Wick Design Research Articles

Multi-Fidelity Design of Porous Microstructures for Thermofluidic Applications

Abstract As modern electronic devices are increasingly miniaturized and integrated, their performance relies more heavily on effective thermal management. In this regard, two-phase cooling methods which capitalize on thin-film evaporation atop structured porous surfaces are emerging as potential solutions. In such porous structures, the optimum heat dissipation capacity relies on two competing objectives that depend on mass and heat transfer. Optimizing these objectives for effective thermal management is challenging due to the simulation costs and the high dimensionality of the design space which is often a voxelated microstructure representation that must also be manufacturable. We address these challenges by developing a data-driven framework for designing optimal porous microstructures for cooling applications. In our framework, we leverage spectral density functions to encode the design space via a handful of interpretable variables and, in turn, efficiently search it. We develop physics-based formulas to simulate the thermofluidic properties and assess the feasibility of candidate designs based on offline image-based analyses. To decrease the reliance on expensive simulations, we generate multi-fidelity data and build emulators to find Pareto-optimal designs. We apply our approach to a canonical problem on evaporator wick design and obtain fin-like topologies in the optimal microstructures which are also characteristics often observed in industrial applications.

Enhanced capillary performance of multiscale ultrathin titanium wicks guided by modified wicking dynamics

Ultrathin titanium vapor chambers with wicks fabricated by ultrafast laser direct writing provide a promising solution for future electronics' lightweight thermal management. However, despite being a powerful tool for wick manufacturing, the application of ultrafast laser is restricted by its low fabrication efficiency and imperfect model for wick design. Herein, a multiscale titanium wick with stable superhydrophilic nano ripples and microgrooves was high-efficiently fabricated by a multi-beam femtosecond laser processing technology. A modified wicking dynamic model was proposed for ultrafast laser manufactured microgroove wick design. It considered the extra capillary force by laser induced nanoripple and viscous resistance of the narrow V-shape grooves, which is ignored in the classic model. The prediction error of wicking performance reduced from 72.9 % to 4.5 %. Compared to the conventional V-shape design the optimized microgrooves wick improved by 56.7 % and exhibited capillary performance parameter (K/Reff) of 1.88 μm at a thickness of 80 μm, which outperformed most of the recently reported data. The modified model, multiscale high-performance wick, and its highly efficient fabrication method propose a universal and handy approach for accurate and quantitative wick design and manufacturing.

Heat transfer enhancement of the ultra-thin flat heat pipe integrated with copper-fiber bundle wicks

The rapid size shrinkage and the increasing power of the chip made it increasingly challenging to dissipate the heat generated by electronics. Ultra-thin flat heat pipes possessed the characteristics of high thermal conductivity, slender shape, and absence of moving parts, making them an effective solution for heat dissipation in small spaces. In this paper, the composite wicks made by copper-fiber bundle were designed and fabricated. The heat transfer performance of the ultra-thin heat pipe was experimentally investigated with a focus on the effects of wick design. The weaving method used in the composited wick affected the thermal performance of the ultra-thin heat pipe. The results indicated that the wicks with heterostructure increased the maximum heat transfer capacity by 6 W when compared with the homogeneous structures. The single-layer copper mesh design resulted in a 13 W increase in maximum heat transfer capacity compared to the double-layer design.

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.

