Wick Structure Research Articles

Experimental and Numerical Analysis of Composite Wick Heat Pipes using Ethanol

Warmth pipes come convenient now-a-days as they work with most noteworthy warmth conductance contrasted with some other method of warmth move and accessible over wide scope parameters. In the present investigation de-ionized water stream in plain thermo siphon, Sintered Copper wick and Helical scored heat pipes with synchronous vanishing, adiabatic and buildup wonder are contemplated utilizing Heat pipe test gear. In this hardware warmth pipe exposed to foreordain heat load an obstruction radiator at its evaporator end water coat with controlled progression water is utilized disperse warmth vitality at condenser end. Every one temperatures are estimated necessary computations are done get rate efficiencies at different stream rates warmth inputs. The exhibition warmth funnels correlation between their efficiencies is done. The sintered copper wick structure pipe have been found capable when stood out from other two with heat inputs beginning from 50 to 800 watts evaporator 30, condenser 72, adiabatic 110, flate heat pipe width 7.6mm, thickness 3.4 mm, first dia 6mm warmth pipe holder thickness in 0.5mm working liquid ethanol wick in view prevailing Capillarity property. The varieties of evaporator and condenser surface temperatures are plotted for changing warmth information sources and stream rate changes at condenser water coat. ANSYS programming is utilized for computational investigation and exploratory outcomes are in great concurrence with the examination.

Experimental study on the heat transfer performance of ultra-thin flattened heat pipe with hybrid spiral woven mesh wick structure

In this work, eight different spiral woven mesh (SWM) structures, namely, SA, SB, SC, SD, SE, SF, SG and SH, were designed to investigate the effect of the SWM weaving method on the heat transfer performance of ultra-thin heat pipes (UTHPs). The SA and SH were SWMs, severed as the control group, and they were woven using only 0.05 and 0.04 mm diameter copper wires, respectively. The other six types were hybrid SWM (HSWM) structures that were woven from the two diameters copper wires. The thermal performance of the UTHPs with different SWM/HSWM wick structures was experimentally investigated. The results indicated that the number and distribution of different-diameter copper wires in every strand of HSWM determine the pore size and total pore volume inside the wick. The staggered arrangement of different-diameter copper wires in the HSWM is conducive to the formation of more multisize pores in the wick, thereby improving its comprehensive hydraulic performance. Compared with the SA and SH UTHPs, the maximum heat transport capacity of the SB and SF UTHPs increased by 33.33–53.85% and the total thermal resistance decreased by 27.53–42.92%, significantly improving the heat transfer performance of UTHP by using the appropriate HSWM wicks.

Thin hybrid capillary two-phase cooling system

A novel hybrid two-phase cooling system was developed that integrated a mechanically pumped two-phase loop with a capillary-driven two-phase cooling device. The latter cooling mechanism was based on evaporation/boiling from wick structures made by sintering copper particles on the interior surfaces of a copper cold plate. The cold plate provided cooling to two surfaces and each of them included four heaters in series. The novelty of the developed technology was preventing flooding of the evaporator wicks by isolating the evaporation surface from the pumped liquid flow that fed them. This arrangement allowed for a high liquid feed flow rate much greater than would be allowed by a capillary pumped system while maintaining a low thermal resistance at the evaporation surface. Using this approach, the cooling system removed over 850 W with a low pumping power below 1.0 W while using R245fa as the working fluid. The equivalent heat fluxes exceeded 970 W/cm2 over areas less than 0.12 cm2. The measured thermal resistance was as low as 0.09 K-cm2/W. The presented thermal management solution enables an increase in the power of high heat flux electronic devices beyond the state-of-the-art.

