比A管高9.56%到11.05%,A管的结构优于B管。     

三维双侧水平强化管R245fa凝结换热性能试验

由于制冷剂R11和R123对臭氧层有破坏作用,为完成环保新工质R245fa对R11和R123的替代工作,对R245fa在内螺纹外斜翅片的三维双侧强化管外的凝结换热性能进行试验。数据处理过程中,采用Wilson图解法获得管内水侧对流换热系数及其计算关联式,再利用热阻分离法获得管外凝结换热系数。研究表明:试验中管内对流换热系数高于管外冷凝换热系数,所以管外侧的传热热阻是占据主导地位的传热热阻;相对于光管,R245fa在三维双侧强化管管内换热强化换热倍率为3.58,管外强化换热倍率为2.48;对实验数据进行拟合,得到管外换热系数的变化规律和凝结换热关联式。     

两种三维双侧强化管管外R245fa凝结换热的实验对比

为研究环保制冷剂R245fa在水平强化管外凝结换热特性及表面结构对管内外换热性能的影响,分别对三维齿结构低肋管(A管)和斜翅管(B管)进行管外凝结换热实验。在数据处理方法上,采用Wilson-Gnielinski图解法获得管内水侧对流换热系数及其计算关联式,再利用热阻分离法获得管外凝结换热系数。实验结果分析得出A管和B管的管内换热系数强化倍率分别为2.04和2.98,管外强化倍率分别为1.77～1.94,1.87～2.14,B管管内外换热性能都优于A管,造成两种强化管内外换热性能差异的主要因素是强化管内的螺纹高度和管外翅化比。     

R410A和R134a在水平强化管内的冷凝换热性能

为研究强化管的冷凝换热性能和强化换热机理,采用实验的方法对R410A在外径6.35和8 mm的光管及内螺纹管(螺旋角为18°和28°)中的冷凝换热性能进行了研究,并与R134a进行对比,实验工况:冷凝温度30和35℃,质量流速400～1 100 kg/(m~2·s)。结果表明:螺纹管冷凝传热系数强化倍率均显著大于内表面扩展倍率;R134a强化因子大于R410A,强化管对粘度、表面张力较大的制冷剂强化效果更显著;8 mm管强化因子大于6 mm,管径较大时,换热提升效果更好;水侧雷诺数为14 000时,8 mm、28°螺纹管在质量流速为500 kg/(m~2·s)时,管内外侧热阻接近,强化效果较好。     

水平单管内换热实验研究

利用隔膜泵作为系统动力输出源,搭建了单管内传热和流动测试实验台,对制冷剂R22在水平单管内的换热性能进行了实验研究,考察了不同蒸发温度和不同冷凝温度对总传热系数、制冷剂表面换热系数和管内压降的影响.实验结果表明:总传热系数和制冷剂表面换热系数均随着蒸发温度和冷凝温度的上升而增大;管内压降随着蒸发温度的上升而减小,随着冷凝温度的上升而增大;对于同一根实验管,在相同的冷却水流量和制冷剂质量流量下,最佳蒸发工况为10℃;冷凝实验中,总传热系数和制冷剂表面换热系数在40℃时高于其他两种冷凝温度时的值,但35℃冷凝时,管内压降高于其他两种工况.     

水平光管内R410A与R134a流动冷凝换热特性的对比研究

为研究R410A与R134a在水平光管内的冷凝换热特性,在管内冷凝换热试验台上进行冷凝试验,分析质量流量、冷凝温度、测试水雷诺数Re、管径和制冷剂物性对换热系数和压降的影响。研究表明:换热系数、压降均随着质量流量的增加而变大,随冷凝温度的升高而减小,换热系数随测试水雷诺数Re的增加而减小,而测试水雷诺数Re对压降的影响相对较小;尽管R410A的换热系数随管径的减小而增大,而管径对R134a换热系数的影响并不显著,R134a与R410A的压降均随管径的减小而增大;单位压降换热系数随质量流量的增加而减小; Cavallini et al.关联式可较好预测R410A与R134a在光管内换热系数,而Shah关联式只能用于预测R134a的换热系数。     

