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。     

内表面烧结型多孔管的流动沸腾换热

采用流动沸腾传热试验平台,研究了2 m长铁基烧结型内表面多孔管竖直管内流动沸腾传热特性,利用流动沸腾传热学基本原理及公式计算了传热过程中的热通量、沸腾传热系数及相关参数,并考察了过热度和流速对多孔管流动沸腾传热性能的影响.结果表明:烧结型表面多孔管的流动沸腾传热能力优于同条件下的光滑管,内表面沸腾传热系数是同尺寸光滑管...     

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。     

垂直布置B30波槽管管外膜状凝结的实验研究

在略高于大气压的凝结条件下,对垂直布置渡槽管管外膜状凝结换热进行了实验研究。实验结果表明：垂直布置时波槽管具有一定的强化传热效果,在实验范围内,最佳波槽管的总传热系数约比光管提高27％-43％,而阻力系数约为光管的2．94-3．48倍,垂直布置的强化传热效果不如水平布置的好。通过对实验数据的回归分析,得到了垂直管管内对流换热、管外凝结换热及阻力系数的实验关联式。     

多孔表面新型复杂结构优化沸腾传热的实验研究

报道了R11在烧结多孔表面开槽时沸腾传热的实验研究，实验发现，与普通槽道和双空隙层多孔表面相比，沸腾换热增强，沸腾表现为液体灌注、槽道起泡、底部蒸干三个区，对特定的多孔层，开槽可获得更好的换热效果。带槽道的多孔表面实验件与均匀多孔表面相比，在相同壁面过热度（θ）条件下，热流密度（q）提高2－10倍，临界热流密度提高2－4倍。     

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。     

螺旋槽管换热过程的三维速度场与温度场耦合数值模拟

根据螺旋槽管换热器结构特点及传热特性,建立了以水为工质的换热器流动与传热的三维几何模型。运用有限元分析软件ANSYS模拟出换热器在换热过程中速度场与温度场的状况,分别得到了螺旋槽管内壁与外壁的对流换热系数。结果表明:槽深越大,随着Re增大,换热性能越好;当Re较小时,螺距越大,换热效果降低。其与该类光管换热器相比,得出螺旋槽管的换热系数是光管的2.5倍左右,强化了传热,为此产品的进一步理论研究和推广应用提供了依据。     

槽式太阳能DSG系统集热管内强化传热的数值模拟

为了强化集热管内蒸汽换热,避免管壁温度过高,对槽式太阳能直接产生蒸汽系统(DSG)采用内螺纹管和光管时过热段的传热及阻力特性进行了数值模拟,分析了螺纹高度、螺距和雷诺数对螺纹管内换热特性和流动阻力特性的影响,并与光管的数值模拟结果进行了比较.结果表明:内螺纹管增强了管内蒸汽扰动,提高了集热管换热性能,降低了管壁温度.螺纹管的努塞尔数Nu是光管的1.6～2.4倍,且螺纹高度越高、螺距越小,换热性能越好;螺纹管的阻力系数f是光管的3.1～8.7倍,且螺纹高度越高、螺距越小,f越大.     

水平微细管道内R717沸腾换热特性研究

搭建了氨(R717)沸腾换热测试台,对内径3 mm水平光管内R717的沸腾换热特性进行了测试,分析热流密度、干度、饱和温度及质量流率对沸腾换热及换热方式的影响。实验热流密度15～40 kW/m~2,质量流率40～160 kg/(m~2·s),饱和温度-5、0和5℃,干度0.1～0.9。结果表明:在氨制冷剂管内沸腾换热的过程中,质量流率过低和热流密度过高会导致干涸传热恶化,换热形式由核态沸腾换热向气态氨制冷剂强制对流换热转变,同时也影响干涸的起始干度;在干涸发生前,沸腾换热系数随着干度的增加而增大,逐渐达到峰值;在干涸发生后,传热恶化导致换热系数急剧降低;饱和温度升高会加快核态沸腾气泡生成速率,强化沸腾换热,但干涸的起始干度随着饱和温度升高而降低。     

管内单相对流强化换热及阻力特性的实验研究

通过实验对光管、内覆丝网管、轨槽管及多头内肋管进行了强迫对流换热和阻力特性的研究。根据工程需要，选用适当的准则对这些管子的强化传热进行了评价，在实验范围内得出了最佳管型。     

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.     

