水基纳米流体在铜丝平板热管中的应用

对使用三种水基纳米流体作为工质的铜丝平板热管的传热特性进行了实验研究.使用的纳米流体分别是平均粒径20 nm的Cu纳米颗粒、平均粒径50 nm的Cu纳米颗粒和平均粒径50 nm的CuO纳米颗粒的水基悬浮液(简称水基20 nm Cu、50 nm Cu、50 nm CuO纳米流体),着重分析了纳米流体种类,纳米颗粒质量分数、运行温度或工作压力对热管传热特性的影响.研究结果表明,使用纳米流体作为工质可以显著提高热管的传热特性;在不同运行温度条件下,不同的纳米流体均在质量分数1.0％时具有最佳传热效果;纳米流体是一种适用于铜丝平板热管的新型工质.     

多壁碳纳米管水基纳米流体重力热管传热特性数值模拟

为揭示多壁碳纳米管水基纳米流体应用在重力热管中的传热特性,基于多相流模型(VOF)建立其重力热管数值模型,并将数值结果与实验数据进行对比验证。以热阻作为性能评价指标,改变加热功率和充液率,讨论二者对热管换热性能的具体影响。通过添加传热传质源项来编写用户自定义函数(UDF)完成内部流体蒸发冷凝过程中的相变模拟。模拟结果表明:该数值模型能够较好模拟多壁碳纳米管水基纳米流体应用重力热管内部复杂的流动与传热过程;在选定的加热功率及充液率参数范围内,该重力热管的整体热阻随蒸发段加热功率的增大而减小,随充液率的增大而增大。     

磁纳米流体热管太阳能集热装置换热性能试验

设计了一套换热系统试验台,分别把磁纳米流体和水作为热管的工质,对玻璃真空管内插热管式太阳能集热器进行对比试验,分析两种集热器处于不同倾角、不同天气条件、不同总辐射量下的光热转换瞬时效率及日平均效率。此外,比较了玻璃真空集热器和热管内插式集热器的平均热损失系数。研究表明:内插热管式集热器的平均热损失系数约为全玻璃真空管集热器的2.32%,远小于全玻璃真空集热管,且工质为纳米流体的热管式玻璃真空管太阳能集热器的热损失系数比水工质热管真空集热管更低,其瞬时效率及日平均效率更高,运行更加高效、安全、稳定。     

开启式重力热管传热特性的实验研究

对φ38 mm×3 mm,长1 600 mm的开启式碳钢-水重力热管进行了传热性能的实验研究.实验为0.02～0.8 MPa,2.5～9.6 kW.对负压下开启武重力热管的传热性能首次进行了研究,发现负压与正压的传热规律相似,但传热性能较正压差.实验结果表明,开启式热管具有较好的传热特性,性能稳定,可作为高温烟气余热锅炉的传热部件.根据实验数据回归了开启式重力热管沸腾换热系数实验关联式,可用于工程实际.     

Cu-H_2O纳米流体流动与强化换热的数值模拟研究

模拟纳米流体在三维管道中的流动和强化传热过程,运用数值计算方法研究纳米流体的流动特性和传热机理,探究不同纳米颗粒体积分数和不同纳米颗粒大小在不同雷诺数(Re)下对纳米流体的流动和传热特性的影响。基于DPM模型对纳米流体在圆管中的对流换热进行了数值模拟研究,研究结果表明,在一定范围内,每增加0.5%的体积分数,纳米流体的传热性能平均增强7.82%。随着纳米颗粒的减小,纳米流体的传热系数不断增加。     

纳米流体强化传热研究分析

文中综述了目前国内外对于纳米流体强化传热技术的研究情况,分析了纳米流体的强化传热机理及添加纳米粒子后对液体的物性参数--粘度、比热、密度、流体流动的影响;说明了石墨/水纳米流体及Fe3O4/水纳米流体导热系数和对流换热系数测量实验的原理及结果,并对结果进行了分析,实验结果表明纳米流体强化了传热.     

