Capillary Wicking Research Articles

Experimental study on the characteristics of loop heat pipe with modified carbon fiber felt wick

In order to achieve accurate and effective thermal management of high-performance electronic components, the performance of a small flat loop heat pipe was investigated. A flexible composite capillary wick material has been applied in a flat loop heat pipe. Surface modification of carbon fiber felt was carried out using arc spraying technology to make it a more suitable material for producing capillary wicks. The physical properties of the modified carbon fiber felts were determined by measuring their wettability, porosity, capillary force, and thermal conductivity. A small flat loop heat pipe assembled with carbon fiber felt wick was designed and fabricated, and experimental testing was conducted. The experimental data indicated that the performance of methanol was better than that of acetone and water. The loop heat pipe with a 60% filling ratio maintained the heat source temperature at 84.47 °C under a load of 120 W (30 W/cm2). Conditions where the heat source starts before the cooling system are more favorable for the quick and stable startup of the loop heat pipe. The experimental results demonstrated that the loop heat pipe with a modified carbon fiber felt wick exhibited excellent operational and heat transfer performance.

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.

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.

Nano-capillary aluminum finned heat sink for ultra-efficient evaporative cooling

Aluminum finned heat sinks are commonly used for cooling electronics and high-power systems through convective heat transfer, but their cooling efficiency per unit volume is a major bottleneck to further improve performance or compactness of device/system. Here we report a new strategy including surface modification and system design that for the first time allowed commercially available aluminum finned heat sinks to perform capillary-driven water evaporation from entire surface of fins for significant cooling enhancement. The capillary wicking ability was attributed to a nanostructured surface resulting from the surface modification. Experimental results showed that the passive cooling power of this nano-capillary evaporative finned heat sink (EF-HS) was about 7.8–8.4 times higher than that of a regular (unmodified) heat sink under natural convection. The new strategy can potentially reduce the fin height, cut material costs and save space without compromising heat dissipation efficiency, which is favorable for both large-scale power systems and compact devices. Using photovoltaic (PV) solar panel cooling as an example, the EF-HS with only 9 mm fin height was able to reduce the PV panel temperature by 15.5 °C (26.4%) in a windless condition and increase the energy conversion efficiency by 12.7%.

Experimental study on the influence of surface modification methods of carbon fiber felt on the operation and heat transfer characteristics of loop heat pipes

Under high heat flux, the heat leakage rate of the compensation chamber in a small loop heat pipe is large, which affects the operational stability and heat transfer efficiency of the loop heat pipe. It was proposed for the first time to locally modify the capillary wick thermal conductivity through surface modification, thereby reducing the heat leakage from the compensation chamber. By spraying a high thermal conductivity metal coating on the surface of a low thermal conductivity carbon fiber felt, the contradiction between the heat dissipation rate of the heat source and the heat leakage of the compensation chamber was solved. Static wicking height showed that the metal coating increased the suction force of the carbon fiber felt. Start-up and operation experiments showed that the modified wick significantly improved the start-up stability of the loop heat pipe and reduced the temperature of the heat dissipation surface. The loop heat pipe with a single-sided-coated wick showed the lowest heat dissipation surface temperature, thermal resistance, and heat leakage rate. The loop heat pipe with an aluminum single-sided-coated wick exhibited a heat source temperature of 82.81 ℃, a thermal resistance of 0.26 K/W, and a heat transfer coefficient of 26.44 kW/(m2·K) under a load of 150 W (37.5 W/cm2). This study has significantly contributed to the future application of loop heat pipes in high heat flux electronic cooling.

