R134a在板式换热器中的凝结换热实验研究

本文对R134a在板式换热器内的凝结换热特性进行了实验研究,通过测量换热器中冷却水及板壁温度获得了局部凝结换热系数随蒸气干度、质量流量及热流密度的变化关系.实验结果表明,凝结换热系数随着蒸气干度增加而增加.文章还将实验结果与部分文献数据进行了比较与分析.本文的研究为换热准则关系式的发展提供了实验数据.     

高浓度CO_2存在时水蒸汽在V形纵槽表面凝结换热的实验研究

本文进行了CO_2-水混合蒸气在不锈钢V形纵槽表面进行了凝结换热实验研究。结果表明,CO_2质量浓度在80%~94%范围内,两V形纵槽表面的凝结换热特性均优于光滑平板。1 mm×2 mm纵槽冷凝表面的凝结换热系数比平板提高了8.51%~15.4%。在20%~94%浓度范围内,1 mm×2 mm冷凝特性均优于0.5 mm×1 mm冷凝块,所以适当增加肋片的高度和肋间距在一定程度上可以强化冷凝传热过程。     

有肋片的倾斜向下表面的凝结换热和表面水平时肋片对液膜稳定性的影响

本文给出了水蒸汽在有肋片的倾斜向下表面的凝结换热实验数据,特别分析了表面水平时肋片对相邻肋片间表面上的液膜的稳定性的影响。分析表明,肋片对液膜有引流作用,可以减小肋根处液体表面的圆角,从而增加肋片间的有效距离,提高液膜的不稳定性,增强水平表面的凝结换热能力,同时肋片本身也有凝结蒸汽的作用,得剑了肋片高度不同的水平向下表面的凝结换热关联式。     

斜翅型冷凝强化换热管传热性能的实验研究

对一种斜翅型外翅片带内螺纹的冷凝强化换热管进行传热性能的实验研究。管外冷凝换热的制冷剂为R134a,管内对流换热的介质为水。分别在定热流密度与定水流速的条件下进行一系列工况的实验,得到相应的实验数据。在定热流密度条件下,利用Wilson图解法得到管内的换热系数数据及相应的计算关联式。在定水流速的条件下,利用分离方法得到管外冷凝换热系数数据及相应的计算关联式。将强化管换热系数数据与光管换热系数的理论计算值进行了比较,结果表明:冷凝强化换热管管内对流换热的强化倍率为2.4,管外凝结换热系数随壁面过冷度的增加而增大,管外凝结换热的强化倍率为:1.78～3.92。     

高效凝结换热可视化实验研究

文中通过对实验数值的分析来研究冷凝传热传质问题,以套管式冷凝器为研究对象,设计出一种对冷凝段凝结换热过程进行可视化研究的实验台。研究的目的在于,寻求蒸汽管内换热由珠状凝结渐变为膜状凝结时的管长与热力参数之间的的特征关系,在相变发生的管长位置设法将凝结液提前排出,以维持较高的对流换热系数。通过研究表明:凝结状态转折点所对应的管长与蒸汽入口速度、流体压降以及蒸汽干度密切相关,并给出了具有指导意义的关联式,且实验值与理论计算值的误差值小于10%。     

氨的添加引发水蒸气凝结换热强化的实验研究

研究了添加极少量氨时,氨-水混合蒸气在水平圆管上的凝结传热特性。结果表明:由于氨的添加引发的Marangoni效应,水蒸气的凝结换热在实验工况范围内基本上都得到了强化。随着表面过冷度的增加,凝结换热系数表现出有峰值点的非线性变化规律。当氨蒸气的浓度为0.38%时,混合蒸气的最大凝结换热系数可达纯水蒸气的1.9倍,从液膜热阻和扩散热阻的角度分析了强化换热的机理。     

冷凝器分液特性实验研究

结合传统的管壳式换热器设计加工出一个冷凝器,冷凝器管箱两端的隔板上开有分液小孔,并通过设计搭建的凝结换热实验台,对冷凝器的换热特性进行了实验研究。通过对实验结果的归纳,结果表明凝结液封稳定的情况下,凝结换热系数随开启漏液管数增加而变大。这是对以后设计高效换热器进行的有益探索。     

