Effect of low thermal conductivity wick on the performance of compact loop heat pipe

The existing work deals with the evaluation of compact loop heat pipe by means of a low thermal conductivity sintered chrysotile wick to avoid large heat leaks as of the evaporator to the compensation chamber. Accordingly, a wick with low thermal conductivity (0.068–0.098 W/mK) chrysotile powder of a mean particle diameter of 3.4 μm is fabricated through sintering. Nine chrysotile wicks are sintered with different compositions of binders (bentonite and dextrin) and pore-forming agent NaCl at sintering temperatures of 500°C, 600°C, and 700°C with a sintering time of 30 min. The wick properties, for instance, porosity, permeability, wettability, and capillary rise are studied owing to sintering temperature. Consequently, it is observed that a pure chrysotile powdered wick at a sintering temperature of 600°C exhibits a high porosity of 61.8% with permeability 1.04 × 10⁻¹³ m² and a capillary rise of 4.5 cm in 30 s and is considered optimal. This optimal wick is used for performance evaluation in compact loop heat pipe and a decrease of 36.1% in thermal resistance is found when compared with copper mesh wick in a loop heat pipe. The lowermost thermal resistance originates to be 0.147 K/W at 120 W with wall temperature 57.7°C. This indicates that loop heat pipe with sintered chrysotile wick can operate at lower heat loads efficiently when compared with copper mesh wick and as heat load increases a chance of dry out condition occurs. The highest evaporative heat transfer coefficient obtained is 65.7 kW/m² K at a minimum heat load.     

Comparative study of the effect of hybrid nanoparticle on the thermal performance of cylindrical screen mesh heat pipe

The thermal performance of a cylindrical screen mesh heat pipe with hybrid nanofluid was experimentally investigated. The hybrid nanofluid was prepared by mixing Al2O3 and CuO nanoparticles with deionised water. The heat pipe was fabricated with straight copper tube of dimensions 300 mm length, 12.5 mm outer diameter and 1 mm thickness. The wick structure in the heat pipe was created by a three layer copper screen mesh of 100 mesh size. The heat input to the heat pipe was varied from 50 W to 250 W in five equal steps. The heat pipe was tested with three hybrid nanofluids made with combinations of Al2O3 and CuO nanoparticle in DI water (Al2O3 75%–CuO 25%, Al2O3 50%–CuO 50% and Al2O3 25%–CuO 75%). The tested hybrid nanofluids were made with 0.1% volume concentration of Al2O3 and CuO nanoparticle combination in deionised water. The results of the investigation showed that for the maximum heat load of 250 W considered in this work, the thermal resistance of the hybrid nanofluid with combination, Al2O3 25%–CuO 75%, showed 44.25% reduction compared to deionised water. The reduction in thermal resistance is due to the formation of porous coating on the wick surface which increases the wettability and surface roughness thereby creating more nucleation sites as seen in the SEM images. From the experimental investigation, it was observed that hybrid nanofluids are alternative to the conventional working fluids in heat pipes for electronic cooling applications.     

Miniature heat-pipe thermal performance prediction tool – software development

The software for flat miniature heat-pipe parameters (Q_max, R_hp, temperature field along the pipe surface, heat transfer coefficients in the evaporator and condenser zones h_e, h_c, etc.) prediction and numerical modeling was developed. The experimental data received for the flat miniature heat pipe (2.5–4 mm thickness, 50–250 mm length, 8–11 mm width) with a copper sintered powder wick saturated with water were compared with the data of numerical analysis and results showed that experimental verification testifies the validity of the software application.     

Effect of copper surface wettability on the evaporation performance: Tests in a flat-plate heat pipe with visualization

The effects of copper surface wettability on the evaporation performance of a copper mesh wick were experimentally studied in an operating flat-plate heat pipe. Different degrees of wettability were obtained by varying the exposure times in air after the wicked plates were taken out of the sintering furnace. Three different working fluids: water, methanol and acetone, which possess different figures of merit, were investigated at the same volumetric liquid charge. The surface wettability was quantified by the static contact angle of sessile water drops on a flat copper surface. While the static contact angles of water drops varied from 10° to 40° for different degrees of wettability, the methanol and acetone drops still fully wetted the copper surface. A two-layer 100 + 200 mesh copper wick, 0.26 mm in thickness, was sintered on a 3 mm-thick copper base plate. A glass plate was adopted as the top wall of the heat pipe for visualization. Uniform heating was applied to the base plate near one end, and a cooling water jacket was connected at the other end. With increasing heat load, the evaporative resistance decreased with liquid film recession until a critical heat load showing the minimum evaporative resistance. Afterwards, partial dryout began from the front end of the evaporator. With decreasing wettability, the evaporating water film receded faster with increasing heat load and the critical heat loads were significantly reduced. In contrast, the critical heat loads for methanol and acetone seemed hardly affected by different wettability conditions. The minimum evaporative resistances, however, remained unaffected by surface wettability for all the three working fluids.     

