Performance degradation of flattened heat pipes

The performance degradation of flattened heat pipes is studied experimentally under a horizontal orientation. The original cylindrical copper/water heat pipes are ?6 mm and 30 cm in length. Tested are the sintered-powder wick and the groove wick. The maximum heat load (Q_max), the evaporator resistance (R_e), the condenser resistance, the overall thermal resistance, and the longitudinal temperature distributions are measured under incremented heat loads. After flattening, R_e is slightly reduced. Q_max is hardly affected when only the evaporator is flattened; but it is greatly reduced for fully flattened heat pipes. Different mechanisms of performance degradation are observed for flattened powdered and grooved heat pipes. With a thicker wick and larger saturate charge, the main degradation mechanism of flattened powdered heat pipes is liquid clogging at the condenser end. This causes malfunction of a powdered heat pipe flattened to 2.5 mm. When flattened to 3 mm, the powdered heat pipe exhibits milder Q_max degradation than a grooved heat pipe because the liquid flow is better protected against the vapor–liquid interfacial shear. In contrast, the serious Q_max degradation of a flattened grooved heat pipe is mainly caused by the interfacial shear which leads to greatly prompted dryout at the evaporator.     

An Investigation of Capillary Flow Effect on Condensation Heat Transfer on a Grooved Plate

A simple model predicting the effect of capillary flows on the condensation heat transfer occurring on a vertical grooved plate is presented. The model includes the effect of capillary flows along the axial direction of the grooves and the direction from the apex to the cornered region of the groove. Numerical results indicated that the capillary flow induced by the capillary force significantly affects the condensation heat transfer, as does the plate height, groove width, and groove angle. In order to verify the theoretical prediction, an experimental investigation was conducted, and the theoretical prediction agrees well with the experimental data. Results of the investigation will assist in optimizing the condensation heat transfer performance in heat pipes.     

Thermal Performance Enhancement of L-Shaped Microgrooved Heat Pipe Containing Water-Based Al2O3 Nanofluids

An experimental study was performed to investigate the thermal performance of an L-shaped grooved heat pipe with cylindrical cross section, which contained 0.5 wt% water-based Al₂O₃ nanofluid as the working fluid. The transient performance of the heat pipe and the effect of cooling water temperature on the heat transfer characteristics of the heat pipe were investigated. The outer diameter and the length of the heat pipe were 6 mm and 220 mm, respectively. Experimental results revealed that the temperature of the cooling water has a significant effect on the thermal resistance of the heat pipe containing nanofluids as its working fluid. By increasing the cooling water temperature from 5°C to 27.5°C, the thermal resistance decreases by approximately 40%. At the same charge volume, test results indicated an average reduction of 30% in thermal resistance of heat pipes with nanofluid as compared with heat pipe containing pure water. For transient conditions, unsteady state time for nanofluids was reduced by approximately 28%, when compared with water as the working fluid.     

Heat transfer and single-phase flow in internally grooved tubes

This study investigates heat transfer and flow characteristics of water flowing through horizontal internally grooved tubes. The test tubes consisted of one smooth tube, one straight grooved tube, and four grooved tubes with different pitches. All test tubes were made from type 304 stainless steel. The length and inner diameter of the test tube were 2 m and 7.1 mm, respectively. Water was used as working fluid, heated by DC power supply under constant heat flux condition. The test runs were performed at average fluid temperature of 25 °C, heat flux of 3.5 kW/m², and Reynolds number range from 4000 to 10,000. The effect of grooved pitch on heat transfer and pressure drop was also investigated. The performance of the grooved tubes was discussed in terms of thermal enhancement factor. The results showed that the thermal enhancement factor obtained from groove tubes is about 1.4 to 2.2 for a pitch of 0.5 in.; 1.1 to 1.3 for pitches of 8, 10, and 12 in., respectively; and 0.8 to 0.9 for a straight groove.     

Modeling of Thin Liquid Film in Grooved Heat Pipes

Axially grooved heat pipes are devices used mainly to dissipate heat flow through the latent heat of phase change of a fluid at a saturation state. Part of the heat injected at the evaporator flows through a micro-region where the meniscus remains hung at the top of each groove. This article presents a steady-state model built to study the heat and mass transfers in this zone. A parametric study has been performed on aluminum/ammonia heat pipes, mainly used for satellite thermal control, and it led to interesting results, especially regarding the incidence of the radius of curvature of the meniscus and the influence of temperatures on the transfers occurring in the problem.     