在螺旋套管冷凝器内R417a凝结换热性能的实验研究

在空气源热泵热水器中,使用混合型制冷工质R417a,冷凝器采用螺旋套管换热器且套管环形空间内制冷剂与内管中的水逆流换热。对在不同工况下,环形通道内R417a的凝结换热特性进行实验研究和理论分析。实验的工况为:水的体积流量为0.60～1.00 m~3/h,水的流速为0.58～0.98 m/s,冷凝器进水温度为20.0～55.0℃。实验结果表明:环境温度为15.0℃,螺旋套管内R417a的凝结传热系数随冷凝饱和温度的升高而减小,局部凝结传热系数随干度的增大而增大。当冷凝器进水体积流量为0.60 m~3/h,饱和冷凝温度由40.0℃增加至60.0℃时,冷凝器制冷剂侧凝结传热系数从3 839.0减小至2 372.0 W/(m~2·K),约减少了38.1%。     

R410A在水平强化管外蒸发换热特性研究

搭建了一个单管管外流动蒸发换热实验台,研究工质R410A在两种双侧强化管外流动蒸发换热特性。实验段分别为一根长2 000 mm,外径为25.4 mm的光滑管和两根相同尺寸的双侧强化管。实验工况:蒸发饱和温度为5～10℃,水的进口温度为8～18℃,水流量为0.6～1.6 m~3/h。在处理数据过程中采用G-W图解法获得管内水侧对流换热系数,再利用热阻分离法获得管外蒸发换热系数。结果表明:与光滑管表面传热系数相比,TLD型管的管内、管外强化倍率分别为3.49～3.7和4.78～8.86;EX2型管的管内、管外强化倍率分别为3.25～3.68和5.9～9.23;EX2型管管外换热性能较好,TLD型管管内换热性能较好。     

混合制冷剂在空气源热泵热水器系统中替代R22的可行性研究

空气源热泵热水器系统所用制冷剂主要包括R410A,R407C,R134a,它们的GWP值较高,会对大气环境造成不利影响。将R1234ze/R152a,R290/R1234ze,R152a/R134a分别按一定比例组成混合制冷剂,通过理论分析混合制冷剂的环境影响指数、热力学性能、安全性以及经济性能等因素,并与R410A,R407C,R134a进行对比。分析结果表明,对于配比为3∶7的R290/R1234ze混合制冷剂,其对环境影响较小,饱和压力线与R22相接近,饱和液体密度为R22的1/2,饱和气体比热容大于R22,COP为R22的90%、压力比比R22低5.2%,排气温度比R22和R410A分别低18,20℃。综上可知,混合制冷剂R290/R1234ze是一种性能良好的近共沸混合制冷剂,可替代R22。     

T型沸腾强化换热管传热性能的实验研究

对一种T型翅片内螺纹沸腾强化换热管进行换热性能实验研究,管外以制冷剂R134a为工质,管内以水为介质,在定热流密度(q=9 000 W/m2)与定水流速(v=1.5 m/s,v=2.6 m/s)的工况下得到一系列实验数据.利用Wilson图解法得到管内外的换热系数,并与理论光管计算值进行比较,得出T型翅片管管内外沸腾换...     

Generalized neural network model of alternative refrigerant (R600a) inside a smooth tube

The correct prediction of refrigerant condensation heat transfer performance is important for design of condensers. A generalized neural network correlation for condensation heat transfer coefficient of alternative refrigerant R600a inside horizontal tube has been developed in this paper. Mass flow rate, vapor qualities, saturation temperature, difference value temperature are selected as the input parameters, while the Nusselt number and heat transfer coefficient as the output. Three-layer network is used for predicting the Nusselt number and the heat transfer coefficient. The number of the neurons in the hidden layer was determined by a trial and error process together with cross-validation of the experimental data evaluating the performance of the network and standard sensitivity analysis. The trained network gives the best values over the correlations with less than 4% mean relative error. The experimental data of the heat transfer coefficients of R600a, a hydrocarbon refrigerant, in a horizontal smooth copper tube with an inner diameter of 4 mm and outer diameter of 6 mm are from Agra et al. [O. Agra, “Condensation of refrigerants in a horizontal tube in annular flow regime”, PhD thesis Yildiz Technical University, 2007]. The condensing heat transfer coefficients obtained from the experimental study were seen to be consistent by ± 20% with the correlations developed by Shah [M.M. Shah, A general correlation for heat transfer during film condensation inside pipes, Int. J. Heat Mass Transfer 22 (1979) 547–556], Travis [D.P. Traviss, W.M. Rohsenow, A.B. Baron, Forced convection condensation inside tubes: a heat transfer equation for condenser design, ASHRAE Trans. 79 (1972) 157–165] and Cavallini-Zecchin [A. Cavallini, R. Zecchin, A dimensionless correlation for heat transfer in forced convection condensation, Proceedings of the Fifth International Heat Transfer Conference, vol. 3, 1974, pp. 309–313]. And it is seen that results from the trained network shows good agreement with the experimental data and better results than the correlations given by Shah, Cavallini and Travis.     

