振荡流热管自激强化传热的可行性分析

围绕自激振荡流热管的工作特性,联系目前国内外纳米流体及非均匀截面结构在强化传热研究方面的最新进展,分析了在自激振荡流热管内的复杂相变换热条件下,通过采用纳米流体工质与非均匀管截面以实现振荡流热管自激强化传热的可行性,并提出了纳米流体的浓度、相容性、充液率以及管截面结构等需要解决的关键性技术问题。为下一步有针对性的强化传热实验研究和新型自激振荡流热管换热器的设计与开发提供了必要的理论依据。     

非均匀截面自激振荡流热管内热传输特性实验研究

针对两种不同截面结构的回路型自激振荡流热管,其中,一种内径为3 mm的均匀截面;另一种是在此均匀截面的基础上,将其加热段和绝热段断面加工成垂直交错布置的椭圆形非均匀截面。运用实验研究的方法,在热管壁面不同部位分别布置了温度测点,在不同功率的激光加热条件下,通过对热管壁温监测数据的采集和分析,研究了这种非均匀截面回路型自激振荡流热管内部的热传输性能,并与均匀截面自激振荡流热管进行了对比分析。结果表明:非均匀截面自激振荡流热管内的脉动机制以及热传输特性与均匀截面有很大不同,在中、高负荷条件下,非均匀截面结构热管可以明显起到强化传热效果,与均匀截面结构热管相比,热传输功率提高了13.6%。     

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.     

间隔热源加热下新型热管冷板内部汽液两相流动与传热的数值模拟

为了有效解决阵列行波管的多热源、高热流散热问题，研制了一种结构全新的热管冷板。建立了热管冷板内部汽液两相流动与传热的两相流模型，并采用IPSA算法对间隔热源加热下热管冷板的内部流动和传热特性进行了数值模拟。通过数值模拟和试验验证，考察了该热管冷板的初步运行性能，为其实际应用提供了依据。图8表1参9     

多通道并联回路型脉动热管运行特性的试验研究

针对回路型脉动热管进行了管路结构形式调整,制作了多通道并联回路型脉动热管并建立试验系统,选用丙酮和无水酒精作为工质,在相近热力工况下通过试验考察多通道并联回路型脉动热管和典型回路脉动热管在不同加热工况下的运行情况,并进行比较.结果表明:多通道并联回路型脉动热管与典型回路型脉动热管具有相似的启动特征,但其在运行中具有更好的稳定性,不易出现干烧现象;其传热效果也优于典型回路型脉动热管,具有较低的运行热阻,低充液率(34%)时的传热效果优于高充液率(51%、68%)时,具有较高的传热极限.     

热管式强化传热燃气锅炉研制

针对气体燃料的燃烧特点,研究开发了热管式强化传热燃气热水锅炉.阐述了此种型式锅炉的基础实验、结构设计及计算要点.实测及运行经验表明,此系列燃气锅炉具有强化传热、洁净燃烧、高效节能、低排放、经久耐用等优点.     

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.     

回路热管的热力特性分析

对回路热管稳定运行的动力和热力特性进行了分析,指出整个循环要顺利完成,主毛细芯产生的毛细压差必须具备的条件以及换热条件.并进而在合理的简化和假设条件下,对该热管建立了热力分析数学模型.用简单的代数运算代替了复杂的微积分计算,在精确度要求不高时,能够简便快速的进行热力分析.应用所提模型和计算方法对两个给定的回路热管进行了热力分析,得出了关键节点的压力、温度,分析了工作温度、环境温度和毛细芯厚度等对回路热管传热性能的影响.     

C型混沌结构中传热强化的数值分析

随着对强化传热的广泛重视与研究,利用混沌对流来强化传热的新技术得到了关注.利用CFD软件Fluent对C型混沌结构内的流体流动与传热进行数值模拟,对比了C型混沌结构与普通平直结构在流体流动场、温度场分布和传热特性等细观信息,分析了C型混沌结构的强化传热性能及特点.分析结果表明,C型混沌结构使流体在较小速度下产生混沌对流,这种流态增加了流体的扰动与湍动,增大主流区或近壁处流动的混合,强化了流道内的传热,使流道横截面上的温度分布均匀化;混沌对流内的传热Nu数和Po数(即fRe值)不再象普通层流为一定值,而随Re数的增大而增大.     

