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。     

近临界压力下R410A在水平螺纹管内的冷凝换热研究

为研究近临界压力下R410A在水平螺纹管内的冷凝换热特性,在质量流率为200 ～ 800 kg/(m2·s)、干度为0.1～0.9、冷凝压力比为0.8和0.9时,对3种不同管径的内螺纹铜管(5、7和9.52 mm)进行单管冷凝换热实验.结果 表明:冷凝换热系数随着质量流率和干度的增加而增加,冷凝压力越接近临界压力,换热...     

多孔表面管沸腾传热试验研究

针对烧制成多孔表面管,进行了传热性能研究,试验表明：多孔管可以显著地强化多孔侧沸腾传热,民同规格光滑管传热性能试验对比,其沸腾给热系数比光滑管提高５－６倍。     

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。     

强化管内沸腾换热实验研究

主要研究在低过热度下微槽对流动沸腾换热特性的影响,分别以单工质甲醇和甲醇与甲苯的混合物为工质对不同流量情况下光管、直槽管和螺旋槽管的流动沸腾换热特性进行了实验研究。研究结果表明：对单工质甲醇来说,螺旋槽管可以明显起到强化传热作用,而且流量越低,强化传热效果越明显。对混合工质来说,当流量较低时,螺旋槽管强化传热效果不明显,而在流量较高时,强化传热效果比较明显。无论是单工质还是混合工质,直槽管在实验所能达到的壁面温度条件下不能起到明显的强化传热效果。还给出了螺旋槽管强化传热的定性解释。     

Boiling Heat Transfer and Pressure Drop of a Refrigerant R32 Flowing in a Small Horizontal Tube

In this study, experiments were performed to examine characteristics of flow boiling heat transfer and pressure drop of a low global warming potential refrigerant R32 flowing in a horizontal copper circular tube with 1.0 mm inside diameter for the development of a high-performance heat exchanger using small-diameter tubes or minichannels for air conditioning systems. Axially local heat transfer coefficients were measured in the range of mass fluxes from 30 to 400 kg/(m²·s), qualities from 0.05 to 1.0, and heat fluxes from 2 to 24 kW/m² at the saturation temperature of 10°C. Pressure drops were also measured in the rage of mass fluxes from 30 to 400 kg/(m²·s) and qualities from 0.05 to 0.9 at the saturation temperature of 10°C under adiabatic condition. In addition, two-phase flow patterns were observed through a sight glass fixed at the tube exit with a digital camera. The characteristics of boiling heat transfer and pressure drop were clarified based on the measurements and the comparison with data of R410A obtained previously. Also, measured heat transfer coefficients were compared with two existing correlations.     

石墨烯纳米制冷剂核态池沸腾换热的实验研究

为分析单层石墨烯纳米片对核态池沸腾换热的影响机理,对基液为R141b、分散相为单层石墨烯纳米片的纳米制冷剂的核态池沸腾换热特征进行了测定,采用Hot Disk热物性分析仪和铂金板法分别测定了石墨烯纳米制冷剂的热导率和表面张力,采用接触角测量仪和扫描电子显微镜（SEM）观测了沸腾后加热表面的润湿性和形貌特征。实验中,单层石墨烯纳米片的质量百分含量（ω）为0.02%~0.50%,实验压力为一个标准大气压,热流密度为20~200 kW/m2。实验结果表明：单层石墨烯纳米片的加入,使制冷剂R141b的核态池沸腾换热得到强化;当ω=0.2%时,换热系数提高比例出现峰值,为57.7%。伴随ω的增加,石墨烯纳米制冷剂的热导率增大、表面张力减小,沸腾表面润湿性增强且微腔数先增后减,综合作用的结果导致存在一个最佳的单层石墨烯纳米片浓度（即ω=0.2%）使换热系数最高。     

“球囊夹紧法”取出锁骨下动脉支架推送杆断裂残端一例

针对水平光滑管和微肋管,基于FLUENT平台对制冷剂管内沸腾传热特性进行了数值模拟,研究质量流量、热流密度及干度等因素对制冷剂R245fa沸腾换热系数的影响。模拟结果表明:沸腾换热系数随着制冷剂质量流速与热流密度的增加而提高;随着干度的增加,换热系数先增加再降低,并在x=0.7时达到极大值;相比光滑管,微肋管内制冷剂的沸腾传热系数能提高10%~25%。     

Enhanced Boiling Heat Transfer of Water/Salt Mixtures in the Restricted Space of the Compact Tube Bundle

