•A novel sintered copper mesh wick was developed for improving the performance of ultra-thin heat pipes.•Capillary force of the deposited wick structures was experimentally examined by comparing with ...normal mesh wicks.•The capillary force of the deposited wick structures was larger than that of a normal wick.
A novel sintered copper mesh wick, fabricated by weaving, chemical deposition, and sintering, was developed for improving the performance of ultra-thin heat pipes. Capillary force of the deposited wick structures was experimentally examined by comparing with normal mesh wicks. In this study, the sintering process was used to enhance the adhesive strength of the surface structure. Capillary rate-of-rise tests with ethanol and acetone were performed to characterize the capillary force of wick structures. An infrared (IR) thermal imaging method was utilized to monitor the capillary rise processes. The effects of deposition time and sintering temperature on the capillary force were investigated. Test results indicate that the capillary force of the deposited wick structures was larger than that of a normal wick, and the rising velocity and capillary rise height increased as the deposition time increased from 5 to 20min. The sintering process maintains good integrity of the surface microstructures even after being subjected to ultrasonic vibrations for 2min, and also has a great influence on the capillary force of the deposited samples. The deposited wicks can achieve optimum operating efficiency by choosing deposition time of about 15min and a sintering temperature of around 500°C.
•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.
•Micro-nanostructured composite mesh wick presents a superior capillary performance.•The composite mesh wick enables the reduction of the liquid film in vapor channels.•Boiling and evaporation are ...promoted by the wick based on the visualization results.•The ultra-thin vapor chamber with a thickness of only 0.35 mm is developed.•The effective thermal conductivity of the vapor chamber is 10,602 W/mK at 3.5 W.
Ultra-thin vapor chambers (UTVCs) show great potential in cooling compact and high-power electronic devices and improving efficiency for energy systems. However, the two-phase flow heat transfer inside UTVCs remains unclear, particularly for those with steam chamber thicknesses of less than 0.4 mm. In this study, the two-phase flow characteristics inside UTVCs with a steam chamber thickness of 0.2 mm are investigated by conducting visualization experiments. Wettability and capillary wicking tests were also carried out to optimize the wicks. The results show that the composite mesh wick enables optimum heat transfer performance for UTVCs due to enhanced capillary wicking, reduced liquid film in vapor channels, and promoted evaporation and boiling. Then UTVCs at a thickness of 0.35 mm with different wicks are fabricated and tested by water cooling and natural convection heat dissipation experiments. The optimum UTVC exhibits the highest effective thermal conductivity of 12,454 W/mK at a heat input power of 3 W, indicating its powerful advantages for the heat dissipation of electronics in limited space. This study sheds light on the unidentified mechanism of two-phase flow inside extremely thin vapor chambers and guides the future design of high-performance UTVCs for cooling compact power devices.
This paper focuses on fouling formation of nanofluid in a gravity-assisted thermosyphon equipped with a mesh screen wick. A transient study has been established on the fouling resistance of ...water-based TiO2 nanofluids at different operating conditions comprising the applied heat flux, mass concentration and inclination of thermosyphon. Nanofluids were prepared using prolonged sonication, stirring, and pH control. Triton X-100 was utilized as a dispersant. The thermosyphon is a copper made tube, which is dimensionally 10.7 mm and 12 mm for inner and outer pipe diameters respectively and total length of 280 mm. Transient results over the extended time reveal a considerable deterioration of heat transfer coefficient and thermal performance as a result of fouling formation inside the wick at evaporator section. According to the results, rate of fouling can be enhanced by increasing the nanofluid mass concentration. Intensification of fouling on wick structure and internal wall of evaporator causes instabilities in thermal performance of thermosyphon over the time, which eventually causes the thermosyphon failure. This point has been ignored by most of previous researchers, which has a considerably negative impact on thermal performance of thermosyphon at longer operating time and higher heat loads. Therefore, a new fouling resistance model has been re-developed combining the microfluidic typical models. This model can predict the fouling resistance of nanofluid inside the thermosyphon with approximate deviation of 30%.
