Wick Performance Research Articles

A dual-height wick to improve capillary performance of vapor chambers

The wick performance is a crucial metric for ensuring the reliable and efficient operation of vapor chambers (VCs) away from dry-out. To enhance the performance of VC, a pitcher plant inspired enhanced wick characteristic dual-height microgrooves (DHMW) was fabricated on 6063 aluminum alloy by one-step picosecond laser texturing without chemical modification. This study describes the two-step capillary behavior of DHMW by high-speed camera and infrared camera. The effects of minor rib number, groove height ratio, length scale, and test liquids (water, acetone, ethanol) on the capillary performance were investigated. When using water as the working fluid, the DHMW achieved a K/Reff of 1.182 µm, which was about 71.3 % larger than that of normal microgroove wicks due to the liquid propagation effect and the flow resistance reduction effect. Furthermore, the maximum K/Reff could reach 1.819 µm of the sample with a minor rib depth of about 70 μm. However, the wetting results showed that the droplets of acetone and ethanol tend to spread laterally after contact with the surface, which is unfavorable for the formation of hierarchical transport in the DHMW. This study demonstrates that laser texturing is an effective and controllable method for the complex shaping of grooved aluminum wicks, with the potential to improve the heat transport capability of VCs.

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 performance of hierarchical dendritic copper surface for ultrathin heat-spreading devices

High-performance electronic systems require efficient heat dissipation devices. Vapor chambers (VCs) are practical thermal solutions for lightweight portable electronics that have limited heat dissipation space. A functional VC requires an interior wick to sustain the capillary circulation of the condensed fluid back to the heated region. The capillary wick performance can be indexed by the ratio of liquid permeability to the effective pore size of the wick structure. This study describes the capillary behavior of a thin hierarchical dendritic copper film (<100 μm thick) prepared by electrodeposition and thermal sintering. The effects of the electrodeposition current density, deposition time, and sintering temperature on the capillary performance of the dendritic copper films were investigated. The relationship between the wicking capability and dendritic morphology, tailored by the electrodeposition process, was explored. A post-deposition sintering treatment was found to be beneficial for improving the structural integrity, adhesion, and capillary performance of dendritic copper wicks. A very high capillary performance (0.81 μm) was realized on a 30-μm-thick dendritic copper wick that matches the need for ultrathin VCs used in highly compact electronic systems.

High-performance electrodeposited copper wicks for heat-spreading vapor chambers

A capillary wick structure is a key component for producing the capillary pressure that drives two-phase circulation in phase-change heat transfer devices such as heat pipes and vapor chambers. This study examines the capillary performance of a porous Cu wick structure fabricated using a new electrodeposition process. This Cu wick exhibits large channels generated by hydrogen bubbles and small pores in the dendritic Cu deposits resulting from high-current electrodeposition. The capillary wick performance can be effectively enhanced by adjusting the morphology of the dendritic Cu deposits. Dendritic Cu wicks with different porous features are prepared using different electrodeposition current densities and Cu sulfate concentrations in the electrolyte. A capillary rate-of-rise method is used to assess the liquid permeability (K), effective pore radius (Reff), and capillary performance (K/Reff) of dendritic Cu wicks with deionized wateras the working fluid. The Cu wick is then thermally treated at 700 °C in ambient N2 for 90 min to improve its structural integrity. Despite partial welding occurring between the fine Cu dendrites, the Cu wick shows a superior K/Reff of 1.5 ± 0.06 μm and excellent structural stability. The high capillary performance can enhance the mass transport rate and effective transport distance of the working liquid in vapor chambers for high-density and large-area heat dissipation applications. A vapor chamber integrated with an electrodeposited Cu wick demonstrates excellent heat-spreading characteristics, with a vapor thermal conductivity of up to 6500 W/m⋅K.

Design Concepts and Performance Characterization of Heat Pipe Wick Structures by LPBF Additive Manufacturing.

Additive manufacturing offers a wide range of possibilities for the design and optimization of lightweight and application-tailored structures. The great design freedom of the Laser Powder Bed Fusion (LPBF) manufacturing process enables new design and production concepts for heat pipes and their internal wick structures, using various metallic materials. This allows an increase in heat pipe performance and a direct integration into complex load-bearing structures. An important influencing factor on the heat pipe performance is the internal wick structures. The complex and filigree geometry of such structures is challenging in regards to providing high manufacturing quality at a small scale and varying orientations during the printing process. In this work, new wick concepts have been developed, where the design was either determined by the geometrical parameters, the process parameters, or their combination. The wick samples were additively manufactured with LPBF technology using the lightweight aluminum alloy Scalmalloy®. The influence of the process parameters, geometrical design, and printing direction was investigated by optical microscopy, and the characteristic wick performance parameters were determined by porosimetry and rate-of-rise measurements. They showed promising results for various novel wick concepts and indicated that additive manufacturing could be a powerful manufacturing method to further increase the performance and flexibility of heat pipes.

