Wick Structure Research Articles

Experimental study on the thermal performance of ultra-thin flat heat pipes with novel multiscale striped composite wick structures

The rapid development of power-intensive and flexible electronic devices requires thinner heat-dissipation devices with better thermal performance. Ultra-thin flat heat pipe (UTFHP) with striped wick structure is a promising candidate for this application, but its wick structure and thermal performance have not yet been thoroughly studied and optimized for the small concentrated heat source, which is commonly encountered in electronics. In this study, several concentrated striped composite wick (CSCW) structures for 0.6 mm thick UTFHPs are proposed and experimentally investigated. The CSCW consists of copper foam with striped passages converging in the heating zone and double layers of copper screen mesh. The thermal performance of UTFHPs with various composite wick structures is experimentally evaluated. UTFHPs with the proposed structures are also compared with a UTFHP with a more conventional parallel passage composite wick structure. Experimental results show that the CSCW with the hollow structure at the evaporation section is preferred, due to the directed liquid working medium reflux and a large vapor-liquid evaporation interface. Besides, the passage width of the copper foam significantly affects the thermal performance. With the best-performing wick structure, the UTFHP gives the lowest thermal resistance of 0.79 °C/W at a heat load of 23.34 W. Its effective thermal conductivity is approximately 7 times that of copper. The proposed striped wick structure for UTFHPs provides an alternative to handle the hot-spot challenge of electronic devices.

Elliptical pillars for the thermal performance enhancement of micro-structured evaporators

In this study, we propose and investigate the employment of elliptical micropillars as a novel evaporator wick structure to boost the performance of capillarity-driven two-phase heat spreaders through high-fidelity numerical modeling that accounts for all pertinent transport mechanisms, including thermocapillary convection. Results reveal that transforming cylindrical micropillars to elliptical ones significantly enhances the overall evaporator performance by improving the hydraulic performance associated with decreased flow resistance and boosted capillary pumping and by improving the thermal performance associated with increased contact (triple) line length. Among the scenarios tested and elliptical aspect ratios swept, it is demonstrated that the heat capacity and heat transfer coefficient can be increased by up to 350% and 52%, respectively. Overall, this study paves the way for the utilization of stretched geometries in the capillary flow direction of pillar-type evaporators by using elliptical micropillars as a demonstrator showcase.

Heat transfer incipience of capillary-driven liquid film boiling

High-power electronic devices bring both unprecedented challenges and great opportunities to two-phase cooling technologies for efficient thermal management. Capillary-driven liquid film boiling with wicking structures combines the advantages of capillary evaporation and nucleate boiling, and thus could be one of the most promising cooling methods. However, a comprehensive understanding of liquid film boiling mechanism is lacking. For example, the current prediction of liquid film boiling incipience is based on the bubble nucleation model utilized in pool boiling, which assumes that bubbles nucleate inside the thermal boundary layer with linear temperature distribution due to heat conduction. In liquid film boiling, heat transfer processes are quite different, including both heat conduction inside the liquid film and evaporation at the liquid-vapor interface. In this work, based on the thermal resistance network considering evaporation at thin-film region near the tri-phase line, we develop a model to predict liquid film boiling incipience. Both visualization and heat transfer experiments are conducted using copper micromesh to validate this model. The model predictions of critical liquid film thickness, surface superheat, and heat flux at the onset of nucleate boiling (ONB) are compared with the experimental data on various wicking structures in the literature, including silicon micropillar, copper micropowder, and copper micromesh. The results show that the heat flux and superheat reduce as the wick thermal conductivity decreases, the wick thickness increases, and the wick porosity increases.

Strategies for thermal performance improvements of polymeric thermal ground planes (TGP)