Mesoscopic numerical study of the startup characteristics of grooved heat pipe under high heat flux

The dry-out phenomenon that occurs in the evaporation section under high thermal load can lead to heat pipe startup failure, which considerably affects the safe and efficient operation of microelectromechanical systems. In this study, the startup characteristics of a grooved heat pipe are investigated with the mesoscopic lattice Boltzmann method. The focus is on the effects of wettability, inclination angle, and liquid filling volume on the evolution of the liquid supply velocity and dry-spot area. Results show that nucleate boiling can be formed by starting the heat pipe at high heat flux, and the vapor jetting generated by bubble bursting can reduce the liquid supply velocity by more than 70%. Capillary pressure can be increased, and the perturbation of bubbles to the meniscus region can be suppressed by enhancing the wettability of the capillary wick, thus promoting the return of the condensate to the evaporation section. The pressure on the liquid side in the evaporation section decreases as the inclination angle increases, which shortens the waiting time for vapor core formation and improves the stability of boiling behavior. A non-monotonic relationship exists between the liquid filling volume and heat transfer limit time of the heat pipe, which reaches the maximum only when the initial liquid layer fills the capillary wick exactly.

Enhanced capillary and heat transfer performance of asymmetric micropost wicks

We developed asymmetric capillary wicks composed of slanted microposts using inclined photolithography. Then we investigated the effects of inclination angle and wicking direction on the capillary and the heat transfer performances. The working fluid accelerates when it flows in the slope direction of the structure (forward direction, FD) and decelerates when it flows in the opposite slope direction (rear direction, RD). We applied the scaling law to the capillary rise experiment data to verify that the inclination angle and the wicking direction affect the capillary performance. The capillary performance parameter was improved by up to ∼39% with FD case and decreased by 21.3% with RD. The heat transfer performance test showed that the wick-CHF (the enhanced critical heat flux due to the formation of the wick) of the asymmetric FD case was increased by 43.3% compared to symmetric ones while maintaining the heat transfer coefficient. This work shows that asymmetric evaporator wicks can enhance the critical heat flux without sacrificing the heat transfer coefficient, which can help develop high-performance thermal management solutions.

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.

Experimental study on 3D printed heat pipes with hybrid screen–groove combined capillary wick structure

The integration of heat pipes as a passive thermal management system for nuclear reactors has been proposed as it provides the advantages of compact design, reliability, efficient heat transfer performance, and enhanced safety. Applying the heat pipe to a nuclear reactor without collision between systems and achieving efficient long-distance heat transfer requires the manufacturing of heat pipes with a high degree of freedom. Using three-dimensional (3D)-printing technology can be a solution for fabricating complex or bent capillary wick heat pipes into an integrated geometry. The study involves the production of a heat pipe using metal 3D-printing technology and an experimental study conducted for thermal performance evaluation. The heat pipe includes a combination of a screen-groove wick in the evaporator section, a groove wick in the adiabatic and condenser sections, and a bent wick structure in the middle. By using 3D printing technology, it is possible to create a heat pipe that has a complex wick design, a consistent porous structure and remains undistorted during the manufacturing process. The surface roughness of the 3D-printed heat pipe was 17 to 1700 times higher than the commercial stainless-steel pipe, which has the benefit for enhance capillary performance. The experimental results showed that the 3D-printed heat pipe can achieve high effective thermal conductivity up to 25.7 kW/m-K and showed 24.2% lower thermal resistance compared to the conventionally manufactured heat pipe. Furthermore, it can be successfully operated at various inclination angles.

Pore structure characteristic and properties of porous Ti3(Al,Si)C2 solid solution prepared by pressureless sintering

MAX phase solid solutions obtained by alloying at M-, A- and X-sites have attracted much attention recently to adjust the lattices and control the properties of MAX phases. In this work, a series of porous Ti3(Al,Si)C2 solid solutions were fabricated by pressureless sintering with Ti/Al/Si/TiC powder mixtures. Pore structures and effects of Si content were systematically characterized. The Si substitution affected the pore structure of porous Ti3(Al,Si)C2 following a V-shaped law: when xSi≤0.4, the porosity decreased together with the reduced pore size, narrow distribution and increased tortuosity factor; when xSi>0.4, the porosity and pore size increased together with the decreased tortuosity factor. Considering the specific application as capillary wick materials, the capillary performance, compressive strength and thermal conductivity of porous Ti3(Al,Si)C2 solid solution were further measured. The results showed that the larger porosity improved the capillary performance, but decreased the compressive strength and thermal conductivity. Compared with porous Ti3AlC2 of similar porosity, porous Ti3Al0.4Si0.6C2 had smaller pore size with more concentrated distribution, but better overall performances (the much better capillary performance, higher compressive strength and lower thermal conductivity). It was demonstrated that the adapt Si substitution achieved the overall promotions of porous Ti3AlC2, and porous Ti3Al0.4Si0.6C2 solid solution could act as the novel wick materials and show broad prospects in loop heat pipes (LHPs).