High-Pressure Pool-Boiling Heat Transfer Enhancement Mechanism on Sintered-Particle Wick Surface

We experimentally study high-pressure pool-boiling heat transfer enhancement using a copper sintered-particle wick structure in water. The wicks are fabricated using the multi-step sintering process using 200 μm sintered copper powder. Thermophysical properties of water change at elevated pressure, therefore changing the bubble dynamics. For the monolayer wick, the Critical Heat Flux (CHF) 179.63 W/cm2, 182.42 W/cm2, and 198.16 W/cm2 are found at 0, 103.4, and 206.8 kPa, respectively. The maximum CHF is found at 206.8 kPa with the wick superheat of 9.89 K. A 110% enhancement is found in the CHF for 206.8 kPa, compared to 0 kPa. While we observe the CHF is 1.8 times higher as compared to the plain copper surface at 0 kPa. The maximum heat transfer coefficient, 252.46 W/cm2 K is found at heat flux 100 W/cm2 for 206.8 kPa. The high-pressure boiling result for the monolayer wick shows that the heat transfer coefficient is enhanced by 100% compared to 0 kPa. We suggest the reasons of enhancements in the pool-boiling performance are primarily due to high rate of bubble generation, high bubble release frequency and reduced thermal-hydraulic length modulation, and enhanced thermal conductivity due to the sintered wick layer. Our analysis suggests that the Rayleigh-critical wavelength decreases by 4.67 % with varying pressure, which may cause the bubble pinning between the gaps of sintered particles preventing bubble coalescence. Similarly, the dominant role of pressure over the wicking layer is also analyzed and found that the critical flow length, λu reduces by three orders of magnitude with 200 μm particles. We suggest that the porous wick layer provides a capillary-assist to liquid flow effect, and delays the surface dry out. The surface modification and the pressure amplify the boiling heat transfer performance. All these reasons may contribute to the CHF, and HTC enhancement in the wicking layer at high pressure.

Application of bio-wick in compact loop heat pipe

Compact loop heat pipes are widely used in high-performance compact electronic devices. The wick structure plays a vital role in the design of compact loop heat pipes. The performance of a wick dramatically influences the performance of the heat pipe. There are different kinds of wick structures used to enhance the performance of heat pipes such as screen wick, sintered wick, and grooved wick. In this study, a natural bio-carbon based wick structure is introduced to study the performance of a compact loop heat pipe. The Carbon material for the wick structure is prepared by carbonizing the Karuvelam wood as it possesses high water-absorbing capacity and ability to withstand high temperatures. The wick parameters such as porosity, capillary rise, pore size, and contact angle are investigated in order to study the effect of heat input, the impact of wick structure on temperature distribution and equivalent thermal resistance of the heat pipe. The results reveal that the temperature difference between the evaporator and condenser decreases effectively so that the thermal resistance decreases. This work reveals that the bio-carbon wick structure shall be a cost-effective alternative solution for a loop heat pipe mechanism.

Experimental study of a Capillary Pumped Loop assisted with a mechanical pump placed at the evaporator inlet

The heat transfer performances of capillary-driven evaporators are still improving while their pumping capacity remains drastically limited by the porous wick structure. The pump assistance allows this capillary limit to be overcome and more largely, gives the opportunity of an important enhancement of capillary two-phase loops operating range. An experimental device made of a Capillary Pumped Loop coupled with a controlled centrifugal pump located at the inlet of the evaporator was proposed. We have shown that the hybrid system acts as it would in a simple capillary-driven regime with an operating loop pressure drop far beyond the capillary limit (more than 60kPa i.e. more than 6 times the evaporator capillary limit with methanol) while preserving the evaporator thermal efficiency due to vaporisation. Moreover, we have found that the pump assistance significantly increases the system robustness during large amplitude heat load step and startup by influencing the liquid subcooling and flow rate at the evaporator inlet.