This paper presents a study on the thermal performance and enhancement techniques of polymeric thermal ground planes (TGPs). The literature features a range of polymeric TGPs that exhibit an effective thermal conductivity up to twice that of copper. However, the TGP effective thermal conductivity does not provide insight into the performance and potential improvements that could be made to a specific TGP design and configuration. Relying only on this metric can be limiting to compare different TGPs performances, since effective conductivity is affected by the device size, layout of heated and cooled zones, and the fabrication materials. In this paper, we adapted a model by connecting the TGP pumping capacity to its thermal resistance to predict the polymeric TGP thermal performances as a function of their geometry, test configuration and material used. This approach enables the development of strategies to optimize the polymeric TGPs performances and identify their limitations rather than relying on effective thermal conductivity as a benchmark for comparison. The present model considers vapor flow through a mechanical structure for the first time in TGPs. Indeed, the polymeric TGP in contrast to the metallic one requires a denser mechanical structure to support the vapor core thickness during vacuum due to the flexibility of the polymeric shell. The generated vapor pressure drop along the TGP has a dual impact on its performance as it affects both the capillary limit and vapor resistance resulting in a tradeoff between the TGP pumping capacity and thermal resistance, for a given TGP thickness. To back up the proposed model, experimental characterization was carried out on a one-step assembled polymeric TGP, to extract simultaneously the total and vapor thermal resistances. The model highlighted a trade-off in designing the vapor core and wicking structure, balancing the polymeric TGP pumping capacity and thermal resistance, for a target TGP thickness. Hence, the current study provides guidelines for defining improvement strategies for TGPs thermal performances based on the geometry and the type of material used during the fabrication process. The recommended approach also permits the identification of polymeric TGPs capabilities and limits rather than depending on the thermal metrics, which may be misleading while comparing performances.

Experimental Investigation on Thermal Performance of Vapor Chambers with Diffident Wick Structures

In this paper, a type of vapor chamber with a gradient pore size wick (VC-G) was developed, and its thermal performance was experimentally tested and compared with two types of VCs with uniform pore size wick (which can be defined as VC-U (200) and VC-U (50) as the powder size of the wick is 50-mesh and 200-mesh, respectively) and a VC without a wick (VC-N). In addition, a VC heat transfer ability experiment platform was built, and the thermal resistance, temperature distribution and thermal response time of the VC with different wick structures were experimentally investigated. The experiment results show that the capillary driving force provided by gradient pore size wick increases gradually from outside, which can not only promote the condensation fluid to gather in the central heat source but also facilitate the vapor to spread around. Therefore, compared with VC-U (200), VC-U (50) and VC-N, VC-G shows the best heat transfer performance, temperature uniform performance and start-up performance.

Capillary enhanced phase change in a microfabricated self-oscillating fluidic heat engine (SOFHE)

This paper reports the design, fabrication, and characterization of a miniaturized version of a self-oscillating fluidic heat engine (SOFHE) for thermal energy harvesting. This new design includes capillary corners of a square cross-section, as well as an etched capillary path on the bottom wall that improves the performance in terms of stability and mechanical power owing to the enhanced phase change. The engine consists of a vapor bubble trapped in a microchannel by an oscillating liquid plug (acting as a piston) set in motion by periodic evaporation and condensation in the vapor bubble. The underlying physics of the oscillations is similar to those of a single-branch pulsating heat pipe. The channel is microfabricated by anodically bonding a grooved glass wafer (top and sidewalls) to a silicon wafer (bottom wall). To further increase the phase change, two more channels are fabricated with an etched capillary path on the bottom wall at two different widths of 25 and 50 µm and a depth of 100 µm. This is the first miniaturized SOFHE that generates a reliable amplitude in the millimeter range. By measuring the change in the volume of the vapor bubble and the frequency, we calculated the change in pressure using the momentum balance on the liquid plug, and then calculated the work, mechanical power, and power density. We observed that the addition of the etched capillary path at a width of 50 µm increased the amplitude (from 1.6 to 4 mm) leading to a fivefold increase in the generated power (from 8 to 40 µW). This study opens a new path towards designing different wicking structures to maximize the amplitude and power density of the SOFHE, making it a promising thermal energy harvester to power wireless sensors.

Experimental study on the thermal performance of a novel two-phase loop thermosyphon under low-heat-flux conditions

Two-phase loop thermosyphons are efficient heat transfer components that do not require extra power and can effectively transfer heat over long distances. In this study, two kinds of novel ethanol loop thermosyphons with and without wicks inside their evaporators were fabricated to experimentally investigate the start-up characteristics and thermal performance of the systems under low-heat-flux conditions and with different filling ratios; in addition, there was no height difference between the evaporator outlet and condenser inlet. As a result, temperature overshoot and geyser boiling were evidenced as start-up phenomena specific situations. Both systems failed to transfer heat when the filling ratio was below 30 % because some temperature measurement points were higher than 100 °C, at which point the limits of the experiment were reached. When the heat input was higher than 10 W, the loop thermosyphon with a wick structure could run and stay stable, while the regular loop thermosyphon system stabilized when the heat input was higher than 25 W. The increase in the filling ratio worsened the uniformity of the temperature distribution in the evaporator. The thermal resistance of the system was at its minimum when the filling ratio was 70 %. Both systems had good heat transfer performance when the heat input was high enough.

