Minimization of entropy generation by asymmetric convective cooling

The uniform internal heating of a solid slab and the viscous flow between two parallel walls, are used to illustrate the possibility of minimizing the global entropy generation rate by cooling the external surfaces convectively in an asymmetric way. The known analytic expressions for the temperature field, in the first case, and the velocity and temperature fields, in the second case, are used to calculate the global entropy generation rate explicitly. In dimensionless terms, this function depends on the dimensionless ambient temperature and convective heat transfer coefficients (Biot numbers) of each surface which, in general, are not assumed to be the same. When the Biot numbers for each surface are equal, the entropy generation rate shows a monotonic increase. However, when the Biot numbers are different this function displays a minimum for specific cooling conditions.     

Nucleate boiling in a flat grooved heat pipe

The present paper is devoted to an experimental study aiming at determining the thermal behavior of a flat plate heat pipe (FPHP) with micro grooves and at focusing especially on the effect of boiling in the grooves. The FPHP is hermetically sealed on its upper face with a transparent plate for observations. Experimental results obtained with methanol are presented. Nucleate boiling in the grooves is observed for relatively small heat fluxes (3 W cm^?2). The presence of nucleate boiling in the grooves improves the thermal performance of the flat heat pipe, and it must be emphasized that it does not constitute an operation limit as it is generally stated. The dry out of the evaporator is observed for heat fluxes much higher than the heat flux of onset of nucleate boiling. The results obtained for different filling ratios show the influence of this parameter that has to be optimized to improve the performance of the device.     

一种热管平板太阳能集热装置的性能研究

为解决平板大阳能集热装置在冬季的防冻问题,设计建造了热管平板集热装置,并进行了实验测试.在实验中测量了系统的初末状态水温、流量、环境温度、辐照强度等数据.通过对数据的分析拟合出了系统效率与初始水温、环境温度以及辐照强度之间的经验公式,并且以该经验公式为依据对系统全年12个月典型晴天气象条件下的系统效率进行了模拟.结合实...     

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.     

Vapor pressure distribution of a flat plate heat pipe

The vapor pressure distributions of an asymmetric vapor flow of a flat plate heat pipe were calculated using the concept of suction and injection principle. Both symmetrical and asymmetrical flow conditions were considered. Comparisons were made with existing results. The agreement was reasonably good.     

热管技术应用于太阳能平板集热器的研究

介绍了将热管技术应用于平板集热器,以克服其易冰冻、管内结垢、腐蚀等常见问题,论述了设计中需关注的影响因素,并针对热管及翅片的结构优化、加工工艺等进行了探讨.     

On Effectiveness and Entropy Generation in Heat Exchanger

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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。     

Numerical analysis reliability control for a double‐pipe heat exchanger with virtual entropy generation method

There are two primary laws including the first and second laws of thermodynamics that should be used to assess a process. Generally, only the first law of thermodynamics is investigated in numerical solutions, so it is possible to exist some numerical results that do not satisfy the second law of thermodynamics because of numerical errors. To achieve reliable numerical outcomes, it is better to apply two indexes of HEAT BALANCE ERROR and VIRTUAL ENTROPY GENERATION, which come from the second law of thermodynamics. In other words, an approach to develop computational fluid dynamics investigations is to take second law of thermodynamics into consideration. In this study, two different models including counterflow double‐pipe heat exchanger and single‐pipe with constant wall temperature are simulated in various cases with different efficiencies and temperature ratios. It is found that 46 cases of total 523 double‐pipe models and 24 cases of total 402 simulations of single‐pipe models had unacceptable results regarding to two mentioned criteria. The results revealed that it is less likely to gain unreliable results in smaller efficiency and lower inlet temperature for double‐pipe heat exchanger and single‐pipe respectively.     

Optimization of a wickless heat pipe flat plate solar collector

The transient thermal behaviour of wickless heat pipe flat plate solar collectors has been analyzed with regard to various parameters. These parameters include global solar radiation intensity, inlet cooling water temperature, absorber plate material and thickness, ratio of pitch distance to wickless heat pipe diameter and ratio of condenser section length to total wickless heat pipe length. The results show that the selection of an absorber plate having a high value of conduction heat transfer rate per unit temperature difference is limited by the pitch distance.     

A Modified Entropy Generation Number for Heat Exchangers

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Study on heat transfer performance for loop heat pipe with circular flat evaporator

A loop heat pipe (LHP) with a circular flat evaporator was designed for cooling electronic devices. The flat evaporator with an outside diameter of 41 mm and a thickness of 15 mm was developed with a copper powder wick. The developed evaporator was examined to improve insufficient subcooling of liquid in a compensation chamber, which decreases an operating limitation of the LHP. Many different orientations of the elevation and direction of the evaporator were also considered during all of the experiments for this system. The active heating area was 3 cm × 3 cm, and water was used in the tests. This LHP generated a heat load in excess of 140 W with a total thermal resistance of 0.39 °C/W.     