Enhanced evaporation performance on a novel microstructured surface with vertical dimension gradient

The wick design is critical for vapor chamber or flat heat pipes based on phase-change heat dissipation. Typically, the high-performance wick is governed by geometry, topology, and the microfabrication constraints. In this article, a novel microstructure incorporating gradient dimension optimization design is proposed to co-enhance the processes of thin film evaporation and capillary pumping. The numerical work systematically studies the parametric effect of wick porosity, contact angle, microstructure layout and Marangoni convection on the performance for different wicks. This novel structure with pyramid-like topology can promote the wick performance because it not only improves the capillary performance, but also shows high effective heat flux not that apparently influenced by contact angle. Meanwhile, the gradient cuboid microstructure and gradient microchannel are fabricated by a cost-effectively one-step hot micro-embossing process and compared with traditional microstructures by macro-scale evaporation test. The temperature distribution and excellent evaporation rate on gradient cuboid surface show satisfactory effect agrees with numerical results.

Multiobjective Optimization of Graded, Hybrid Micropillar Wicks for Capillary-Fed Evaporation.

As electronic device power densities continue to increase, vapor chambers and heat pipes have emerged as effective thermal management solutions for hotspot mitigation. A crucial aspect of vapor chamber functionality depends on the properties of the microporous wick that drives heat and mass transport within the device. While many prior studies have focused on the optimization of these porous structures to increase the maximum capillary-limited dryout heat flux, an equally important aspect of porous wick design is the minimization of the thermal resistance above heated areas. Segmented wicks with geometries that vary along the length of the wick are attractive candidates that can potentially be used to fulfill these simultaneous design goals. Previous studies on bisegmented wicks with only two distinct adiabatic and heated region geometries, however, have shown mixed results regarding the degree of performance benefit over homogeneous wicks. In this work, we present a systematic modeling approach to investigate the optimal composition of segmented micropillar wicks comprising multiple, discrete regions of graded geometry. Using a genetic algorithm, we generate Pareto fronts of optimal segmented wick distributions that maximize the dryout heat flux and minimize the thermal resistance for a given heating configuration. We find that optimal, graded segmented wicks are capable of dissipating dryout heat fluxes more than 200% higher than baseline homogeneous wicks with significantly lower thermal resistance. The sensitivity of the wick performance to the total number of geometry segments is found to vary depending on the desired heat flux and thermal resistance operating regimes.

Biomorphous Silicon Carbide as Novel Loop Heat Pipe Wicks

Biomorphous silicon carbide is tested as a novel wick material for flat evaporator loop heat pipes (LHP). The machinable ceramic gives the possibility to improve the wick performance by macroporous channel networks without closing the structure inherent microporosity. A minimum thermal resistance of 0.4236 °C W−1 is obtained for unmodified SiC at a power input of 200 W. Modification with channels parallel to the evaporating surface results in an improved thermal resistance of 0.3792 °C W−1. Modification with channels perpendicular to the evaporating surface increases the thermal resistance to 0.4469 °C W−1, but reduces the necessary heat load for stable start‐up from 50 to 25 W.

Analysis of a Vertical Flat Heat Pipe Using Potassium Working Fluid and a Wick of Compressed Nickel Foam

Heat at high temperatures, in this work 400–650 °C, can be recovered by use of cooling panels/heat pipes in the walls of aluminum electrolysis cells. For this application a flat vertical heat pipe for heat transfer from a unilateral heat source was analyzed theoretically and in the laboratory, with special emphasis on the performance of the wick. In this heat pipe a wick of compressed nickel foam covered only the evaporator surface, and potassium was used as the working fluid. The magnitudes of key thermal resistances were estimated analytically and compared. Operating temperatures and wick performance limits obtained experimentally were compared to predictions. Thermal deformation due to unilateral heat flux was analyzed by the use of COMSOL Multiphysics®. The consequences of hot spots at different locations on the wick were analyzed by use of a numerical 2D model. A vertical rectangular wick was shown to be most vulnerable to hot spots at the upper corners.

Investigating the effect of powder-mixing parameter in biporous wick manufacturing on enhancement of loop heat pipe performance

This study investigated the use biporous wick with two pore sizes to enhance the performance of loop heat pipe (LHP). In the manufacturing process of a biporous wick, this paper varied the mixing ratio between high polymer PMMA, which was used to make large pores, and nickel powder, which was used to make small pores. This paper used PMMA as the sacrificial material to form large pores; the pore size and content of PMMA was varied, with powder sizes of 177–210μm, 250–297μm and 350–420μm, and contents of 30vol%, 35vol%, and 40vol%. After mixing with nickel powder and the sintering process to produce the biporous wick, the product was put into LHP for performance testing and measurements. Experimental results indicated that a higher content of PMMA particles corresponds to better performance, but beyond a maximum and optimal content the performance worsens; results also indicated that larger powder size leads to better vapor transport and evaporation, but beyond a certain point the large pores can cause weakened structure. Therefore, we found that 250–297μm and 35vol% was the optimal PMMA powder diameter and content, respectively; under these parameters, the maximum LHP heat load was 800W, the thermal resistance was around 0.095°C/W, the evaporator heat transfer coefficient was 131kW/m2C, and the porosity was 86%. Compared with using a monoporous wick, the wick performance was enhanced by about 130%, reducing the LHP thermal resistance by about 60% and, for the first time, to less than the order of 10−2°C/W.

