水平小管内R290的凝结换热特性实验

以管内径为4 mm的水平光滑铜管,质量流速100～250 kg/(m~2·s),热流密度5～10 kW/m~2,饱和温度分别为40、50、55℃,干度0～1,对R290(丙烷)进行凝结换热实验。结果表明:提高质量流速或增大热流密度,均可增大凝结换热系数;而饱和温度升高则会使凝结换热系数减小;此外,随着凝结过程的进行,干度逐渐降低,凝结换热系数通常也随之减小,仅在热流密度过大时出现先增后减的现象。最后,选取6种经典的凝结换热关联式计算凝结换热系数,并与实验结果对比,Cavallini关联式和Bohdal关联式的预测效果较好。     

带有横向微沟槽的矩形紧凑式通道换热器内超临界液化天然气工质换热强化特性模拟研究

提出了一种用于超临界液化天然气换热的微小通道换热器整体性能提高的被动式强化技术并进行了数值模拟验证和设计优化。在普通的矩形微小通道内利用3D激光打印技术在壁面加工横向圆弧形微沟槽以强化换热能力。首先对圆弧形微沟槽的槽深、槽宽和相邻两槽道中心距等几何尺寸进行了优化计算,然后讨论了在使用强化技术后工质温度在跨越临界温度的120K-250K范围内的换热强化和流动特性,进一步考察了工质温度、质量流量（雷诺数）和进口压力对换热系数（努塞尔数）、摩擦因子和综合效益系数的影响。此外,通过微沟槽附近的局部流动特性分析强化换热机理,数值模拟结果表明带有横向微沟槽的紧凑式换热器的综合换热效益得到30%左右增加,显示了优异的换热强化综合效果     

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。     

R32在水平强化管内的流动沸腾换热特性研究

为研究制冷剂在内螺纹管内的换热情况,搭建集蒸发冷凝性能测试于一体的实验台,研究了替代工质R32在水平内螺纹管内的流动沸腾换热特性和压降特性。实验测试管外径分别为7和8 mm,进口过冷度和出口过热度为3～5℃,制冷剂质流密度为300～700 kg/(m~2·s),饱和温度保持在5～10℃,实验段水侧雷诺数为12 000～20 000,热平衡误差保持在5%以内。结果表明:制冷剂侧表面换热系数随制冷剂质流密度的增大而增大,随饱和温度的增大而减小,而水侧雷诺数Re对其并无影响;总传热系数随制冷剂质流密度、水侧雷诺数Re的增大而增大,随饱和温度的增大而减小;试验段压降随制冷剂质流密度的增大而增大,随饱和温度的增大而减小,水侧雷诺数对其无影响;在相同工况下,7 mm管比8 mm管的制冷剂侧换热系数以及总传热系数都大,但是其压降也比8 mm管大。     

太阳能吸热器换热管蓄热数值模拟与试验研究

对以高温共晶盐LiF—CaF2为相变材料(PCM)和以干空气为工质的相变蓄热系统，采用焓方法建立了以控制体单元为对象的单管相变蓄热模型，并对系统进行了数值分析，得到了循环工质气体出口温度、相变材料容器最高温度和平均壁温等参数的瞬态变化曲线，实验研究了吸热器换热管的蓄傲热性能，分析了工质进口温度、输入热流级工质流量对工质出口温度、PCM容器平均壁温及最高壁温的影响。计算结果和试验表明单元换热管的蓄傲热性能达到了设计要求，试验结果与数值计算吻合良好。     

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。     

R245fa有机工质在Turbo-EHP管外流动沸腾换热实验研究

结合沸腾传热问题,通过凝练并设计实验系统,研究有机工质R245fa在高效蒸发换热管Turbo-EHP管外流动沸腾传热的性能,分析有"孔穴"结构的表面强化管的强化机理。实验中通过改变热源流量、温度以及工质流量等参数,研究其与局部以及平均沸腾换热系数之间的关系。实验结果表明,Turbo-EHP型管前、中、末3段的强化效果不同,在前段气泡热阻对换热性能有一定的影响,中段换热效果达到最优,末段由核态沸腾转为气与壁面的对流换热,换热效果减弱;且随着工质流量的增大,沸腾性能显著增强。     

螺旋槽管传热与污垢性能的实验研究

进行了螺旋槽管的传热性能实验，得出了螺旋槽管管内强制对流换热关联式。以硬度800mg／L的人工硬水作为工质，在0．24m／s，管外水浴60℃和相同管内工质入口温度的条件下，进行了螺旋槽管及其对应光管管内污垢的对比实验。结果表明，螺旋槽管有较好的传热性能，但阻垢性能却弱于光管。     