Biomimetic Micropillar Wick for Enhanced Thin-Film Evaporation

Sustainable liquid cooling solutions are recognized as the future of thermal management in the chip industry. Among them, phase change heat transfer devices such as heat pipes and vapor chambers have shown tremendous potential. These devices rely on the physics of capillary-driven thin-film evaporation, which is inherently coupled with the design and optimization of the evaporator wicks used in these devices. Here, we introduce a biomimetic evaporator wick design inspired by the peristome of the Nepenthes alata that can achieve significantly enhanced evaporative cooling. It consists of an array of micropillars with multiple wedges along the sidewall of each micropillar. The efficacy of the wedged micropillar is evaluated based on a validated numerical model on the metrics of dryout heat flux and effective heat transfer coefficient. The wedge angle is chosen such that wedged micropillars cause liquid filaments to rise along the micropillar vertical walls. This results in a significant increase in thin-film area for evaporation. Additionally, the large mean curvature of the liquid meniscus produces strong capillary pumping pressure and simultaneously, the wedges increase the overall permeability of the wick. Consequently, our model predicts that the wedged micropillar wick can attain ∼234% enhancement of dryout heat flux compared to a conventional cylindrical micropillar wick of similar geometrical dimensions. Moreover, the wedged micropillars can also attain a higher effective heat transfer coefficient under dryout conditions, thus outperforming the cylindrical micropillar in terms of heat transfer efficiency. Our study provides insight into the design and capability of the biomimetic wedged micropillars as an efficient evaporator wick for various thin-film evaporation applications.

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.

Topology optimization of vapor chamber internal structures consisting of evaporator and condenser

This paper studies the integrated topology optimization of vapor chambers consisting of evaporator and condenser. Firstly, the patterning is regarded as a fundamental access-maximization problem and classified into two general categories: (1) “area to point” (AP) flow problem for condenser wick design and (2) “area to line” (AL) flow problem for evaporator wick design. Then, the branch-patterned disc with first order assembly of flow channels is deduced, where the design domain is divided into 18 identical sectors for both condenser and evaporator. A fluid dynamic model is established to analyze the flow characteristics of the working fluid driven by capillary force. Based on this, a generative topology optimization model with a bifurcation rule is developed to produce the hierarchical capillary flow channels that minimize the global resistance for AP flow problem and AL flow problem, respectively. Unlike the commonly used Eulerian framework for topology optimization, the generative topology optimization used in this paper is carried out essentially in a Lagrangian framework, where capillary flow channels undergoing changes of volume, of positions, of inclined angles and of shapes are described by a set of parameterized level-set surfaces which can move, deform, overlap and merge freely upon the underlying FEM grids. The generated constructal wick structures exhibit dichotomous, hierarchical and branching characteristics. Numerical comparisons show that the new wick structures can contribute to the vapor chambers with better thermal performance.

Vapor generation via porous nanochannel wicks

Vapor generation rate in nano/microstructured-based interfacial vapor generation devices is limited by water supply. Here, we report an innovative design for enhancement in vapor generation using porous nanochannel wicks. Vapor generation rate of 1.18 kg m − 2 h -1 is achieved in a dark environment (without heat input) and of 17.12 kg m − 2 h -1 is achieved with a heat input of 5.08 W at low temperature of 62°C, significantly higher than a typical vapor generation rate reported for interfacial solar vapor generation systems. We also demonstrate our wick’s performance under solar heat flux of 1.25 sun and attain a vapor generation rate of 4.87 kg m − 2 h -1 at surface temperature below 35°C. Thus, we present an unconventional approach of separating surfaces of vapor generation and heat supply that would provide additional flexibility in the choice of energy source, such as solar energy and waste heat from exhaust gases. This approach can be consequential in low-grade waste heat recovery. • Wick made of cross-connected buried nanochannels with micropores at intersections • Capillary-induced passive water supply with simultaneous evaporation at micropores • Thin film evaporation causes high vapor generation rate at low surface temperature Vapor generation rates are limited in passive nano/microstructure-based interfacial vapor generation devices. Ranjan et al. report the performance of a porous nanochannel wick design that can achieve high vapor generation rate with solar heat flux, with waste heat, and even in the absence of heat supply.

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.

