工质和管材及管径对强化管内冷凝传热影响研究

为评估制冷工质和管材及管径对强化管的冷凝传热影响规律,采用实验方法对R410a和R22在内螺纹管内的冷凝传热进行了测试。所采用的管外径包括7 mm和9.52 mm,管材料包括铝和铜。制冷剂的冷凝温度为47℃,质量流速为200—400 kg/(m²·s),入口干度从0.1—0.8变化,出口干度比进口干度低0.1。研究结果表明,波状分层流和环状流的转变干度介于0.4—0.5之间。R22的冷凝压降显著高于R410a,且压降增速快于R410a;当干度介于0.2—0.4时,R410a和R22的冷凝传热系数较为接近。干度与PF呈负相关关系,干度的增加并没有带来PF的改善,R410a和R22的PF比较接近。7 mm铜管和铝管管壁导热热阻与制冷剂冷凝热阻之比小于2%,制冷剂侧冷凝热阻占主导地位;管径对冷凝传热的影响远高强化表面结构,随着管径的减小,剪切力和表面张力逐渐取代重力,成为主导力,有利于去除和稀释底部的液膜。     

R134a与R410A在空调工况下的性能比较

传统制冷剂R22的替代已是大势所趋.本文着重对R22的替代制冷剂R134a与R410A的传热、阻力压降特性进行实验比较研究,认为R410A在传热、阻力压降特性方面比R134a优良,作为R22的替代制冷剂,R410A应比R134a更有竞争力.     

采用新制冷剂R134a和混合制冷剂R410A的板式蒸发器性能研究

采用分布参数法对波纹型通道板式蒸发器建立数学模型,并进行了数值模拟.通过计算板内局部蒸发传热系数和压降可以简化板式蒸发器内复杂三维网状流动的传热特性.针对应用较广的R134a和R410A制冷剂来比较和分析板式蒸发器在小的温差下的传热性能.在3种不同的计算工况下简要分析了各种热力参数的变化对蒸发器整体传热性能的影响.不同的制冷剂,其传热系数和压降差别较大,相同工况下采用R410A替代R22,板式蒸发器的传热性能可提高8.5%～10.0%,且压降可大幅降低.     

R410A和R22在小管径水平管内冷凝换热特性研究

本文搭建了冷凝换热实验台,对R410A和R22管内冷凝换热系数性能进行对比研究,实验工况为质量流速200~800kg/(m^2·s)、饱和温度40℃、干度0~1、5 mm外径水平光滑铜管,分析了质量流速和干度对管内冷凝换热的影响,并将应用于传统管道的关联式与实验所得数据进行对比。结果表明:冷凝换热表面传热系数与质量流速和干度呈正相关,高干度区域时的冷凝换热表面传热系数增幅显著;M. M. Shah[4]关联式来预测实验数据的效果并不理想,与实际值相比偏差最大可达60%,但是预测低质量流速和低干度区的数据较为理想;当质量流速较小(G=200 kg/(m^2·s))时,R410A的冷凝换热表面传热系数要低于R22;随着质量流速的增大(G=400 kg/(m^2·s)),二者冷凝换热表面传热系数的差距减小;当达到中高质量流速(G=600kg/(m^2·s))时,R410A的冷凝换热表面传热系数与R22的相似;当质量流速继续增大(G=800 kg/(m^2·s))时,R410A的冷凝换热表面传热系数随着干度的增大开始高于R22的。     

R22替代工质冷凝器的性能研究

本文基于传热单元法,建立了冷凝器的稳态分布参数模型.分别以R22及其3种替代产品(R407C,R410A,R134A)为工质,运用该模型详细比较了在流量变化及风量变化两种情况下2排管冷凝器的换热和流动特性.结果表明:无论是随着管内冷媒流量的变化,还是随着迎风面风速的变化,换热量及进出口压降的变化趋势基本一样.四种工质中,R410A性能较好(换热量最大,压降最低),但其冷凝压力比R22高出60%左右,R134A由于压降较大,两者都不是R22的理想替代物.R407C与R22在换热量及压降方面最为接近,是其理想的替代工质.     

