【作者】 白鹏飞；

【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， 2010， 博士

【摘要】 利用微细通道内的强迫对流换热已被证实是最具发展潜力的高效冷却解决方案之一。本文基于流-固强化换热原理,采用模压固相烧结技术制造多孔复合微细通道强化传热结构。鉴于微细尺度下,流-固表面结构对流体的影响巨大,而本文涉及多孔结构表面复杂,因此对多孔微细通道的传热性能进行了重点研究。主要内容如下:(1)基于固相烧结技术的强化换热结构的优化设计与制造根据微细尺度下强迫对流换热的特点,设计了四种多孔微细通道结构。提出采用模压固相烧结法制造多孔型微细通道换热芯的制作方法及工艺流程,并通过实验总结对各加工参数进行了优化。鉴于铜粉难以制造高孔隙的多孔材料,提出利用多齿切削法制作的铜纤维来制造低孔隙率的多孔烧结微通道换热芯。实验证明采用固相烧结法制造的多孔型通道结构具有精度高、可控性好、工艺简单等优点,而使用铜纤维制备的烧结多孔材料,孔隙率调节范围宽、热导率高,非常适合作为微通道换热芯。(2)微通道流-固耦合传热数值模拟针对微细尺度下,通道传热特性与流-固接触表面张力、流体粘性以及通道直径的限制,综合采用变流体物性、低雷诺数k-ε湍流模型以及混合沸腾相变模型对微通道内单相及沸腾局部传热特性进行数值模拟。模拟结果表明:流体物性强烈影响微通道内的对流及传热特性;采用变物性方法,对单相流态下的通道入口发展段、换热系数、压降特性等的数值模拟,均很好地符合实验结果;利用混合相变模型可以有效捕捉到通道内的气-液相分布特性,对换热器的设计制造有很好的指导意义。(3)多孔复合沟槽微细通道换热芯的传热性能测试多孔复合沟槽微细通道换热结构,通道底部烧结一层不规则铜粉。单相流条件下,多孔层颗粒度最大的换热芯传热性能最好,相对于平滑通道传热系数提高了约30%;多孔复合通道的摩擦系数高于平滑通道,并利用现有数据拟合了它们的摩擦系数关联式。沸腾两相流通道中,流体出现剧烈的波动现象,并观测到了多种两相流态,其中环状流是占主导地位的流态;多孔层有利于降低通道沸腾过热度,在通道干度较小时有利于提高其传热性能,但强烈沸腾的通道中无强化传热效果。(4)不同孔隙率及骨架材料的多孔微细通道传热性能测试?对铜粉及铜纤维构成四种孔隙率的烧结多孔微细通道换热芯进行对比测试。实验发现,相同流速下,孔隙率低的铜粉通道传热性能最好,但其压降也更大。耗费相同泵功下,孔隙率为70%铜纤维通道传热效率最高。沸腾发生时,多孔微细通道中没有观测到强烈的压降波动现象。从小型强迫对流换热器的角度总结了多孔复合沟槽型及多孔型两种微细通道换热结构的传热特点。较低泵功下多孔复合沟槽通道传热效率高;而较高泵功下低孔隙多孔通道传热效率高,且更适合沸腾相变传热。更多还原

【Abstract】 To use forced convective heat transfer in the micro-channel has been already proven to be one of the most promising high efficient cooling solutions. Based on flow-solid enhanced heat transfer theory, mould pressing solid-phase sintering technology was used to fabricate the enhanced heat transfer structure of porous composite micro-channel. Due to huge influence of flow-solid surface structure to heat transfer in micro scale and complex structure of porous material, the heat transfer performances were also researched in detail. The major contents are as following:(1) Optimization design and manufacture of enhanced heat transfer structural based on solid-phase sintering technologyAccording to the forced convection heat transfer characteristics in micro-scale, four kinds of porous micro-channel structures were designed. The fabrication methods and technics processes of using molded compression solid-phase sintering to fabricate porous micro-channel heat transfer core were proposed. The processing parameters were optimized through experiments summarizations. Since it is difficult to manufacture high-porosity porous material by using the copper powder, copper fibers made through the multi-tooth cutting method were proposed to fabricate low-porosity porous sintered micro-channel heat exchanger core. It is proved by the experiment that porous channel structure manufacturing by solid state sintering is of high precision, controllable, and easy process, and so on. The copper fiber sintered porous materials is of wide porosity adjustment range, high thermal conductivity, very suitable used for micro-channel heat exchanger.(2) Heat transfer numerical simulation of flow-solid coupling in micro-channelFor the limitations of the channel heat transfer characteristics and fluid-solid contact surface tension, fluid viscosity and the channel diameter in micro scale, numerical simulation of single-phase and boiling part heat transfer characteristics in micro-channel was taken by synthetically using variable fluid properties, low Reynolds number k-e turbulence model and hybrid boiling phase change model. The results of the simulation show that: fluid properties strongly affect the convection and heat transfer characteristics in the micro-channel; at single-phase flow, numerical simulation of fluid entry development section, heat transfer coefficient, pressure drop characteristics can meet well with the experimental results; using mixed-phase transition model can effectively capture the gas-liquid phase distribution in the channel, which would be a good guide for design and manufacture of heat exchanger. ?(3) Heat transfer performance test of porous composite grooved micro-channel heat transfer coreIn the heat transfer structure of porous composite micro-channel, a layer of irregular copper powder was sintered in the channel bottom. Under the single-phase flow conditions, the heat transfer core with the largest size of porous layer particle has the best heat transfer performance, which was increased by about 30% relative to the heat transfer coefficient of smooth channel; friction coefficient of porous composite channel is higher than that of smooth channel. The correlation of these two friction coefficients were developed by using the existing data.In the boiling two-phase flow channel, the phenomenon of sharp fluctuations occurred in fluid, and multiple two-phase states were observed, of which the annular flow is the main flow. Porous layer is helpful to reduce the channel boiling superheat and improve the heat transfer performance when the dryness of the channel is small, but has no effect to enhance heat transfer in a strong boiling heat transfer channel.(4) Heat transfer performance test of porous micro-channel with different porosity and skeleton materialsTests were taken to compare four kinds of porosity sintered porous micro-channel heat transfer cores fabricated by copper powder and copper fibers. It was found in the experiment that at the same flow rates, copper powder channel with low porosity had the best heat transfer performance, but the pressure drop was greater. At the same pump power cost, copper fiber channel with the porosity rate of 70% had the highest heat transfer efficiency. No strong fluctuation phenomenon of pressure drop was observed in the porous micro-channel while boiling occurred.The heat transfer characteristics of porous composite grooved micro-channel heat transfer structure and porous micro-channel heat transfer structure were summarized. Porous composite grooved channel under lower pump power had higher heat transfer efficiency; while porous channel with low porosity under higher pump power had higher heat transfer efficiency, and is more suitable for boiling phase change heat transfer.更多还原