Performance Of Wick Research Articles

Enhanced capillary performance of nanostructures copper woven mesh wick for ultrathin heat pipes

Multiscale microstructure mesh wicks (MMWs) are widely used in the fabrication of ultrathin heat pipes (UTHPs) due to their excellent capillary properties. In this study, multiscale structures, including superhydrophilic nanosheet structures and microparticles, were prepared on copper meshes by a simple chemical etching method to enhance the capillary properties of wicks. The effects of chemical etching time on wick morphology, composition and capillary rise were investigated, and the effects of high-temperature sintering on the capillary performance of wicks were compared to provide application guidance for evaluating the heat transfer performance of UTHPs. The results indicated that the chemical etching caused the self-growth of superhydrophilic CuC2O4 nanosheet structures on the copper mesh surface compared with the untreated surface, and the maximum capillary rise velocity and the wicking coefficient could reach 8.4 mm/s and 404.457 mm2/s, respectively. After high-temperature sintering, the superhydrophilic CuC2O4 on the wick surface turns into CuO and Cu through reduction and pyrolysis reactions, and the nanosheet structure also turns into particle structure and bridges into microchannels at the dense particles. Due to the change of surface structure and chemical composition, the capillary performance of the wicks decreased, and the maximum capillary rise height could reach 80.6 mm and the maximum capillary performance parameter could reach 117.419 mm2/s, which were approximately 11.43% and 70.97% lower than that without high-temperature sintering, respectively.

The effect of layer thickness on the geometry and capillary performance of strut-based heat pipe wicks manufactured by laser powder bed fusion

Additive manufacturing (AM) can be used to fabricate heat pipes and two-phase heat sinks which have integrated wicking structures. These devices can be customized and have superior performance when compared with devices with conventionally fabricated wicks. The current work investigates the impact of build rate on the capillary performance of AM wicks fabricated by laser powder bed fusion (LPBF). The build rate was examined by varying the layer thickness from 30 µm to 80 µm for four different strut-based wick geometries: (i) body-centered cubic (BCC), (ii) face-centered cubic (FCC), (iii) simple cubic (SC), and (iv) fluorite. The mass rate-of-rise method was used to quantify the wick hydraulic parameters (namely wick permeability, K, and effective pore radius, reff). Layer thickness has a significant influence on wick permeability; layer thickness of 40 µm result in the highest value for all configurations but have a small effect on the effective pore radius. Layer thickness of 40 µm and 60 µm reached the highest K/reff ratio, primarily because of the permeability increase. We attribute this to poor build quality such as missing struts and less defined edges which facilitate low resistance to fluid flow. The SC 60 and BCC 40 configurations achieved the maximum capillary performance with K/reff of 1.34 µm and 1.42 µm, respectively. Overall, it was found that while increasing the build rate by varying layer thickness may affect the part build quality, it promotes hydraulic performance for some configurations by creating random rough surface morphologies and arteries, which help increase permeability.

Characterization of high-performance nanostructured wick for heat pipes

To investigate the best performance wick forms to support the design and fabrication of high-performance heat pipes. This paper investigates the comprehensive performance of wicks by nanosurface technology and experimental methods to predict heat pipe performance rapidly. In this paper, two kinds of nanostructured wick were prepared by the oxidation-reduction method. They are stainless steel fiber mat-type nanostructured and nickel foam-type nanostructured wicks. The effect of surface nanostructure on the wick performance was investigated by comparing the surface characteristics, capillary properties, permeability, and evaporation properties. In this study, the competitive relationship between the characteristic parameters of the wick is measured by the capillary performance factor M: K/reff, which is used as a measure of the comprehensive performance of the wick, and its maximum value is pursued industrially. The result shows that the nanostructured wick presented excellent super hydrophilic properties. A liquid-lift visualization experimental method for measuring the capillary performance factor M was proposed, and it was verified to be correct by the separation effect of the capillary pumping experiment under a vacuum environment. With the increasing oxidized degree, the densification of nanoparticles on the wick surface increased the capillary performance factor M from 8.68 μm to 13.81 μm, with a maximum enhancement of 59.1%. The effective capillary radius decreased from 47.0 μm to 40.1 μm, with a maximum reduction of 14.9%. The permeability increased from 433 μm2 to 583 μm2, with a maximum enhancement of 34.7%. The increase in porosity made the effective capillary radius dominate the competition for permeability, and the capillary performance factor decreased by 17.1%. The thermal resistance of the wick decreased with the enhancement of the capillary performance factor, which fell from 20.3 cm2·K/W to 12.6 cm2·K/W. The current study provides a reference for the future design and preparation of high-performance heat pipe wicks.