Nucleate pool-boiling enhancement outside a horizontal bank of twisted tubes with machined porous surface

Nucleate pool boiling of refrigerants is of important application in the flooded evaporator of refrigeration and air-conditioning system. Many surface geometries involve machined porous surface have been adopted to enhance the nucleate pool boiling heat transfer of refrigerants. Nucleate pool-boiling performance of R134a and R142b outside a horizontal bank of twisted tubes with machined porous surface (T-MPS tubes) was investigated in this paper. The experimental results showed that the T-MPS tube bank could enhance boiling heat transfer evidently. The enhancement ratios of R134a from the T-MPS tube bank were 1.4–1.7 and the maximum enhancement ratio of R142b could reach up to 4.4. Analyzing the tube bank effects of boiling heat transfer for R134a and R142b, the overall trend showed that the boiling heat transfer performance of the T-MPS tube bank was inferior to that of single T-MPS tube slightly.     

Heat transfer and pressure drop penalization terms (exergy losses) during flow boiling of refrigerants

Heat transfer coefficient and pressure drop correlations are used to analyse the boiling heat transfer performance potentials in plain evaporator tubes of several conventional refrigerants and two newer fluorinated propene isomers possessing low global warming potentials. These correlations are used to calculate two penalization quantities expressed in terms of the refrigerant saturation temperature drop due to pressure drop and the driving temperature difference. These penalization terms are combined into a single Performance Evaluation Criterion dubbed Total Temperature Penalization (TTP). Using the two penalization terms and the TTP, several refrigerants, including the newer alternatives R1234yf (CF₃CF=CH₂) and R1234ze(E) (CF₃CH=CHF ) , are evaluated for their boiling heat transfer performance potentials in plain evaporator tubes. Furthermore, the usefulness of the technique is illustrated through several examples of the optimization of evaporator tube length. Copyright © 2012 John Wiley & Sons, Ltd.     

缩放管管外流动沸腾换热的数值模拟与场协同分析

本文利用FLUENT软件对制冷剂R134a在光管和缩放管水平管外沸腾传热进行了三维数值模拟,得到了其饱和泡状沸腾过程中体积含汽率的分布规律,并比较了它们的换热系数,结果表明:缩放管外侧能够很好地强化沸腾传热.此外,通过改变边界条件分析了质量流量、热流密度的变化对缩放管管外沸腾换热系数的影响.最后应用场协同理论,从局部换热角度分析其强化机理.研究表明:缩放管水平管外沸腾换热得到强化的原因是其凹槽前后的速度场与温度梯度场之间夹角更小,协同程度更好.     

Experimental Study on Pool Boiling Heat Transfer for R22, R407c,and R410a on a Horizontal Tube Bundle With Enhanced Tubes

An experimental test rig for study of the pooling-boiling heat transfer performance of pure and mixed refrigerants was designed and established. The test section is a horizontal tube bundle evaporator with nine mechanically fabricated porous surface tubes in a triangular layout. With this test system, the heat transfer coefficients of the nucleate boiling in the evaporator were measured for R22, R407c, and R410a. Extensive experimental measures were made for those pure and mixed refrigerants at different heat fluxes from 10 kW m^?2 to 43 kW m^?2 at saturation temperature of 9°C. Comprehensive measured data are presented in this paper. From experimental results, it is found that the pool boiling heat transfer coefficient increases with increasing the heat flux. It is also found that boiling heat transfer coefficients for R410a are 1.25–1.81 times and 6.33–7.02 times higher than that for R22 and R407c, respectively. The experimental correlations for the pool boiling heat transfer coefficients of R22, R407c, and R410a on the present enhanced tubes bundle are developed. The thermal resistance analysis reveals that the thermal resistance of the water side is a controlling factor for the evaporator for R22 and R410a. However, for R407c, the thermal resistance of the refrigerant side is slightly higher than that of the water side. To further improve the overall heat transfer coefficient in the evaporator of R22 and R410a, the enhancement for both the inside and outside is equally important, and the effectively enhanced boiling surface must be developed for the evaporator of R407c.     