•Application of TiO2 water based nanofluid in a thermosyphon is studied.•Fouling resistance causes deterioration in thermal performance.•By using nanofluids, evaporator depreciation in a transient study is seen.•In steady state study, nanofluids enhance the thermal performance of heat pipe.
•Set up a visual experiment platform for ultra-thin flat heat pipe.•Study the temperature fluctuation period and working medium flow period of ultra-thin flat heat pipe.•Study the phase change mode ...of working medium of ultra-thin flat heat pipe under different heat loads.•Combine temperature fluctuation analysis with phase transition mode analysis.
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.
•Ultra-thin flattened heat pipes with thickness of 1.0 mm were developed.•Copper screen mesh with various mesh structure were used for wick structure.•The effect of mesh structure on the operating ...performance of UTHPs were investigated.•Multi-orientations test were conducted to study the anti-gravity performance of the UTHPs.
With the fast development of microelectronic devices, high-efficient heat dissipation in narrow spaces faces significant challenges. Ultra-thin flattened heat pipe (UTHP) technology is proposed as a breakthrough in this field; however, some improvement, especially in optimizing the capillary wick, is required before UTHPs can be further applied. In the present study, the thermal performance of UTHPs (thickness of 1 mm) with mesh wicks in various mesh structure have been investigated. Results demonstrate that a higher mesh number of the mesh wick can not only accelerate the start-up response but also reduce the thermal resistance in steady-state operation. However, too high mesh number has disadvantages. The corresponding permeability will decrease as the mesh number increasing, resulting in a large working fluid flow resistance. The mesh structure should be optimized for sufficient capillary pumping power and high permeability to insure the best thermal performance of the UTHP. Within the mesh number range of 120 in−1 to 220 in−1, the 180 in−1 mesh wick should be a better choice to achieve the optimum performance for the UTHP. Moreover, through multi-orientations tests, it is revealed that the gravity plays an important role in the thermal performance of the UTHP. The mesh wick with 200 in−1 mesh number can effectively help the UTHP to maintain good anti-gravity stability.
•Heat pipes with sintered composite mesh-groove wick are tested at various tilt angles.•Two stages exist for horizontal composite wick before full dryout in the heated zone.•Tilted at 90°, Qmax for ...the composite wick attains 39 W, followed by full dryout.•The composite mesh-groove wick excels groove and mesh wicks in heat pipe performance.•No boiling is observed in all tests; heat pipe performance is capillary-limited.
Visualization experiments are performed on flat-plate heat pipes with a sintered composite copper mesh-groove wick at different tilt angles. Deionized water is used as the working fluid. The composite wick consists of a layer of 200-mesh copper screen sintered over parallel semi-circular grooves with a width of 0.18 mm and depth of 0.09 mm. Also investigated are a groove wick with the same groove size and a sintered double-layer 200-mesh (2 × 200 mesh) wick. The effective length of the heat pipes is 101 mm. The heat pipe performance of the composite mesh-groove wick excels the other two wicks in the maximum heat load (Qmax) under the horizontal orientation and in the anti-gravity ability. Visualization reveals two evaporation stages for the horizontal orientation before full dryout in the heated zone of the composite wick. In the first stage with Q up to about 40 W, the wick at the heated zone is filled with water; in the second stage, partial dryout in the grooves occurs and expands with increasing heat load. Up to about 60 W, the heated zone becomes fully dried. In contrast, the Qmax of the 2 × 200 mesh wick is 21–25 W, and full dryout prevails at 14 W for the groove wick. When the tilt angle is between 30° and 90°, the Qmax for the composite wick may reach 39–49 W, followed by drastic increase in the evaporator resistance. The 2 × 200 mesh wick suffers serious dryout at Q = 14 W under α = 45°; the groove wick cannot operate under α ≥ 30°. No nucleate boiling is observed in all the tests for water.