Enhanced capillary-driven thin film boiling on cost-effective gradient wire meshes for high-heat-flux applications

As high-power electronics continue to advance rapidly, the pursuit of efficient thermal management has emerged a critical challenge for their further high-performance and large-scale applications. Capillary-driven thin film boiling phase-change cooling, harnessing the high latent heat of vaporization, presents an effective strategy to efficiently dissipate waste heat and meet the escalating cooling requirements of high-heat-flux applications. In this study, a cost-effective gradient wicking structure was proposed to enhance the thermal performance of the wick by promoting bubble dynamics and optimizing the balance between capillary pressure and permeability in homogeneous wicks. The comprehensive capillary properties of the homogeneous wicks were analyzed through theoretical calculations, and the capillary-driven thin film boiling heat transfer performance of four wick samples was systematically studied experimentally. Additionally, the effects of reduced pressures on the thermal performance of the gradient wick were investigated. Notably, the gradient porous wicking structure demonstrated exceptional thermal performance, with a critical heat flux reaching as high as 202.8 W/cm2 and a maximum heat transfer coefficient of 121.9 kW/m2·K achieved at 88.5 W/cm2. The findings of this study offer valuable design guidelines for the wick structures utilized in phase change equipment, with potential wide-ranging applications in advanced thermal management.

Enhancing capillary performance of copper wicks by electrodeposition of superhydrophilic dendritic structures

Capillary wick is a critical component that sustains working fluid circulation in heat-spreading vapor chambers. A preferred wick layer requires a porous structure of multiscale porosity to meet the requirements of high liquid permeability (K) and large capillary pressure (ΔP). In this study, we controlled the etched substrate topography to modulate the morphology and capillary dynamics of the electrodeposited porous films on a specifically treated Cu substrate. The effective pore radius (Reff) of porous Cu films depends on the size and density of the dendritic bushes grown on Cu substrates treated with sodium persulfate/sulfuric acid (SPS) solution. An analytical approach was used to quantitatively analyze the dendritic deposits grown on the SPS-treated Cu substrates. The surface profile and roughness of Cu substrates were investigated by alpha-step profilometer. Scanning electron microscopy, contact angle analyzer and x-ray diffractometer were used to characterize the surface morphology, wetting behavior and oxide formation of the electrodeposited Cu films, respectively. The capillary performance (K/Reff) of the electrodeposited Cu film was measured using the capillary rate-of-rise test. The effect of dendritic bush distribution on fluid transport properties was investigated. A high K/Reff value of 0.975 μm was obtained for the electrodeposited Cu film on a Cu substrate with an optimal SPS etching treatment.

Dependence of Particle Size and Geometry of Copper Powder on the Porosity and Capillary Performance of Sintered Porous Copper Wicks for Heat Pipes

Permeability and capillary performance are the most important parameters relating to the thermal performance of heat pipes. These parameters are deeply linked to pore structure, which has been influenced by the starting powder utilized. In this paper, the effect of particle size and geometry of copper powder on the porosity and capillary performance of porous wicks were systematically studied. Sintered porous wicks were made from different-sized spherical (58 μm, 89 μm, 125 μm) and dendritic (59 μm, 86 μm, 130 μm) Cu powders. The results demonstrated that the porosity and capillary performance of both types of copper powder increase with particle size due to an increase in the connectivity between internal pores. In comparison to the spherical powder, the dendritic powder demonstrated superior capillary efficiency as well as greater porosity. Additionally, a model was proposed for the capillary performance and permeability of sintered porous copper. The predicted results were quite comparable to the experimental data, demonstrating the effect of the starting powder. These findings suggest that porosity and capillary performance of porous wicks are strongly related to powder geometry as well as particle size.

Effect of spiral woven mesh liquid pumping action on the heat transfer performance of ultrathin vapour chamber

In this paper, ultrathin vapour chambers (UTVCs) with two kinds of wick, namely, NO.250 copper mesh + 1 spiral woven mesh (SWM) and NO.250 copper mesh, were fabricated by brazing process. Capillary rise rate experiment was carried out using infrared (IR) camera method to characterize capillary performance of wicks and verify the SWM liquid pumping action. Effect of filling rate and SWM liquid pumping action on the heat transfer performance of UTVC were studied experimentally, and a model to predict the capillary limit of UTVC was established. The results show that the average rising speed of composite wick increased by 38.89%. The optimal filling rate of composite wick UTVC and copper mesh wick UTVC were 100%, and the maximum heat transfer capacities were 6 W and 5 W, respectively. The heat transfer performance of UTVC was greatly improved by SWM liquid pumping action, the maximum heat transfer capacity of UTVC increased by 20%–60% under different filling rate. The error of growth rate caused by SWM liquid pump action between capillary limit Qcl and maximum heat transfer capacity Qmax is less than 5%.