Vapor chamber with two-layer liquid supply evaporator wick for high-heat-flux devices

Modern electronic devices with high heat flux increasingly exceed the capability of thermal management method. Two-phase thermal dissipation devices such as vapor chambers offer effective method for high heat flux cooling. Various new structural designs of vapor chambers are used as effective thermal control methods for high heat flux devices. However, the optimization criteria for the capillary wick structures are different. The combinations between the structural surface parameters and pore size of the capillary wick are different, while their influence on the heat transfer performance need to be studied. In this work, it is reported that the fabrication and thermal performance of vapor chambers with multi-layer sintered wicks and multi-artery wick design for liquid supply. We numerically and experimentally characterize the two types of sintered multi-artery wick for replenishing liquid and escaping vapor effectively with high heat flux. Three kinds of combinations of different copper particle sizes were applied to the sintered wicks in order to balance the liquid collecting rate and flow resistance. The copper particles of large particle size (60 μm/100 μm/150 μm) in the upper layer wick and copper particles of small particle size (30 μm/50 μm/100 μm) the lower layer wick were applied in the vapor chambers. In the present study, as an evaluation criteria for capillary wick designs, the relationship between surface porosity, surface area and critical heat flux was analyzed to obtain the experimental optimization of structural design and pore size combination as 50/100 μm. It was found that with similar multi-artery structure wicks, higher surface area porosity can effectively increase the critical heat flux of the vapor chambers. Judging from the experiment results, the minimum through-plane direction heat transfer resistance of the vapor chamber is 0.1426 K/W, the minimum in-plane direction heat transfer resistance is 0.016 K/W, while the critical heat flux of the vapor chamber is 262 W/cm2.

Capillary-Assisted Evaporation/Boiling in PDMS Microchannel Integrated with Wicking Microstructures.

To enable rapid development of flexible microelectronic systems, effective thermal management is needed. Flexible polydimethylsiloxane (PDMS)-based microchannel flow boiling may provide a desirable solution. However, the heat transfer performance of PDMS-based microchannels is diminished by its poor thermophysical properties. The development of PDMS wick is proposed to address this dilemma. Herein, a new PDMS wick structure is designed and integrated in the microfluidic device to significantly enhance its thermal performance by promoting capillary-driven flow. Furthermore, to achieve highly efficient vapor removal, a dedicated vapor pathway is designed in the microfluidic device. Experiments have been conducted to investigate the capillary-assisted evaporation/boiling for mass flux ranging from 70 to 245 kg/m2 s on dielectric fluid HFE-7100. The capillary-assisted sustainable and stable thin film evaporation and efficient vapor removal have been demonstrated through the visualization studies. With this new wick design, the liquid rewetting at global and local levels is facilitated through the capillary-driven flow and the efficient vapor removal. This hybrid liquid rewetting mechanism is experimentally demonstrated to significantly enhance the capillary-assisted evaporation/boiling in PDMS-based microchannel. A critical heat flux (CHF) of 14.7 W/cm2 is achieved at a mass flux of 245 kg/m2 s. The heat transfer coefficients (HTC) range from 2000 to 9800 W/m2 K. These values are comparable to that in copper/silicon microchannels, with the aforementioned benefits of flexibility. Equally importantly, stable two-phase transport is achieved as well.

Heat spreading performance of SiC-based power module with bonded vapour chamber for electric powertrain integration

After evolving side by side over the past three decades, insulated gate bipolar transistors (IGBTs) and metal–oxide–semiconductor field-effect transistor (MOSFETs) become two main options in high power semiconductor market of Hybrid Electric Vehicle (HEV) or Battery Electric Vehicle (BEV) systems. Recent improvements on high power performance are focused on the reduction of power losses and the enhancement of system reliability. Power losses of Si-based semiconductor are dissipated in the form of heat, which must be transferred away from the switching junction effectively. In this paper, we evaluate the new vapour chamber wick design and its heat spreading performance on a SiC-based MOSFETs, the results could be used as guides in order to provide a solution to tackle with thermal imbalance incurred by switching current. The selected cases showed that Silicon carbide (SiC) based MOSFETs could operate at more uniform temperature gradient with minimum 1.4 °C temperature difference across the whole bonding interfaces, and hotspots can be constrained below 120 °C by means of phase changing cooling methods. In addition, the replacement of conventional baseplate by vapour chamber could further reduce overall thermal resistance from junctions to coolant fluids. The simulation results show that wicks design with sintering methods could perform better than groove and mesh design in such high power application. At last, under low coolant flow rate chanlleges at 0.5 l/min, an integrated vapour chamber with ellipse pin–fin 16 × 20 is preferred with achieved temperature at 123.3 °C.