Enhanced boiling heat transfer in restricted spaces of a compact tube bundle with enhanced tubes

In flooded-type tube bundle evaporators with smooth tubes and general tube gaps, both wall superheat and heat flux are generally quite low and boiling cannot occur on the heated tubes. But when the tube gap is quite small or the enhanced heat transfer tubes are employed, the incipient boiling can occur at low heat flux levels and results in a significant heat transfer enhancement effect. This study investigates experimentally enhancement effects by the restricted space comprising the compact tube bundle and the enhanced tubes for boiling heat transfer of pure water and salt-water mixtures under atmospheric pressure. The experimental results show that the small tube gaps can greatly enhance boiling heat transfer for the compact enhanced tube bundle.     

Effects of heat exchanger tube geometries on nucleate pool boiling heat transfer in a scaled in-containment refueling water storage tank

To determine the combined effects of the heat exchanger tube geometries of advanced light water reactors (ALWRs) passive residual heat removal system (PRHRS) on the nucleate pool boiling heat transfer in a scaled in-containment refueling water storage tank (IRWST), a total of 1,966 data (1,076 with horizontal tubes and 890 with vertical tubes) for q″ versus ΔT has been obtained using various combinations of tube diameters, surface roughness, and tube orientation. The experimental results show that (1) for both horizontal and vertical tubes, increased surface roughness enhances heat transfer whereas increased tube diameter decreases heat transfer, (2) both effects of the surface roughness and the tube diameter on the nucleate pool boiling heat transfer are significantly greater for vertical tubes than horizontal tubes, (3) the effectiveness of two heat transfer mechanisms, i.e., enhanced heat transfer dur to liquid agitation by bubbles generated and reduced heat transfer by the formation of large vapor slugs and bubble coalescence, depends on the combined effects of the heat flux, surface roughness, and the tube orientation. In addition, two different forms of empirical heat transfer correlations are obtained that fit present experimental data within +35 and −20%.     

Enhancement of boiling heat transfer in a 5 × 3 tube bundle

The results of an experimental investigation on nucleate boiling heat transfer in an electrically heated 5 × 3 in-line horizontal tube bundle under pool and low cross-flow conditions of saturated water near atmospheric pressure are presented here. It is observed that the heat transfer coefficient is minimum on bottom row tubes and increases in the upward direction with maximum values on top row tubes. Also, heat transfer coefficient on central column tubes was found to be slightly higher than those on the corresponding side tubes. Further, a Chen-type relation has been used to determine the local boiling heat transfer coefficient on a tube in a heated tube bundle.     

Pool boiling heat transfer with nano-porous surface

Anodizing technique has been recognized as an efficient way to grow the well-ordered oxide nano-structures on metal substrate. In the present experimental study, the nucleate pool boiling heat transfer coefficient and long-term performance of nano-porous surface fabricated by the cost-effective and simple anodizing technique were investigated with water. The incipient wall superheat of pool boiling in nano-porous surface was lower than that in non-coating surface. The nucleate boiling heat transfer coefficient of nano-porous coating surface appeared higher than that of non-coating surface particularly at the low heat flux condition. The higher coefficient remained throughout 500 h of operation.     

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.     

Effect of pore size on the nucleate pool boiling of structured enhanced tubes

In this study, pool boiling test results are provided for the structured enhanced tubes having pores with connecting gaps. The surface geometry of the present tube is similar to that of Turbo-B. Three tubes with different pore size (0.20 mm, 0.23 mm and 0.27 mm) were manufactured and tested using R-11, R-123 and R-134a. The pore size which yields the maximum heat transfer coefficient varied depending on the refrigerant. For R-134a, the maximum heat transfer coefficient was obtained for the tube having 0.27 mm pore size. For R-11 and R-123, the optimum pore size was 0.23 mm. One novel feature of the present tubes is that their boiling curves do not show a ‘cross-over’ characteristic, which existing pored tubes do. The connecting gaps of the present tube are believed to serve an additional route for the liquid supply and delay the dry-out of the tunnel. The present tubes yield the heat transfer coefficients approximately equal to those of the existing pored enhanced tubes. At the heat flux 40 kW/m² and saturation temperature 4.4° C, the heat transfer coefficients of the present tubes are 6.5 times larger for R-11, 6.0 times larger for R-123 and 5.0 times larger for R-134a than that of the smooth tube     

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.     