小型分离式热管工作温度与传热特性的实验研究

本文对具有短管束的小型分离式热管工作温度与传热特性进行了实验研究。实验装置的蒸发段和冷凝段都是由5根直径20 mm的无缝钢管短管束组成,管子长度为152 mm,带有紧套的钢帛环形肋片结构尺寸如下:肋片外径40mm、厚1 mm、片间距4 mm。工作温度140-220℃,热流密度21.2-40.2 kW/m2。试验结果表明,在本实验条件下,小型分离式热管最佳充液率按蒸发段总容量计为48％-63％,按管束总容量计为20％-40％。根据实验结果,总结了最佳充液率下(25％,按管束总容量计)的蒸发段内部平均沸腾换热系数和冷凝段内部凝结换热努赛尔数综合关系式。     

分离式热管换热器传热特性的实验研究

本文在自行设计分离式热管实验装置的基础上,对其传热特性进行了实验研究。其工作温度为170～250℃,热流密度为25～50 kW/m~2。蒸发段和冷凝段构成相同,均是由7根直径30 mm的无缝钢管短管束组成,管长为160 mm,带有紧套的钢帛环形肋片结构尺寸为:外径45 mm、厚1 mm、片间距4 mm。实验结果表明,在本实验条件下,分离式热管的最佳充液率按管束总容量计为18%～38%。根据实验结果拟合了最佳充液率(24%)下蒸发段内部平均沸腾换热系数和冷凝段内部凝结换热努塞尔数综合关系式。     

蒸气凝结相关问题探讨

讨论了几个与蒸气凝结相关的问题,指出壁面上球冠形液滴的内外压差和临界半径同样遵循经典的Laplace公式和Kalvin公式;蒸气在冷壁上的冷凝形态主要由后退接触角决定;空气中的水蒸气在换热器表面呈膜状冷凝时换热器的性能优于呈滴状冷凝时换热器的性能。     

Investigation of Thermal Performance of Perforated Finned Heat Exchangers

In this study, the thermal performance of perforated finned heat exchangers with angle of rotation θ was experimentally investigated. Six-millimeter-diameter holes that were opened on each circular fin on a heating tube have a potential to reduce the thickness of the boundary layer that is formed on the circular fins placed on the heating tube, thus increasing heat transfer through convection in this area. The experiments were carried out at six different angular locations to determine the best angular location. In addition, a perforated finned heater was compared with an imperforate finned heater. For the finned heater at 60°, the effectiveness is 18% higher and the pressure drop is 1.16% lower than other angular positions. In this respect, it can be concluded that the best angular position is 60°. In addition, results show an increase in effectiveness with an increasing number of transfer units.     

The Effect of Heat Exchanger Type on Two-Phase Heat Transfer Coefficient and Pressure Drop

Three different types of heat exchangers were tested experimentally to investigate two-phase heat transfer coefficient and pressure drop during the condensation process of CO₂ gas. Experimental results revealed that the convection heat transfer coefficient was enhanced by a factor of four due to the existence of porous media and by a factor of seven due to the use of micro-pipes when compared to the normal macro-tubes. The pressure drop was measured and noticed only in porous tubes and micro-pipes, reaching about 17.5 kPa/m and 8.4 kPa/m, respectively. Comparisons between experimental and correlated results were conducted.     

不凝气体对R123凝结换热的影响

对氟利昂 R123 在水平单管外的凝结换热性能进行了试验研究,试验管为光管和五根强化管.目的是获得不凝气体对 R123 蒸气凝结时最佳肋密度的影响.试验结果表明:光管管外 Nusselt 理论值与实验数据偏差在±5%以内.对于含 8%不凝气体的 R123 在低肋管外的凝结换热,在肋密度为 1475 翅/米时可以获得最佳的换热性能.含不凝气体的 R123凝结换热系数显著下降,其管外换热系数约为纯蒸气的 20%～25%.随着肋密度的减小,不凝气体对凝结换热的影响逐渐减弱,但其最佳肋间隙仍保持不变,均为 0.32 mm.     

Parameters Affecting Heat Transfer During Condensation Inside Micropipes: Surface Tension Effect. Paper II

Experimental and analytical investigation was carried out to show the effect of surface tension on the heat transfer coefficient of gas flow inside micropipe heat exchangers during the condensation process. An empirical correlation was formulated for the coefficient of the convective heat transfer. A comprehensive comparison between the experimental results with published correlations in the literature and that obtained by the developed correlation was conducted. The results showed that the predicted heat transfer coefficient of the proposed correlation was closer to the experimental results than any other correlation, a finding related to the consideration of the surface tension.     