Surface evaporation enhancement using porous metasurfaces: 3-D multiscale, open-system wick evaporators

In open-system water-vapor production by direct heat supply, wick evaporators are promising for efficient use of the supplied heat. However, designing a suitable wick structure for achieving high evaporator performance had proven challenging. Here, novel, high-performance porous metasurfaces (unit-cell based capillary structures) are designed, fabricated and tested, using sintered copper powder. In addition, the wicks optimized for yet higher performance are presented. Vertically placed wicks are partially submerged in a pool of water, while heated through a copper substrate by Joule heating. The baseline wick is a monolayer wick, and three different particle diameters (78, 100, and 130 μm) are used. The next variations are bilayer wicks which improve the particle packing and the maximum capillary pressure. The most improvement is achieved by adding strips of capillary arteries (1 mm thick and 1 mm wide, with an interartery gap of 1 mm) on the monolayer. These unit-cell based 3-D porous metasurfaces allow for tailoring the surface design to achieve an optimal thermal-hydraulic performance. Analyses of the wick capillary-viscous dryout limit and the extended-surface heat transfer predict the performance of the fabricated wicks under the test conditions. Overall, an evaporation efficiency of unity and an evaporation enhancement by 50% are recorded.

The effect of different variables and using of the internal adiabatic wall in the construction and performance of thermosiphon heat pipes: Experimental investigation

Heat pipes are a practical and powerful tool for recovering thermal energy and conserving energy sources. Thermosiphon is one of the most widely used devices that can transfer large amounts of heat at high rates between hot and cold sources without the use of external energy. The amount of vacuum in the pipe, the percentage of fluid filling, the type of operating fluid, the pipe’s length and the quantity of heat flux are the factors affecting the efficiency and effectiveness of the thermosiphon heat pipe. In this paper, the effects of different variables in the construction of heat pipes such as working fluid, pipe length, the use of mesh screen wick structure and the use of internal adiabatic wall on the thermosiphon heat pipes performance are investigated. The results show that using of an internal adiabatic wall eliminates and reduces limitations such as boiling, evaporator drying, thermosiphon flooding and vapor pressure and significantly improves the heat pipe’s performance. So that, the effective thermal conductivity (K) is increased up to 350% using the internal adiabatic wall. However, in some nanofluids, such as water/multi-walled carbon nanotubes (MWCNT), with increasing the nanofluid’s mass fraction, the startup speed in heat pipes with internal adiabatic wall is reduced by up to 20%.

A numerical approach to investigate the effect of various fluids on the performance of a heat pipe

The present study is carried out using CFX, a commercial computational fluid dynamics (CFD) software, to estimate the thermal characteristics of a heat pipe. A heat pipe consists of three concentric circular tubes with three different split sections: Evaporator, adiabatic, and condenser sections. A detailed parametric study takes water and methanol as working fluids – Additionally, a study investigates the heat pipe with condenser section subjected to ambient air. The complete analysis is performed for an existing heat pipe with given design characteristics such as orientation, diameter and length, wick structure, and the stated circumstances and intake flow to the evaporator. The results are reported and compared in graphical form. It is observed that the water improves the heat pipe thermal features in terms of temperature than that of methanol. The maximum surface temperature is recorded in the evaporator zone at 132 oC, while the lowest temperature was measured at the conclusion of the condenser portion at 87.6 oC. The condenser portion's extreme surface temperature is 79.93 oC, whereas the evaporator portion's extreme surface temperature is 127 oC. When attributes are compared at atmospheric pressure, water outperforms methanol in terms of merit. This shows that compared to methanol, water has greater heat transfer properties.

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 heat transfer performance in 3D-printed integrated vapor chamber using composite structures

The demand for lightweight heat dissipation devices in aerospace and various other fields has spurred the development of aluminum-based vapor chambers. However, the surface of the aluminum particles is covered by a dense alumina layer, which poses a challenge in preparing the aluminum-based porous wick structure by sintering aluminum powder. In this study, two aluminum-based vapor chambers (60 mm × 50 mm × 4 mm) with distinct wick structures were integrated with the container utilizing 3D technology. The wicks of the vapor chamber included an aluminum-based grooved wick structure (GVC) and an aluminum-based grooved-porous composite wick structure (GPVC). A comprehensive experiment was conducted to evaluate the heat transfer characteristics of the vapor chamber, including startup performance, temperature uniformity, and thermal resistance, under different filling ratios and wick structure parameters. The working fluid was acetone. The results indicated that the GPVC demonstrated excellent startup performance under both low and high heat loads. The startup time of the GPVC was found to be 230 s, 30.3% less than that of the GVC under a heat load of 80 W. Furthermore, at a 60% filling ratio, the GPVC demonstrated lower thermal resistance and better temperature uniformity compared to the GVC. The GPVC achieved a minimum thermal resistance of 0.033 K/W with a maximum temperature difference of only 4.51 °C. This resulted in a reduction of thermal resistance by 47.5% and an improvement in temperature uniformity by 69%. The findings of this study provide valuable insights to the application of 3D printing technology to the production of aluminum-based composite porous wick structures.