Thermal performance of an ultra-thin flat heat pipe with striped super-hydrophilic wick structure

• The UTFHP with striped super-hydrophilic wick structure was developed. • The mesh number of N = 200 and passage width of P = 1.5 mm is the best wick structure. • The UTFHP could tolerate 8 W heating load with the minimum thermal resistance of 0.26 K/W. The development of high-performance electronic devices requires high heat transfer components with small thickness and high thermal performance. This paper presented an ultra-thin flat heat pipe (UTFHP), and the total thickness and inner height is respectively 0.68 mm and 0.48 mm. The striped-channel structure was developed to withdraw the deformation of UTFHP and reduce the flow resistance. In order to improve the capillary performance of mesh wick, the oxidation treatment was finished by the thermal oxidation method. The thermal performance of UTFHPs was investigated under natural convection cooling condition. The effects of mesh structure, passage width and filling ratio on the thermal resistance and temperature distribution were analyzed. It is found that the UTFHP with mesh number of N = 200 and passage width P = 1.5 mm showed the best thermal performance under the filling ratio of φ = 57%. The minimum thermal resistance was 0.26 K/W with a maximum temperature of 74.07 °C at 8 W heating load, indicating that the proposed UTFHP was able to be a reliable candidate for the thermal management of electronic devices with high heat transfer rates.

Experimental and theoretical analysis of the thermal performance of aluminum and polytetrafluoroethylene wicks

Experimental and theoretical analysis of the thermal performance of aluminum and polytetrafluoroethylene wicks

Fabrication and capillary performance of a novel composite wick for ultra-thin heat pipes

The rapid development of high-performance portable electronics requires ultra-thin heat pipes (UTHPs) with higher heat transfer capacity to meet their heat dissipation needs. Wick structure, the key component of a heat pipe, makes a decisive role in the heat transfer capacity of the heat pipe. However, since the wall thickness of UTHPs is typically < 0.1 mm, it is difficult to machine deep enough grooves on the shell to increase the permeability of the wicks like conventional heat pipes. Therefore, a novel composite wick structure with high permeability for UTHPs is proposed in this study. It is found that the wick with larger pores formed by parallel woven spiral meshes have higher permeability, resulting in more working fluid in the wick, which helps delay the drying out at the evaporation end of heat pipes. To further improve the capillary performance of wicks, the effect of thermal oxidation suitable for mass production on the capillary pumping performance of wicks is systematically studied for the first time. Compared with the single sintered wick, the capillary rise height after oxidation at 400 °C, 500 °C, 600 °C, and 700 °C for 150 min increased by about 32.2%, 19%, 19.5%, and 12.9%, respectively. Considering that the copper oxidation may limit the choice of working fluid for the heat pipe, the wicks are heated to 400 °C for reduction reaction for 1 h (with 95% N2+5% H2 gas). The results show that the capillary rise height of the wicks after oxidation at 400 °C, 500 °C, 600 °C, and 700 °C and reduction treatment is about 110%, 127%, 114%, and 91.4% of that of the single sintered wick, respectively. The experimental results show that the maximum heat load power of the UTVCs with 5 layers #200 mesh, single sintered composite wick and 500 °C oxidation–reduction wick are 16 W, 29 W and 38 W, respectively.

Recent advances in the optimization of evaporator wicks of vapor chambers: From mechanism to fabrication technologies

For the challenge of cooling hotspots in the burgeoning electronics/optoelectronics equipment, vapor chambers provide a promising solution based on a distinctive passive two-phase cooling technology. However, as the most pivotal component in vapor chambers, evaporator wicks involve complex multi-physical and multi-scale processes in porous structures, which has attracted extensive attention. This work comprehensively reviews the fundamental understanding of the inherent mechanisms of evaporator wicks, optimization strategies of macro/micro-scale structures, as well as fabrication technologies and their characteristics. Further, exploration reveals that the trend of improving the thermal performance of evaporator wicks can be described as: relying on advanced fabrication technologies to synergistically optimize capillary wicking capacity, critical/dryout heat flux and thermal resistance by considering multi-scale effects, such as patterned structures, hierarchical structures, and nano-textured surfaces. Finally, this review concludes its findings and put forward prospects for future research directions.