Phase change heat transfer characteristics of an additively manufactured wick for heat pipe applications

This study presents the phase change heat transfer characteristics of an additively manufactured wick for heat pipe applications. For this study, a flat heat pipe is considered that is made of stainless steel 316L utilizing a fabrication process that yields a wick structure with multi-scale features. Using laser powder bed fusion, relatively large pore structures, similar to screen-mesh structures, are laser molten with micro-scale laser-sintered features on top of this – we refer to these as multi-scale features in this paper. To characterize the phase change processes at heat pipe operating conditions, an experimental facility was developed. For this aim, a test cell with an internal volume of 40 × 20 × 6 mm3 was fabricated. Different filling ratios with water as working fluid and heat inputs ranging from 0.75 to 82.5 kW/m2 were tested. Additionally, a thermal network model to predict the overall heat transfer performance of an additively manufactured flat heat pipe is presented. The model is based on the heat conduction through the wick and wall in both axial and radial directions. Validation by comparison to experimental data shows an excellent agreement for the operating temperature (adiabatic or vapour temperature). The fabricated wick structure exploits enhanced evaporation heat transfer; thin film evaporation occurs due to the multi-scale sintered powder features on top of the fabricated wick surface. Additionally, the optimal filling ratio for which the device has maximal thermal performance is determined. The evaporation superheat is determined and discussed for different filling ratios. Experimental results show the optimum thermal performance of the flat heat pipe for a filling ratio of 110%. Compared to conventional wick structures, additively manufactured wick design exhibits improved thermal performance at higher heat fluxes. The experimental results suggest that additive manufacturing is a promising technology to fabricate freeform porous structures for heat pipes in general.

Fabrication and experimental investigation of the bionic vapor chamber

With the rapid development of photoelectric products, their miniaturization and high integration have intensified the problem of heat dissipation. In this paper, a new type of bionic vapor chamber is designed and manufactured. The wick structure for the condenser combines the fractal network microchannels and copper foam, which are used to imitate the macroscopic leaf vein network and microscopic mesophyll tissue, respectively. Furthermore, the permeability gradient distribution between the leaf veins and mesophyll tissue is added, presented as the porosity gradient distribution of the copper foams around the fractal network microchannels. Then the fractal network microchannels on the wick are processed by laser engraving and mould coining, respectively. The wick structure for the evaporator is homogeneous copper foam. The minimum value of the maximum temperature difference on the upper surface of the condenser of the bionic vapor chamber is only 1.21 K and that of the overall thermal resistance of the bionic vapor chamber is only 0.094 K/W. Compared with the existing novel vapor chambers, when using deionized water as the working fluid, the maximum through-plane effective thermal conductivity and minimum in-plane effective thermal conductivity are 9.52 W/m·K and 10500 W/m·K, respectively, which are higher than most existing vapor chambers.

Experimental Analysis of Atypically Long Finned Oscillating Heat Pipe for Ventilation Waste Heat Recovery Application

Oscillating heat pipes (OHP) which are constructed from a serpentine-arranged capillary tube possess a desirable aerodynamic form factor and provide for relatively high heat transfer rates via cyclic evaporation and condensation of an encapsulated working fluid with no internal wicking structure required. In last two decades, OHP has been extensively investigated for its potential application in thermal management of various applications. This study presents an experimental investigation on the heat transfer performance of an atypically long finned OHP. The heat transfer performance of the proposed OHP was analyzed and compared with a bare tube OHP with similar overall dimensions. Results show that a unit row of finned OHP filled with n-pentane with fill ratio of 70% can recover up to (400±40) W of heat from a typical waste exhaust air stream. The additional pressure drop due to fins was estimated to be (6.8±2) Pa resulting in an increase of 1–2 W of fan power consumption. The average heat recovery rate via finned OHP was found to be almost 80% more than bare tube OHP filled with same working fluid with same fill ratio.

