Experimental Investigations On Heat Transfer Characteristics Of Multi-scale Wick Structure Flat Heat Pipe

With the development of semiconductor technology,all kinds of optoelectronic chips and electronic devices continuously toward high power and miniaturization direction,which led to a sharp increase in heat production per unit volume.Once the heat can not be discharged in time,it will affect the optoelectronic chip and electronic devices usage and working performance.However,traditional air forced convection cooling method has reached its limitation,thus the heat dissipation problem becomes a major bottleneck restricting the industry development of large heat flux optoelectronic chips and electronic devices.In this thesis,the heat transfer characteristics of different heat pipes,including multi-scale wick flat heat pipes,the match of hydrophilic evaporator and hydrophobic condenser flat heat pipes,were investigated by combining experimental and theoretical analysis methods.The main research contents and conclusions are summarized as follows:1.According to the micro convex structure of the natural hydrophilic plant,conical wick was sintered,using the mean particle diameter of 556 nm and 73.8 μm spherical copper powder as sintering material,and new flat heat pipes were constructed.Experiments were conducted to study the effect of heating power(heat flux),inclination angles,wick with thermal oxidation and porogen adding ratio of the new flat heat pipes with deionized water as the working fluid.The results showed that:(1)The conical wick has multi-scale: the wick wituout oxidation has three scales and the wick with oxidation has four scales.(2)The conical wick has a good liquid suction ability,and the ability enhanced greatly after wick oxidation.(3)The thermal oxidation has a good effect on the conical wick: improved its hydrophilicity and enlarged its surface area,which greatly improved the thermal performance of the flat heat pipe.(4)For the four flat heat pipe samples,the influence of the angle can be divided into four categories.(5)Because of the conical wicks touch the flat heat pipe condenser directly,thus not only shorten the liquid flow path and return time,but also thinning the thickness of the liquid flim,which strengthen the condensation heat transfer significantly.(6)The multi-scale wick flat heat pipe has well temperature uniformity.(7)During the process of wick sintering,appropriate amount of porogen can enhance the thermal performance of flat heat pipe.2.To further optimize the thermal performance of flat heat pipe,conical wick was sintered again,using the mean particle size of 75 μm dendritic copper powder as sintering material,and new flat heat pipes were constructerd.Experiments were conducted to study the effect of heating power(heat flux),inclination angles,wick with hydrogen peroxide oxidation and condenser with super-hydrophobic treatment of the new flat heat pipes with deionized water as the working fluid.The results showed that:(1)After hydrogen peroxide oxidation,nano-micron-millimeter multi-scale structure was formed,which greatly increased the surface roughness,improved its hydrophilicity,and enhanced the liquid absorbent capacity.(2)Gravity almost has no effect on the new flat heat pipes.(3)Condensation heat transfer will change from film condensation to drop condensation for the super-hydrophobic surface,which greatly thinning the thickness of the liquid film and strengthen the condensation heat transfer.(4)Both evaporation and condensation heat transfer coefficients increase with heat flux(heating power)increases,and the match of super-hydrophilic evaporator and super-hydrophobic condenser flat heat pipe has the maximum increment.