Augmentation of heat transportation by an oscillatory flow in grooved ducts

This paper deals with heat transportation by oscillatory flow in grooved ducts. The heat transportation rate, work rate, heat transportation efficiency, and dispersions of fluid particles and heat were analyzed with the computer code FLUENT. The frequency and tidal amplitude of the oscillatory flow were 0.05 Hz and 45 mm, respectively. The internal diameters of the contraction and expansion sections of the grooved ducts were 6 and 12 mm, respectively. The groove lengths varied from 0 to 40 mm and the pitch of the grooves was fixed at 10 mm. We found that: (1) The grooved duct with S=15 mm had the highest heat transportation rate, which was about 4.5 times that of the smooth round pipe. (2) The grooved duct with S=20 mm had the greatest heat transportation efficiency, which was about 6.4 times that of the smooth round pipe. (3) Enthalpy transportation by the dispersion motion of fluid particles played a substantial role in the heat transportation of the grooved ducts with 5 mm≤S≤40 mm. (4) The grooved duct with S=10 to 15 mm had the greatest dispersion of fluid particles, which explained their having the greatest heat transportation rate. © 2008 Wiley Periodicals, Inc. Heat Trans Asian Res, 37(2): 68–85, 2008; Published online in Wiley InterScience ( www.interscience.wiley.com ). DOI 10.1002/htj.20192     

Flow visualization in turbulent free convection over horizontal smooth and grooved surfaces

The present paper discusses the flow visualization for turbulent free convection in a tank of water with the bottom surface being a smooth or a grooved surface and the top of the water surface exposed to ambient. The grooved surface is of parallel 90° V-grooves with groove height of 10 mm and groove width of 20 mm. The experiment is carried out with aspect ratio (AR) of 2.9 and Rayleigh number (Ra) in the range, 1.3 × 10⁷ − 4 × 10⁷. Here AR is the aspect ratio (= width of fluid layer/height of fluid layer). Heat flux at the bottom surface is from electrical heating. From the pH-dye visualization, interesting flow structures are observed and these structures are analyzed with the help of plumes dynamics and temperature variations with time.     

Analytical and Experimental Investigations on Axially Grooved Aluminum-Ethane Heat Pipe

Heat pipes are promising devices to meet the needs of thermal management in spacecraft. In the present work, a dual core axially grooved aluminum-ethane heat pipe was developed and tested for its heat transport characteristics over the temperature range 150–240 K. An analytical model has also been developed to yield maximum heat transport, taking into account the effect of liquid-vapor interfacial shear stress, liquid meniscus variation, and the amount of working fluid in the heat pipe. The model results were compared with experimental results and are found to be in reasonably good agreement.     

A mathematical model for analyzing the thermal characteristics of a flat micro heat pipe with a grooved wick

A mathematical model is developed for predicting the thermal performance of a flat micro heat pipe with a rectangular grooved wick structure. The effects of the liquid–vapor interfacial shear stress, the contact angle, and the amount of liquid charge are accounted for in the present model. In particular, the axial variations of the wall temperature and the evaporation and condensation rates are considered by solving the one-dimensional conduction equation for the wall and the augmented Young–Laplace equation, respectively. The results obtained from the proposed model are in close agreement with several existing experimental data in terms of the wall temperatures and the maximum heat transport rate. From the validated model, it is found that the assumptions employed in previous studies may lead to significant errors for predicting the thermal performance of the heat pipe. Finally, the maximum heat transport rate of a micro heat pipe with a grooved wick structure is optimized with respect to the width and the height of the groove by using the proposed model. The maximum heat transport rate for the optimum conditions is enhanced by approximately 20% compared to existing experimental results.     

Heat pipes for thermal control of ISS solar battery drive

This paper considers design and testing aspects of ammonia axial grooved heat pipes (AGHPs) made of aluminum alloy 6063 intended for cables thermal control of the solar battery drive integrated into the International Space Station.The heat removal system includes four heat pipes with a diameter of 10 mm and length of 2 m. Each heat pipe shall transfer 50 W in temperature range of 0–50 °C with a maximum temperature differential of 5 °C.Great attention is paid to the requirements of reliability and prolonged lifetime (19 years: 15 years of operation and four years of storage). For this purpose several heat pipes made 10 years ago of the same initial materials were subjected to accelerated lifetime testing (testing duration is 4250 h at a temperature of 100 °C).AGHP performance obtained before and after accelerated testing and also acceptance test results are presented in this paper. The determined data shows that AGHP performance is in compliance with the statement of work requirements.     