Optimization design of grooved condenser wick structures in a vapor chamber for electronic cooling applications

This article focuses on the layout design of grooved condenser wick structures to improve the surface temperature uniformity for vapor chamber heat spreaders. The design is formulated as a general “area-to-point” (AP) heat conduction problem in disk-shaped configuration. The fundamental relationship between the heat transfer performance and the grooved wick layouts is discussed from the perspective of constructal optimization. The first-order constructs of the heat generating disk are theoretically optimized when the constraint of minimum length scale is imposed for manufacturability of the high conductivity channels (HCCs) (wick grooves). According to the first-order constructs, the disk domain is divided into a number of identical sectors whose boundaries are all adiabatic. A new growth simulator is then developed to obtain the full constructal layouts, where the HCCs are treated as being alive and the optimization is viewed as an adaptive growth procedure. In each sector, the HCCs sprout from the disk center, extend toward the perimeter, and finally converge at a unique topology that exhibits dichotomous, hierarchical, and branching characteristics. Numerical comparison indicates that the new wick structure improves both the temperature uniformity and the capillary pressure, which can contribute to the vapor chamber heat spreaders with better thermal performance.

Optimization design of grooved evaporator wick structures in vapor chamber heat spreaders

This article proposes a novel optimization method for designing the layout of grooved evaporator wick structures in vapor chamber heat spreaders. Firstly, the design is transformed into a general ‘Area-to-Line’ (AL) constructal problem with conductive heat transfer in a disc configuration. Then, constructal optimization of the first order high conductivity channels (HCCs) with strict length scale is carried out based on entransy dissipation rate (EDR) minimization. According to the optimal constructs, the uniformly heated disc is divided into a series of identical sectors whose perimeter is set as the heat sink and other boundaries are all adiabatic. Based on this, a new growth simulator is developed from the inspiration of natural branching phenomena, and applied to obtain the full constructal layouts of the HCCs. In such simulation, the HCCs are described by parameterized level-set surfaces, which start at the heat sink line and grow toward the disc center according to the design sensitivities. The evaporator wick structure derived by growth simulation shows a unique topology exhibiting dichotomous, hierarchical and branching characteristics, which make the capillary pressure improve, and the temperature gradient more homogeneous. The proposed method contributes to the vapor chamber heat spreaders with better thermal performance.

Fabrication and Thermal Characterization of Composite Cu-CNT Micropillars for Capillary-driven Phase-Change Cooling Devices

ABSTRACTThis paper presents the fabrication, testing, and modeling of an array of composite copper-carbon nanotubes (Cu-CNT) micropillars as a wick structure for potential application in passive phase-change cooling systems. This novel wick structure has a larger spacing at the base of the micropillars to provide a higher liquid permeability and mushroom-like structures on the top surface of the micropillars with a smaller spacing to provide a greater capillary pressure. The composite Cu-CNT micropillars were fabricated by an electrochemical deposition method on a patterned copper template. Cauliflower-like nanostructures were then grown on the top surface of the micropillars using chronoamperometry technique to improve the capillary pressure and thermal performance of the wick structure. After successful fabrication of the micropillars, a series of tests were conducted to quantify the thermal performance of the wick structures. The results demonstrate superior thermal and corrosion performances for composite Cu-CNT micropillars compared to those of copper micropillars. Additionally, a thermal resistance network analysis was conducted to model the thermal performance of the fabricated mushroom-shaped micropillar array. Model predictions were compared with the experimental results and good agreement was observed.

Fabrication and performance evaluation of flexible flat heat pipes for the thermal control of deployable structure

In this work, we report the fabrication and thermal performance evaluation of flexible flat heat pipes by using laser ablated casing material with aluminum compound packing film and using gradient wetting functional copper meshes covered with nanowires as the wick structure. Deionized water was chosen as working fluid and three different filling ratios (20%, 30%, 40%) of working fluid were loaded into the heat pipe to investigate its impact on thermal performance. The fabricated flexible flat heat pipes can be bended from 0° to 180° in the horizontal operation mode and demonstrated consistently low thermal resistance of 0.525 °C/W or effective thermal conductivity of 1499 W/m·K after repeated bending. Simultaneously, theoretical analysis reveals that bending disturbs the vapor flow from evaporator to condenser in the flexible flat heat pipe, which leads to increased liquid–vapor interfacial thermal resistance in the evaporation section. The purpose of this study is to apply the flexible flat heat pipe to thermal control of deployable structures.