In desalinization devices and some heat exchangers making use of low-quality heat energy, both the wall temperature and the heat flux of the heated tubes are generally quite low, hence cannot cause boiling in flooded-type tube bundle evaporators with a large tube spacing. But when the tube spacing is quite small, incipient boiling can occur in the restricted space and results in higher heat transfer than that in a falling-film evaporator or during pool boiling at the same heat flux. This study experimentally investigates the effects of the tube spacing, the positions of tubes, and the salt-water concentration on bundle boiling heat transfer of salt water in the restricted space of the compact tube bundle evaporator under atmospheric pressure. The experimental results provide a restricted space boiling database for salt water in the compact tube bundle. Of particular importance is information concerning the influences of the tube spacing of the tube bundle and the concentration of salt water in desalination evaporators.     

强润湿性液体池沸腾传热的实验研究和机理分析

对强润湿性液体的池沸腾传热实验而言,本文提出了行之有效的实验程序,并严格按照实验程序进行了R113池沸腾传热的实验研究,具体研究了表现老化和液体过冷度对池沸腾传热曲线及起沸点的影响,实验中观察到了三个反常现象,最后,从强润湿性液体的沸腾传热机理的角度对其给出了相应的解释。     

Experimental Investigation of M-Shape Heat Transfer Coefficient Distribution of R123 Flow Boiling in Small-Diameter Tubes

This article presents a study of flow boiling of R123 in two small-diameter silver tubes with inner diameters of 1.15 mm and 2.3 mm. The experiments have been accomplished for a wide range of quality variation (0.01–0.9), mass flow rate (650–3000 kg/m²s), and heat fluxes (40–80 kW/m²). The saturation temperature ranged from 30 to 70°C. In the experiments a peculiar distribution of heat transfer coefficient leading to development of two maxima in its distribution with respect to quality has been observed. Such behavior was seen in both sizes of tubes.     

水平螺旋管外冷凝换热的理论分析与实验研究

用改进的Nusselt—Rohsenow方法分析了水平螺旋管外的层流膜状冷凝换热。考虑了粘性和重力的影响以及离心力对液膜流动和换热的影响。通过分析,得到了较通用的无量纲冷凝方程,并导出适合于其它几何形状换热元件的冷凝换热方程。经数值计算得到了在不同条件下的局部Nusselt数(Nu)和平均Nusselt数(?)。理论结果得到了实验数据的验证,并推荐了实用的传热计算公式。     

微细通道内R290流动沸腾换热特性实验研究

对R290制冷剂在微细通道内的流动沸腾换热特性进行了实验研究。研究管径分别为1和2 mm,热流密度为20~65 k W/m~2,质量流率为100~200 kg/m~2·s,饱和温度为15和25℃,干度范围为0.1~0.9。通过实验数据分析管径、热流密度、质量流率、饱和温度对流动沸腾换热的影响。结果表明:随着管径的下降,换热系数呈现出大幅上升的趋势,其平均增幅为31%;随着热流密度的上升,换热系数呈现出大幅上升的趋势,其平均增幅达到了131%;随着质量流率的上升,换热系数呈现出小幅上升的趋势,其平均增幅为14%;随着饱和温度的上升,大部分换热系数呈现出小幅上升的趋势,其平均增幅为12.6%。     

Falling Films on Arrays of Horizontal Tubes with R-134a,Part I: Boiling Heat Transfer Results for Four Types of Tubes

A new falling film heat transfer test facility has been built for the measurement of local heat transfer coefficients on a vertical array of horizontal tubes, including flow visualization capabilities, for use with refrigerants. Presently, the facility has been used for evaporation tests on four types of tubes at three tube pitches and three nominal heat flux levels for R-134a at 5°C. A new method for determining local heat transfer coefficients using hot water heating has been applied, and test results for a wide range of liquid film Reynolds numbers have been measured for arrays made of plain, Turbo-BII HP, Gewa-B, and High-Flux tubes. The results show that there is a transition to partial dryout as the film Reynolds number is reduced, marked by a sharp falloff in heat transfer. Above this transition, the heat transfer coefficients are nearly insensitive to the film Reynolds number, apparently because vigorous nucleate boiling is always seen in the liquid film. The corresponding nucleate pool boiling data for the four types of tubes were also measured for direct comparison purposes. Overall, about 15,000 local heat transfer data points were obtained in this study as a function of heat flux, film Reynolds number, tube spacing, and type.     

