Ultra-thin Flattened Heat Pipes Research Articles

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.

Liquid plug characteristics and heat transfer performance of a silicon-based ultra-thin flat heat pipe at different incline angles

Silicon-based ultra-thin flat heat pipes (UFHPs) that can be integrated with semiconductor devices provide an opportunity for electronic thermal management. To meet the requirements of various operation states, the impacts of gravity on the liquid plug characteristics and heat transfer performance of an UFHP are investigated by a visualization experiment conducted at different incline angles. Condensate flows back through microgrooves under the capillary pressure and gravity under a low heating power (capillary flow stage), and then excess condensate flows back along the edges of the UFHP under a higher heating power (gravitational flow stage). Over a certain heating power (Q ≥ 14 W), liquid plug fluctuations caused by vapor entrainment are observed with periodic dry-out at the evaporator under large incline angles (α ≥ 60°), corresponding to temperature fluctuations. Intense fluctuations of the liquid plug enhance sensible heat transfer, although the liquid plug occupies the condenser area and impedes the vapor flow. Meanwhile, evaporation at the evaporator benefits from corner-film evaporation until a large area of dry-out occurs. Due to the increased gravity and buoyancy, a larger incline angle is conducive to thermal performance enhancement, but this effect becomes restricted as the heating power rises. At a cooling-water temperature of 20 ℃ and an incline angle of 90°, the lowest thermal resistances of the UFHP yield a value of 1.14 ℃/W at 4 W in the steady stage and a value of 1.22 ℃/W at 18 W in the fluctuation stage, respectively. The heat transfer limit of UFHP with overfilling reaches 24 W, compared with the theoretical heat transfer capacity of 8.9 W at an incline angle of 90°. Additionally, a higher cooling-water temperature is beneficial for the thermal performance in the capillary flow stage, but not in the gravitational flow stage and fluctuation stage. This study provides guidance for the operational design of silicon-based UFHP for electronic thermal management.

An Experimental Study of a Composite Wick Structure for Ultra-Thin Flattened Heat Pipes.

As the thickness of an ultra-thin flattened heat pipe (UTHP) decreases, the fabrication difficulty increases exponentially, and the thermal performance deteriorates rapidly. In this study, three types of composite wicks were developed for UTHPs with a 0.6 mm thickness: copper foam and mesh wick (CFMW), two layers of different mesh wick (TDMW), and three layers of the same mesh wick (TSMW). The startup and steady-state performances of the UTHPs with liquid filling ratios of 60% to 120% were investigated. The findings indicated that the CFMW UTHP with a filling ratio of 100% exhibited the best startup performance, with the highest equilibrium temperature of 58.37 °C. The maximum heat transport capacities of the CFMW, TDMW, and TSMW UTHP samples were 9, 8, and 8.5 W, respectively, at their corresponding optimum filling ratios of 110%, 90%, and 100%. The CFMW UTHP exhibited the lowest evaporation and condensation thermal resistances of 0.151 and 0.189 K/W, respectively, which were 24.67% and 41.85% lower than those of the TSMW UTHP. CFMW can be used to improve the thermal performance of UTHPs. This study provides important guidelines for the structural design, fabrication technology, and performance improvement of high-performance UTHPs used in portable electronic devices.

PREPARATION METHOD AND THERMAL PERFORMANCE OF A NEW ULTRA-THIN FLEXIBLE FLAT PLATE HEAT PIPE

Ultra-thin flat plate heat pipes must provide a degree of flexibility to meet foldable electronics heat dissipation requirements. In this paper, a new flexible ultra-thin flat plate heat pipe with a thickness of 0.75 mm has been designed and fabricated. Compared with the traditional flexible ultra-thin flat heat pipes, the innovation lies in the flexible insulation section formed by epoxy resin pouring of the shell. The design of the shell ensures that the flexible ultra-thin plate heat pipe can respond quickly to the external temperature change, and also has good flexibility, which provides a new choice for the material and structure design of the flexible ultra-thin plate heat pipe shell. The gas-liquid coplanar type mesh is used as the capillary wick to reduce the flow resistance of steam inside the heat pipe, and the wick is hydrophilically modified to improve its capillary pumping performance; a sandwich support structure is used to prevent the steam chamber from collapsing. The thermal performance of the three liquid filling ratios of 0.3, 0.4, and 0.5 was tested at different tilt angles and bending angles. The results show that in the cases of filling ratios of 0.3, 0.4, and 0.5, the ultra-thin flexible flat plate heat pipe with the liquid filling ratio of 0.3 has the best heat transfer performance under different working conditions; the tilt angle has different effects on the heat transfer performance and starting speed of the ultra-thin flexible flat plate heat pipe with different filling ratios, and the bending angle changes the steam condensation position inside the ultra-thin flexible flat plate heat pipe and increases the thermal resistance.