流路布置对R22替代工质冷凝器性能的影响研究

基于相关文献提出的冷凝器分布参数模型,分别以R22的3种替代产品（R407C,R410A,R134A）为工质,分析了4种不同流路布置的2排管冷凝器的换热和流动特性,并与以R22为工质的冷凝器进行了性能比较。结果表明：采用R22的3种替代工质时,冷凝器性能的变化规律基本一样,4种流路布置中,在随着管内冷媒流量的变化和随着冷凝器迎风面风速的变化两种工况下,逆流换热效果最好,其次是错流,顺流最差;在与R22为工质的冷凝器性能比较中,采用3种替代工质的冷凝器换热量及进出口压降的变化趋势基本一样;在3种替代工质中,R410A性能较好,换热量最大、压降最低,但其冷凝压力比R22高出60%左右,R134A压降较大,这两种都不是理想替代物,而R407C与R22在换热量及压降方面最为接近,是其理想的替代工质。     

R22替代工质回热循环的性能分析

R22的替代工质的制冷性能通常比R22差，采用回热循环是改善循环性能的一种方法，但是增加回热器会带来成本的增加，而且不同的制冷剂在回热循环中的COP及容积制冷量的变化也是不同的。针对上述问题，分析回热循环的特性和6种制冷剂（R290，R1270，R134a／R1270（0．45／0．55），R134a／R290（0．6／0．4），R407C和R410A）在回热循环中的容积制冷量和COP的变化特点。研究结果表明，R134a,／R1270，R290和R134a／R290系统使用回热器后，性能改善较大，R410A系统只有在高冷凝温度、高过热度时才有必要使用回热器，其余替代工质系统使用回热器，其系统性能改善不明显。     

R22和R410A在水平微肋管内冷凝性能的实验研究

建立无润滑油的实验台,以R22和R410A为工质,测试微肋管的传热系数,并将其结果进行比较。从实验数据可以得出,R22传热系数最高,R410A的压降值最小,该管较好地验证了实验的正确性,同时说明了实验管的高效性。     

板式蒸发器换热性能的数值模拟Ⅱ:结果及分析

在已建立的数学模型的基础上,对板式蒸发器换热能力进行了数值模拟.针对应用较广的R134a和R410A制冷剂来比较和分析板式蒸发器在小的温差下的换热性能.在三种不同的计算工况下简要分析了各种热力参数的变化对蒸发器整体换热性能的影响.不同的制冷剂,其换热系数和压降差别较大,相同工况下采用R410A替代R22,板式蒸发器的换热性能可提高8.5%～10.0%,且压降可大幅降低.     

小通道钎焊板式冷凝器性能仿真与实验研究

本文针对当量直径为1.5 mm的小通道钎焊板式冷凝器的换热和压降特性进行了仿真和实验研究。采用有限体积法建立了一维稳态分布参数模型,对R134a和R1234yf两种制冷剂在板间冷凝换热的性能进行仿真模拟,并对模型进行了实验验证。实验结果表明:本文所建立的仿真模型精度较高,换热性能平均误差为4%,压降平均误差为16%,可用于分析换热器的整体性能。最后用此模型仿真对比了R134a和R1234yf在小通道钎焊板式换热器内的冷凝换热特性,结果显示,在相同工况下,用R1234yf替代R134a,传热系数平均下降9%,压降平均下降8%。     

Condensation and evaporation heat transfer of R410A inside internally grooved horizontal tubes

Heat transfer coefficient and pressure drop were measured for condensation and evaporation of R410A and HCFC22 inside internally grooved tubes. The experiments were performed for a conventional spiral groove tube of 8.01 mm o.d. and 7.30 mm mean i.d., and a herring-born groove tube of 8.00 mm o.d. and 7.24 mm mean i.d. To measure the local heat transfer coefficients and pressure drop, the test section was subdivided into four small sections having 2 m working length. The ranges of refrigerant mass flow density was from 200 to 340 kg/(m² s) for both condensation and evaporation of R410A and HCFC22, and the vapour pressure was 2.41 MPa for condensation and 1.09 MPa for the evaporation of R410A. The obtained heat transfer data for R410A and HCFC22 indicate that the values of the local heat transfer coefficients of the herring-bone grooved tube are about twice as large as those of spiral one for condensation and are slightly larger than those of spiral one for the evaporation. The measured local pressure drop in both condensation and evaporation is well correlated with the empirical equation proposed by the authors.     