Fabrication and capillary characterization of multi-scale micro-grooved wicks with sintered copper powder

Capillary wick is a key component of loop heat pipes, and its structure design is essential to improve the thermal performance of the devices. In this work, a novel micro-grooved wick with multi-scale structures was fabricated by powder sintering technique and two-step chemical treatment processes. The effect of structure parameters on the capillary performance of sintered-powder wicks was investigated including the particle size and microstructural features, i.e., nanograss and microcavities. The results indicated that the microstructure features had a minimal effect on the permeability of samples. Under the same particle size, compared with the plain sintered-powder wicks (PCPWs) and sintered-powder wicks covered by nanograss (NGPWs), the composite wick covered by microcavities (MCPWs) showed a higher capillary pumping amount and higher wicking velocity in the capillary pumping process. For the nine sintered-powder wicks, MCPW2 and PCPW1 showed the maximum and the minimum capillary pumping amount, respectively, leading to a 73% of increasement in capillary pumping amount. In the comprehensive evaluation of capillary performance, MCPW2 showed the optimal capillary performance, which increased by more than 260% compared with PCPW1. Finally, we predicted the critical size of the microscale gap to enhance the boiling heat transfer and obtained an optimal structure of capillary wick. The optimized capillary wick could significantly improve the capillary limit, providing an ideal option for capillary wicks of loop heat pipes.

Experimental Investigation on the Vapor Chambers with Sintered Copper Powder Wick

High power electronics units generate high-density heat flux, which poses a significant threat to the reliability of these devices. The vapor chamber (VC) has a very high heat transfer rate and has a wide range of applications in the heat dissipation of electronic products. VCs with sintered copper powder wicks sized as irregular shape copper powder 50 µm (I-50 µm), I-75 µm, I-110 um, I-150 µm were studied in this paper. The effect of liquid filling ratio was discussed. The results indicated that the thermal resistance of all VCs decreased with the increase of heating power. The capillary performance of wick was the dominant factor for the heat transfer performance of VCs with different kinds of wick. A capillary performance factor was proposed to evaluate the capillary performance of VCs. The capillary performance factor of VC with the wick of I-75 µm was much higher than that of the other types of VC in this study. The heat source surface temperature of VCs could be affected by filling ratio and wick structure. But, it should be insensitive to the filling ratio when a better capillary performance factor was obtained. With the same heating area and similar structure, the maximum heat flux density of the VC would decrease as the height of vapor cavity decreased.

Evaporation heat transfer characteristic of porous wick in an open capillary evaporator

An open capillary evaporator is proposed to study the evaporation heat transfer performance of porous wick. Both temperature and evaporating mass are measured in this system. Besides, the temperature and relative height of the supplied fluid can be adjusted. A composite porous wick is tested through the constant and variable heat load operation, and it reaches a maximum heat load of 320 W, which corresponds to a heat flux of 25.5 W/cm2. Two types of the evaporator wall temperature curve are observed, and the effect of the initial condition is considered. Intermittent fluid supply appears and causes a temperature overshoot of the evaporator wall. But continuous intermittent fluid supply and small amount of fast-replenishing fluid does not cause the temperature overshoot. The influence of adding water on temperature overshoot is also studied. The boiling and evaporation occur at different heat loads, and the two heat transfer modes coexist at moderate heat load. The transition in heat transfer modes leads to that the evaporation heat transfer coefficient first increases and then decreases with the increase of heat load. This study is helpful to analyze the evaporation heat transfer performance of porous wick and understand the operating characteristics of loop heat pipe.

Development of lotus-type porous material for heat pipe application using centrifugal slip casting process

Heat pipe is a heat conductor which has a large conduction capacity using a special fluid-filled pipe as a conductor of heat from the hot end (evaporator) to the cold end (condenser) used for thermal management. In the heat pipe, there is a sintered wick which transfers refrigerant fluid from condenser to evaporator. The objective of this research is to improve capillary performance by making a straight pore called as lotus-type porous material (LTP) using centrifugal slip casting technique. Copper powder is used as wick materials due to its high thermal conductivity. A starch solution is used as binder material to make the copper slurry. Nylon wire is applied as straight pore mold. Freeze drying is applied to curing and demolding. Vacuum sintering is conducted to increase metallurgical bonding between the particle. The results show that centrifugal casting machine is capable to make a wick with LTP structure. On the performance department, capillary pumping is affected by rotation speed, powder size, and powder loading. The optimum parameters were copper powder with 260.54 μm powder size, 50% Powder Loading with 688 rpm rotation speed. The LTP wick is proved to have a higher capillary pumping capacity compared to the conventional wick. Further work is necessary to verify the performance of LTP wick in the heat pipe.