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 the parameters of the sintered loop heat pipe

The purpose of this paper is to establish an experimental formula for sintered dendritic nickel powder. For this reason, wick structures with different porosity ranging from 65 to 80% were fabricated by cold pressing sintering process at fixed porosity and their parameters that included porosity, pore radius, and permeability were also measured. According to both the capillary limitation and the present experimental formula of the sintered dendritic nickel powder, the wick structure parameters that would affect the heat transfer capacity of the loop heat pipe (LHP) were analyzed theoretically and then investigated experimentally. The results showed that there exists an optimal combination of wick structure parameters by which the performance of the LHP would achieve optimization. The maximum heat transfer capacity was up to 500 W and the thermal resistance was 0.12^°C/W at the allowable working temperature 80^°C. © 2004 Wiley Periodicals, Inc. Heat Trans Asian Res, 33(8): 515–526, 2004; Published online in Wiley InterScience ( www.interscience.wiley.com ). DOI 10.1002/htj.20034     

A microscale model for thin-film evaporation in capillary wick structures

A numerical model is developed for the evaporating liquid meniscus in wick microstructures under saturated vapor conditions. Four different wick geometries representing common wicks used in heat pipes, viz., wire mesh, rectangular grooves, sintered wicks and vertical microwires, are modeled and compared for evaporative performance. The solid–liquid combination considered is copper–water. Steady evaporation is modeled and the liquid–vapor interface shape is assumed to be static during evaporation. Liquid–vapor interface shapes in different geometries are obtained by solving the Young–Laplace equation using Surface Evolver. Mass, momentum and energy equations are solved numerically in the liquid domain, with the vapor assumed to be saturated. Evaporation at the interface is modeled by using heat and mass transfer rates obtained from kinetic theory. Thermocapillary convection due to non-isothermal conditions at the interface is modeled for all geometries and its role in heat transfer enhancement from the interface is quantified for both low and high superheats. More than 80% of the evaporation heat transfer is noted to occur from the thin-film region of the liquid meniscus. The very small Capillary and Weber numbers resulting from the small fluid velocities near the interface for low superheats validate the assumption of a static liquid meniscus shape during evaporation. Solid–liquid contact angle, wick porosity, solid–vapor superheat and liquid level in the wick pore are varied to study their effects on evaporation from the liquid meniscus.     

Thermal performance of screen mesh wick heat pipes using water-based copper nanofluids

Fairly stable surfactant free copper–distilled water nanofluids are prepared using prolonged sonication and homogenization. Thermal conductivity of the prepared nanofluid displays a maximum enhancement of ～15% for 0.5 wt% of Cu loading in distilled water at 30 °C. The wall temperature distributions and the thermal resistances between the evaporator and the condenser sections of a commercial screen mesh wick heat pipe containing nanofluids are investigated for three different angular position of the heat pipe. The results are compared with those for the same heat pipe with water as the working fluid. The wall temperatures of the heat pipes decrease along the test section from the evaporator section to the condenser section and increase with input power. The average evaporator wall temperatures of the heat pipe with nanofluids are much lower than those of the heat pipe with distilled water. The thermal resistance of the heat pipe using both distilled water and nanofluids is high at low heat loads and reduces rapidly to a minimum value as the applied heat load is increased. The thermal resistance of the vertically mounted heat pipe with 0.5 wt% of Cu–distilled water nanofluid is reduced by ～27%. The observed enhanced thermal performance is explained in light of the deposited Cu layer on the screen mesh wick in the evaporator section of the heat pipe.     

A Feasibility Study About Using SiO2 Nanofluid Screen Mesh Wick Heat Pipe for Cooling of High-Power LEDs

Effective and timely heat removal from high-power light-emitting diodes (LEDs) is crucial to their performance and lifetime. The strategy of using a screen mesh wick heat pipe with SiO₂ nanofluid as the working fluid for LED heat dissipation is comprehensively evaluated. An experimental system is set up to study the heat transfer performance of the heat pipe. The obtained experimental results give optimal conditions/parameters for the heat pipe: 60% charging ratio, 30° incline angle, and 1wt% concentration of the nanofluid. Compared with a heat pipe using the secondary distilled water as the working fluid, the thermal resistance of the heat pipe using the SiO₂ nanofluid as the working fluid is generally reduced by around 35–40% for the investigated heat load range of 1–60 W. Based on an equivalent heat conductivity of the SiO₂ nanofluid heat pipe derived from the experimental results, an Icepak modeling effort for the cooling system of a 60-W LED lamp is then expended. The numerical results show that the temperature of the LED lamp remains low and quite uniform across the LED chip region, indicating the technical feasibility of using this class of heat pipes for cooling of high-power LEDs.     

