【作者】 周述璋；

【导师】 汤勇；

【作者基本信息】 华南理工大学 ， 机械制造及其自动化， 2014， 博士

【摘要】 传热强化是现代科学技术面临的一个主要问题，和单相对流传热相比，气/液两相沸腾传热可以带走很多热量，是今后散热与热管理的主要解决方案。池沸腾传热不需要外力驱动，液体内部的传热主要通过汽化产生气泡并由此运动带来周围液体的扰动从而强化传热。通过沸腾相变过程中的两相传热可以带走很多热量，具有可以传递高热流的潜力，但对于普通表面仍不能满足要求，因此需要研究出新型的表面结构来强化沸腾传热。改善沸腾传热的表面结构是沸腾传热强化的主要方法，优异的沸腾传热强化表面应该具有更大的表面积和更多的活化孔穴。为了得到更大的扩展面积，采用烧结模具制造出烧结多孔结构，在高温条件下通过颗粒状铜粉粉末的融合与基体紧密结合在一起，并且颗粒之间存在微小的孔隙。由于具有几何复杂外形的多孔表面密集的核化孔隙可以产生较多的蒸汽通道，以及微小孔隙能够提供较大的毛细作用力，更容易使液体补充，从而使气液界面形成更加复杂的流动。同时具有几何外形的多孔结构更能够增加固体和流体的接触面积，使得多孔表面具有更多的汽化核心，因而可以显著提高池沸腾的传热效率，减小沸腾过热度，推迟临界热流密度（CHF）的到来。烧结多孔柱复合结构不仅增加了液体与烧结表面的接触表面积，而且烧结多孔柱还可以在沸腾时提供液体的补充隧道，从而有利于液体吸入多孔结构通道，同时还具有阻碍加热表面形成覆盖蒸汽膜的作用，在高热流密度阶段可以有效分离气液相对运动。烧结多孔微槽道结构不仅增加了沸腾时烧结多孔结构的表面积，还能够提供蒸汽通道，保证蒸汽从阻力较小的开槽通道中溢出，易于使气泡产生脱离，而液体则通过毛细力的作用在多孔层内流动，这样，液体剧烈沸腾时就可以提供气液两相通道，分离气液相对流动，气液流动就会变得更加有序，从而使多孔表面的沸腾传热更加稳定。设计和制造了四种烧结多孔强化沸腾传热结构：1、烧结型表面多孔管结构；2、烧结多孔交错微槽道复合结构；3、烧结多孔柱复合结构；4、烧结多孔径向微槽道复合结构，并分析了四种强化沸腾传热结构的传热机理。分析了烧结多孔结构的制造工艺包括模具的制造方法、基体的加工、烧结工艺和烧结方法、电火花线切割加工工艺和方法等。研究了烧结工艺性能如铜粉形貌、烧结厚度、收缩率、孔隙率、渗透率、导热率等，同时以超声波振动方式研究了烧结多孔结构的结合强度。设计了沸腾传热强化的实验测试装置，分析了装置的原理和测试方法。搭建了实验测试系统，介绍了实验数据的采集过程，并对采集数据的结果进行了误差分析。最后对不同的烧结多孔结构进行了实验测试。烧结型表面多孔管在酒精中的强化传热效果达到了光滑管的3-10倍左右，烧结管的热流密度和传热系数明显大于光滑管，烧结管比光滑管更容易进入沸腾状态；实验过程中发现烧结铜粉粒径越小，烧结管的热流密度越大，其传热性能越好。烧结多孔表面开槽强化了沸腾传热效果，烧结多孔交错微槽道复合结构的传热系数与光滑表面相比可提高2-3倍。双向交叉槽道比单向微槽道具有更好的传热效果，能够形成一个稳定的液体补充和气体逸出网络。因此在烧结多孔表面上加工槽道形成微型复合结构，使得烧结多孔结构更加复杂和具有三维立体性，是强化沸腾传热的新方向。烧结多孔柱复合结构在所测定的热流密度范围内0~120w/cm2，不同厚度0.5mm，1mm的底部多孔层基础上烧结柱状结构有着不同的性能，对于底部厚度为1mm的多孔柱状结构可以强化其传热效果，但对于底部厚度为0.5mm的多孔结构其沸腾传热强化效果并不明显。对于底部多孔层厚度为1mm，柱高度为2mm的多孔柱结构与烧结多孔无柱结构相比，传热效率可达后者的1.5倍，但柱高度也并非越高越好。同时对不同的柱直径进行了分析，发现柱直径对沸腾传热效果的影响较小。对于底部厚度为1mm的烧结多孔径向微槽道复合结构，多孔微槽道结构的传热效果明显好于无槽道多孔表面，分析了不同微槽道个数和微槽道角度对传热性能的影响，微槽道数目越多强化效果越明显，90°交叉槽道传热效果好于45°。对于底部厚度为2mm的多孔微槽道结构，分析了微槽道深度和宽度的影响，实验证明微槽道深度为2mm的传热效果好于1mm，微槽道宽度为0.8mm的传热效果好于0.4mm。最后对不同热流密度下烧结多孔复合结构沸腾传热过程中的气泡形态进行了分析。更多还原