EXPERIMENTAL INVESTIGATION ON THERMAL PERFORMANCE OF ULTRA-THIN FLATTENED HEAT PIPE WITH MIDDLE HEATING FOR ELECTRONICS COOLING

For cooling the electronics in limited space, this study proposes ultra-thin flattened heat pipes (UTFHP) with two working modes, i.e., a short UTFHP with single-end heating and single-end cooling (SHSC) and a long UTFHP with middle heating and dual-end cooling (MHDC). The length of the short UTFHP is half that of the long UTFHP. The effects of input heat load and cooling temperature on the thermal performance of the short UTFHP with SHSC and long UTFHP with MHDC have been studied for the performance comparison. The input heat load ranges from 0-38 W and the cooling temperature ranges from 15°C to 65°C. The results show that the two layers wrapped 200 in<sup>-1</sup> screen mesh can provide adequate capillary pressure; hence, both of the two UTFHP working modes show good temperature uniformity. The short UTFHP with SHSC shows better thermal performance compared to the performance of long UTFHP with MHDC. In addition, the thermal resistances of both UTFHPs decrease with the increase of the input heat load and the decrease of the cooling temperature under the ranges of operating conditions.

Thermal and hydrodynamic characteristics of an ultra-thin flattened centered-wick heat pipe: Experiments and numerical analyses

An ultra-thin flattened heat pipe with a centered-wick structure is a promising option for the thermal management of ultra-slim electronic devices. However, while experimental studies are being actively conducted, the thermal and hydrodynamic characteristics of the ultra-thin flattened centered-wick heat pipe have not been deeply analyzed. In the present study, therefore, experiments and three-dimensional numerical analyses were conducted to clarify the characteristics specific to this type of heat pipe. Because a semi-transparent flat-plate centered-wick heat pipe with inner height of 0.5 mm was fabricated in the authors’ previous study, the present study was performed based on this semi-transparent heat pipe. The experiments were conducted to obtain experimental results for comparison with numerical results. In the numerical analyses, the heat pipe was represented by a model comprising vapor, liquid–wick, upper wall, and lower wall regions; the numerical results were obtained for the velocity, pressure, and temperature distributions within the heat pipe. The experimental and numerical results confirmed that their good agreement was obtained within ± 6 % when a condensation end position was set 20 mm from the inlet of the condenser section. In contrast to conventional heat pipes, the present centered-wick heat pipe contains vapor-wick-wall contact lines, which causes the concentration of heat flux. The numerical results clarified that the concentration of heat flux occurred in the wick structure within the range of 0.2 mm from the bottom. This range needs to be considered to improve the wick structure of the centered-wick heat pipe. Simple one-dimensional equations were derived to discuss the vapor pressure difference within the heat pipe. Agreement was obtained within ± 10 % between the vapor pressure differences from one- and three-dimensional calculations.

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

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

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.

Comprehensive optimization design of ultra-thin flat heat pipe based on pressure-bearing and heat transfer performance

With the development of high-power consumption and miniaturization of electronic devices, the high heat flux in limited space requires thinner thickness and better heat transfer performance of ultra-thin flat heat pipe (UFHP). Due to the decrease of UFHP thickness, the thickness of wall and vapor chamber, and the arrangement of support structure have completely different effects on pressure-bearing and heat transfer performance. Therefore, in order to reasonably design the structure of UFHP, it is necessary to consider both the pressure-bearing and heat transfer capacity. In this study, the deformation, thermal resistance and pressure drop of UFHP with support strips are analyzed. It is found that with the decrease of vapor chamber thickness, under the constraint of the maximum wall deformation, the rapid increase of thermal resistance and pressure drop caused by thin thickness could be reduced by optimizing the vapor chamber size. The size optimization model of UFHP based on pressure-bearing and heat transfer performance is established to guide the design of UFHP with support strips.