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.     

Heat transfer with flow and phase change in an evaporator of miniature flat plate capillary pumped loop

An overall two-dimensional numerical model of the miniature flat plate capillary pumped loop (CPL) evaporator is developed to describe the liquid and vapor flow, heat transfer and phase change in the porous wick structure, liquid flow and heat transfer in the compensation cavity and heat transfer in the vapor grooves and metallic wall. The entire evaporator is solved with SIMPLE algorithm as a conjugate problem. The effect of heat conduction of metallic side wall on the performance of miniature flat plate CPL evaporator is analyzed, and side wall effect heat transfer limit is introduced to estimate the performance of evaporator. The shape and location of vapor-liquid interface inside the wick are calculated and the influences of applied heat flux, liquid subcooling, wick material and metallic wall material on the evaporator performance are investigated in detail. The numerical results obtained are useful for the miniature flat plate evaporator performance optimization and design of CPL.     

Experimental study of the effects of a transversal air-flow deflector in electronics air-cooling

This paper presents an experimental investigation of the influence of a transversal flow deflector on the cooling of a heated block mounted on a flat plate.The deflector is inclined and therefore it guides the air flow to the upper surface of the block.This configuration is simulating the air-cooling of a rectangular integrated circuit or a current converter mounted on an electronic card.The electronic component is assumed dissipating low heat power,as such,air forced convection is still a sufficient cooling way even without fan or heat sink on the component.The measurements are given by hot and cold wires anemometers and by an InfraRed camera.The results give details of the effects of the deflection on the hydrodynamic and the thermal fields on and over the block for different inclination angles.They show that the deviation caused by the deflector may significantly enhance the heat transfer from the component.Deflection is also able to avoid local overheating of the electronic component.Optimum heat transfer rate and homogenised temperature are shown to be obtained with an inclination angle =30°.     

3D heat transfer analysis in a loop heat pipe evaporator with a fully saturated wick

A practical quasi three-dimensional numerical model is developed to investigate the heat and mass transfer in a square flat evaporator of a loop heat pipe with a fully saturated wicking structure. The conjugate heat transfer problem is coupled with a detailed mass transfer in the wick structure, and incorporated with the phase change occurring at the liquid–vapor interface. The three-dimensional governing equations for the heat and mass transfer (continuity, Darcy and energy) are developed, with specific attention given to the wick region. By comparing the results of the numerical simulations and the experimental tests, the local heat transfer mechanisms are revealed, through the obtained temperature distribution and the further derived evaporation rates along the liquid–vapor interface. The results indicate that the model developed herein can provide an insight in understanding the thermal characteristics of loop heat pipes during steady-state operation, especially at low heat loads.     

Experimental investigation of a flat plate heat pipe performance using IR thermal imaging camera

This paper presents results and analysis of an experimental investigation into determining the thermal performance of a flat plate heat pipe using infra red (IR) thermal imaging camera. Steady state and transient temperature distribution of the evaporator surface of the flat plate heat pipe were measured using a single heat source with varied heat flux inputs. For performance comparison, the experimental measurements were also carried out on an identical flat plate heat pipe with a defect and on a solid copper block of similar dimensions. It was shown that temperature excursion on the surface of the fully functioning flat plate heat pipe is less than 3 °C for operating temperatures up to 90 °C and heat flux inputs ranging from 4 to 40 W/cm². Furthermore, the thermal spreading resistance of the flat plate heat pipe was found to be about 40 times smaller than that of the solid copper block and flat plate heat pipe with a defect.     

Design of miniature loop heat pipe

In the present study, the loop heat pipe (LHP) was miniaturized for application to electronic cooling. According to the capillary limitation, the wick structure parameters that would affect the heat transfer capacity were analyzed theoretically. Among the various wick parameters, this study especially investigated the effect of wick thickness, which has rarely been mentioned in the literature. Here, various thicknesses were analyzed theoretically and then tested experimentally. The results showed that the temperature on the evaporator wall dropped with decreasing wick thickness. This effect would lead to the raising of heat transfer capacity and the descending of thermal resistance. According to the analysis and the practical demand for electronic cooling, a miniature LHP was fabricated with the evaporator outer diameter of 13 mm and the evaporator length of 50 mm. The testing results showed that, at the allowable working temperature of 80 °C, the maximum heat transfer capacity was up to 200 W and the thermal resistance was 0.17 °C/W. © 2003 Wiley Periodicals, Inc. Heat Trans Asian Res, 33(1): 42–52, 2004; Published online in Wiley InterScience ( www.interscience.wiley.com ). DOI 10.1002/htj.10133     

