Experimental Study On Heat Transfer Characteristics Of Flatten Heat Pipe Based On Spiral Woven Mesh Wick

Ultra thin flatten heat pipe has the characteristics of self-driving,high equivalent thermal conductivity,light weight and compact structure.It is widely used in the heat dissipation of electronic components in narrow space.Facing the increasing demand for high-power heat dissipation,the current ultra-thin heat pipe has the problem of insufficient heat dissipation capacity.As the core component of ultra-thin flatten heat pipe,the capillary performance of the wick determines the heat dissipation capacity of the heat pipe.Therefore,improving the capillary limit by optimizing the wick structure has become the key method to improve the heat transfer capacity of ultra-thin flatten heat pipe.In order to balance the capillary pressure,porosity,permeability and wick thickness of the capillary wick,the spiral woven mesh with the high capillary pressure,low thickness,high porosity and low cost is selected as the wick material,and the micro and nano scale capillary structure is formed on the surface of the wire mesh through super hydrophilic modification,At the same time,the modified hydrophilic wire mesh core was compounded with polyethersulfone(PES)to obtain the composite structure wick.The heat transfer performance of three ultra-thin heat pipes based on spiral woven mesh wick,hydrophilic spiral braided woven mesh wick and composite structure wick are studied experimentally.The following conclusions are obtained:Through the surface morphology characterization and anti-gravity performance test of three wicks: spiral woven mesh wick,hydrophilic treated spiral woven mesh wick and PES wick,it is found that many micron flow channels are formed between the wick fibers of spiral woven mesh,providing capillary force for liquid transportation;The nano grass like structure grows on the surface of the hydrophilic wire mesh fiber,which greatly enhances the wettability of the wire mesh wick surface and reduces the contact angle from 53° before treatment to °;PES wick provides capillary force through irregular pores with high porosity.After super hydrophilic modification,the anti-gravity height of spiral woven mesh wick is increased from 38 mm to 89 mm,and the height of composite structure wick is increased to91 mm.The visual heat transfer test setup of ultra-thin flatten heat pipe is designed and built(the internal cavity thickness of flatten heat pipe is 1mm).The effects of liquid filling ratio,inclination angle,working medium type and wick type on the heat transfer performance are tested and compared.It is found that the performance of hydrophilic modified wick heat pipe is the best at 40% liquid filling ratio,and the maximum heat transfer power is 26 W under horizontal orientation,When the heat pipe is installed along the gravity direction,its heat transfer limit is further increased to 32 W,and the minimum thermal resistance is 1.04 ° C /W.Comparing the heat transfer for different working fluids,it can be found that R134 a can reach a higher heat transfer limit than that for water,but the temperature difference for the working fluid of water within 6.5W～20W is lower.For the heat transfer characteristics of different wicks,it is found that the hydrophilic wick is suitable for low and medium power heat dissipation cases.The super hydrophilic modification can increase the equivalent thermal conductivity of ultra-thin heat pipe by 23.6%,while the composite structure wick is more suitable for medium and high power heat dissipation occasions,and the equivalent thermal conductivity can reach 8900 w /(m·K).The two-phase flow of the three wicks under different tilt angles and liquid filling ratio is further observed visually.The results show that the working fluid changes in the form of evaporation when the heat pipe is placed horizontally and anti-gravity angle;When placed along the gravity angle,the working medium first undergoes boiling phase transition due to the accumulation of liquid at the evaporation end.With the increase of power,the liquid gradually decreases and the working medium gradually changes to evaporation phase transition.Comparing the heat transfer processes of the three wicks,it can be found that the dry interface on the surface of the unmodified wire mesh wick is not obvious,while the hydrophilic wick and the composite wick have a more obvious dry interface,and the dry interface will gradually move to the condensation end with the increase of power.Comparing the dry interface position of hydrophilic treated wick and composite wick under the same power,it is found that the dry interface of composite wick is later than hydrophilic treated wick under the same power because of its higher capillary force.The ultra-thin flat heat pipe samples were made with unmodified mesh wick and hydrophilic modified mesh wick as capillary wick respectively.The effects of different liquid filling ratio and inclination angle on the performance of heat pipe were tested.It was found that the super hydrophilic modified wick significantly improved the heat transfer limit and anti-gravity heat transfer performance of heat pipe.When placed horizontally,the maximum heat dissipation power of unmodified mesh wick flatten heat pipe can reach 10 W and the minimum thermal resistance is 0.52°C W.The ultimate heat transfer power of super hydrophilic modified mesh wick flatten heat pipe when placed horizontally can reach 12 W,and the minimum thermal resistance is 0.402°C/ W.when operating under anti-gravity conditions,the heat transfer limit can be increased by 50%.