Sintered Nickel Powder Wicks for Flat Vertical Heat Pipes

The fabrication and performance of wicks for flat heat pipe applications produced by sintering a filamentary nickel powder has been investigated. Tape casting was used as an intermediate step in the wick production process. Thermogravimetric analysis was used to study the burn-off of the organic binder used and to study the oxidation and reduction processes of the nickel. The wicks produced were flat, rectangular and intended for liquid transport in the upwards vertical direction. Rate-of-rise experiments using heptane were used to test the flow characteristics of the wicks. The wick porosities were measured using isopropanol. The heat transfer limitation constituted by the vapour static pressure and the capillary pressure was discussed. The influence on wick performance by using pore former in the manufacturing was studied. When Pcap/Psat &gt; 1, the use of a pore former to increase the wick permeability will always improve the wick performance. When Pcap/Psat &lt; 1, it was shown that if the effective pore radius and the permeability increase with an equal percentage the overall influence on the wick capacity is negative. A criterion for a successful pore former introduction is proposed and the concept of a pore former evaluation plot is presented.

Performance of compressed nickel foam wicks for flat vertical heat pipes

The fabrication and performance of wicks for vertical flat heat pipe applications produced by compression of nickel foams has been investigated. The permeabilities and the effective pore radii for the wicks were estimated from rate-of-rise experiments, using the model fluid heptane. The porosities of the wicks were measured using isopropanol. The results, which are new and of vital importance for optimum use of such wicks, show that the permeabilities and the effective pore radii are in the upper range for heat pipe use. The joining pressure required during the sintering of the wicks was determined, and it was discovered that the nickel foams turned hydrophilic during the sintering.

Heat transfer and capillary performance of dual-height superhydrophilic micropost wicks

We propose dual-height superhydrophilic (DHS) micropost evaporator wicks with improved heat transfer performance for thermal management applications such as micro heat pipes. The heat transfer coefficient and capillary performance are characterized with a numerical model that accounts for the finite curvatures of liquid menisci by varying solid fraction of micropost arrays from 0.18 to 0.54. The DHS wicks vertically stretch a thin (<2μm) evaporative film region with low thermal resistance, which enhances the heat transfer coefficient up to ∼300% compared with the previously reported single-height superhydrophilic (SHS) micropost wicks with the same solid fraction. The stretch of thin film region does not significantly affect the capillary pressure and permeability determined from microscopic menisci. The optimum solid fraction maximizing the heat transfer coefficient occurs since the maximum height ratio between tall and short posts satisfying the pinning criteria decreases with increasing solid fraction, while the total perimeter of microposts increases. The vertical stretch of thin film also lowers the sensitivity of performance to liquid fill charge over 60%. This work suggests that DHS micropost wicks can provide significantly higher heat transfer coefficient in more robust way compared with previous SHS wicks without scarifying the maximum heat transfer performance.

Influence of packing density on performance of porous wick for LHP

In the present work, experimental trials were carried out to investigate the influence of packing density on performance of porous wick for loop heat pipe (LHP). Carbonyl nickel powders with an average size of 1.19μm were used as raw materials. During the filling of powders the graphite mold was put on a mechanical vibrator for different times to get different packing densities. The results show that the porosity, average pore size and permeability decrease with the increasing packing density, respectively. However, the capillary pressure increases with it. And there are no obvious effects on micro-structure and shrinkage ratio. Moreover, the packing density has complex effects on thermal property. After the optimization of parameters, the cylindrical wick with porosity of 59%, average pore size of 1.41μm, permeability of 1.25×10−13m2, capillary pressure of 204kPa and excellent thermal property could be realized.

Study on the capillary performance of micro-post wicks with non-homogeneous configurations

In this study, the capillary performance of the micro-post wicks with non-homogeneous configurations is investigated and compared with a conventional micro-post wick with homogeneous configuration. The experimental and numerical studies show that the capillary performance of the micro-post wick can be significantly enhanced, compared with the homogeneous type wick, by employing non-homogeneous configurations: a groove type wick and a square array type wick have 35% and 31% higher capillary performances, respectively, compared with the homogeneous type wick having equal post size. The capillary performance is shown to be primarily a function of the solid fraction and increases linearly with decreasing solid fraction, regardless of the wick configuration. However, the capillary performance is found to be significantly reduced when the solid fraction falls below a threshold value.