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.     

Thermal performance of inclined grooved heat pipes using nanofluids

An experimental study was performed to investigate the thermal performance of an inclined miniature grooved heat pipe using water-based CuO nanofluid as the working fluid. This study focused mainly on the effects of the inclination angle and the operating pressure on the heat transfer of the heat pipe using the nanofluid with the mass concentration of CuO nanoparticles of 1.0 wt%. The experiment was performed at three steady sub-atmospheric pressures. Experimental results show that the inclination angle has a strong effect on the heat transfer performance of heat pipes using both water and the nanofluid. The inclination angle of 45° corresponds to the best thermal performance for heat pipes using both water and the nanofluid. The present investigation indicates that the thermal performance of an inclined miniature grooved heat pipe can be strengthened by using CuO nanofluid.     

Numerical analysis on flow pattern and heat transfer characteristics of flow boiling in the mini-channels

Abstract

Flow pattern and heat transfer of flow boiling for different flow orientation, mini-channel width and height were presented in this work. The data were obtained by the numerical simulation with the coolant of R141b flow in a vertical mini-channel. Orientation includes upward and downward. A constant heat flux was loaded at the wall of the channel, of which the width ranges from 1?mm to 3?mm, and a length changes from 200?mm to 400?mm. Subsequently the impact of those parameters that referred to heat flux, inlet temperature and inlet temperature of the coolant could be investigated by carrying out the numerical calculation. In addition, a validation for the model was illustrated in comparison with the previously experimental data.     

Flat and U-Shaped Heat Spreaders for High-Power Electronics

To achieve a high heat-flux level and reduce manufacturing costs associated with conventional heat pipes, the concept of network heat spreaders employing a boiling heat-transfer mechanism in a narrow space had been proposed, and several flat-plate wickless heat spreaders had been designed and fabricated. The heat spreaders had been tested under different working conditions and orientations relative to gravity with very good results. The previously tested network heat spreaders, however, were based on plates with a relatively large size for general heat spreading purposes. In the present study, network heat spreaders with overall dimensions of 78 2 62 2 3.2 mm are designed and fabricated. Spreaders of this size are intended for use as heat sinks of high-power electronic components. External cooling fins are attached to enhance air-cooling heat transfer rate. The network heat spreaders are tested under various working conditions with water as the working fluid. The maximum heat input rate achieved is about 150 W with a corresponding heat flux of 60 W/cm 2 . Compared to the performance of a solid copper plate having the same overall size as the spreader, the maximum temperature difference over the surface is reduced from about 32°C to 3.3°C. The heat transfer performance of the spreader is also largely dependent on the filling ratio of the working fluid and the boiling heat transfer in the narrow space. For these reasons, boiling heat transfer mechanisms in a narrow space are analyzed, and a spreader design that would improve the performance in a horizontal position is described.     

An Experimental Study of Small-Diameter Wickless Heat Pipes Operating in the Temperature Range 200°C to 450°C

An experimental investigation is reported for medium-temperature, wickless, small-diameter heat pipes charged with environmentally sound and commercially available working fluids. The wickless heat pipes (thermosyphons) studied have many applications in heat recovery systems since their operational temperature range is between 200°C and 450°C. The heat pipes investigated had an internal diameter of 6 mm and a length of 209 mm. The lengths of evaporator and condenser sections were 50 mm and 100 mm, respectively. The working fluids tested were diphenyl based: Therminol VP1 and Dowtherm A. High-grade stainless steel was chosen as the shell material for the heat pipes to provide chemical compatibility between heat pipe casing material and working fluids at elevated temperatures. Thermal resistances of less than 0.4 K/W have been achieved at working temperatures of up to 420°C with an effective thermal conductivity of 20 kW/m-K, which corresponds to an axial heat flux of 2.5 MW/m². Even for such small-diameter heat pipes, the experimental data for the evaporator showed good agreement with Rohsenow's pool boiling correlation.     