重力热管单线内螺纹分布强化换热实验研究

采用实验方法,研究了不同的内螺纹分布和油浴温度等因素对热管换热特性的影响。实验选用的热管材料为紫铜,外径16 mm,壁厚3 mm,长度为200 mm,传热工质为水,充液率为20%。实验结果表明:在同一油浴温度下,内螺纹重力管的启动特性要优于光滑重力热管。对比不同油浴温度下,布置内螺纹能够有效地降低热管的工作温度。实验选型的内螺纹仅布置在蒸发段不会提高热管的换热系数,而在绝热段和冷凝段布置内螺纹则能够使换热系数显著提升,且随油浴温度的增加,换热系数线性增加。     

扁平热管的传热性能实验

一、前言圆形热管或热虹吸管在工业余热回收中获得了广泛应用。并取得了显著的节能效益和经济效益。但是在工程中有许多烟气和废气含尘量较大,容易使换热器产生积灰。影响换热器的传热效果。甚至造成阻塞影响工厂的正常生产。为此研究人员根据物体绕流的特点,提出了采用流线形管材.如椭圆管、扁平管.或者倾斜布置圆形管(沿流体流动方向投影为一椭圆形).减弱物体绕流的脱体尾涡.以减轮换热器的积灰程度。此外,截面为扁平的热管还具有良好的管外流动特性。用这样的热管组成的换热器具有阻力小、结构紧凑等优点。本文对     

自激振荡流热管内Cu-水纳米流体的传热特性

对工质为Cu-水纳米流体的自激振荡流热管在不同激光加热功率下的热传输特性进行了实验研究,并对工质为蒸馏水的自激振荡流热管的传热性能进行了比较.通过对不同的充液率、Cu纳米颗粒份额的Cu-水纳米流体自激振荡流热管实验结果分析发现:自激振荡流热管内Cu-水纳米流体的热传输具有一定的特殊性,在一定条件下纳米流体可以起到强化传热的作用,但决定纳米流体自激振荡流热管热传输性能的参数应是充液率.     

Experimental research on heat transfer of pulsating heat pipe

Experimental research was conducted to understand heat transfer characteristic of pulsating heat pipe in this paper, and the PHP is made of high quality glass capillary tube. Under different fill ratio, heat transfer rate and many other influence factors, the flow patterns were observed in the start-up, transition and stable stage. The effects of heating position on heat transfer were discussed. The experimental results indicate that no annular flow appears in top heating condition. Under different fall ratios and heat transfer rate, the flow pattern in PHP is transferred from bulk flow to semi-annular flow and annular flow, and the performance of heat transfer is improved for down heating case. The experimental results indicate that the total heat resistant of PHP is increased with fill ratio, and heat transfer rate achieves optimum at filling rate 50%. But for pulsating heat pipe with changing diameters the thermal resistance is higher than that with uniform diameters.     

氧化石墨烯对脉动热管传热性能的影响

实验选用外径为4mm、内径为2mm的铜质脉动热管研究了氧化石墨烯对以去离子水和体积分数为50%的乙醇溶液为工质的脉动热管传热性能的影响。实验分别采用加有少量氧化石墨烯的去离子水溶液(简称氧化石墨烯水溶液)和体积分数为50%的乙醇溶液(简称氧化石墨烯乙醇溶液),氧化石墨烯质量分数均为0.03%。实验发现：氧化石墨烯对以去离子水为工质的脉动热管传热性能具有强化作用,对以体积分数为50%的乙醇溶液为工质的脉动热管传热性能的影响较差,但都和脉动热管的加热功率密切相关。对于以去离子水为工质的脉动热管,在加热功率低于20W时,氧化石墨烯对脉动热管的强化作用较弱；当加热功率在30～60W之间时,氧化石墨烯对脉动热管的强化作用较强,在3.71～11.33%之间,且强化作用随加热功率的增大呈逐渐增强趋势；但随着功率继续增大,氧化石墨烯的强化作用逐渐减弱,当加热功率达到80W后,热管传热性能减弱,原因可能是氧化石墨烯颗粒出现了沉降现象。     

环路型脉动热管的工质流动和传热特性实验研究

建立了部分可视化的环路型铜-乙醇脉动热管试验台,研究了充液率、倾斜角度、环路数目等因素对脉动热管传热性能的影响。结果表明:不能形成脉动效应时工质的流型是间歇振动,形成脉动效应时工质的流型是弹状流或环状流;最佳倾角为70°~90,°最佳充液率在50%左右;热阻随着环路数目的增加而减小。     

Experimental study and analysis on heat transfer coefficient of radial heat pipe

The experimental study was performed on five eccentric radial heat pipes with two outer-tube diameters.The test range can be given as follows,working fluid filling ratio Ω=44%～83%,heat flux q=10000W/m2～32000W/m2,and working temperature tv=50 ℃～120 ℃.The correlations between radial heat pipe heat transfer performance and filling ratio,heat flux,working temperature were studied in the experiment.Based on linear regression of experimental data,the relationship between heat pipe equivalent heat resistance R and working temperature tv,heat flux q and filling ratio Ω was obtained.     