An investigation into the heat transfer characteristics of spiral wall with internal rib in a supercritical sliding-pressure operation once-through boiler

Within the pressure range of 9–28 MPa, mass velocity range of 600–1 200 kg/(m²·s), and heat flux range of 200–500 kW/m², experiments were performed to investigate the heat transfer to water in the inclned upward internally ribbed tube with an inclined angle of 19.5 degrees, a maximum outer diameter of 38.1 mm, and a thickness of 7.5 mm. Based on the experiments, it was found that heat transfer enhancement of the internally ribbed tube could postpone departure from nucleate boiling at the sub-critical pressure. However, the heat transfer enhancement decreased near the critical pressure. At supercritical pressure, the temperature difference between the wall and the fluid increased near the pseudo-critical temperature, but the increase of wall temperature was less than that of departure from nucleate boiling at sub-critical pressure. When pressure is closer to the critical pressure, the temperature difference between the wall and the fluid increased greatly near the pseudo-critical temperature. Heat transfer to supercritical water in the inclined upward internally ribbed tube was enhanced or deteriorated near the pseudo-critical temperature with the variety of ratio between the mass velocity and the heat flux. Because the rotational flow of the internal groove reduced the effect of natural convection, the internal wall temperature of internally ribbed tube uniformly distributed along the circumference. The maximum internal wall temperature difference of the tube along the circumference was only 10 degrees when the fluid enthalpy exceeded 2 000 J/g. Considering the effect of acute variety of the fluid property on heat transfer, the coreelation of heat transfer coefficient on the top of the internally ribbed tube was provided. Translated from Proceedings of CSEE, 2005, 25(16): 90–95 [译自: 中国电机工程学报]     

Numerical investigation of heat transfer and pressure drop in enhanced tubes

This study investigates passive heat transfer enhancement techniques to determine the distribution of temperature and static pressure in test tubes, the friction factor, the heat flux, the temperature difference between the inlet and outlet fluid temperatures, the pressure drop penalty and the numerical convective heat transfer coefficient, and then compares the results to the experimental data of Zdaniuk et al. It predicts the single-phase friction factors for the smooth and enhanced tubes by means of the empirical correlations of Blasius and Zdaniuk et al. This study performed calculations on a smooth tube and two helically finned tubes with different geometric parameters also used in the analyses of Zdaniuk et al. It also performed calculations on two corrugated tubes in the simulation study. In Zdaniuk et al.'s experimental setup, the horizontal test section was a 2.74 m long countercurrent flow double tube heat exchanger with the fluid of water flowing in the inner copper tube (15.57–15.64 mm i.d.) and cooling water flowing in the annulus (31.75 mm i.d.). Their test runs were performed at a temperature around 20 °C for cold water flowing in the annulus while Reynolds numbers ranged from 12,000 to 57,000 for the water flowing in the inner tube. A single-phase numerical model having three-dimensional equations is employed with either constant or temperature dependent properties to study the hydrodynamics and thermal behaviors of the flow. The temperature contours are presented for inlet, outlet and fully developed regions of the tube. The variations of the fluid temperature and static pressure along tube length are shown in the paper. The results obtained from a numerical analysis for the helically tubes were validated by various friction factor correlations, such as those found by Blasius and Zdaniuk et al. Then, numerical results were obtained for the two corrugated tubes as a simulation study. The present study found that the average deviation is less than 5% for the friction factors obtained by the Fluent CFD program while Blasius's correlation has the average deviation of less than 10%. The corrugated tubes have a higher heat transfer coefficient than smooth tubes but a lower coefficient than helically finned tubes. The paper also investigates the pressure drop penalty for the heat transfer enhancement.