Ultra-thin Heat Pipe Research Articles

A steady-state distributed parameter ultrathin heat pipe model considering the unsaturated wick

Ultrathin heat pipes have become mainstream to address the heat dissipation issues of small electronic devices. Numerous numerical studies based on the saturated wick assumption have been proposed to analyze their heat transfer performance. In order to achieve a more accurate assessment, this study presents a three-dimensional model that quantitatively reflects the relationship between capillary pressure and medium volume, along with a corresponding correlation. Based on this, a steady-state distributed parameter ultrathin heat pipe model considering the retreating of medium volume in the wick and the unsaturation vapor-liquid interface was developed, whose accuracy was verified by the experimental data. The results demonstrated that the capillary pressure increased with the decrease of medium volume and contact angle. The evolution of the overall meniscus with medium volume was categorized into two stages, where capillary pressure dependent on the main and micro meniscus at different stage. The medium volume distribution, corresponding to capillary pressure, will automatically adjust to match the overall medium pressure drop. Comparative analysis against experimental data, the proposed model reduced the maximum temperature difference error by 8.43 % compared to the saturated wick assumption at a heat load of 6 W, revealing that the unsaturated liquid wick model exhibited higher accuracy.

An inverse opal complex wick for high-performance ultrathin heat pipes

An inverse opal complex wick for high-performance ultrathin heat pipes

Preparation and thermal properties of biomimetic polymer-based flexible ultra-thin flat heat pipe

Inspired by the structure of the human spine, a flexible ultra-thin flat heat pipe featuring a biomimetic human spine design has been devised. This flat heat pipe, equipped with a biomimetic human spine structure, is capable of undergoing repeated bending at large angles. An experimental study was conducted to investigate the primary influencing factors. The results revealed that the optimal liquid filling ratio for the flexible ultra-thin flat heat pipe is 0.3. The dip angle is positively correlated with the heat transfer performance of the flexible ultra-thin flat heat pipe, and the bending angle is negatively correlated with the heat transfer performance of the flexible ultra-thin flat heat pipe. The maximum thermal conductivity of the flexible ultra-thin flat heat pipe can reach 1457.68 W/(m·K). The flexible ultra-thin flat heat pipe with liquid filling rate of 0.3 and bending angle of 0 degree has the fastest start-up speed. A visualized experimental platform was further designed to observe the internal flow of the working fluid within the flexible ultra-thin heat pipe in various bending states. The visual experimental results demonstrate that bending initiates the premature condensation of steam within the flexible section, and the condensed working medium subsequently generates resistance to the flow of subsequent steam, leading to an inability for the latter to be smoothly transported to the condensing section, thereby creating a vicious cycle.

Study on the flow characteristics of microscale copper inverse opal wick structures

Ultra-thin heat pipes are extensively used to address heat dissipation challenges in compact electronic devices with limited space. Research on new types of micro-porous wick is crucial for enhancing the heat transfer of ultra-thin heat pipes. The hydraulic transport properties of micro-porous wick structures are crucial factors for the applications. The inverse opal (IO) micro-porous metal structure has gained extensive attention for its ordered periodic arrangement and high porosity characteristics. In the paper, the copper inverse opal (CIO) and gradient CIO are fabricated by a template-assisted deposition method. Experimental measurements are conducted to study the fluid ascent process within the CIO structure. In addition, a face-centered cubic geometric model is established based on the structure information obtained by a scanning electron microscope (SEM). The calculation employs a porous media model and takes into account the resistance characteristics in the initial phase. The relative error between the simulation and experimental results is less than 7.7%. Furthermore, the fabricated gradient CIO structure is uniform and exhibits a higher permeability than single-pore CIO. The permeability of this gradient CIO is about 1.9–3.2 × 10−14 m2. Understanding the microscopic liquid transport process within CIO structures is of great importance in the selection and optimization of capillary wick structures.