以R32和R245fa为工质的重力热管传热特性

对以R32和R245fa为工质的长3m、管内径为40mm、管外径为44mm的重力热管开展了传热性能实验研究。热源为恒温水浴,冷源为固定进口温度和流量的冷水。实验得出了不同热源温度下的热管传热负荷、工质蒸发换热系数和冷凝换热系数,并与经典拟合公式的预测值进行了对比。实验分析对比表明,R32更适宜工作在热源温度为常温到50℃的工况,R245fa更适合工作在50~100℃的热管工况。     

电场对管内凝结液膜厚度的影响

以R123为工质,对圆柱形电极进行了电场强化管内凝结换热的试验研究,提出并计算了当量液膜厚度。试验换热管为垂直套管式,外层为冷却水路,内层为工质回路。电压范围为0～30 kV,热流密度范围为4～6 kW/m2。实验观察到了凝结液膜的减薄,在给定的实验条件下,当电压超过5 kV后,随着电压的增加,凝结换热系数增加,当量液膜厚度减小。实验结果表明,液膜厚度的减薄是电场强化凝结换热的主要原因。     

Pool boiling heat transfer of propane, isobutane and their mixtures on enhanced tubes with reentrant channels

Pool boiling heat transfer experiments were carried out on a conventional smooth tube and four enhanced tubes with reentrant surfaces using propane, isobutane and their mixtures as working fluids for six saturation temperatures. The heat transfer performance is very different for different surface-fluid combinations. Compared to the smooth tube, the mixture boiling heat transfer degradation is more significant for the enhanced tubes. The current data are compared with available literature data for the same fluids and also with data for R12 and R134a. Experimental results of boiling hysteresis and for twin-tube bundles are also provided. Further explanations for the different heat transfer performances is provided by means of visualization in an accompanying paper [Y. Chen, M. Groll, R. Mertz, R. Kulenovic, Visualization and mechanisms of pool boiling of propane, isobutane and their mixtures on enhanced tubes with reentrant channels, submitted to Int. J. Heat Mass Transfer (H/S 04016)].     

In-tube passive heat transfer enhancement in the process industry

Enhanced heat transfer surfaces are used in heat exchangers to improve performance and to decrease system volume and cost. In-tube heat transfer enhancement usually takes the form of either micro-fin tubes (of the helical micro-fin or herringbone varieties), or of helical wire inserts. Despite a substantial increase in heat transfer, these devices also cause non-negligible pressure drops.By making use of well-proven flow pattern maps for smooth tubes and the new ones for smooth and enhanced tubes, it is shown from the refrigerant condensation data that flow patterns have a strong influence on heat transfer and pressure drop. This is done for data obtained from in-tube condensation experiments for mass fluxes ranging from 300 to 800 kg/m² s at a saturation temperature of 40 °C, for refrigerants R-22, R-134a, and R-407C. The flow regimes, pressure drops, heat transfer coefficients, and the overall performance of three different tubes, namely a smooth-, 18° helical micro-fin-, and a herringbone micro-fin tube (each having a nominal diameter of 9.51 mm), are presented and compared to the performance of smooth tubes with helical wire inserts (with pitches of 5 mm, 7.77 mm and 11 mm corresponding to helical angles of 78.2°, 72°, and 65.3°, respectively).     

Investigation of Enhanced Boiling Heat Transfer from Porous Surfaces

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Falling film evaporation heat transfer of water/salt mixtures from roll-worked enhanced tubes and tube bundle

An experimental study is carried out for enhancement of falling film evaporation heat transfer of pure water and water/salt mixtures on horizontal smooth tube and two kinds of structured tube bundles under atmospheric pressure. The experimental results show that the low-cost roll-worked tube can greatly enhance the evaporation heat transfer performance of the falling film, and make it comparable to that of expensive commercial enhanced tubes such as GEWA-T tubes, TE tubes and HF tubes, even at low and moderate heat flux levels. The average evaporation heat transfer coefficients for the roll-worked tube bundle are basically independent from the parameters tested such as flow and heating conditions, salt-concentrations, as well as geometries of the tube bundles. The present experimental data result in a constant heat transfer coefficient; α≈20 kW/m² K, in the convective heat transfer range of the heat fluxes <10⁵ W/m².     

Strategy for selection of elements for heat transfer enhancement

The present paper points out that the selection of elements for heat transfer enhancement in heat exchangers requires a methodology to make a direct comparison of the performances of heat exchanger surfaces with different elements. Methods of comparison used in the past are, in many respects, approximate and hence fail to predict accurately the relative performance of conventional heat exchanger surfaces operated with different heat exchanger elements. Owing to the direct use of the Colburn factor for performance assessment, these methods over-predict the relative performance of heat exchangers. In the present paper, a more consistent comparison method is presented and is demonstrated to work by comparison of the performance of an experimentally investigated pin fin heat exchanger with that of a smooth pipe heat exchanger. The method yields results that belong to the volume goodness factors group. It represents a practical approach, as it is applicable to all kinds of heat exchanger surfaces and does not require the conversion of the experimental data in terms of Nusselt number and friction factor for comparison purposes. The present work demonstrates that the suggested method can also be used for performance comparison of existing heat exchanger surfaces with available heat transfer and pressure loss data.     