Analysis of thermal characteristics of the heat pipes with segmented composite wicks

In the present study, the thermal characteristics of two different porous media (SWM (spiral woven mesh) and SCP (sintered copper powder)) at different length proportions, different particle sizes of SCP, and different heating directions under different input power were explored to enhance the thermal performance of UTHP (ultra-thin flattened heat pipe). To clarify, the length of the wick was divided into 10%, 30%, 50%, 70%, and 90% proportions of SCP and SWM, respectively. After determining the optimal wick ratio, SCP with particle sizes ranging from 0.38–0.25, 0.25–0.18, 0.18–0.15, and 0.15–0.12 mm were combined with the SWM. The study then investigated and compared the effects of different particle sizes of SCP on the performance of UTHP with the thermal performance of a single structure. Some conclusions can be drawn. SCP and SWM have different performances when compounded with different proportions. Although the best performance was observed when the length proportion of SCP was 50%, it is important to note that the composite structure is not always superior to the single-wick structure. With appropriate parameters, UTHP with a segmented wick showed an increase in maximum heat dissipation capacity by 16.7% and a reduction in thermal resistance compared to the UTHP with a single wick structure.

Influence of the liquid plug on the heat transfer performance of the ultra-thin flat heat pipe

Under extreme size, the ultra-thin flat heat pipe will form a liquid plug connecting the upper and lower surfaces due to the size effect and surface tension, and the liquid plug occupies and separates the vapor flow path. To study the influence of the liquid plug on the heat and mass transfer performance of the ultra-thin flat heat pipe, the ultra-thin flat heat pipe with a thickness of 1 mm and a filling rate of 30% is theoretically analyzed and visualization research. The results show that the liquid plug divides the vapor flow path into two separate sections, obstructing the continuous vapor flow. And the two sections are evaporated and condensed, respectively. The liquid plug will affect the temperature gradient distribution. In the hot end bubble, the temperature gradient decreases in the region where the condensing phase change occurs and increases in the region adjacent to this. The liquid plug needs to be pushed from the evaporator to the condenser during the heating process, resulting in an increased start-up time for the ultra-thin flat heat pipe. At low heating power of 4 W and 8 W, the liquid plug leads to the wick of the evaporator being completely covered by the liquid plug or liquid film, and the thermal resistance is higher than that without the liquid plug. When the heating power rises to 12 W and 16 W, the evaporator enters the nucleated boiling stage, and the liquid plug will replenish the working fluid reflux in the wick, and the thermal resistance decreases compared to the case without the liquid plug.

Numerical Simulation of Heat Transfer Performance for Ultra-Thin Flat Heat Pipe

Numerical Simulation of Heat Transfer Performance for Ultra-Thin Flat Heat Pipe

Experimental Research on Thermal Performance of Ultra-Thin Flattened Heat Pipes

Experimental Research on Thermal Performance of Ultra-Thin Flattened Heat Pipes

Thermal performance of an ultra-thin flat heat pipe with striped super-hydrophilic wick structure

• The UTFHP with striped super-hydrophilic wick structure was developed. • The mesh number of N = 200 and passage width of P = 1.5 mm is the best wick structure. • The UTFHP could tolerate 8 W heating load with the minimum thermal resistance of 0.26 K/W. The development of high-performance electronic devices requires high heat transfer components with small thickness and high thermal performance. This paper presented an ultra-thin flat heat pipe (UTFHP), and the total thickness and inner height is respectively 0.68 mm and 0.48 mm. The striped-channel structure was developed to withdraw the deformation of UTFHP and reduce the flow resistance. In order to improve the capillary performance of mesh wick, the oxidation treatment was finished by the thermal oxidation method. The thermal performance of UTFHPs was investigated under natural convection cooling condition. The effects of mesh structure, passage width and filling ratio on the thermal resistance and temperature distribution were analyzed. It is found that the UTFHP with mesh number of N = 200 and passage width P = 1.5 mm showed the best thermal performance under the filling ratio of φ = 57%. The minimum thermal resistance was 0.26 K/W with a maximum temperature of 74.07 °C at 8 W heating load, indicating that the proposed UTFHP was able to be a reliable candidate for the thermal management of electronic devices with high heat transfer rates.