开式热管传热特性的实验研究

对新型换热元件开式热管的传热特怀进行了实验研究，结果表明开式热管存在两个传热转折点。其实际工作的最佳范围应在两个传热转折点之间。本文给工度经、小孔直径及小孔位置变化工式热管传热能力的实验结果。为合理设计开式热管理提供了参考实验数据。     

Heat transfer in a second grade fluid through a porous medium from a permeable stretching sheet with non-uniform heat source/sink

In the present article an analysis is carried out to study the boundary layer flow and heat transfer characteristics of a second grade, non-Newtonian fluid through a porous medium. The stretching sheet is assumed to be permeable so that suction effects come into play. The effects of viscous dissipation, non-uniform heat source/sink on heat transfer are addressed. The basic boundary layer equations for momentum and heat transfer, which are non-linear partial differential equations, are converted into non-linear ordinary differential equations by means of similarity transformation. Analytical solutions are obtained for the resulting boundary value problems. The effects of viscous dissipation and non-uniform heat source/sink, Prandtl number, Eckert number and suction/injection on heat transfer are shown in several plots for two different heating processes (CST and PST cases). Dimensionless surface temperature gradient is tabulated for various values of the governing the parameters.     

Effect of Convection Heat Transfer on Performance of Waste Heat Thermoelectric Generator

Efficiency of energy conversion processes can be improved if waste heat is converted to electricity. A thermoelectric generator (TEG) can directly convert waste heat to electricity. The TEG typically suffers from low efficiency due to various reasons, such as ohmic heating, surface-to-surrounding convection losses, and unfavorable material properties. In this work, the effect of surface-to-surrounding convection heat transfer losses on the performance of TEG is studied analytically and numerically. A one-dimensional (1-D) analytical model is developed that includes surface convection, conduction, ohmic heating, and Peltier, Seebeck, and Thomson effects with top and bottom surfaces of TEG exposed to convective boundary conditions. Using the analytical solutions, different performance parameters (e.g., heat input, power output, and efficiency) are calculated and expressed graphically as functions of thermal source and sink temperatures and convection heat transfer coefficient. Finally, a two-dimensional (2-D) mathematical model is solved numerically to observe qualitative results of thermal and electric fields inside the TEG. For all calculations, temperature-dependent thermal/electric properties are considered. Increase in thermal source temperature results in an increase in the power output with adiabatic side wall conditions. A change in boundary condition to convection heat transfer from adiabatic boundary has a large impact on thermal efficiency.     

电场作用下肋表面电对流现象的可视化研究

电场可强化池态沸腾换热已经得到研究者的普遍认可.以往研究者主要针对光滑表面、加热管或加热线表面的电场强化换热进行研究.在光滑表面增加肋结构可以改变加热面附近的电场分布,从而影响加热面的换热效果.以R113作为实验工质,搭建了可视化的池沸腾实验台,采用高速摄像机记录了肋高为2 mm的加热面在电场作用下的电对流现象.实验发现,电对流随着施加电压的升高而增强,电对流加速了加热面附近流体的混合,破坏了热边界层,使得加热面温度降低从而强化了换热.     

非均匀热流下水平管内混合对流大涡模拟研究

针对以槽式太阳能集热器为背景的高密度、高度非均匀热流下水平管内的混合对流换热问题,采用大涡模拟方法,研究了热流密度非均匀性对水平管内混合对流瞬态涡结构、脉动强度、湍流热通量及局部平均壁温的影响;揭示了非均匀热流下自然对流对管内湍流特性的影响规律;提出了适用于不同热边界条件下管内混合对流换热的强化措施。结果表明:均匀热流时,自然对流会抑制管顶部的湍流脉动,使流动层流化,造成传热能力局部恶化;非均匀热流时,随着自然对流的增强,近壁面速度脉动强度先减小后增大,二次流逐渐增强,换热能力逐渐提高,故管内换热能力受湍流脉动与二次流协同影响;在自然对流影响下,均匀加热时管顶部可采用针对层流的强化换热措施,非均匀加热时需着重提高管底部高热流区域的湍流脉动与涡强度。     