换热表面直接与间接测温的对比性实验研究

进行了沸腾换热表面的间接测温与直接测温的对比性实验研究。以水和乙醇作工质,对大气压力下的池沸腾换热平表面,用间接测温法和直接测温法同时测量壁面过热度,对两种测温方法的结果进行了比较研究。同时,用直接测温法进行三种不同管径的光管沸腾试验,并把实验结果与Rohsenow公式进行了比较。大量实验结果表明,在一定操作条件下,直接测温法测量沸腾换热表面过热度误差较小,是一种简便而又行之有效的方法。     

Enhancement of boiling heat transfer in restricted spaces in compact horizontal tube bundles

ln desalinization devices and some heat exchangers making use of low‐quality heat energy, both wall temperatures and heat fluxes of heated tubes are quite low and generally cannot cause boiling in flooded‐type tube bundle evaporators with a large tube spacing. But when the tube spacing is very small, boiling in restricted spaces can occur and induce a higher heat transfer than that of a falling film or pool boiling at the same heat flux. This study investigated experimentally the effects of tube spacing, positions of tubes, and heating status of tubes as well as surface status (smooth and roll‐worked) on boiling in restricted spaces in compact horizontal tube bundle evaporators under atmospheric pressure. The experimental results provide a restricted space boiling database for water in smooth and enhanced surface tube bundles. Of particular importance is information concerning the influence of tube spacing of flooded‐type tube bundle evaporators, especially for the case of zero pitch, when the neighboring tubes are contacting each other. © 2001 Scripta Technica, Heat Trans Asian Res, 30(5): 394–401, 2001     

Performance investigation of plain and finned tube evaporatively cooled heat exchangers

The performance of two evaporatively cooled heat exchangers is investigated under similar operating conditions of air flow rates and inlet hot water temperatures. The heat exchangers are plain and plate-finned circular tube types which occupy the same volume. Spray water, which is circulated in a closed circuit, is injected onto the exposed surfaces of the tubes and fins. The contact between air and spray water results in evaporative heat transfer. The tubes are copper, 10 mm o.d. The finned configuration is constructed by introducing 0.5 mm thick copper plates between the tubes, with a total area ratio of four. A substantial increase in heat transfer takes place for the plate-finned tubes. The increase is 92–140% for air velocities from 1.66 to 3.57 m s⁻¹. A model is used to calculate the thermal performance of the plain and finned tubes assuming a constant spray water temperature in the heat exchanger. The wet-finned surfaces show low fin efficiency compared with dry surfaces. An energy index defined as the ratio of volumetric thermal conductance to air pressure drop per unit length is found to be close for the two heat exchangers. This reveals higher thermal utilisation of the occupied volume by the finned tubes with the same energy index.     

A new set of correlations for EHD enhanced condensation heat transfer of tubular systems

An improved set of correlations for evaluating the heat transfer coefficient of EHD assisted condensation heat transfer for tubular systems is presented in this paper. The correlations were developed by curve fitting a wide range of experimental data and then optimising the empirical constants so that the percentage deviation is less than ±30%. The experimental data was classified in four categories: (I) condensation outside horizontal smooth tubes, (II) condensation inside horizontal smooth tubes, (III) condensation outside vertical tubes and (IV) condensation inside vertical tubes. The experimental data in each category included results obtained using various refrigerants and various electrode systems. The improved correlations were used to predict a total of 166 experimental data points.

A comprehensive literature review of all available correlations for EHD assisted condensation heat transfer is also described in this paper. Two correlations that were identified to be simple were used to predict the above-mentioned data bank. These are the correlations of Choi and Reynolds and Didkovesky and Bologa. The Choi and Reynolds correlation was found to successfully predict heat transfer rates of condensation outside horizontal and vertical tubes and of condensation inside vertical tubes when high intensity electric field is applied. However, the Didkovesky and Bologa correlation was found to successfully predict only the results of condensation heat transfer outside vertical surfaces.     

The effect of the number of tube rows on heat,mass and momentum transfer in flat-plate finned tube heat exchangers

The effect of the number of tube rows on heat, mass and momentum transfer is experimentally investigated for flat-plate, finned-tube heat exchangers which consist of aluminum fins and copper tubes. Four flat-plate finned-tube heat exchangers are identical except for changes in the number of tube rows (1 to 4). Heat-transfer coefficients for wet and dry surface conditions are obtained for both heating and cooling of moist air flowing over finned tubes. The air velocity was varied from 0.9 to 4 m/s. Heat transfer, Colburn and friction factors are determinated for different tube rows numbers while the Reynolds number were being warried. It is found that the values of Colburn and friction factors for wet surfaces are higher than for dry surfaces and for both conditions the Colburn and friction factors decrease with an increase in the tube row numbers.