平板型环路热管温度波动现象研究

环路热管(loop heat pipe,LHP)是一种靠蒸发器内毛细芯产生毛细力驱动回路运行,利用工质相变来传递热量的高效传热装置.研制了一套小型平板式蒸发器、风冷式冷凝器的环路热管(mLHP),mLHP的毛细芯为500目不锈钢丝网,工质为丙酮和甲醇.蒸发器、冷凝器以及所有管路均由紫铜制成.着重研究了平板型mLHP在不同热负荷条件下的温度波动特性.实验结果表明,平板式mLHP在某些热流密度区间容易发生温度波动;同时,重点研究了工质对mLHP系统温度波动的影响,并给出相应的合理解释.     

平板式微型热管的温度特性实验研究

平板热管是一种较好的电子散热元件 ,本文对平板热管不同工质、不同的输入功率及不同的充液率的换热特性进行试验。结果表明平板热管有较好的散热效应 ,可以有效地降低物体温度。     

Thermal response of a flat heat pipe sandwich structure to a localized heat flux

The temperature distribution across a flat heat pipe sandwich structure, subjected to an intense localized thermal flux has been investigated both experimentally and computationally. The aluminum sandwich structure consisted of a pair of aluminum alloy face sheets, a truncated square honeycomb (cruciform) core, a nickel metal foam wick and distilled water as the working fluid. Heat was applied via a propane torch to the evaporator side of the flat heat pipe, while the condenser side was cooled via natural convective and radiative heat transfer. A novel method was developed to estimate experimentally, the heat flux distribution of the torch on the evaporator side. This heat flux distribution was modeled using a probability function and validated against the experimental data. Applying the estimated heat flux distribution as the surface boundary condition, a finite volume analysis was performed for the wall, wick and vapor core regions of the flat heat pipe to obtain the field variables in these domains. The results were found to agree well with the experimental data indicating the thermal spreading effect of the flat heat pipe.     

Analysis of flow and heat transfer in evaporator porous wicking structure of a flat heat pipe

With a specified pressure distribution, an analytical investigation was conducted to explore the flow and heat transfer characteristics in an evaporator porous wicking structure of a flat heat pipe. The boundary effect on the flow rate is more significant than the inertia, and both the boundary and inertia effects exert very little influence on fluid layer thickness and velocity distribution. The bottom of the porous layer is at a quite uniform temperature, and the heat flux is almost normal to the solid boundary. © 2008 Wiley Periodicals, Inc. Heat Trans Asian Res; Published online in Wiley InterScience ( www.interscience.wiley.com ). DOI 10.1002/htj.20237     

Effect of maximum and minimum heat capacity rate on entropy generation in a heat exchanger

This paper presents a theoretical analysis of a heat exchanger with a negligible fluid flow pressure drop to determine whether it is better to operate the heat exchanger with the minimum or maximum heat capacity rate of the hot fluid from entropy generation point of view. Entropy generation numbers are derived for both cases, and the results show that they are identical, when the heat exchanger is running at a heat capacity ratio of 0.5 with heat exchanger effectiveness equaling 1. An entropy generation number ratio is defined for the first time, which has a maximum value at ε = 1/(1+R) for any inlet temperature ratio. When R equals 0.1, 0.5 and 0.9, the entropy generation number ratio receives a maximum value at an effectiveness equaling 0.91, 0.67 and 0.526, respectively. When R=0.9, the entropy generation number ratio is the same for all inlet temperature ratios at ε=0.8. The results show that the entropy generation number ratio is far from 1 depending on the inlet temperature ratio of the cold and hot fluid. The results are valid for parallel‐flow and counterflow heat exchangers. Copyright © 2010 John Wiley & Sons, Ltd.     

Operational characteristics of two biporous wicks used in loop heat pipe with flat evaporator

Two special biporous wicks are adopted in stainless-steel–ammonia loop heat pipes (LHPs) with flat evaporator to enhance their heat transfer performances. The experimental results demonstrate that thermal and hydraulic characteristics of the wick with porosity of 69% (in LHP 2) are better than that of the wick with porosity of 65% (in LHP 1). The maximum heat loads of LHP 1 and LHP 2 could, respectively, reach 120 W (heat flux 11.8 W/cm²) and 130 W (12.8 W/cm²) at the allowable evaporator temperature below 60 °C. Meanwhile, they can start up at heat load as low as 2.5 W. The LHPs show very fast and smooth response to heat load and operate stably without obvious temperature oscillation. The total thermal resistances of the LHPs vary between 1.47 and 0.33 °C/W at heat load ranging from 10 to 130 W.