Stainless steel flame wick design with green and safety concerns

Stainless steel flame wick design with green and safety concerns

Numerical exploration of optimal microgroove shape in loop-heat-pipe evaporator

The heat and mass transfer in a loop heat pipe evaporator with microgrooves are investigated in a three-dimensional numerical model. The design of the microgroove wick must consider the pressure loss of the vapor. The simulation obtains the detailed pressure distribution of the vapor flow in the grooves. The simulated heat flux distribution in the evaporator revealed that the applied heat flux concentrates along the three-phase contact line (TPCL) within the casing, wick, and grooves. The effect of TPCL length was investigated in three microgroove wicks with circumferential and axial grooves and a classical wick with only axial grooves. The heat-transfer coefficient initially increased with TPCL length increasing, but thereafter decreased because when the TPCL becomes too long, a large pressure loss occurs in the grooves. The developed model was validated experimentally. In both the model and experiment, the heat-transfer coefficient was locally maximized at a certain TPCL length. The proposed method is expected to provide a simple approach for wick design; especially, the wick shape can be optimized merely by varying the TPCL length. The effect of thermal conductivity of the wick material is also discussed.

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.

The role of vapor venting and liquid feeding on the dryout limit of two-layer evaporator wicks

Vapor chambers developed for high-heat-flux operation require advanced evaporator wick designs that can sustain capillary flow when boiling occurs over the heater region. A two-layer evaporator wick integrates a thin base wick layer that is supplied with liquid from a thick cap layer through an array of vertical feeding posts distributed over the heated area. This design allows boiling to occur within the thin base layer, while separating the incoming liquid feeding and outgoing vapor venting pathways. In our prior work, boiling in two-layer wicks was experimentally demonstrated to provide high-heat-flux dissipation over larger heater areas and at low thermal resistance. The current study experimentally explores the effect of two-layer wick design parameters, specifically the dimensions that alter the area available for liquid feeding and vapor venting, on the thermal performance and dryout limit of the wick, using water as the working fluid. Four different two-layer wick designs are fabricated over a 1 cm2 evaporator area by sintering 180–212 μm copper particles. Increasing the vapor-venting area from 7% to 16% of the total evaporator area yielded a significant increase in the dryout limit, from 315 W/cm2 to 405 W/cm2. Increasing the liquid-feeding area using wider posts increased the dryout limit further. Finally, a parametrically optimized design with fewer but larger posts and vents resulted in better performance compared to a design with denser features. With this two-layer wick design, we demonstrate an extremely high dryout limit of 512 W/cm2 over the large 1 cm2 heated area at a thermal resistance of 0.08 K/W.

Area-scalable high-heat-flux dissipation at low thermal resistance using a capillary-fed two-layer evaporator wick

A two-layer sintered porous evaporator wick for use in vapor chambers is shown to offer very high performance in passive high-heat-flux dissipation over large areas at a low thermal resistance. The two-layer wick has an upper cap layer dedicated to capillary liquid feeding of a thin base layer below that supports boiling. An array of vertical posts bridges these two layers for liquid feeding, while vents in the cap layer provide an unimpeded pathway for vapor removal from the base wick. The two-layer wick is fabricated using a combination of sintering and laser machining processes. The thermal resistance of the wicks during boiling is characterized in a saturated environment that replicates the capillary-fed working conditions of a vapor chamber evaporator. Thermal characterization tests are first performed using conventional single-layer evaporator wicks to analyze the effect of sintered particle size on capillary-fed boiling of water. Of the particle size ranges tested, wicks sintered from 180 to 212 μm-diameter particles provided the best combination of high dryout heat flux and a low boiling resistance. A two-layer evaporator wick comprising particles of this optimal size and a 15 × 15 array of liquid feeding posts yielded a maximum heat flux dissipation of 485 W/cm2 over a 1 cm2 heat input area while also maintaining a low thermal resistance of only ∼0.052 K/W. The thermal performance of the two-layer wick is compared against various hybrid and biporous evaporator wicks previously investigated in the literature. While previous wick designs are typically restricted to small areas and low power levels or high surface superheats when dissipating such heat fluxes, the unique area-scalability of the two-layer wick design allows it to achieve an unprecedented combination of high total power and low-thermal-resistance heat dissipation over larger areas than were previously possible.