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 > 1, the use of a pore former to increase the wick permeability will always improve the wick performance. When Pcap/Psat < 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.

Investigating the effect of double-layer wick thickness ratio on heat transfer performance of loop heat pipe

This study investigated the effect of double-layer wick thickness ratio on the heat transfer performance of loop heat pipe (LHP). With the outer layer of the wick being biporous to allow vapor to travel and the inner layer being monoporous to provide capillary force, the wick used in this study eliminated the problems with wick's structural strength and difficulty in vapor release encountered when using a monoporous wick. By changing the double-layer wick thickness ratio, the LHP heat transfer performance was enhanced.Under a fixed total wick thickness, the double-layer wick thickness ratio was varied by adjusting the biporous and monoporous layers' thicknesses; higher thickness ratio corresponds to the wick having more biporous wick characteristics, and lower thickness ratio corresponds to the wick being more like a monoporous wick. In this study, the ratios investigated were 0.28, 0.42, 0.57, 0.71, 0.86, and 1. Results showed that at 0.57, the highest heat load under 85 °C was 1060 W, the total thermal resistance was 0.065 °C/W, the heat flux was 50 W/cm2, the heat transfer coefficient was 188 kW/m2 °C, and the porosity was 82%. Compared with the double-layer wick performance reported thus far, performance was increased by about 50%, and compared with that of the monoporous wick, the performance increase was about 200%. The best thickness ratio was successfully determined, and the critical heat load reached, for the first time, the order of kW. A trend line and empirical equation for LHP performance results for monoporous (thickness ratio 0) [1], double-layer (thickness ratio 0.28 ∼ 0.86), and biporous (thickness ratio 1) wicks were fitted and established, providing a reference for future designs.

Monoporous micropillar wick structures, II-optimization & theoretical limits

This article, the second of a two-part study, is aimed at finding optimal micropillar wick geometries and their theoretical performance limits. For the first time, the theoretical optimization results are experimentally verified. The permeability and capillary pressure models determined in the first part of this study are utilized in the optimization process. First, the existence of optimality is ascertained through constrained parametric sweeps of the individual variables in the wick performance model, which reveal the presence of optimal values of each variable for a specific combination of the other variables. Then, the overall optimization of wicks is accomplished through a genetic algorithm. It is established that a unique optimal geometry, i.e. a combination of diameter, spacing and height, exists at each wicking length. Finally, these results are used to determine the theoretical performance limits of micropillar array wicks. The results suggest that the theoretical performance limit is 3–4 times greater than the highest performance reported in literature.

A novel wick structure of vapor chamber based on the fractal architecture of leaf vein

A new pattern of wick in vapor chamber is designed and analyzed in this study. The wick is a kind of leaf-vein-like fractal architecture with polygon loops which are composed of many Y-shaped bifurcations, for this reason that the condensable area is enlarged efficiently. The polygonal loops play a unique role in avoiding blocking. And the sucking capacity is introduced to evaluate the wick performance. The permeability in both the length and height direction as well as the resistance in length direction are calculated, and compared with the parallel structure. It shows the permeability in both directions is far greater than that of the parallel structure under the same power source condition. Not only that, the resistance is far less than that of the parallel structure as the reasonable width ratio was chose. The analytical results indicate that the leaf-vein-like fractal architecture has better performance than the parallel structure.

Effect of Filling Powder Volume Rate in Wick Manufactured for Loop Heat Pipes

This investigation studies the effect on pressure of the filling volume rate in a manufactured nickel powder capillary structure (wick) for a loop heat pipe (LHP). The filling volume ratio is an index of pressure from 1 to 1.3 to investigate the influence on filling weights of the internal parameters of a wick (permeability, effective pore radius and porosity) and heat transfer performance for LHP. The wick is manufactured, based on the standard and non-pressurized filling weights in the manufactured wick mold. The conversion of a pressure change to a weight change used to investigate the relationship between the change in filling volume of pressure to the internal parameters of the wick and the heat transfer performance. As the filling volume rate increases, the capacity of the wick increases, but an excessive filling volume rate makes the wick too dense, affecting its permeability and heat transfer performance in LHP. An experimental test demonstrates that the permeability and heat transfer performance are optimal at a filling volume ratio of 1.2.

High-Temperature Heat Pipes for Waste-Heat Recovery

Operation of heat pipes in air at temperatures above 1200/sup 0/K has been accomplished using SiC as a shell material and a chemical vapor deposit (CVD) tungsten inner liner for protection of the ceramic from the sodium working fluid. The CVD tungsten has been used as a distribution wick for the gravity assisted heat pipe through the development of a columnar tungsten surface structure, achieved by control of the metal vapor deposition rate. Wick performance has been demonstrated in tests at approximately 2 kW throughput with a 19-mm-i.d. SiC heat pipe. Operation of ceramic heat pipes in repeated start cycle tests has demonstrated their ability to withstand temperature rise rates of greater than 1.2 K/s.