带有横向微沟槽的矩形通道内超临界液化天然气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120.000～250.000 K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)和进口压力对传热系数(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。     

Analytical and experimental investigation on the operational characteristics and the thermal optimization of a miniature heat pipe with a grooved wick structure

A mathematical model for heat and mass transfer in a miniature heat pipe with a grooved wick structure is developed and solved analytically to yield the maximum heat transport rate and the overall thermal resistance under steady-state conditions. The effects of the liquid-vapor interfacial shear stress, the contact angle, and the amount of initial liquid charge have been considered in the proposed model. In particular, a novel method called a modified Shah method is suggested and validated; this method is an essential feature of the proposed model and accounts for the effect of the liquid-vapor interfacial shear stress. In order to verify the model, experiments for measuring the maximum heat transport rate and the overall thermal resistance are conducted. The analytical results for the maximum heat transport rate and the total thermal resistance based on the proposed model are shown to be in close agreement with the experimental results. From the proposed model, numerical optimization is performed to enhance the thermal performance of the miniature heat pipe. It is estimated that the maximum heat transport rate of outer diameter 3 and 4 mm heat pipes can be enhanced up to 48% and 73%, respectively, when the groove wick structure is optimized from the existing configurations. Similarly, the total thermal resistance of these heat pipes can be reduced by 7% and 11%, respectively, as a result of optimization.     

Performance studies on solar collector with grooved absorber tube configuration using aqueous ZnO–ethylene glycol nanofluids

Solar flat plate collector (SFPC) is regarded as one of the best renewable energy devices to acquire hot water for domestic usage. Though its theoretical efficiency is projected at higher scale, its conversion efficiency is observed to be low due to various collector losses. Hence, to achieve improved performance of collector different techniques for heat transfer augmentation in circular pipes can be adopted. Among passive and active techniques available, passive is preferred over active due to economics and saving in exergy. Hence, in the present study absorber tube configuration is modified with internal grooves in the collector setup to enhance the rate of heat transfer. Also, fluids dispersed with metal oxide particles provide an increase in thermal conductivity such that ZnO-based aqueous EG mixture nanofluid is used as working medium to analyze the performance of collector setup. Also, the change in heat transfer rate and temperature profile of grooved tubes under forced laminar conditions for different working fluids are compared and reported.     

Heat transfer enhancement in a grooved channel with curved vanes

We visualize unsteady temperature fields in the grooved channel with curved vanes using holographic interferometry. The heat transfer performance of the investigated channel is compared with that of the basic grooved channel. The addition of curved vanes above the downstream end of the heated block redirects the flow from the main channel into the groove. Heat transfer shows an increase by a factor of 1.5-3.5, when compared to the basic grooved channel, mainly due to increased flow velocities in the groove region. Flow transition from steady to oscillatory occurs around Re=450 and flow oscillations contribute to heat transfer enhancement. The pressure drop is 3-5 times higher than in the basic grooved channel.     

Thermal analysis on a groove‐enhanced minichannel application

A grooved surface feature is considered as a thermal enhancement for electronics cooling with single‐phase flow in minichannels. The influence of the groove structure and groove depth‐to‐width ratio on the flow and heat transfer in the minichannels has been numerically investigated. A two‐dimensional turbulent flow model was employed to optimize the groove structure in the minichannel. The effect of the groove geometric shape on the heat transfer performance in the minichannel was analyzed by evaluating the fluid thermophysical parameter and Nusselt number. It is found that the average Nusselt number ratio ($Nu_{avg}^{*}$ ) changes with an increase in groove depth‐to‐width ratios from d/W = 0.1 to d/W = 0.5. The groove heat transfer unit number (NTU) integrating heat transfer area (A) and heat transfer coefficient (h) were defined. The change of the NTU^* for the minichannel with a triangular groove is different from that of the minichannel with a cylindrical groove while the groove depth‐to‐width ratio varies from d/W = 0.1 to d/W = 0.5. In addition, the flow pressure loss across the groove and the effects of the Reynolds number in the minichannels were also investigated. All the results should be taken into account for a better design of a minichannel with groove. © 2012 Wiley Periodicals, Inc. Heat Trans Asian Res; Published online in Wiley Online Library ( wileyonlinelibrary.com/journal/htj ). DOI 10.1002/htj.20413