Effect of key structure and working condition parameters on a compact flat-evaporator loop heat pipe for chip cooling of data centers

Loop heat pipe has the advantages of large heat transfer coefficient without extra power input, therefore it has a good application potential in chip cooling of data centers. In this paper, a compact flat-evaporator loop heat pipe is designed and the effects of key structural parameters and working conditions on the dynamic and steady-state performance are studied. The results show that increasing heat load or the filling ratio, arranging vapor channels in the middle of the wick (rather than at the bottom) or lengthening the condenser can all reduce the temperature fluctuation; A small inclination angle of 5° and 10° can reduce the temperature fluctuation and the stable temperature, while large inclination angle of 15° will instead bring higher stable temperature; When the heat load increases step by step, the stable temperature (average temperature for fluctuation conditions) decreases and then increases for all filling ratios and structures; Minimized thermal resistance is achieved by optimizing wick and condenser structure under low and high heat load, respectively.

Numerical analysis of high temperature potassium heat pipe under marine condition

A heat pipe (HP) is a two-phase heat transmission device with very high effective thermal conductivity, extensively employed in multiple industrial fields. In the nuclear engineering area, numerous experimental and numerical studies are widely produced for its thermal behavior performance in high temperatures. In this paper, a HP that potassium as the working fluid with a capillary wick structure was taken into consideration under marine conditions through numerical simulations. The physical properties of potassium and the additional force method of rolling motion are utilized with user-defined functions (UDFs). This CFD model verified with available experimental results, is employed to investigate the influence of HP surface wall temperature distribution and thermal resistance by several parameters including inclination angle, filling ratio (FR), and rolling period. Besides, the charts of phase change and wall temperature distributions are visible with the help of CFD simulation under static and rolling condition. Therefore, the high temperature HP application of design and evaluation is provided as a reference in present work under marine condition.

Capillary performance of vertically grooved wicks on laser-processed aluminum surfaces with different wettability

Ultra-thin heat pipes have been employed extensively for the thermal management of electronic devices. Their capacity for heat transfer is significantly influenced by the capillary performance of the wicking structure in the pipes. In this study, superhydrophilic (SHPi) and superhydrophobic (SHPo) background surfaces were prepared on aluminum sheets using a nanosecond fiber laser. SHPi grooves with widths ranging from 0.1 to 0.4 mm were then produced on the surfaces with the same laser. The effects of the background wettability on the capillary performance of the grooves were investigated. The fastest ascent of the water in the grooves on the SHPo background surface occurred when the groove widths were between 0.1 and 0.2 mm. As the groove width increased to 0.3–0.4 mm, the water level rose most quickly in the groove on the SHPi surface. Furthermore, water absorption was always larger on the SHPi background surface for grooves of the same width as on the other two surfaces. As a result, a wettable background surface that matches the application requirements should be selected. The SHPo background surface should be used when a rapid water rise in the groove is desired. The SHPi background surface, on the other hand, increases water absorption capacity. Water absorption achieved a maximum of 6.8 mg with a groove width of 0.4 mm, and capillary performance parameters reached 4.62 × 10–7 N, which was 117.9% higher than the pristine background surface. This study presents fresh suggestions for increasing the capillary performance of vertically grooved wicks.

Biomimetic Copper Forest Structural Modification Enhances the Capillary Flow Characteristics of the Copper Mesh Wick