R1234yf Flow Boiling Heat Transfer Inside a 2.4-mm Microfin Tube

ABSTRACT

This paper presents an experimental study on R1234yf flow boiling inside a mini microfin tube with an inner diameter at the fin tip of 2.4 mm. R1234yf is a new refrigerant with an extremely low global warming potential (GWP <1), proposed as a possible substitute for the common R134a, whose GWP is about 1300. The mass flux was varied between 375 and 940 kg m^?2 s^?1, heat flux from 10 to 50 kW m^?2, and vapor quality from 0.1 to 1. The saturation temperature at the inlet of the test section was kept constant and equal to 30°C. The wide range of operative test conditions permitted highlighting the effects of mass flux, heat flux, and vapor quality on the thermal and hydraulic behavior during the flow boiling mechanism inside such a mini microfin tube. The results show that at low heat flux the phase-change process is mainly controlled by two-phase forced convection, and at high heat flux by nucleate boiling. The two-phase frictional pressure drop increases with increasing both mass velocity and vapor quality. Dry-out was observed only at the highest heat flux, at vapor qualities of around 0.94–0.95.     

Pool Boiling Heat Transfer Performance of a Dielectric Fluid With Low Global Warming Potential

Due to growing concerns over anthropogenic effects on the climate, there is increasing need to replace engineered fluids of high global warming potentials (GWPs), such as hydrofluorocarbons (HFCs) and perfluorocarbons (PFCs), with more environmentally friendly alternatives in thermal management systems. This article presents experimental data and compares various correlations for predicting the pool boiling heat transfer coefficient of a new low-GWP fluid, FK-649. Using a pressurized boiling facility with a smooth aluminum heater, the critical heat flux (CHF) and heat transfer coefficient were measured for the pool boiling of FK-649 at various saturation conditions. The commonly used refrigerant tetrafluoroethane (R-134a) is tested in the same pressurized facility to act as a benchmark for the new fluid. While R-134a exhibited a higher heat transfer coefficient and CHF, this behavior is expected from the fluid properties. Two-phase heat transfer performance of FK-649 is expected to be similar to that of the fluorocarbon FC-72. Experimental data are compared to predictions using the Rohsenow, Borishanskii–Mostinski, Stephan and Abdelsalam, Cooper, and Leiner correlations. Where applicable, empirical constants are obtained by using a least-squares fit to experimental data. The Rohsenow correlation yielded the best result with a new surface–fluid constant C _s,_f of 0.0037 for FK-649.     

Simulation Study on Flow and Heat Transfer of Cross-Arrangement Tube Bundle

The air precooler of SABRE engine(Synergetic Air-Breathing Rocket Engine) is a tube bundle type of heat exchanger consisting of thousands of small tubes.The cold helium gas flows in the small tube bundle,and the high-speed hot air circulates outside the tubes and exchanges heat with the helium in the tubes.In this study,the cross-arrangement tube bundle consisting of small tubes with outer diameter of 1mm was taken as the research object.The system simulation calculation and data sorting were ca...     

Prediction of Local Bundle Boiling Heat Transfer Coefficients: Pure Refrigerant Boiling on Plain,Low Fin,and Turbo-BII HP Tube Bundles

New correlations predicting the local boiling heat transfer coefficients for vertical flow over a tube bundle have been developed. These correlations were developed for three different classes of tubes: smooth, low-finned, and Turbo-BII HP. These correlations are based on experiments with refrigerants R-134a, R-507A, and R-410A, at a saturation temperature of 4.4°C (40°F). Each one accounts for the local flow structure and/or local geometry particular to each tube. Each one also uses a void fraction model designed specifically for vertical two-phase flow in a tube bundle as an integral part of the correlation. The correlations have been compared to recent local bundle boiling data and show good agreement to within ± 20%.     

水平三维内微肋管在局部蒸干区的沸腾换热及其关联式

为了得到不同流型下的换热性能 ,以 R1 3 4a为实验工质在一种水平三维内微肋管内进行了流动沸腾换热实验研究 ,通过可视化等措施对得到的主要流型及其转换曲线表示在 G-x图上。对局部蒸干区的沸腾换热特点进行了讨论 ,并根据此区域换热的特点 ,沿周向管壁分成两个部分 ,即 :蒸干部分和非蒸干部分。对于非蒸干部分又分为淹没微肋的底部液体 ,且认为同环状流换热机理相同 ,而另一部分认为液休带领在沟槽中 ,从而得到了此区域的换热实验关联式 ,此换热关联式与实验值的最大偏差在± 1 6%以内