复杂结构微通道热沉流动可视化及传热过程热力学分析

发布时间：2018-01-11 09:14

本文关键词：复杂结构微通道热沉流动可视化及传热过程热力学分析　出处：《北京工业大学》2015年博士论文　论文类型：学位论文

【摘要】：在传热学领域中,存在着许多散热问题,比如微型设备的散热,具有瞬态热流密度高及传热面积小的特点,严重制约着能源动力、航天航空、生物化工、军工核能及微型电子技术等领域的发展。随着大规模集成电路技术的迅速发展,微型电子芯片单位面积的散热功率不断升高,甚至高达107W/m2。为了保持微型电子器件表面温度的恒定,必须及时地去除多余热量。因此,如何解决微型设备的散热问题成为亟需解决的关键。随着微加工工艺的发展,微通道热沉由于面体比大、结构紧凑、散热效率高等优点成为解决高热流密度问题行之有效的方法之一。本文采用实验、模拟与理论相结合的方法,针对具有高热流密度发热面微型设备的冷却问题,综合考虑通道结构及工质种类两方面因素,设计具有结构紧凑、高散热性能的复杂结构微通道热沉,并建立复杂结构微通道热沉强化传热过程的热力学模型及结构优化的数学模型,为微通道热沉强化换热研究提供理论基础。主要包括以下几方面内容:首先,为了解决凹穴型微通道在低雷诺数下散热不明显的问题,在此通道的基础上,在两个凹穴之间加入内肋,形成结构更为复杂的凹穴及内肋组合的微通道。并且,用数值模拟的方法研究了不同形状的凹穴及内肋组合的微通道的综合传热性能。模拟结果表明,在凹穴区,由于面积突扩流速减小,容易引起层流分层,形成旋涡,有利于冷热流体充分混合;而在肋区,由于面积突缩流速增大,流体质点能保持相对大的动能快速地流过凹穴段,防止形成层流滞止区。总的来说,凹穴及内肋组合的微通道能明显增强内部扰动,起到强化传热作用。在低雷诺数时,三角形凹穴及梯形内肋组合的微通道的综合传热性能最优;而在高雷诺数时,三角形凹穴及三角形内肋组合的微通道的综合传热性能最优。其次,根据热力学第一及第二定律,推导出复杂结构微通道流动与传热过程的熵产模型,并结合场协同原理,从热力学及传热学角度共同分析影响微通道热沉强化传热的本质原因。理论分析表明,降低流体温度梯度的净值能起到强化传热作用。并指出用强化传热因子、场协同数、熵产增大数及热能传输效率这四个无量纲数均能评价通道内部的传热性能,但由于定义不同,评价的侧重点也不同。再次,使用先进的流动可视化手段——Micro-PIV(Micro-Particle Image Velocimetry)系统,观察凹穴及内肋组合的微通道热沉内部详细的流动情况,从流动角度分析及验证该类型热沉的强化传热作用。实验结果表明,该类型通道的轴向速度呈波峰及波谷相互交替的周期性分布趋势;在低雷诺数时,凹穴区没有出现旋涡,内肋区的流体具有较大的动能带走凹穴区的流体,减少流体的滞止时间,提高了通道的换热性能;而在高雷诺数时,凹穴区容易形成旋涡,二次回流有助于提高通道的换热性能。接着,使用两步法制备不同体积分数的Al2O3纳米流体,并用实验手段测量其导热系数及动力粘度值。并把它作为冷却工质,流经本课题组前期提出的凹穴型微通道热沉,目的是对比分析不同种类工质对热沉散热能力的影响。实验结果表明,纳米流体的导热系数随着表面活性剂SDS浓度的增大而减小。说明表面活性剂不利于传热,其作用是使纳米粒子均匀地分散在基液中,因此表面活性剂的添加量要衡量传热与均匀性两方面因素;传热实验结果与单相模拟结果不吻合,说明单相模型不再适合用于模拟纳米流体流动与传热的问题;与纯水相比,使用纳米流体作为工质时热沉的传热性能更优,并随雷诺数及体积分数的增大而增大,但压降也随之增大;用性能评价图综合分析热沉的传热性能,表明纳米流体的确起到强化传热的作用。再接着,设计一种新型的复杂结构双层微通道热沉,并建立其熵产模型;然后提出双层微通道热沉的整体封装方式,并分析各种入口流动方式对热沉总性能的影响。结果表明,当体积流量较小时,双层微通道所产生的不可逆损失较大,不宜采用该类型热沉对微电子设备散热;当体积流量较大时,采用逆流布置方式的复杂结构双层微通道热沉进行散热,其底面温度分布更均匀,能有效地根除热应力作用。而且,下层通道的流体所产生的熵产率是引起总熵产率增大的主要原因。不同的入口布置方式对热沉整体性能的影响也很明显。最后,在给定热沉总尺寸及散热功率下,由单相流体对流传热模型,设计满足要求的微通道热沉;并以多目标遗传优化算法为基础,建立通道结构优化的数学模型,以热阻及泵功值两个目标函数同时最小为优化目标,根据遗传算法优化出传热性能优良的通道结构。以给定散热功率100W及散热面积10mm×10mm为例,根据要求设计出满足散热要求的热沉,并用性能评价图分析不同尺寸热沉的综合传热性能。结果表明,通道具有中等宽高比αc的热沉的综合传热性能最好;然后,用遗传算法优化微通道热沉的结构参数,以热阻及泵功为优化目标参数,得到一系列不同泵功下热阻最优的通道结构尺寸。
[Abstract]:In the field of heat transfer, there are many problems such as heat dissipation, micro heat dissipation equipment, has the characteristics of high heat flux density and heat transfer area is small, seriously restricting the power, aerospace, chemical, nuclear and military development in the field of micro electronic technology. With the rapid development of large-scale integrated circuit technology, power dissipation miniature electronic chip unit area increasing, even as high as 107W/m2. in order to maintain a constant temperature on the surface of micro electronic devices, must promptly remove excess heat. Therefore, how to solve the problem of heat dissipation of the micro devices become the key to solve. With the development of micro fabrication technology, micro channel heat sink due to surface ratio, compact structure the higher cooling efficiency, has become one of the effective methods to solve the problem of high heat flux. This paper uses the method of simulation and experiment, the combination of theory, aiming at With the cooling of high heat flux heating surface micro devices, considering two aspects of tunnel structure and working fluid type factors, the design has the advantages of compact structure, complex structure and high heat dissipation performance