In this paper, we present a new analytical model to investigate the maximum heat transfer rate of a thin vapor chamber (TVC) with multiple heat sources and sinks. The model can specifically consider ...different heat flux conditions for each heat source. Both capillary limitations and allowable maximum temperature constraints were employed to determine the maximum heat transfer rate. The liquid and vapor pressure distributions within the TVC were analytically derived using the Brinkman-extended Darcy equation and the Hagen–Poiseuille equation, respectively. Additionally, the theoretical wall temperature distribution was calculated based on the 3D energy equation, considering different heat flux conditions for multiple heat sources with a weighting factor. Our results demonstrate that the heat flux conditions applied to the heat sources significantly impact the internal flow pattern of the TVC. These changes in flow patterns influence the pressure distributions of the liquid and vapor, thereby affecting the maximum heat transfer rate. Furthermore, the effects of wick parameters on the maximum heat transfer rate under various heat flux conditions were examined.
Fairly stable surfactant free copper–distilled water nanofluids are prepared using prolonged sonication and homogenization. Thermal conductivity of the prepared nanofluid displays a maximum ...enhancement of ∼15% for 0.5 wt% of Cu loading in distilled water at 30 °C. The wall temperature distributions and the thermal resistances between the evaporator and the condenser sections of a commercial screen mesh wick heat pipe containing nanofluids are investigated for three different angular position of the heat pipe. The results are compared with those for the same heat pipe with water as the working fluid. The wall temperatures of the heat pipes decrease along the test section from the evaporator section to the condenser section and increase with input power. The average evaporator wall temperatures of the heat pipe with nanofluids are much lower than those of the heat pipe with distilled water. The thermal resistance of the heat pipe using both distilled water and nanofluids is high at low heat loads and reduces rapidly to a minimum value as the applied heat load is increased. The thermal resistance of the vertically mounted heat pipe with 0.5 wt% of Cu–distilled water nanofluid is reduced by ∼27%. The observed enhanced thermal performance is explained in light of the deposited Cu layer on the screen mesh wick in the evaporator section of the heat pipe.
► Cu–distilled nanofluids display thermal conductivity enhancement of ∼15% at 30 °C. ► Thermal performance of heat pipes containing screen mesh wick is investigated. ► Effect of nanoparticle concentration and heat pipe inclination are studied. ► Evaporator–condenser thermal resistance is reduced by ∼27% for vertical heat pipe.
The influence of the heat pipe orientation and the working fluid type of the screen mesh wick heat pipe was investigated experimentally. A hybrid nanofluid using three proposals for composite ...nanoparticles scattered in DI Water was used as the working fluid. In each case, seven directions (0°-90°, deviation 15°) were investigated. Thermal properties were evaluated by the average temperature gap between the evaporator and condenser surfaces, thermal resistance, thermal conductivity (effective), and overall HTC. Experimental results imply that the tilt angle and the hybrid nanofluid used have a significant impact on heat transfer of heat pipe. By the time the tested heat pipes come into contact with 75% CuO-25% ZnO/DI water at an angle of 60°, they have the best heat transfer properties of any other type of fluid. Furthermore, it was found that the thermal characteristic of the heat pipes tested were hampered by the expansion of heat loads in the range 60-160 W.
•Heat transfer of cylindrical and flattened heat pipes are compared.•Influence of tilt angle, mesh size of wick and cross section of heat pipe studied.•Reliable operation of heat pipe in anti-gravity ...confirmed with repeated tests.
A comparative study of cylindrical and flattened heat pipes having different screen mesh size wicks is conducted at various inclinations (−90° to +90°) for a heat load range of 10–60W. The effect of flattening of heat pipes having same screen mesh wick at various inclinations are also analysed. Anti-gravity (−90°) tests are also repeated at different time intervals to analyze the reliability of these heat pipes if they have to remain idle for a certain period. Experiment results show that the thermal performance of the heat pipes are influenced by inclination angle, mesh size of the wick, cross section of the heat pipe and heat input. The inclination at which the maximum heat transfer occurs is not the same for three heat pipes tested. The lowermost thermal resistance witnessed is 0.46K/W, for cylindrical heat pipe, at an inclination of −45° for an applied heat input of 60W. The maximum evaporator and condenser heat transfer coefficient values observed are 3876W/m2K and 1698W/m2K respectively. The anti-gravity repeatability tests shows that these heat pipe works well even after some idle period and the variation in evaporator temperature is found to be less than ±7.5%.
Thermal performance evaluation of TiO
2
-coated copper mesh wick in a cylindrical heat pipe with graphite nanofluid is experimentally analyzed at various inclinations, nanoparticle concentrations and ...power levels. Boiling heat transfer from the evaporator of a heat pipe depends on both thermal conductivity of the nanoparticle and suspension stability of the nanofluid. The lower the density of the nanoparticle, the better the suspension stability. Spherical graphite nanoparticles having lower density and good thermal conductivity quicken the heat transfer rate and hence the vaporization of base fluid. A hydrophilic coating of TiO
2
is done on the copper wick structure to reduce the contact angle of graphite nanofluid. Results showed that the heat pipe worked well at 60° inclination compared to the other tested orientations. For this optimum condition, a reduction in 168.75% of thermal resistance is obtained compared with DI water with uncoated wick at horizontal position and also an improvement in thermal efficiency of 94.07% for 1.0 mass% particle loading. The enhancement in equivalent thermal conductivity is 90.87% for 1.0 mass% compared with DI water. Results from the repeatability test also confirm that the hydrophilic coating over the wick is stable, and results are repeatable.