新型制冷机冷凝器壳侧的传热强化研究

针对现行制冷机冷凝器的不足 ,设计了适用于冷凝传热的新型折流杆冷凝器的壳程内部结构 ,并分析了其传热强化机理。在重力控制条件下 ,对不同内部支承结构与管束组合的冷凝器进行了传热实验研究 ,得到了水蒸汽在冷凝器壳侧的冷凝传热特性曲线。采用冷凝传热强化因子的概念 ,建立了预测该种折流杆冷凝器壳侧冷凝传热膜系数的计算公式 ,为折流杆冷疑器在制冷系统中的设计应用提供了理论依据。     

螺旋扁管管外蒸汽冷凝双侧强化传热试验研究

为促进螺旋扁管在冷凝换热装置上的应用,对螺旋椭圆管管外蒸汽冷凝工况下的传热特性进行了试验研究。研究结果表明,螺旋椭圆管在强化管内无相变对流传热的同时也可以强化管外冷凝传热。相同工况下,同圆管相比,所用螺旋椭圆扁管的总传热系数高11%-16%,管内传热系数高约18%,管外冷凝传热系数高约9%。并从二次流减薄传热边界层及冷凝表面利于排除冷凝液的角度,分析了螺旋椭圆扁管的双侧强化传热机理。     

Compact heat exchangers based on individually finned flat multichannel tubes

Attempts to design prospective aluminium heat exchangers based on individually finned flat multichannel tubes manufactured according to a waste-free technology of obtaining surfaces by undercutting and flanging thin layers of metal (fins) aer considered in this paper. Technological peculiarities of arranging and assembling radiators based on individually finned tubes, experimental data on aluminium radiators and heat exchangers based on heat tubes of capillary structure, and their analysis are also presented.     

Condensation Characteristics of Flue Gas/Steam Mixtures with Fine Lignite and Ash Particles along Horizontal Finned Tube Bundles

The condensation behavior for a gas/steam mixture with fine lignite particles and lignite ash particles is experimentally investigated as the particles flow over horizontal finned tube bundles. The effects of the gas velocity, inlet temperature of cooling water, excess air coefficient, and particle dimension are discussed. The total mass flow rate of the condensate and the condensation heat transfer coefficient for flue gas including particles are higher than those of flue gas excluding particles when Reynolds number is higher than 2,300. The area covered by ash depositions tends to grow from the leeward toward the windward side with increasing particle diameter.     

R410A与R22在水平微翅管内流动沸腾传热特性研究

建立了水平管流动沸腾试验台,采用恒热流加热方法,对 R410A 在水平微翅管内流动沸腾特性进行了实验研究,分析了影响 R410A 在水平微翅管内换热系数的因素,考察了工质质量流量、热流密度、质量干度以及微翅管的几何参数对工质的流动沸腾换热性能的影响关系.通过对比 R410A 与 R22 的实验数据,分析比较二者的换热系数,结果表明R410A 与 R22 相差不大,R22 比 R410A 的换热系数大约高 7.5%.     

Flow condensing heat transfer of R410A,R22, and R32 inside a micro-fin tube

An experimental study of condensation heat transfer characteristics of flow inside horizontal micro-fin tubes is carried out using R410A, R22, and R32 as the test fluids. This study especially focuses on the influence of heat transfer area upon the condensation heat transfer coefficients. The test sections were made of double tubes using the counter-flow type; the refrigerants condensation inside the test tube enabled heat to exchange with cooling water that flows from the annular side. The saturation temperature and pressure of the refrigerants were measured at the inlet and outlet of the test sections to defined state of refrigerants, and the surface temperatures of the tube were measured. A differential pressure transducer directly measured the pressure drops in the test section. The heat transfer coefficients and pressure drops were calculated using the experimental data. The condensation heat transfer coefficient was measured at the saturation temperature of 48°C with mass fluxes of 50–380 kg/(m²s) and heat fluxes of 3–12 kW/m². The values of experimental heat transfer coefficient results are compared with the predicted values from the existing correlations in the literature, and a new condensation heat transfer coefficient correlation is proposed.