Thermal Performance of a Closed-Loop Pulsating Heat Pipe With Multiple Heat Sources

A closed-loop pulsating heat pipe with multiple heat sources (CLPHP w/MHS) was invented to be used as a heat transfer medium between a number of heat sources to a single heat sink. However, an issue on the suitable heat source arrangement that causes the heat pipe to have the highest thermal performance was suspicious. The CLPHP w/MHS was made of a copper capillary tube with 32 turns. There were three heat sources with nonidentical input heat flux installed along a longitudinal axis in the evaporator section. Experimental investigations were conducted by permuting the heat sources into six unduplicated arrangements. For the vertical CLPHPs, the highest thermal performance is achieved when heat sources are arranged in consecutive order ascending from the lowest heat flux at the inlet of the evaporator section, since working fluid is promoted to circulate in complete one direction and then the heat can transfer more continuously. Finally, for the horizontal CLPHPs, the highest thermal performance is achieved when the heat sources are arranged in opposite order to the case of vertical CLPHPs, that is, descending from the highest heat flux, since working fluid pulsates with no intermission stop and this causes the heat transfer to be not interrupted.     

A Study of Bubble Behavior and Boiling Heat Transfer Enhancement under Electric Field

The effect of a D.C. electric field on nucleate boiling heat transfer for refrigerants, R11, R113, and FC72, was investigated experimentally in a single-tube shell/tube heat exchanger by using the temperature control method of wall superheat. Also the behavior of bubble under nonuniform electric field produced by wire electrodes was studied by numerical calculation. For R11, the electrohydrodynamic (EHD) enhancement for boiling heat transfer was observed for all ranges of wall superheat tested. However, the enhancement in boiling heat transfer disappeared if the wall superheat exceeded 13°C for R113, and no electric field effect on the boiling heat transfer was observed for FC72. An application of approximately 5 kV was enough to eliminate the boiling hysteresis for R11 and R113. Numerical study of the electric field in a single medium has hinted that the bubbles are forced away from the heating surface and toward the electrostatic stagnation point by the dielectrophoretic force. Such modified bubble motion turns out to promote the boiling heat transfer if one uses proper electrode configuration.     

加热方向对方腔内相变石蜡储热性能影响研究

为探究方腔内相变石蜡的储热性能,基于等效热容法和Boussinesq假设,建立相变石蜡融化储热计算模型,并针对加热方向及约束形式等因素对相变石蜡的储热性能的影响进行研究,并开展相变石蜡融化试验,验证计算模型的正确性。结果表明：相变石蜡融化储热过程是由热传导和自然对流传热综合决定的,其中自然对流传热在相变石蜡融化储热过程中起着极为显著的促进作用;不同加热方向下,相变石蜡表现出截然不同的融化储热效率,其中顶、底、侧边单独加热下的自然对流传热效应依次使储热效率提升了0.01,27.9和13.1倍,即底部热源的储热效率最高;在四面加热下,固相因无约束而下沉至底部,并抑制底部热壁面的自然对流传热效应,此时顶、底、侧热壁面的储热贡献率分别为17.3%,37.3%和22.7%;当固相运动被预埋热电偶等因素限制时,将形成钟型融化前缘,该形态包含了各热壁面单独加热下的融化储热特征,此时顶、底、侧热壁面的储热贡献率分别为19.2%,29.8%和25.5%。     

Heat transfer in a viscoelastic boundary layer flow over a stretching sheet

Studies are made on the viscoelastic fluid flow and heat transfer characteristics over a stretching sheet with frictional heating and internal heat generation or absorption. The heat transfer analysis has been carried out for the cases of prescribed surface temperature (PST) and prescribed surface heat flux (PHF). The momentum equation is decoupled from the energy equation for the present incompressible boundary layer flow problem with constant physical parameters. Exact solution for the velocity field and the skin-friction are obtained. Also, the solutions for the temperature and heat transfer characteristics are obtained in terms of Kummer’s function. The work due to deformation in energy equation, which is essential while formulating the viscoelastic boundary layer flow problems, is considered. This paper examines the effect of viscoelastic parameter, Eckert number, Prandtl number and non-uniform heat source/sink parameter on temperature distribution, wall temperature gradient for PST-case and wall temperature for PHF-case.