Numerical study of the heat transfer and flow characteristics of heat pipes

In this work, a three-dimensional (CFD) analysis is used to investigate the heat transfer and fluid flow of a heat pipe using water and nanofluids as working fluids. The study focused mainly on the effects of volume concentrations of nanoparticles and the operating temperature on the heat transfer performance of the heat pipe using the nanofluids, but there is revealed also aspects concerning flow characteristics of fluids. The analysis was performed for water and two types of nanofluids, two volume concentrations of nanoparticles (1.0% and 4 vol.%) and three operating temperatures (70 °C, 80 °C and 90 °C). Nanofluids were composed of water and SiC and TiC nanoparticles with the diameter of 10nm. The numerical results indicate that the volume concentration of nanoparticles has an important effect in reducing of the temperature difference between evaporator and condenser. The present study shows also that the thermal performance of a heat pipe can be significantly improved by using of both wick structure and nanofluids.

Multiscale Simulation of a Novel Leaf-vein-inspired Gradient Porous Wick Structure

With the rapid development of photoelectric products, their miniaturization and high integration have intensified the problem of heat dissipation. Vapor chamber is a special type of heat pipe that is a particularly effective heat spreader for electronics. In this paper, a novel Leaf-vein-inspired Gradient Porous (LGP) wick structure is designed macroscopically and the LGP design is verified using a general model. After that, the gradient porous design Model 1G is selected for the subsequent mesoscopic modeling. Then a connected 2D random LGP wick model presenting porosity gradient is generated by the expanded quartet structure generation set method. Using the mesoscopic Lattice Boltzmann Method (LBM), the flow and heat transfer in the LGP wick model is analyzed. For verification, FLUENT based on the macroscopic finite volume method is used as a benchmark. Finally, the microscopic flow behaviors in the 2D random LGP wick model are analyzed using the LBM developed. Observing the entire flowing process from the inlet to outlet, it is possible to explain the mesoscopic and macroscopic phenomena well based on the microscopic flow behaviors.

Novel Production and Performance of Aluminum-Based Ultrathin Flat Heat Pipes Based on Preforming and Multi-Tooth Plowing

This paper presents a novel method combining the preforming and multi-tooth plowing processes to fabricate aluminum-based ultrathin flat heat pipes (UTFHPs). The use of a preforming process makes it more effective to machine arrayed microgrooves on the inner wall of UTFHPs. The multi-tooth plowing process helps greatly increase the capillary pressure and hydrophilicity of the heat pipe wick. The morphologies of the wick structures are characterized and a self-designed experimental platform is constructed to investigate the thermal performances of UTFHP samples under incremental heat loads. The results indicate that the multi-tooth plowing process is able to significantly reduce the minimum thermal resistance of the heat pipe by 83%. The thermal resistance of the UTFHP with a unilateral porous wick is smaller than that based on a central porous wick at the same liquid filling rate. The UTFHP has excellent radial temperature uniformity with the minimum radial temperature difference of 1.2 °C, because the structures made by multi-tooth plowing can effectively reduce the temperature difference.

Numerical analysis on the thermal hydraulic performance of a composite porous vapor chamber with uniform radial grooves

Composite porous vapor chamber (CPVC) with good thermal performance in temperature uniformity and high heat-flux limit was recently developed, and its wick structure consisted of a condenser wick and an evaporator wick with uniform radial grooves. However, the underlying mass and heat transfer mechanisms of the CPVC are unclear, hindering its further development. A simplified numerical model is presented in this paper to study the thermal hydraulic performance of the CPVC. By analyzing of the thermal performance, liquid/vapor velocity and pressure distributions of the CPVC with the input heat flux of 6 × 105–28 × 105 W/m2, the influence of the wick structures, the wick porosity configuration and the powder size on the thermal hydraulic performance of the CPVC is investigated. Results show that the wicks can provide radial multi-artery channels for liquid backflow and heat conductive passages for heat transfer. The wick porosity affects the performance of the CPVC more than the powder size does. To obtain better performance, the configuration of the evaporator wick porosity and the condenser wick porosity should make the maximum pressure drop in the wicks slightly less than the maximum capillary pressure. However, the optimal porosity configuration varies with the powder size. Larger powder size results into smaller optimal porosity, and vice versa. In addition, the wick porosities should be as close as possible to the lower bound of their range. To facilitate the fluid flowing through the multi-artery channels, the evaporator wick porosity should be slightly larger than the condenser wick porosity. The work is useful for optimizing CPVC.