Heat generation and transfer simulation of a high temperature heat pipe under induction heating based on a coupled model

High temperature heat pipes are highly efficient heat transfer devices, and induction heating is often utilized in their performance tests. However, issues exist in evaluating the equivalence between induction heating and surface heating encountered in typical application scenarios, and in quantifying the accurate input heat flux of induction heating. In order to identify the key influencing factors and investigate their effects on heat generation and transfer within high temperature heat pipes under induction heating, a finite element thermal-electrical coupled model is developed based on the pseudo wick thermal conductivity model and electromagnetic theory. The effects of frequency (10 kHz, 100 kHz, 300 kHz), loaded current (10 ∼ 890A), cooling coefficient (10 ∼ 250Wm-2K−1) and temperature (300 ∼ 1000 K) are investigated. Over 98% of the induction heat is found to be generated in the evaporator section, with a considerable proportion generated in the annular channel (38 ∼ 52%) and the capillary wick (23 ∼ 26%) at low frequency (10 kHz). The wall temperature of the evaporator section is slightly lower than the temperature of the annular flow channel or even the capillary wick at weak cooling boundary conditions (<130Wm-2K−1@10 kHz, <50Wm-2K−1@100 kHz, <30Wm-2K−1@300 kHz). Quantitative calibration formulas of induction heat are established in relation to frequency, current, and temperature via two sets of quadratic polynomial fitting. The results can be used as reference for design and evaluation of transient and steady-state tests of heat pipes under induction heating.

A novel flat heat pipe for anti-gravity orientations: Leaf fractal evaporator and bi-directional transport capillary channel

Flat heat pipes (FHPs) could be an innovative solution for thermal management in aerospace applications; without the aid of gravity, FHPs could still transport high density energy fluxes with superior temperature control and almost zero energy consumption. Inspired by the powerful transpiration and liquid transport ability of leaf vein structure of plants, a novel FHP design with bionic grading evaporator structured surface was proposed. In addition, a bi-directional transport capillary structure was built inside this FHP, reducing the vapor–liquid flow path and enhancing condensed liquid return simultaneously, and thereafter promoting phase change intensity of FHPs. In the present research, heat transfer performance of FHP with self-wetting fluid as a coolant has been experimentally investigated under anti-gravity, concerning on heat inputs, filling ratios, diameter ratios, porosities of hybrid capillary wick, and two representative tree-like evaporator plate structures (H type and Y type). Our experimental results indicated that heat transfer capability of FHP showed a minimal temperature difference, which was suitable for multi-gravity working conditions. Interestingly, minimal thermal resistance value of 0.45 ℃/W and average enhancement ratio in heat transfer coefficient of 56.39% were achieved simultaneously at anti-gravity orientations for a novel FHP with the 70 PPI and αMF = 0.2 of hybrid capillary wick. Superior vapor flow and diffusion and permeability capability of FHP were further obtained as filling ratio of 30%. Moreover, due to the Marangoni effect, the addition of SRWFs could reduce the axial and radial thermal resistance of the FHP by 5–7 % under anti-gravity condition. Compared with traditional heat sink, the novelty of this passive heat transfer application was capable for realizing high heat transport performance for aerospace flights and applications.