Comprehensive Optimization Design of Ultra-Thin Flat Heat Pipe Based on Pressure-Bearing and Heat Transfer Performance

Comprehensive Optimization Design of Ultra-Thin Flat Heat Pipe Based on Pressure-Bearing and Heat Transfer Performance

Influence of the Liquid Plug on the Heat Transfer Performance of the Ultra-Thin Flat Heat Pipe

Influence of the Liquid Plug on the Heat Transfer Performance of the Ultra-Thin Flat Heat Pipe

Heat and mass transfer characteristics of ultra-thin flat heat pipe with different liquid filling rates

The visualization method is used to study the heat transfer performance of ultra-thin flat heat pipe (UTFHP) with a vapor chamber thickness of only 0.8 mm under different liquid filling rates, and observe the change of vapor–liquid interface in mesh wick and the flow characteristics of working fluid in vapor chamber, which has important reference value and guiding significance for determining the optimal liquid filling rate, improving heat transfer performance and analyzing the flow of working medium in ultra-thin flat heat pipe. The experimental results show that the optimum filling rate is 15%, the working medium just fills the wick, and the thermal resistance is the smallest, which is 1.2 ℃/W. At the excessive liquid filling rates, the boiling phenomenon is observed in the heating area at 20 W, but the effect of excessive liquid filling on improving the heat transfer limit is very limited, which only increases the heat transfer limit by 4 W. Capillary limit is the key factor restricting the heat transfer limit of ultra-thin flat heat pipe. The excessive liquid forms a liquid bridge in the vapor chamber, which will hinder the vapor flow. Meanwhile, it will destroy the capillary flow of the condensing working medium, resulting in liquid accumulation in the condensing area and increasing the thermal resistance of the plat heat pipe. Under the condition of slight inclination angle, the condensed working medium accumulated in the condensation area is obviously reduced, at 45% liquid filling rate, the heat transfer limit reaches 28 W.

Visualization study on boiling heat transfer of ultra-thin flat heat pipe with single layer wire mesh wick

In this paper, an experimental study on the ultra-thin flat heat pipe with a single-layer wire mesh core is carried out through a visual experimental platform. Combined with the analysis of temperature and phenomena, it is found that the peak value of temperature fluctuation of ultra-thin flat heat pipe is affected by thermal load, and the period of temperature fluctuation is an integral multiple of the working fluid flow. The working modes of the ultra-thin flat-plate heat pipe under different heat loads include evaporation heat transfer, subcooled boiling heat transfer, nucleated boiling heat transfer and the drying zone beyond the heat transfer limit. When the boiling working medium in the ultra-thin finite cavity enters the nucleate boiling zone, the thermal resistance in the evaporation section will decrease and the thermal resistance in the condensation section will increase slightly. The overall thermal resistance shows certain regularity, with the minimum value about 21.7% lower than the maximum value. Under the condition of sufficient condensation reflux, the heat resistance of the ultra-thin flat heat pipe reaches the minimum when the oscillation intensity of the vapor-liquid two-phase working medium reaches the maximum.

Design and experimental study on a new heat dissipation method for watch-phones

In this work, a new heat dissipation method based on ultra-thin flattened heat pipe (UTHP) is developed for cooling watch-phones. The UTHP with a 0.4 mm thickness was designed to be a three-dimensional shape according to the watchcase shape. The UTHP cooling module (HPM) was soldered by three copper plates and a UTHP. The heat dissipation performance of the copper sheet cooling module (CSM) with the same dimensions was made for comparison with the HPM. The heat dissipation performance of the HPM was experimentally studied. The results indicate that the maximum heat dissipation powers of CSM and HPM were 0.8 and 0.9 W, respectively, at the surface temperature of the ceramic heater not exceeding 65 °C. At this time, ΔTc and Rhc of them were 9.18 °C and 9.86 °C/W, 3.98 °C and 2.63 °C/W, respectively. When the cooling module was cooled naturally from 65 to 37 °C, the heat dissipation time of HPM was 25 s shorter than that of CSM. The UTHP cooling module has a better heat dissipation performance, which can quickly reduce the chip temperature, eliminate the chip hotspot, and solve the heat dissipation issue of watch-phone.