Enhanced capillary performance of nanostructures copper woven mesh wick for ultrathin heat pipes

Multiscale microstructure mesh wicks (MMWs) are widely used in the fabrication of ultrathin heat pipes (UTHPs) due to their excellent capillary properties. In this study, multiscale structures, including superhydrophilic nanosheet structures and microparticles, were prepared on copper meshes by a simple chemical etching method to enhance the capillary properties of wicks. The effects of chemical etching time on wick morphology, composition and capillary rise were investigated, and the effects of high-temperature sintering on the capillary performance of wicks were compared to provide application guidance for evaluating the heat transfer performance of UTHPs. The results indicated that the chemical etching caused the self-growth of superhydrophilic CuC2O4 nanosheet structures on the copper mesh surface compared with the untreated surface, and the maximum capillary rise velocity and the wicking coefficient could reach 8.4 mm/s and 404.457 mm2/s, respectively. After high-temperature sintering, the superhydrophilic CuC2O4 on the wick surface turns into CuO and Cu through reduction and pyrolysis reactions, and the nanosheet structure also turns into particle structure and bridges into microchannels at the dense particles. Due to the change of surface structure and chemical composition, the capillary performance of the wicks decreased, and the maximum capillary rise height could reach 80.6 mm and the maximum capillary performance parameter could reach 117.419 mm2/s, which were approximately 11.43% and 70.97% lower than that without high-temperature sintering, respectively.

Numerical Investigations on Fluid Flow and Heat Transfer Characteristics of an Ultra-Thin Heat Pipe with Separated Wick Structures

Numerical Investigations on Fluid Flow and Heat Transfer Characteristics of an Ultra-Thin Heat Pipe with Separated Wick Structures

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.

The thermal analysis of the heat dissipation system of the charging module integrated with ultra-thin heat pipes

• A hybrid heat dissipation system of charging module is performed. • An application of ultra-thin heat pipes (UTHPs) in charging module. • Effects of heat dissipation factors are compared quantitatively. • The HTUPs greatly improve the temperature uniformity. Electric vehicles (EV) played an important role fighting greenhouse gas emissions that contributed to global warming. The construction of the charging pile, which was called as the "gas station" of EV, developed rapidly. The charging speed of the charging piles was shorted rapidly, which was a challenge for the heat dissipation system of the charging pile. In order to reduce the operation temperature of the charging pile, this paper proposed a fin and ultra-thin heat pipes (UTHPs) hybrid heat dissipation system for the direct-current (DC) charging pile. The L-shaped ultra-thin flattened heat pipe with ultra-high thermal conductivity was adopted to reduce the spreading thermal resistance. ICEPAK software was used to simulate the temperature and flow profiles of the new design. And various factors that affected the heat dissipation performance of the system were explored. Simulation results showed that the system had excellent heat dissipation capacity and achieved good temperature uniformity. Rather than solely relied on the fans, this new design efficiently dissipated heat with a lower fan load and less energy consumption.

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.

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.

Optimization of vapor-liquid channel parameters for ultrathin heat pipe with limited internal cavity

Ultrathin heat pipes (UTHPs) have become core devices for the thermal management of portable electronic devices, but guidance is lacking on how to allocate vapor–liquid channels in UTHPs. In this paper, a transient 2D numerical model was established to investigate the effect of vapor-liquid channel changes in an UTHP on its thermal performance. The optimal configuration of vapor-liquid channels was determined by adjusting the percentage of wicks in a fixed space. The results showed that the larger the proportion of wicks was, the higher the temperature of the evaporation section. The temperature difference of the UTHP was proportional to the vapor pressure drop. As the wick ratio increased, the total pressure drop of UTHP first decreased and then increased. The total pressure drop of UTHP was minimum when the wick ratio was 30%. UTHPs with an internal thickness of 0.26 mm and a wick ratio of 30.77% were fabricated and tested for heat transfer performance. The experimental results were compared with the numerical results. At normal operating conditions, the numerical model can effectively predict the temperature of the evaporation section with an error <0.5 K. Finally, an analytical model was developed to quickly obtain the optimal wick ratio for UTHPs. The trends in the analytical and numerical models agreed well. The optimal wick ratio for the UTHP studied was 26%. The optimal wick ratio was independent of the input power and the UTHP length, and it increased with internal thickness and operating temperature. The results of this study provide a reference for designing UTHPs and improving their performance.