Analytical study of the heat transfer limits of a novel loop heat pipe system

A novel loop heat pipe system was designed for use in solar hot water heating and an analytical model was developed to investigate its thermal performance and determine six major limits to system operation, i.e. capillary limit, entrainment limit, viscous limit, boiling limit, sonic limit, and filled liquid mass limit. Relations among the limits and several associated parameters, i.e. the heat pipe operating temperature, wicks type, heat pipe diameter, and height difference between the absorbing pipes array and condenser (heat exchanger), were established through a comprehensive analyses. It was found that the levels of capillary, entrainment, viscous, sonic, and filled liquid mass limits increased with the increasing temperature; however, the boiling limit was in the adverse trend. It was also found that the mesh screen wicks were able to obtain a higher capillary limit than sintered powder wicks, whilst other limits remained same. Larger pipe diameters would lead to higher operating limits. The height difference between the condenser (heat exchanger) and absorbing pipes (absorber) was the most important factor impacting on heat transfer capacities of the system, and largely affected the capillary limit of the system. It was noted when the pipe (inner) diameter increased to 5.6 mm or above, the governing limit of the system switched from entrainment to capillary. Relationship between the system governing limit, i.e. capillary limit, and the above addressed parameters were analysed. Adequate system configuration and operating conditions were suggested, which were summarized as follows: 6 mm of pipe inner diameter with mesh screen wicks, 58°C of heat pipe operating temperature, and 1.3 m height difference between absorber and condenser (heat exchanger). Copyright © 2010 John Wiley & Sons, Ltd.     

微槽群平板热管散热器传热性能的研究

分别选择不同的翅片间距和高度,对一种新型微槽群平板热管散热器的翅片结构进行优化,得到了热管散热器的最佳整体结构。结果表明:翅片的间距为14mm、高度为60mm时,平板热管散热器的传热性能最好。将热管、管脚以及翅片的温度与实验结果进行对比,结果吻合良好。     

Study on forced convective heat transfer of non-newtonian nanofluids

This paper is concerned with the forced convective heat transfer of dilute liquid suspensions of nanoparticles (nanofluids) flowing through a straight pipe under laminar conditions. Stable nanofluids are formulated by using the high shear mixing and ultrasonication methods. They are then characterised for their size, surface charge, thermal and rheological properties and tested for their convective heat transfer behaviour. Mathematical modelling is performed to simulate the convective heat transfer of nanofluids using a single phase flow model and considering nanofluids as both Newtonian and non-Newtonian fluid. Both experiments and mathematical modelling show that nanofluids can substantially enhance the convective heat transfer. Analyses of the results suggest that the non-Newtonian character of nanofluids influences the overall enhancement, especially for nanofluids with an obvious non-Newtonian character.     

Experimental study of heat transfer of buried finned pipe for ground source heat pump applications

Ground heat exchangers have vital importance for ground source heat pump applications. Various configurations tried to improve heat transfer in the soil. A new kind of aluminium finned pipe buried in the soil for this aim. In order to compare effectiveness of the Al finned pipe over the traditional PPRC pipe an experimental study carried out. The experimental GSHP system was installed at Y?ld?z Technical University Davupasa Campus on 800 m² surface area with no special surface cover. Temperature data were collected using thermocouples buried in soil horizontally and vertically at various distances from the pipe center and at the inlet and the outlet of the ground heat exchanger. Experimental results were compared with results from analytical study. To compare effectiveness of the Al finned pipe and PPRC pipe a new parameter defined as transferred amount of heat per unit mass of working fluid per unit time for this aim. It is found that Al finned pipe has higher heat transfer values than the traditional PPRC pipe.     

Experimental investigation of heat transfer enhancement in a double pipe heat exchanger with a twisted inner pipe

In the present work, an experimental investigation is conducted to address the influence of inner pipe twisting on the overall performance of a double pipe heat exchanger. With the fluid to fluid heat exchange, both parallel and counter flow directions are examined as well. In addition to the original elliptical pipe, three pipes with different numbers of twisting (3, 5, and 7 twists per unit length) constructed from the elliptical pipe are considered where the heat transfer rate and pressure drop are addressed. All tests are carried out in the turbulent flow regime where the Reynolds number (Re) ranging from 5000 to 26,000 and water is used as the working medium. The obtained outcomes show that for both flow directions, there is an enhancement in the heat exchanger overall performance with all considered twisting pipes. The maximum enhancement in the Nusselt number is found to be 1.8 for the parallel flow and around 2.2 for the counter flow compared with the original pipe. The inner pipe with 7 twists, however, improves the overall performance the most, where a maximum performance enhancement factor of 1.63 and 1.9 are observed at Reynolds number of 26,000 in the parallel and counter flow configurations, respectively.