High performance vapor chamber enabled by leaf-vein-inspired wick structure for high-power electronics cooling

The boom of the advanced technologies, such as microprocessors, 5G, and big data, has led the thermal management to be a crucial factor in determining the operating speed, reliability, and efficiency as well as lifetime of electronic devices. As passive heat transfer devices utilizing the liquid–vapor phase change, vapor chambers are greatly attractive due to the high thermal conductivity and passive operation. Here, we present a novel vapor chamber with leaf-vein-inspired wick structure, which could effectively facilitate the condensate to flow back to the evaporation region. Besides, the capillary wick functioned as the supporting structure instead of traditional solid supporting columns to withdraw the deformation of the vapor chamber. The heat transfer performance of the proposed vapor chamber was experimentally investigated under forced water cooling condition, and the effects of the cooling water temperature and mass flow rate were analyzed systematically. The results showed that the vapor chamber could effectively tolerate a wide heat load range from 20 W to 500 W with no noticeable performance degradation, and a minimum thermal resistance of 0.029 °C/W was attained at 200 W. Additionally, increasing the cooling water temperature and decreasing the cooling water mass flow rate could lead to the increase of the total thermal resistance at all heat loads. Finally, compared to other reported works, the vapor chamber featured a lowest thickness and vapor chamber thermal resistance, demonstrating a promising solution for cooling high-power and miniaturized electronics.

Dynamics and heat transfer of water droplets impacting on heated surfaces: The role of surface structures in Leidenfrost point

Textured surface has shown great potential in inhibiting Leidenfrost phenomenon for the thermal management of high heat flux devices. Although diverse textured surfaces have been designed to achieve multi-fold increases in Leidenfrost point, the dominant physical mechanism of different surface structures remains elusive. In this study, three aluminum-based surfaces including bare aluminum surface, superhydrophilic micro/nanostructured surface, superhydrophilic macro-pillar arrayed surface are fabricated by simple, low-cost methods. The effects of surface structures with various scales on Leidenfrost point and the dynamic characteristics of water droplets are elucidated. The experimental results indicate that micro/nanostructures cause intermittent liquid-solid contact and trigger more vapor dispersion into micro/nanocavities. A thicker vapor layer generated above micro/nanostructures reduces the vapor pressure that supports the droplet and increases Leidenfrost point from 188 ℃ (for bare aluminum surface) to 428 ℃. A higher Leidenfrost point over 500 ℃ (the maximum heating temperature) is achieved by macro-pillar arrayed surface due to strong capillary wicking and high permeability. Theoretical models that integrate the influence of surface structures on Leidenfrost point are established through balancing vapor pressure with capillary pressure and gravitational pressure. The prediction of present model shows good agreement with the experimental results. Impact and heat transfer regime maps are also provided, where a unique explosive bouncing with a significant reduction in contact time and an extended temperature span of transition boiling are observed on structured surfaces. Furthermore, macrostructures are found to induce a larger and longer liquid-solid contact, thus demonstrating an excellent cooling capability in the transition boiling.

Experimental study on a dual compensation chamber loop heat pipe with a ceramic wick

The dual compensation chamber loop heat pipe (DCCLHP) has excellent application prospects in the future field of space thermal management. However, DCCLHP has some problems, such as startup difficulty with a small heat load and serious heat leakage from the back of the capillary pump. To ensure the reliable startup and efficient heat transfer of DCCLHP in the space microgravity environment, a kind of DCCLHP with an eccentric structure of silicon nitride capillary wick was manufactured and verified. The experimental results show that the DCCLHP can be started with a 2 W small heat load, with two different starting conditions. The heat transfer capacity exceeds 450 W in the 2.0 m heat transfer range. The starting temperature overshoot of DCCLHP presents an inverted “V” shape relationship with the heat load. The smaller the temperature difference between the two compensation chambers is, the heat transfer performance of DCCLHP will be promoted. This study has significant reference value for applying DCCLHP in space microgravity thermal management.

Heat Transfer Characteristics of Pool Boiling with Scalable Plasma-Sprayed Aluminum Coatings