【Abstract】 Heat transfer enhancement is a major problem faced by modern science and technology,comparing to single-phase convective heat transfer, gas/liquid two-phase boiling heattransfer can take a lot of heat. It is the main solution to the future of cooling and thermalmanagement. Pool boiling heat transfer does not require external drivers. The heat transfer inthe liquid is mainly through producing bubbles which caused by vaporization and thismovement disturbances surrounding liquid thus enhanced heat transfer. It can take a lot ofheat through two-phase heat transfer during boiling phase transition process to enhancedboiling heat transfer. It also has the potential for high heat flow can be passed. But it still cannot meet the requirements for the average surface. Therefore it is need to come up with a newtype of surface structure to strengthen the boiling heat transfer.Improving surface structure of boiling heat transfer is the main method of enhancementof boiling heat transfer. Excellent surface of enhancement boiling heat transfer should have agreater surface area and more activation cavity. In order to obtain greater expansion area,using sintering mold to produce a sintered porous structure in high temperature conditionsclose together with the substrate by fusion of granular copper powder. Also there are smallpores between particles. Due to the dense core porosity of the porous surface which hasgeometry complex shape can produce more steam channel, and tiny pores can provide a largercapillary forces which make it easier for liquid replenishment. Thereby it makes the gas-liquidinterface form a more complex flow. At the same time a porous structure having a geometryshape can more increase the contact area between solids and fluids, make the porous surfacehaving more core of vaporization. Therefore it can significantly improve the heat transferefficiency of the pool boiling and reduce the boiling superheat. And it can delay critical heatflux (CHF).Sintered porous column composite structure not only increases the contact surface areabetween the liquid and the sintered surface, and the sintered porous columns can also provideadditional tunnels to the liquid while boiling. Thus contributing to liquid is sucked into porousstructure channel. It also has a role in hindering the heating surface forming the coveringvapor film. It can be effectively separated from the gas-liquid relative motion at high heatflux.Sintered porous micro channel composite structure not only increases the surface area ofthe sintered porous structure during boiling, can also provide a steam channel, ensure thesteam to escape from the channel of the slot with less resistance. And it is easy to make bubbles out. While the liquid flows in the porous layer through the action of capillary forces.Thus, it can provide gas-liquid two-phase channel when the liquid is violently boiling,separate gas-liquid relative flow. Gas-liquid flow will become more orderly. So that theboiling heat transfer of the porous surface is more stable.Four kinds of sintered porous structure enhancement of boiling heat transfer are designedand manufactured:1, sintered surface porous tube structure;2, sintered porous interleavedmicro-channel composite structure;3, sintered porous column composite structure;4, sinteredporous Radial Micro channel composite structure. And the heat transfer mechanism of fourkinds of enhancement of boiling heat transfer structure is analyzed.The manufacturing process is analyzed, including mold manufacturing method ofsintered porous structure, processing the substrate, sintering process and sintering method,EDM wire cutting technology and methods. The sintering properties are studied such ascopper powder morphology, sintering thickness, shrinkage, porosity, permeability, thermalconductivity, etc., and the bond strength of sintered porous structure was studied by the waysof ultrasonic vibration. The testing device of boiling heat transfer enhancement experiments isdesigned. The principle of the apparatus and test method are analyzed. Overall experimentaltest platform is build. The collection process of experimental data is introduced. And the erroranalysis of the results of data collection is done. Finally, the experimental tests of the differentsintered porous structure are carried out.The enhancement heat transfer effect of sintered surface porous tube in alcohol reachedabout3-10times the smooth tube. Heat flux and heat transfer coefficient of sintered tube wassignificantly greater than the smooth tube. Sintered tube is easier than the smooth tube intothe boiling state. During the experiment it is found that the smaller the sintered copper powderparticle size, the greater the heat flux of the sintered pipe, the better the heat transferperformance.Sintered porous surface grooving strengthens the effect of boiling heat transfer. The heattransfer coefficient of sintered porous interleaved micro-channel composite structure can beincreased2-3times compared to the smooth surface. Bidirectional cross-channel has betterheat transfer effect than unidirectional micro channel. It can form a stable liquidreplenishment and gas escape network. Therefore, it is a new direction for enhanced boilingheat transfer that machining channels on a sintered porous surface to form a micro-compositestructure, making sintered porous structure more complex and having a three-dimensionalnature.Within the range of0~120w/cm2the measured heat flux density, sintering columnar structures have different properties at the bottom on the basis of the different thickness of theporous layer such as0.5mm or1mm. For a bottom thickness of1mm porous columnarstructure it can enhance heat transfer effect. But as for a bottom thickness of0.5mm porouscolumnar structure，its boiling heat transfer enhancement effect is not obvious.To the porouscolumn structure, whose bottom of the porous layer having a thickness of1mm, porouscolumn height of2mm, the heat transfer efficiency comes up with to1.5times the lattercomparing to the porous layer which having no porous column. But the column height is notas high as possible. At the same time different column diameters were analyzed and foundcolumn diameter having little effect on boiling heat transfer effects.To the sintered porous radial micro channel composite structure, whose bottom of theporous layer having a thickness of1mm, the heat transfer effect of the porous micro channelstructure is significantly better than non-channel porous surface. The effect of differentmicro-channel number and micro-channel angle on heat transfer is analyzed. The more thenumber of micro-channel, the enhancement effect is more obvious. The heat transfer effect of90°cross-channel is better than45°. To the porous micro channel structure, whose bottom ofthe porous layer having a thickness of2mm, the impact of micro-channel depth and width isanalyzed. Experiments show that the heat transfer effect of the depth of2mm micro channel isbetter than that of1mm. The heat transfer effect of the width of0.8mm micro channel is betterthan that of0.4mm. Finally, the bubble shape of sintered porous composite structure duringboiling heat transfer process under different heat flux is analyzed.更多还原