A study on thermal and hydraulic performance of ultra-thin heat pipe with hybrid mesh-groove wick

High power electronics require ultra-thin heat pipe (UTHP) with more efficient heat transfer capabilities to meet thermal management challenges. And the design of the wick structure is crucial to the heat transfer performance improvement of the UTHP. At the present work, a thermo-hydraulic model is proposed for UTHP with composite mesh-grooved wick structure and the potential applications of the hybrid wick with non-full coverage by mesh is analyzed. According to the different mesh coverage areas, the wick structures are classified into three types, including evaporator covered, evaporator-adiabatic section covered, and full covered. The results show the flow characteristics and thermal performance of UTHPs is closely related to mesh coverage area and vapor core thickness. The reduction in mesh coverage area causes an expansion in vapor space, the vapor velocity and the vapor pressure drop both decreases, the mass flow rises with the higher vapor-liquid circulation efficiency. The liquid pressure drop is positively related to working fluid mass flow. Moreover, a theoretical model to predict the heat transfer limit of the heat pipe with composite wick was established and verified by experimental results with a maximum error of 3.63%.

Numerical Investigation on Internal Structures of Ultra-Thin Heat Pipes for PEM Fuel Cells Cooling

Proton exchange membrane fuel cell (PEMFC) powered propulsion has gained increasing attention in urban air mobility applications in recent years. Due to its high power density, ultra-thin heat pipe technology has great potential for cooling PEMFCs, but optimizing the limited internal cavity of the heat pipe remains a significant challenge. In this study, a three-dimensional multiphase model of the heat pipe cooled PEMFC is built to evaluate the impact of three internal structures, layered, spaced, and composite, of ultra-thin heat pipes on system performance. The results show that the heat pipe cooling with the composite structure yields a lower thermal resistance and a larger operating range for the PEMFC system compared to other internal structures because of more rational layout of the internal cavity. In addition, the relationship between land to channel width ratio (LCWR) and local transport property is analyzed and discussed based on composite structural heat pipes. The heat pipe cooled PEMFC with a LCWR of 0.75 has a significant advantage in limiting current density and maximum power density compared to the LCWRs of 1 and 1.33 as a result of more uniform in-plane distributions of temperature and liquid water within its cathode catalyst layer.

A study on thermophysical properties of printed wick structures and their applications in ultrathin devices

The printed wick shows great potential in ultrathin heat pipe and vapor chamber fabrications. The recipe of printing paste is crucial to the thermophysical properties of the printed wick structures and the resulting heat transfer performance of the terminal devices. In this study, the dynamics of wettability, water absorption, and evaporation of two typical sintered wick structures with printing pastes A and B were investigated. Their typicality lies in that the wick structure A has uniform small pores, while the wick structure B has a bimodal porous structure. In our intended applications, the liquid line is 35–50 mm long. Therefore, the water absorption behavior in this range is of our interest. Wick A absorbs water faster in this range due to its higher capillary force, promising more suitable for liquid line construction. Wick B has higher evaporation cycling rate due to its bimodal design, which makes it more suitable for evaporator construction. As a comparison, the performance of copper mesh slumps during this range, greatly increasing the failure risk, and is thus unusable. In the end, we applied the printing pastes to the ultrathin loop heat pipe and distributed loop vapor chamber, revealing their potential in constructing increasingly complicated patterns for ultrathin heat dissipation devices.

Stress analysis and thermal performance of ultra-thin heat pipes for compact electronics

The ultra-thin heat pipe (UTHP) has been established as a highly effective thermal control component for cooling electronics with large heat generation rate and limited space for cooling. However, UTHP has been severely limited in application because of buckling or breaking on the shell of UTHP during the flattening process. In this study, a series of high-accuracy UTHPs with a thickness of 0.38–0.47 mm were fabricated by the phase-change flattening process. An internal vapor pressure model of the heat pipe during flattening was constructed and the heat transfer performances of UTHPs under different working conditions were experimentally investigated. According to the internal vapor pressure model, the pipe wall stress after flattening was the first to reach the yield strength of the material, and the upper limit heating temperature of UTHPs decreased with the decrease of flattened thickness. The maximum heat transfer capacity decreased with the decreased flattened thickness. UTHP with a thickness of 0.47 mm has a maximum heat transfer capacity of 4.5 W and a minimum thermal resistance of 0.64 °C/W. Moreover, gravity appeared to impose minimal effects on the heat transfer performance of UTHPs and the heat transfer capabilities of the UTHPs fluctuated within ±0.5 W for different gravity orientations.