Numerical Investigation of the Performance of a U-Shaped Pulsating Heat Pipe

In this research the performance of a U-shaped pulsating heat pipe (PHP) was investigated using numerical methods. This heat pipe consists of two sections: The evaporator is set at the two ends of the pipe, and the middle part of the pipe comprises the condenser section. This heat pipe is a type of open looped pulsating heat pipe. The governing equations are derived analytically from the continuity, momentum, and energy equations and are solved implicitly. In this model, considering the liquid mesh, the rate of convection and boiling heat transfer in the U-shaped PHP, which has not been investigated as of yet, are examined. The effect of the evaporator temperature on the pulse amplitude and frequency, rate of convection, and boiling heat transfer is also investigated. The results show that by increasing the evaporator temperature, due to the increase in pulse amplitude and frequency, the rate of heat transfer due to convection and boiling in the pipe will increase too. Furthermore, it is derived that by increasing the evaporator temperature, the share of boiling heat transfer will increase. In order to validate the results, the calculated heat transfer is compared to experimental and analytical results, and it is seen that the suggested model correctly predicts the rate of heat transfer within a precise range.     

Development of Composite Wicks for Heat Pipe Performance Enhancement

This study examines the enhancement of heat pipe thermal performance through the employment of composite wicks. Wicks were fabricated from a biporous structure comprised of fine nickel metal powders sintered onto layers of coarse pore copper mesh. Wick structures were designed to explore the effects of both enhanced evaporation heat transfer at the liquid/vapor interface and the extension of the capillary limit. A number of composite wick heat pipe configurations were fabricated and tested to assess performance improvements in comparison to conventional designs. Horizontal, gravity-assisted, and against-gravity tests were conducted to determine whether these designs were orientation-dependent. At various heat inputs, some configurations achieved thermal performance levels greater than three times higher than those of conventional heat pipes. During against-gravity tests, virtually all composite designs exhibited improved performance over the conventional heat pipe at all heat inputs. These results clearly demonstrated that significant improvements in heat pipe performance were achieved through composite wick design.     

Study on heat transfer characteristics of porous metallic heat sink with conductive pipe under bypass effect

The work investigated the forced convection heat transfer of the heat sink situated in a rectangular channel by considering the bypass effect. The fluid medium was air. The relevant parameters were the Reynolds number (Re), the relative top by‐pass gap (C/H), and the relative side by‐pass gap (S/L). The size of the heat sink was 60 mm (L)×60 mm(W)×24 mm(H). Two heat sinks were employed as test specimens: (A) the 0.9‐porosity aluminum foam heat sink and (B) the 0.9‐porosity aluminum foam heat sink with a 20 mm diameter copper cylinder. The copper cylinder was used as a conductive pipe of heat sink. The average Nusselt number was examined under various forced convection conditions. Experimental results demonstrate that increasing by‐pass space decreased the Nusselt number. Besides, the average Nusselt number of mode B heat sink was higher than that of mode A heat sink by 30% for the case without by‐pass flow. The heat transfer enhancement by the copper cylinder would decline as the by‐pass space grew. © 2009 Wiley Periodicals, Inc. Heat Trans Asian Res; Published online in Wiley InterScience ( www.interscience.wiley.com ). DOI 10.1002/htj.20247     

Modulated wick heat pipe

In heat pipes, modulation of evaporator wick thickness provides extra cross-sectional area for enhanced axial capillary liquid flow and extra evaporation surface area, with only a moderate increase in wick superheat (conduction resistance). This modulated wick (periodic stacks and grooves over a thin, uniform wick) is analyzed and optimized with a prescribed, empirical wick superheat limit. A thermal-hydraulic heat pipe figure of merit is developed and scaled with the uniform wick figure of merit to evaluate and optimize its enhancement. The optimal modulated wick for the circular and flat heat pipes is found in closed-form expressions for the viscous-flow regime (low permeability), while similar results are obtained numerically for the viscous-inertial flow regime (high permeability which is also gravity sensitive). The predictions are compared with the experimental result of a prototype (low permeability, titanium/water pipe with the optimal design) heat pipe which gives a scaled figure of merit of 2.2. Good agreement is found between the predicted and measured performance. The maximum enhancement is limited by the pipe inner radius (tapering of the stacks), the wick effective thermal conductivity, and the prescribed wick superheat limit.     

Experimental studies of rotating heat pipes

Thermal characteristics of a rotating heat pipe were measured under steady state at moderate rotational speeds. Copper‐water rotating heat pipe with copper screen mesh wick was fabricated for testing at various heat loads. An experimental test rig with a water‐cooled condenser section was fabricated to study the heat transfer in the rotating heat pipe (RHP) for various heat loads and various rotational speeds ranging from 1000 rpm to 2000 rpm. A heat transfer correlation was developed for the condensing heat transfer coefficient and compared with the experimental results. © 2009 Wiley Periodicals, Inc. Heat Trans Asian Res; Published online in Wiley InterScience ( www.interscience.wiley.com ). DOI 10.1002/htj.20265