Experimental Study Of Heat Transfer Characteristics Of Novel Ultra-thin Heat Pipe

The miniaturization,highly integrated,and high power of electronic equipment have put forward new challenges to the heat dissipation technology.Ultra-thin heat pipe(UTHP),as a passive phase change heat transfer device,is an ideal tool for solving the heat dissipation problem of electronic devices in small space.The wick is the most important part of the heat pipe.But at present,the wick of the UTHP is simple in structure and single in scale,and the heat transfer characteristics of UTHP are limited easily,which can't meet the heat dissipation demand of higher power electronic devices.Based on this,a new type of UTHP with a total thickness of 1.30 mm was developed in this paper.The internal wick is a multi-scale composite wick structure consisting of porous layer(PL)and porous wire(PW).Chemical methods were used to modify the wick and the condensation surface.In this paper,four experimental samples were set:#1 sample belongs to the traditional copper wire flat heat pipe and#2?#4 samples have novel multi-scale composite wicks.The wick and condensation surfaces of#2 sample are unmodified which are hydrophilic;The wick surface of#3 sample has nanostructure which has super-hydrophilic properties,while the condensation surface of#3 sample is not modified.The surfaces of the wick and condensation of#4 sample have different nanostructures,which have the properties of super-hydrophilic and super-hydrophobic respectively.The processing of these four samples was described in detail,and its internal microstructure was characterized.The absorptive capacity of the wick was analyzed by water absorption experiment,and the influence of gravity,nanostructure and wick structure on water absorption were investigated.The effects of the wick structure,nanostructure,liquid filling ratio and incline angle on the thermal performance were studied by thermal experiments,and the temperature distribution performance of the UTHP was analyzed.The novel multi-scale composite wick structure developed in this paper realizes vapor-liquid phase separation inside the UTHP and balances the contradiction between multiple parameters.In this paper,a new processing technology of PL was proposed,and the process of composite wick forming and heat pipe packaging was optimized.Compared with the traditional processing technology,the process is simple and efficient,which has commercial value.The internal microstructures of the UTHP have been observed.The nanostructures change the wettabilityand structural properties of the wick and condensation surface,and the multi-scale structure of nano-micron-centimeter is formed in the UTHP.The water absorptive capacity of the wicks is the result of the combination of wettability,capillary force and viscous resistance.Water absorption experiments showed that:The wick designed in this paper can absorb liquid by antigravity and absorb liquid faster under the condition of gravitational force;The nanostructure enhances the wettability of the wick,and reduces the effective pore size to a certain extent,which enhances capillary force of the porous medium,but also increases the viscous resistance.When the wick is dry,the viscous resistance has a weak effect and the nanostructure can promote the liquid absorption speed.The speed of liquid absorption shows the features of fast in early period and gradual decrease in later period.The droplet is absorbed by the PW for the longest time,and the PL for the shortest time,and the absorptive time of the composite wick is between the above.The heat transfer characteristics of the UTHP are determined by the heat transfer characteristics of the evaporation section and the condensation section.The combined effects of the evaporation and condensation section make the curve of total thermal resistance with the change of power of#2 and#3 samples almost identical.The influence of nanostructure of the wick surface on the thermal performance is related to the liquid filling ratio.When the amount of liquid in the wick is low,the nanostructures promote the boiling heat transfer and the refluxing efficiency of the condensation liquid,and improve the heat transfer performance of the UTHP.When the liquid in the wick is sufficient,the nanostructures will increase the vapor spill resistance and liquid flow resistance in the wick,limiting the heat transfer performance of the UTHP.The condensation mechanism of#1 to#3 samples is filmwise condensation,and the thickness of the condensate film affects the condensation heat transfer of the UTHP.The super-hydrophilic wicks can promote the absorption of condensed liquid,reduce the condensate film thickness and promote the condensation heat transfer.However,the condensation mechanism of#4 sample is dropwise condensation,which shortens the path of the condensation liquid reflux and improves the efficiency of condensate recirculation.The#4 sample is able to withstand higher heat flux density at any filling ratios,but the self-assembled layer of the condensation surface,as an additional thermal resistance,increased the total thermal resistance of#4 sample.Samples without nanostructures are suitable for operation at high filling ratios,while samples containing nanostructures perform better at smaller fillling ratios.The influence of gravity on the thermal performance of the UTHP is great.When the evaporation section is located directly below the condensation section,the heat transfer performance of the UTHP is the best.The condensation surface temperature gradients of#2 and#3 samples are small,and the temperature difference between condensation section and adiabatic section is not large.But the condensation surface temperature gradient of#4 sample is large,and there is a large temperature difference between the condensation section and the adiabatic section.The heat transfer performance of the novel UTHPs designed in this paper is very good,and the maximum power is 9.5?16.4 times of the ordinary copper wire flate heat pipe,and the maximum equivalent thermal conductivity is 28.9?58.5 times of the pure copper plate.The optimal heat transfer performance of sample#2 is optimal in all samples,and its maximum power can reach 83.7 W,the minimum total thermal resistance is only 0.1651 K/W,and the maximum equivalent thermal conductivity coefficient is 23288.7 W/(m·K).