•We experimentally study heat transfer performance of LHPs with different wicks.•The sintered wick LHP performs a little better than the mesh wick LHP.•The sintered wick LHP can start up at very low ...heat load.•The main reason may be the sintered wick is of smaller pore size than the mesh wick.
Heat transfer performance of loop heat pipe (LHP) is tightly related with the wick positioned between its evaporator and compensation chamber. Experiments were carried out to investigate the effects of wick on LHP heat transfer performance. Two wicks, a sintered copper powder wick and a copper mesh wick, were considered for comparison. The former has larger porosity; its pore size spans within a wide range, but smaller than that of the latter. The measured temperature data indicate that the sintered wick LHP starts up faster and operates more stably. The overall thermal resistance of the sintered wick LHP is also slightly lower than that of the mesh wick LHP. Moreover, the sintered wick LHP is found to be able to start up with heat load as low as 5 Watts.
Ultra-thin vapor chambers (UTVCs) are widely used to cool high-power electronics due to their excellent thermal conductivity. In this study, a UTVC of 82 mm × 58 mm × 0.39 mm with composite wick was ...prepared. The composite wick is composed of two layers of copper mesh and multiple spiral-woven meshes (SWMs), and the composite wick was applied in UTVC to improve liquid replenishment performance and temperature uniformity. Furthermore, the thermal performance of UTVCs with different support column diameters, filling ratios (FRs), and SWM structures was experimentally studied. The results found that the equivalent thermal conductivity (ETC) decreases as the diameter of the support column increases; the UTVC with 0.5 mm support column diameter has the highest ETC, at 3473 W/(m·K). Then, the effect of FR on the heat transfer performance of UTVCs with SWM numbers of 0, 1, 2, and 3 (0 SWMs, 1 SWM, 2 SWMs, 3 SWMs) is consistent, the 30% FR UTVC with 3 SWMs having the highest ETC, at 3837 W/(m·K). Finally, the increased number of SWMs can significantly improve the ultimate power of the UTVCs, the UTVC with 3 SWMs having the highest ultimate power, at 26 W. The above experimental studies indicate that the designed and manufactured UTVCs have great potential advantages in thermal dissipation for electronics.
This work presents visualization and measurement of the evaporation resistance for operating flat-plate heat pipes with sintered multi-layer copper-mesh wick. A glass plate was adopted as the top ...wall for visualization. The multi-layer copper-mesh wick was sintered on the copper bottom plate. With different combinations of 100 and 200 mesh screens, the wick thickness ranged from 0.26
mm to 0.8
mm. Uniform heating was applied to the base plate near one end with a heated surface of 1.1
×
1.1
cm
2. At the other end was a cooling water jacket. At various water charges, the evaporation resistances were measured with evaporation behavior visualized for heat fluxes of 16–100
W/cm
2. Quiescent surface evaporation without nucleate boiling was observed for all test conditions. With heat flux increased, the water film receded and the evaporation resistance reduced. The minimum evaporation resistances were found when a thin water film was sustained in the bottom mesh layer. With heat flux further increased, partial dryout appeared with dry patches in the bottom mesh holes, first at the upstream end of the heated area and then expanded across the evaporator. The evaporation resistance re-rose in response to the appearance and expansion of partial dryout. When the fine 200 mesh screen was used as the bottom layer, its thinner thickness and stronger capillarity led to smaller minimum evaporation resistances.
Growing technological developments to improve the thermal efficiency of heat pipes resulted in many innovative techniques. The use of nanofluids with superior thermophysical properties compared to ...conventional fluids is one such technique. However, the service life of nanofluid driven heat pipes is affected by agglomeration and stability issues, demanding an alternate solution. Boiling heat transfer rate of a heat pipe depends on the availability of working fluid at the evaporator, which is decided by the wick wettability and filling ratio. Wick wettability is improved with a hydrophilic coating using sol-gel dip coating process. Coating improves wick capillarity at the expense of wick permeability. Hence, an accurate balance of wick coating thickness and evaporator filling ratio are essential for the enhanced performance. The present work aims to demonstrate the benefits derived from optimal wick coating over these issues posed by nanofluids and to provide a better understanding on device level thermal performance. Thermal performance of coated mesh wick cylindrical heat pipe is experimentally studied for various coating thicknesses, evaporator filling ratios and inclinations. The existence of an optimum evaporator filling ratio for each coating thickness in boosting the limits of heat transfer is established. For the optimized values of inclination, coating thickness and evaporator filling ratio, a maximum reduction of 38.49% in thermal resistance and an improvement of 25% in thermal efficiency are obtained at 150 W heat input. The results from the repeatability test confirms coating stability.