Performance Assessment of a Three-Dimensional Printed Porous Media Produced by Selective Laser Melting Technology for the Optimization of Loop Heat Pipe Wicks

The primary wick in a loop heat pipe device is a key component that is central to the operation of the device. Both high permeability and capillary pumping capacity, two properties highly dependent on wick structure, are strongly desirable for a satisfactory thermal performance. In this paper, selective laser melting (SLM), a three-dimensional (3D) printing technology, is used to create a primary wick for an 80 W heat transfer application. The permeability and capillarity values of this wick, experimentally measured, are compared with those built with the most widely used technologies nowadays, such as powder sintering and meshes. In this study, the SLM scaffold is shown to satisfy the minimum values required by the application in terms of capillarity and permeability: 0.031 mm/s and 4 × 10−12 m2, respectively. Our comparative study revealed that the wick produced with the SLM technology presented higher values of permeability, by two orders of magnitude, and slightly higher capillary figures than those corresponding to powder sintering for such application. However, it had capillary values well below those of a stainless-steel mesh. The hydraulic behavior of the SLM wick was better than that of the sintered copper powder, because it not only met the above-mentioned specifications, but it also improved its performance.

Design, Fabrication, and Performance Evaluation of a Hybrid Wick Vapor Chamber

The growing electrification of transportation systems is dramatically increasing the waste heat that must be dissipated from high-density power electronics. Transformative embedded heat spreading technologies must be developed in next-generation systems to enable air cooling of power semiconductors with heat fluxes exceeding 500 W/cm2 over large hotspot areas up to 1 cm2. In this study, vapor chamber heat spreaders, or thermal ground planes (TGPs), with customized wick structures are investigated as one possibility. A 10 cm × 10 cm TGP with hybrid wick, which is a blend of a biporous wick with a standard monoporous wick, was designed. The TGP was tested in combination with a straight pin fin heat sink under air jet impingement and a 1 cm2 size heat source. The experimental performance of the hybrid wick TGP was compared under the same air-cooled conditions with an off-the-shelf TGP of the same size from a commercial vendor and a TGP with a biporous wick only. The customized hybrid wick TGP exhibits ∼28% lower thermal resistance compared with a traditional commercial TGP, and the capillary limit heat flux is measured as 450 W/cm2. Technical challenges in extending this capillary limit heat flux value and TGP integration into packaged electronics are described.

Experimental study on the capillary performance of aluminum micro-grooved wicks with reentrant cavity array

In order to enhance the capillary performance for two-phase heat transfer apparatus, the aluminum micro-grooved wick with reentrant cavity array (MGRA) was developed by using orthogonal Ploughing-Extrusion (P-E) method. Different MGRAs with variation in the P-E pitch and depth were designed to study the effect of the fabrication parameters on the capillary performance and obtain the optimal wick structures. The IR-thermal imaging method and forced liquid flow approach were used in the capillary rise and permeability tests, respectively. It was found that the reentrant cavities became more visible and bigger with a larger pitch or smaller depth. The results manifested that the capillary rise height decreased with the increasing longitudinal P-E pitch and depth. Besides, the permeability increased with increase of pitch but decreased with increases of depth. The capillary parameter K·ΔPcap was taken as a comprehensive index to evaluate the wicks. The sample with the largest pitch and smallest depth in the tests obtained the maximal value of K·ΔPcap due to the good balance between the permeability and capillary pressure. Therefore, this sample may be the optimum structural design in this study for the application as enhanced wick for two-phase heat transfer apparatus.