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.     

Condensation of refrigerants in a multiport tube with rectangular minichannels

This study investigated the condensation heat transfer and pressure drop characteristics of refrigerants R134a, R32, R1234ze(E), and R410A in a horizontal multiport tube with rectangular minichannels, in the mass velocity range of 100–400 kg m⁻² s⁻¹ and saturation temperature set at 40 and 60 °C. The effect of mass velocity, vapor quality, saturation temperature, refrigerant properties, and hydraulic diameter of rectangular channels on condensation characteristics is clarified. A new correlation is proposed for predicting the frictional pressure drop for condensation flow in minichannels. A heat transfer model for condensation heat transfer in rectangular minichannels is developed considering the flow patterns and effects of vapor shear stress and surface tension. Then, based on this model, a new heat transfer correlation is proposed. The proposed correlations successfully predict the experimental frictional pressure drop and heat transfer coefficients of the test refrigerants in horizontal rectangular minichannels.     

Condensation heat transfer and flow pattern inside a herringbone-type micro-fin tubeTransfert de chaleur lors de la condensation et configuration de l'écoulement à l'intérieur d'un tube à microailettes à chevrons

Condensation heat transfer and pressure drop of R410A and R22 in a newly proposed herringbone-type micro-fin tube are measured and compared to those of a helical micro-fin tube and a smooth tube. The heat transfer coefficient of the herringbone micro-fin tube is higher than that of the helical micro-fin tube in the high mass velocity region, while it has slightly lower value in the low mass velocity region. Pressure drop of the herringbone micro-fin tube is, however, higher than that of the helical micro-fin tube. Flow patterns of the herringbone micro-fin tube are observed and the heat transfer enhancement mechanism is discussed. The heat transfer coefficient and pressure drop of the helical micro-fin tube is predicted well with previously proposed correlations, while those of the herringbone-type micro-fin tube has higher value than the predicted values. Preliminary correlations for the pressure drop and the heat transfer coefficient are proposed for the herringbone micro-fin tube.     

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%.     

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.     

R404A和R410A应用于平行流冷凝器的模拟分析比较

采用分布参数法对平行流冷凝器建立数学模型，对目前广泛使用的制冷剂R134a和低温制冷剂R404A和R410A在平行流冷凝器中的换热和流动性能进行模拟计算和分析比较。分别在相同和不同工况下。比较3种制冷剂的换热系数及压降等换热和流动性能参数。结果表明，在采用平行流冷凝器的汽车空调工况范围内，R410AR404A的流动和传热性能均优于R134a，更适宜用于汽车空调用平行流冷凝器。     

Condensation of pure and near-azeotropic refrigerants in microfin tubes: A new computational procedure

Microfin tubes are widely used in air cooled and water cooled heat exchangers for heat pump and refrigeration applications during condensation or evaporation of refrigerants. In order to design heat exchangers and to optimize heat transfer surfaces, accurate procedures for computing pressure drops and heat transfer coefficients are necessary. This paper presents a new simple model for the prediction of the heat transfer coefficient to be applied to condensation in horizontal microfin tubes of halogenated and natural refrigerants, pure fluids or nearly azeotropic mixtures. The updated model accounts for refrigerant physical properties, two-phase flow patterns in microfin tubes and geometrical characteristics of the tubes. It is validated against a data bank of 3115 experimental heat transfer coefficients measured in different independent laboratories all over the world including diverse inside tube geometries and different condensing refrigerants among which R22, R134a, R123, R410A and CO₂.