Theoretical Analysis of Optimal Subcooling for Single Vapor-Compression Refrigeration Systems

A finite temperature difference heat transfer method and irreversibility analysis have been developed for investigating the effects of subcooling on coefficient of performance, cooling water pressure drop of condenser, and heat exchanger area for R1234yf, R1234ze, R22, R134a, and R410A in a single vapor-compression refrigeration system. In order to satisfy the increasing cooling load for subcooling in a condenser, the heat exchanger size or cooling water pumping power that corresponds to initial cost or operating cost, respectively, is increased. The optimum degree of subcooling in a refrigeration system with superior performance and least initial cost or operating cost is obtained numerically. The results show that the maximum coefficient of initial cost saving and coefficient of operating cost saving and their corresponding optimum degree of subcooling increase with condensation temperature. At a higher inlet temperature of cooling water, the optimum degree of subcooling turns out to be smaller for all refrigerants. The results are expected to facilitate the prospective design of a vapor-compression refrigeration system for using alternative refrigerants.     

Influence of hydrodynamic instability on the heat transfer coefficient during condensation of R134a and R404A refrigerants in pipe mini-channels

This paper presents the results of an investigation of the influence of hydrodynamic instabilities on heat transfer intensity during the condensation of R134a and R404A refrigerants in pipe mini-channels. The heat transfer coefficient h is a measure of the effectiveness of the condensation process. It is particularly important to determine the value of the coefficient in the two-phase condensation area in a compact condenser. In other condenser areas (i.e., precooling of superheated vapor and subcooling of condensate), the heat efficiency is substantially smaller. Hydrodynamic instabilities of a periodic nature have an influence on size changes in these areas. A decrease in the heat transfer coefficient h in the two-phase area results in decreased intensity of the heat removal process in the whole condenser.The experimental investigations were based on the condensation of R134a and R404A refrigerants in horizontal pipe mini-channels with internal diameters of d = 0.64; 0.90; 1.40; 1.44; 1.92; 2.30 and 3.30 mm. Disturbances of the condensation process were induced with a periodic stop and a repetition of the flow of the refrigerant.In the range of frequencies, f = 0.25–5 Hz, of the periodically generated disturbances, an unfavorable influence on the intensity of the heat transfer during the condensation process in pipe mini-channels was identified. The reduction in the intensity of the heat transfer during the condensation process, which was induced with hydrodynamic instabilities, was presented in the form of the dependence of the heat transfer coefficient h on the vapor quality x and the frequencies f of the disturbances.The influence of the refrigerant, the diameter of the mini-channels and the frequency f on the damping phenomenon of the periodical disturbances in the pipe mini-channels was identified.     

Comparative investigations of the condensation of R134a and R404A refrigerants in pipe minichannels

The present paper describes the results of experimental investigations of heat transfer and pressure drop during the condensation of the R134a and R404A refrigerants in pipe minichannels with internal diameters d = 0.31–3.30 mm. The results concern investigations of the local heat transfer coefficient and a pressure drop in single mini-channels. The results were compared with calculations according to the correlations proposed by other authors. Within the range of the examined parameters of the condensation process in mini-channels produced from stainless steel, it was established that the values of the heat transfer coefficient may be described with Akers et al. and Shah correlations within a limited range of the mass flux density of the refrigerant and the mini-channel diameter. A pressure drop during the condensation of these refrigerants is described in a satisfactory manner with Friedel and Garimella correlations. On the basis of the experimental investigations, the authors proposed their own correlation for the calculation of local heat transfer coefficient α_x.     

高效换热管中R22传热特性的试验研究与分析

针对海水冷却的特点和要求,研究了Ｒ２２在高效换热管管内蒸发和管外冷凝的传热和压降特性,并就传热特性与相关文献报道进行了比较。实验结果表明,耐腐蚀铜合金高效换热管作为冷凝时,其传热性能比普通换热管要高出一倍左右。这些研究和分析对热泵空调换热器优化设计及海水高低温热源的应用具有重要意义。