of the micro channel heat sink, and to establish the mathematical model of complex structure of micro channel heat sink thermal strengthening heat transfer model and optimize the structure of micro heat sink enhancement of heat transfer and provide a theoretical basis. It mainly includes the following aspects: first, in order to solve the problems of cavity type micro channel at low Reynolds number the heat is not obvious, based on the channel, between the two recesses in ribs, forming more complex structure and inner cavity the micro channel rib combination. And by using numerical simulation method to study the comprehensive heat transfer of different shapes and internal cavity rib combination of micro channel performance. The simulation results show that in the concave area, because the area expansion velocity decreases, Easy to cause the formation of vortices, stratified, conducive to the hot and cold fluid mixing; and in the costal area, because the area of sudden contraction velocity increases, the fluid particle can maintain a relatively large kinetic energy quickly through the cavity, prevent the formation of laminar flow stagnation zone. In general, the recess and inner rib combined micro channel can enhanced internal disturbance, the heat transfer effect. At low Reynolds number, the optimal heat transfer performance of micro channel combined triangle and trapezoid rib cavity inside; while in the high Reynolds number, the optimal heat transfer performance of micro channel rib combination recess in the triangle and triangle. Secondly, according to the first and the two law entropy model to derive the complex structure of micro channel flow and heat transfer process, and combined with the field synergy principle, from the perspective of thermodynamics and common analysis of the impact of micro channel heat sink nature theoretical analysis shows that the enhancement of heat transfer, To reduce the fluid temperature gradient of the net can play a role. And pointed out that with the enhancement of heat transfer enhancement factor, field synergy number, entropy generation number is increased and heat transfer efficiency of the four dimensionless numbers are the heat transfer performance of the internal evaluation of channel, but because of different definitions, the evaluation focus is also different. Once again, the use of flow visualization method advanced Micro-PIV (Micro-Particle Image Velocimetry) system, observe the notch and inner rib combination of micro channel heat sink with internal flow, and verify the type of heat sink heat transfer enhancement effect from the analysis of flow angle. The experimental results show that the axial velocity of the type of channel with periodically distributed trend peaks and troughs alternating with each other; at low Reynolds number when the recess region does not appear vortex, fluid rib area have more energy to take a recess fluid area, reduce fluid stagnation time, improve the channel The heat transfer performance; while in the high Reynolds number, the recess is easy to form a vortex, two backflow helps to improve the heat transfer performance of the channel. Then, the Al2O3 nanoparticles prepared with different volume fraction using the two step method, and measured the thermal conductivity and viscosity values. And take it as the coolant flows through, ourgroup