In this work, a study to evaluate the effect of the concentration of nanoparticles of an Al2O3/water nanofluid on the capillary limit of a heat pipe with a screen mesh wick was performed. For the ...first time, a numerical model has been developed to couple the hydrodynamical equations that describe the flow and the phase change of the working fluid in the wick of a screen mesh heat pipe, with a population balance which account for the agglomeration of Al2O3 nanoparticles and the deposition of them on the surface of the wick. The model has been validated with experimental data of temperature and capillary limit.
Experimental results showed that capillary limit is reached at higher heat input when nanoparticles are added to the working fluid. The best improvement of the capillary limit was around 30–40%. At high nanoparticle concentration, the improvement of the capillary limit was more variable, and the thermal resistance was higher than even the working fluid without nanoparticles. The improvement of the capillary limit has been found to be due to the modification of the wettability of the wick instead of reduction of effective pore radius or enhancing of the thermal conductivity. Numerical results fitted in good agreement the experimental data, but some limitations were found when high nanoparticle concentration was simulated because model can not predict the non-uniform deposition observed at microscopic level. Finally, it was found that there is an optimal concentration of nanoparticles to delay the occurrence of capillary limit. In this work, that optimal concentration was Al2O3/water 0.5% w/w.
•The wetting angle is reduced by nanoparticles deposition.•Population balance in the model allows for considering wettability variation.•Capillary limit is higher when nanoparticles are added to the working fluid.•There is an optimal concentration of nanoparticles to reduce the thermal resistance.
•Fins in evaporator improves heat transfer.•Different diameter transport lines prevent reverse flow of vapour.•Optimum filling ratio is 30% of total internal volume of mLHP.•mLHP worked well up to ...380W without any sign of dry out.•Significant reduction in evaporator wall temperature observed.
The effect of different filling ratios on the heat transfer performance of a novel miniature loop heat pipe (mLHP) having different diameter transport lines is experimentally studied. The different diameter transport lines are used to prevent reverse flow of vapour through the liquid line. The miniature loop heat pipe made of copper having square flat evaporator is tested using distilled water as the working fluid for the filling ratios of 20%, 30% and 50% in a heat load range of 20–380W. The experimental study shows that the filling ratio has significant impact on the heat transfer performance of miniature loop heat pipe. Filling ratio of 30% is identified as optimum filling ratio for this heat pipe. The thermal resistance values varied between 1.15K/W and 0.106K/W for three filling ratios in the tested heat load range. The optimum filling ratio gave the lowest evaporator wall temperature at all heat loads and at the highest heat load of 380W it had a lowest value of 94.3°C. The thermal efficiency of the miniature loop heat pipe lies in the range of 78–87% for the optimum filling ratio. The value of figure of merit is also calculated to understand its variation with filling ratios and heat loads. The experimental results confirm the suitability of the new design for cooling applications in a heat load range of 20–380W.
In order to cope with high heat flux of miniaturized electronic devices in narrow space, vapor chamber has been developed towards ultra-thin thickness. In this study, a novel design of thin screen ...mesh with superhydrophilic micro structure was firstly proposed to enhance the thermal performance of thin vapor chamber (TVC). A layer of 0.12 mm thick screen mesh was employed as evaporator wick. The central area of the thin wick above heat input region was treated by laser etching to form superhydrophilic micro structure for improving capillary capability and providing nucleation sites. The upper sheet without wick was stamped to form support columns for preventing deformation and providing axial thermal conduction passage. Three TVCs (70 × 70 × 1.5 mm3) with various laser-etching area ratio (LEAR) of 0%, 66.6% and 83.3% were fabricated to study thermal performance of the novel TVCs. The results indicated that the novel evaporator mesh wick could remarkably enhance thermal performance of the TVCs. With LEAR increasing from 0% to 83.3%, the lowest thermal resistance RVC of TVC dropped from 0.431 K/W to 0.328 K/W, i.e. decreased 23.9%, and the maximum heat transfer capacity increased from 100 W to 120W, i.e. increased 20%.
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