Regulatable thermal conductivity and excellent mass transport of water-filled carbon nanotube as capillary wicks

• The heat and mass transfer of single wall carbon nanotubes as capillary wicks are studied by molecular dynamics simulations. • The regulatable thermal conductivity can be beneficial to both start-up and overheating operation of heat pipe. • The thermal conductivity of carbon nanotube is reduced due to the thermal motion of water molecules and vdW force. Passive liquid cooling of ultrathin heat pipe has received considerable attention due to its excellent performance of removing heat from hot spot of electronic devices. However, achieving controllable thermal conductivity of the capillary wick materials to reduce the start-up time and total thermal resistance remains a challenge. Here we propose the capillary wick of carbon nanotube array to regulate its heat transfer performance via the water filling ratio. Molecular dynamics simulation results found that the thermal conductivity of water-filled carbon nanotubes is reduced by 17.8%-23.8%, which is benefit to start-up operation. At overheating operation, the high thermal conductivity of empty carbon nanotube can reduce the temperature of the endothermic surface. Importantly, the thermophoretic movement speed of water can reach up to 74 m/s and radial mass exchange occurs driven by temperature gradient. Phonon spectral analysis and heat flow decomposition revealed that the decrease of water-filled carbon nanotube thermal conductivity originates from the low-energy coupling of van der Waals force between water molecules and carbon nanotube and the thermal motion of two water molecules layers. The regulatable thermal conductivity and excellent mass transport performance demonstrate the promising potential of carbon nanotube as next-generation capillary wick of ultrathin heat pipe.

Research on the Manufacturing Process and Heat Transfer Performance of Ultra-Thin Heat Pipes: A Review.

This paper reviews the manufacturing process of ultra-thin heat pipes and the latest process technologies in detail, focusing on the progress of the shape, structure, and heat transfer mechanism of the wick. The effects of the filling rate and tilt angle on the heat transfer performance of the ultra-thin heat pipe, as well as the material selection of ultra-thin heat pipes, is sorted out, and the surface modification technology is analyzed. Besides, the optimal design based on heat pipes is discussed. Spiral woven mesh wick and multi-size composite wick have significant advantages in the field of ultra-thin heat pipe heat transfer, and comprehensive surface modification technology has huge potential. Finally, an outlook on future scientific research in the field of ultra-thin heat pipes is proposed.

Opportunities and Challenges for the Development of Ultra-Thin Heat Pipe

Opportunities and Challenges for the Development of Ultra-Thin Heat Pipe

Fabrication and capillary performance of a novel composite wick for ultra-thin heat pipes

The rapid development of high-performance portable electronics requires ultra-thin heat pipes (UTHPs) with higher heat transfer capacity to meet their heat dissipation needs. Wick structure, the key component of a heat pipe, makes a decisive role in the heat transfer capacity of the heat pipe. However, since the wall thickness of UTHPs is typically < 0.1 mm, it is difficult to machine deep enough grooves on the shell to increase the permeability of the wicks like conventional heat pipes. Therefore, a novel composite wick structure with high permeability for UTHPs is proposed in this study. It is found that the wick with larger pores formed by parallel woven spiral meshes have higher permeability, resulting in more working fluid in the wick, which helps delay the drying out at the evaporation end of heat pipes. To further improve the capillary performance of wicks, the effect of thermal oxidation suitable for mass production on the capillary pumping performance of wicks is systematically studied for the first time. Compared with the single sintered wick, the capillary rise height after oxidation at 400 °C, 500 °C, 600 °C, and 700 °C for 150 min increased by about 32.2%, 19%, 19.5%, and 12.9%, respectively. Considering that the copper oxidation may limit the choice of working fluid for the heat pipe, the wicks are heated to 400 °C for reduction reaction for 1 h (with 95% N2+5% H2 gas). The results show that the capillary rise height of the wicks after oxidation at 400 °C, 500 °C, 600 °C, and 700 °C and reduction treatment is about 110%, 127%, 114%, and 91.4% of that of the single sintered wick, respectively. The experimental results show that the maximum heat load power of the UTVCs with 5 layers #200 mesh, single sintered composite wick and 500 °C oxidation–reduction wick are 16 W, 29 W and 38 W, respectively.

A review of heat pipe technology for foldable electronic devices

Abstract The advent of foldable and wearable devices, such as the Samsung Galaxy Fold (2019) and Huawei Mate X (2019), require increased battery size and processor performance to drive their larger flexible displays. There is a growing need to develop flexible thermal management solutions to cool these foldable electronic devices. This research presents a comprehensive review of the state-of-the-art for both rigid and flexible ultra-thin heat pipe technology. This review discusses various types of heat pipes, their thermal performance, novel manufacturing processes, and the open research questions, challenges, together with the potential future directions of this research area.