Design of an Area-Scalable Two-Layer Evaporator Wick for High-Heat-Flux Vapor Chambers

A hybrid two-layer evaporator wick is proposed for passive, high-heat-flux dissipation over large areas using a vapor chamber heat spreader. For such applications, the evaporator wick layer must be designed to simultaneously minimize the device temperature rise and minimize the flow resistance to a capillary feeding of the wick. This requires a strategy that exploits the benefits of a thin wick for reduced thermal resistance and a thick wick for liquid feeding. In the present design, a thick cap layer of wick material evenly routes liquid to a thin, low-thermal-resistance base layer through an array of vertical liquid-feeding posts. This two-layer structure decouples the functions of liquid resupply (cap layer) and capillary-fed boiling heat transfer (base layer), making the design scalable to heat input areas of ~1 cm2 for operation at 1 kW/cm2. A reduced-order model is developed to demonstrate the potential performance of a vapor chamber incorporating such a two-layer evaporator wick design. The model comprises simplified hydraulic and thermal resistance networks for predicting the capillary-limited maximum heat flux and the overall thermal resistance, respectively. The reduced-order model is validated against a higher fidelity numerical model and then used to analyze the performance of the vapor chamber with varying two-layer wick geometric feature sizes. The two-layer wick design is found to sustain liquid feeding at higher heat fluxes, without reaching the capillary limit, compared to single-layer evaporator wick designs.

Comparison of Hydoponic System Between Wick Design With NFT Design on Kangkung Growth Ipomoea aquatica

This study aims to find out the comparison of the growth on wick and NFTdesigns and to know the hydroponic design more effective between the two systems tooptimize the growth of kangkung plants. The method used is quantitative experiment witht-Test test data analysis: Paired Two Sample For Means (two paired samples). The studywas conducted for two weeks with one week of seeding and one week of measurement.The parameters observed were number of leaves, leaf length, leaf width, and stem height.The results showed that the growth of kangkung plants in hydroponic wick design isbetter than the NFT design. The growth on wick design can be said better because in thisdesign each hydroponic place is only filled by one kangkung plant so that the absorptionof nutrient solution can occur more optimally. This study proves that there aredifferences in the growth of kangkung plants in wick and NFT design.

Thermal performance analysis of flat heat pipe with graded mini-grooves wick

Thermal management of micro electronic devices is one of the major issues for electronics application. Mini-grooves flat heat pipe is suitable to be used for small space electronics rapid cooling due to its high thermal conductivity, simple structure and good temperature uniformity. In this paper, an axial fluid flow and heat transfer mathematical model of mini-grooves flat heat pipe in steady state is built to study the axial distributions of key parameters such as pressure, velocity and wall temperature. As the wick structure plays an important role in the mini-grooves flat heat pipe heat transfer enhancement, the influence of mini-grooves size and wall with grooves along the axial direction on the heat transfer coefficient of mini-grooves flat heat pipe is studied. It is found that aiming at designing the optimum mini-grooves flat heat pipe, the local wick needs to be close to in saturated state, and the vapor chamber needs to be large. In this way, a novel graded-grooves wick design is put forward for flat heat pipe, which can increase the effective thermal conductivity coefficient by 12.4% with heat transfer rate 5.0 W. And the grooves wick in slope type with simple fabrication accords with practical application.