proposed cavity type micro channel heat sink, to comparative analysis of effects of different kinds of refrigerant on the heat sink capacity. Experimental results show that the thermal conductivity of nanofluids decreases with increasing surfactant concentration of SDS. It shows that the surfactant is not conducive to the heat transfer. The effect is to make the nanoparticles are uniformly dispersed in the liquid medium, the addition of surfactants to measure two aspects of heat transfer and uniformity factor; the experimental results and the simulation results of single-phase heat transfer does not match, that model is no longer suitable for single-phase mode The flow and heat transfer of nanofluids; compared with water, the use of nano fluid as the heat transfer performance of refrigerant heat sink is more excellent, and increases with the increase of Reynolds number and volume fraction, but the pressure drop increases; the heat transfer performance evaluation chart comprehensive analysis of heat sink performance shows that the nano fluid plays the effect of heat transfer enhancement. Then, the design of the complex structure of a new double microchannel heat sink, and to establish the entropy model; and then put forward the overall package of double microchannel heat sink, and analyze all kinds of entrance flow can affect the total precipitation of heat. The results show that when the volume flow rate is low double micro channel generated by the larger irreversible loss, should not use this type of heat sink of microelectronic device cooling; when volume is larger, the layout of the complex structure of the dual countercurrent micro channel heat sink for cooling, the bottom surface The temperature distribution is more uniform, can effectively eliminate the effect of thermal stress. Moreover, the entropy production fluid produced by the lower channel is mainly caused by the increase of the total entropy generation rate. The influence of different ways of entrance layout of heat sink performance is also very obvious. Finally, the total power dissipation in the sink size and given heat. The single-phase convective heat transfer model, micro channel heat sink design to meet the requirements; and the multi-objective genetic algorithm as the foundation, established the mathematical model for the optimization of channel structure, two objective functions at the same time as the optimizing goal and pump power on the thermal resistance value, according to the genetic algorithm to optimize channel structure. With excellent heat transfer performance given 100W cooling power and cooling area of 10mm * 10mm for example, according to the design meet the cooling requirements of the heat sink, and performance evaluation analysis of heat transfer performance of different size heat sink. The results show that the channel has The comprehensive heat transfer performance of the heat sink with a medium height to width ratio of alpha C is the best. Then, the genetic algorithm is used to optimize the structural parameters of the microchannel heat sink, and the thermal resistance and pump power are taken as the optimization target parameters, and a series of optimal channel structure sizes of different pump power are obtained.

【学位授予单位】：北京工业大学
【学位级别】：博士
【学位授予年份】：2015
【分类号】：TK124

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