【作者】 郭威；

【导师】 唐大伟；

【作者基本信息】 大连理工大学 ， 能源与环境工程， 2022， 硕士

【摘要】 超薄平板热管具有自驱动、等效导热系数高、质量轻、结构紧凑的特点,被广泛应用于狭小空间内的电子元件散热。面对日益增加的高功率散热需求,当前的超薄热管存在散热能力不足的问题,吸液芯作为超薄平板热管中的核心部件,其毛细性能决定了热管的散热能力。因此,通过优化吸液芯结构提升毛细极限,成为了提高超薄平板热管传热能力的关键方法。为了兼顾毛细芯的毛细压力、孔隙率、渗透率以及芯体厚度之间平衡关系,本文选择了具有毛细泵压力高、厚度低、孔隙率高、成本低廉等特点的螺旋编织丝网作为超薄热管的芯体材料,并通过超亲水修饰在丝网表面形成了微米和纳米尺度共存的毛细结构,同时将超亲水修饰后的丝网芯体与高分子材料聚醚砜(PES)进行复合得到复合结构丝网芯。并采用实验方法分别研究了基于螺旋编织铜丝网吸液芯、亲水处理螺旋编织铜丝网吸液芯以及复合结构吸液芯的三种超薄热管的传热性能,得到如下结论:通过对螺旋编织丝网吸液芯、亲水处理的螺旋编织丝网芯、PES吸液芯三种芯体材料进行表面形貌表征和反重力性能测试,发现螺旋编织丝网芯纤维间形成了众多微米级流道,为液体输运提供毛细力;亲水处理的丝网编织芯铜纤维表面上生长出的纳米草丛状结构,极大增强了丝网芯表面的润湿性,使得接触角由处理前的53°降为0°;PES芯通过高孔隙率的不规则孔道提供毛细力。螺旋编织丝网芯经过超亲水修饰后,反重力高度由38mm提升到89mm,复合结构丝网芯的高度则提高到91mm。通过设计并搭建超薄平板热管可视化传热实验台(平板热管内部腔体厚度为1mm),对比了充液率、倾斜角度、工质种类、芯体类型等因素对超薄平板热管传热性能的影响规律。实验研究发现,在40%的充液率下,超亲水修饰丝网芯热管传热性能最佳。在水平放置条件下,最大传热功率达26W,热管以顺重力角度运行时其传热极限进一步提升至32W,此时最低热阻为1.04°C/W。对比不同传热工质可以发现,相对于水工质,R134a可以达到更高的传热极限,但水工质在6.5W～20W内的温差更低。通过对比不同芯体的传热特性,发现超亲水处理丝网芯适用于低中功率的散热场合,超亲水修饰使得超薄热管的等效导热系数提升23.6%,而复合结构芯体更适用于中高功率的散热场合,等效导热系数最高可达8900 W/(m·K)。通过可视化手段进一步观察三种芯体在不同倾斜角度及充液率下运行时的两相流动情况,结果表明热管以水平角度与反重力角度放置时工质以蒸发形式发生相变;而顺重力角度放置时由于液体堆积在蒸发端导致工质先进行沸腾相变,随功率增大液体逐渐减少,工质逐渐转变为蒸发相变。对比三种芯体的传热过程可以发现未修饰丝网芯表面的干涸界面并不明显,而超亲水处理丝网芯与复合芯具有更加明显的干涸界面,且干涸界面会随着功率的增加逐渐向冷凝端移动。对比相同功率下亲水处理丝网芯与复合芯的干涸界面位置发现复合芯由于其毛细力更高,相同功率下干涸界面较亲水处理丝网芯靠后。本文以未修饰丝网芯和超亲水修饰丝网芯为毛细芯分别制作了超薄平板热管样品,测试了不同充液率及倾斜角度对热管性能的影响。研究发现超亲水修饰后的芯体显著提升了热管的传热极限和反重力传热性能,水平放置时未修饰丝网芯平板热管的最大散热功率可达10W,热阻最低为0.52°C/W。而超亲水修饰丝网芯平板热管在水平放置时的极限传热功率可达12W,热阻最低为0.402°C/W,反重力条件下运行时,传热极限可提升50%。更多还原

【Abstract】 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%.更多还原