To ensure adequate reliability in two-phase cooling systems involving boiling, it is essential to enhance the heat transfer coefficient and maximize the critical heat flux (CHF) limit. A key technique to avoid surface burnout and increase the CHF limit in pool boiling is the frequent coolant supply to the probable dry-out locations. In the present work, we have explored the plasma-spray coating as a surface modification technique for enhancing heat transfer coefficient and CHF value in pool boiling applications. Three plasma-coated aluminum surfaces (C-15, C-20, and C-25) are fabricated on a copper substrate at three different plasma powers of 15, 20, and 25 kW, respectively. Detailed surface morphologies of the plasma-sprayed coatings are presented, and their roles in pool boiling heat transfer mechanisms are analyzed. Plasma-coated surfaces exhibit wickability characteristics and enhanced wettability compared to the plain copper surface. Saturated pool boiling experiments are performed with DI (deionized) water at atmospheric pressure. Plasma spray-coated surfaces show favorable boiling incipience with less wall superheat and more active nucleation sites than the plain copper surface. Compared to the plain copper surface, enhancement values of nearly 68, 60.7, and 55.5% in the heat transfer coefficient are observed for C-15, C-20, and C-25 plasma-coated surfaces, respectively. Experiments could not be performed beyond the heat flux of 197 W/cm2 due to repeated failure of the cartridge heaters. Based on the experimental measurement of wickabilities, the CHF values of plasma-coated surfaces have been theoretically calculated. Compared to the plain copper surface, a maximum 2.39 times higher CHF value is observed for C-15 plasma-coated surface. Improved wettability and wickability are responsible for CHF enhancement in the case of plasma-coated surfaces. At higher heat flux, capillary wicking and frequent rewetting of the dryout locations delay the burnout phenomenon, enhancing CHF in plasma-coated surfaces. The plasma-spray coating is a robust and scalable process, which can be a potential candidate for high heat flux dissipation in various industrial applications.

Experimental study on developing in-situ hierarchical micro/nanocrystals for improved capillary wicking

Experimental study on developing in-situ hierarchical micro/nanocrystals for improved capillary wicking

Boiling heat transfer on two-tier hierarchical structured surface

In this paper, pool boiling on two-tier hierarchical structured surface is investigated with three-dimensional LB method. Two-tier pillar structures with different scales are designed, termed as primary and secondary pillar, respectively. On the hierarchical surface with upward-orientation secondary pillars, the bubble departure is promoted. And, the heat transfer can be improved by increasing the primary pillar spacing and enhancing surface wettability. Increasing the height of secondary pillar is favorable for enhancing capillary wicking, but can also bring greater flow resistance. On the hierarchical surface with lateral-orientation secondary pillars, the heat transfer is limited by the large flow resistance. With the enlargement of primary pillar spacing, the impact brought by flow resistance is moderated, and the heat flux can be enhanced. Finally, the boiling performance regarding the effect of wall temperature is concerned, where the structured surface with upward-orientation secondary pillars show the better boiling performance than other structured surfaces.

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.

Pool boiling performance on the textured hemi-wicking surfaces fabricated by nanosecond laser ablation

Today the problem of the boiling improvement is directly related to the heating surface modification at the micro- and nanoscale. As was shown by different authors the hemi-wicking surfaces demonstrate extremely high enhancement of heat transfer (HTC) and critical heat flux density (CHF) during boiling. In this paper, the laser ablation technique was used to fabricate textured hemi-wicking silicon surfaces. This surface modification technique is one of the most promising and discussed techniques today due to its high accuracy, implementation ease and high stability of the fabricated surfaces. The effect of the laser type on the silicon surface properties and boiling improvement was studied in detail. The texturing was performed using lasers with different wavelength - infrared (1064 nm) and visible (532 nm), but with the same number of pulses per unit area and laser spot diameter. The experiments showed that the water pool boiling performance differs significantly depending on the type of laser treatment. In particular, while the usage of infrared laser results in the HTC enhancement up to 78% compared to the untreated surface, the visible laser-textured surface in contrast shows the deterioration of the heat transfer rate up to 40%. At the same time, the visible laser-textured surface demonstrates the maximum CHF value of 1806 kW/m2, which is more than 2 times higher than for the untreated surface. The comparison with the models showed, that such an enhancement is explained by the highest capillary wicking of this surface. Based on the analysis of departure diameters, nucleation frequencies and the conditions for the vapor bubbles formation the observed difference in the heat transfer rate during boiling was revealed. Also using the high-speed thermography the dynamics of reversible and irreversible dry spots was studied during boiling on laser-textured hemi-wicking surfaces for the first time.