In a two-phase heat transfer device, achieving a high capillarity of the wick while reducing flow resistance within a limited space becomes the key to improving the heat dissipation performance. As a commonly used wick structure, mesh is widely employed because of its high permeability. However, achieving the desired capillary performance often requires multiple layers to be superimposed to ensure an adequate capillary, resulting in an increased thickness of the wick. In this study, an ultra-thin biomimetic copper forest structural modification of copper mesh was performed using an electrochemical deposition to solve the contradiction between the permeability and the capillary. The experiments were conducted on a copper mesh to investigate the effects of various conditions on their morphology and capillary performance. The results indicate that the capillary performance of the modified copper mesh improves with a longer deposition time. The capillary pressure drops can reach up to 1400 Pa when using ethanol as the working fluid. Furthermore, the modified copper mesh demonstrates a capillary performance value (ΔPc·K) of 8.44 × 10−8 N, which is 1.7 times higher than that of the unmodified samples. Notably, this enhanced performance is achieved with a thickness of only 142 μm. The capillary limit can reach up to 45 W when the modified copper mesh is only 180 μm. Microscopic flow analysis reveals that the copper forest modified structure maintains the original high permeability of the copper mesh while providing a greater capillary force, thereby enhancing the overall flow characteristics.

Experimental study on thermal performance of ultra-thin heat pipe with a novel composite wick structure

The rapid advancement of high-performance electronic devices and high-capacity energy storage has prompted the development of ultra-thin heat pipes with a higher heat transfer limit, faster start-up speed, and lower thermal resistance. In the present study, a novel double-layer wick structure comprising sintered copper powder and spiral woven mesh was proposed to analyze how various combinations of porous media properties, structures, and heating surfaces influence the thermal performance of ultra-thin heat pipes. The wick structure consists of two layers: a layer of sintered copper powder with four different particle sizes and a layer of spiral woven mesh. To improve the starting characteristics, the starting time under different thermal loads was studied. Additionally, a comparison study was conducted between the ultra-thin heat pipe with a single type wick structure and the proposed wick structure. The results revealed that the ultimate power in the two horizontal states was 50 W. The thermal resistance of Horizontal-1 (heat applied to the sintered copper powder side) was 0.069 W/°C, and for Horizontal-2 (heat applied to the spiral woven mesh side), it was 0.093 W/°C, both measured at their respective ultimate thermal loads. Additionally, among all four structures in Horizontal-1, the shortest start-up time occurred at a power of 30 W, with a duration of 42 s. Finally, when compared to the single wick structure, the proposed double-layer wick structure exhibited an 11.1% increase in ultimate power.

A mass rate-of-rise model for additively manufactured wick structures

The hydraulic performance of heat pipes is typically characterized by the ratio of the wick's permeability to effective pore radius, K/reff. To experimentally quantify these values, mass rate-of-rise (mROR) testing is performed by dipping the tip of the wick into a liquid reservoir to track liquid uptake; the experimental data is then fitted to an ROR model. For conventional wicks, the gravity-based m-t model is free of user decisions and the most reliable. However, here we show that this model poorly fits ROR data for additively manufactured (AM) wicks, resulting in underprediction of K/reff. Experimental tests were conducted using 2 AM wicks—grooved and simple cubic—fabricated by laser powder bed fusion. Visual observations revealed that AM wicks exhibited a superposition of two wicking behaviors: a primary flow and microgroove/corner flow. The primary flow quickly fills the innermost area of the wicking structure. In contrast, the secondary flow gradually creeps up the 3D-printed strut surface through the printed fine scale features. A third term was introduced to improve the data fitting, based on previous research on capillary rise in corners. The revised model measured the AM grooved and simple cubic wicks' K/reff values as 1.23 μm and 1.02 μm, respectively.

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.

Molecular dynamics simulation of the evaporation of thin liquid sodium film on the conical nanostructure surface

Deeply understanding the evaporation of the nanoscale thin liquid film on the nanostructure substrates is significant for further constructing the novel wick structure with nanostructure surfaces, which would be used in the optimal design of the sodium heat pipe. For this purpose, this paper investigates the evaporation of thin liquid film on conical nanostructured surfaces by the molecular dynamics method. A cuboid evaporation system is constructed. It consists of the bottom solid substrate and thin liquid sodium film. A flat solid wall is set as the reference. By changing the nanoscale cone's number and height, five conical nanostructure surfaces are built. The number of gas atoms and the total energy of fluid atoms are observed respectively. The evaporation rate and the average heat flux (q) of six cases are compared. The conical nanostructure suppresses evaporation. With the roughness of the conical nanostructures, the evaporation rate decrement maximum can reach 83% and the q decrement maximum can reach 58%. Based on the thermal resistances in the solid-liquid contact region, the D (Self-diffusion coefficient), and the peak values of RDF (Radial distribution function), the mechanisms of the conical nanostructure surfaces are analyzed. The suppressing effects of the conical nanostructure surfaces are achieved by weakening the heat conduction in the solid-liquid contact region and mitigating the collision heat transfer of liquid atoms.