【作者】 郭永献；

【导师】 贾建援；

【作者基本信息】 西安电子科技大学 ， 机械电子工程， 2009， 博士

【摘要】 现有电子器件的热流密度越来越高,如何对芯片级、系统级的电子器件进行有效散热成为提高电子器件可靠性的一个重要因素。作为在电子器件换热领域最有希望取得突破的三项技术之一,喷雾冷却(Spray Cooling)技术正在受到越来越多的关注。当前的研究工作大多集中在实验研究方面,然而由于不同研究人员实验条件的不同使得各种研究结论有所差异甚至相互矛盾,有关喷雾冷却的理论研究相对更加缺乏,因此针对电子器件的喷雾冷却技术的理论和实验研究都尚有广阔的发展空间。基于电子器件喷雾冷却技术的研究现状和研究热点,本文对喷雾在热源表面形成的冲击液膜流动及厚度模型、基于CHF(Critical Heat Flux,临界热流密度)准则的倾斜喷射喷嘴轨迹、喷嘴距热源换热最优时的高度、倾斜喷射实验以及针对动态喷射的电磁驱动器件的吸合问题等方面进行了研究:针对圆锥形轴对称喷雾冷却系统,在对喷雾区域进行划分的基础上,引入滑流边界条件,在提出圆锥状喷雾假设的基础上,建立了喷雾区域雾滴密度连续介质等效模型。基于分离变量法求解液膜流动动力学微分方程组,寻找到合理的动量方程组相容关系,应用数学物理方法求得液膜内速度场的分离变量级数解。在假设液膜出口颈缩截断边界条件的基础上,通过联立其它边界条件,求得待定系数,同时得到了液膜流动场和液膜厚度的求解方程,实现了液膜边界未知的薄膜流动问题求解。仿真结果表明:①当喷雾速率均匀时,液膜为平膜。当速率非均匀时,液膜可以出现中心凸起和中心下凹两种情况,具体形状与液膜表面喷雾的速率分布有关;②靠近热源表面的薄层内的液膜流速较大,而其余大部分液膜流速较慢;使液膜减薄的方法有两种:增加喷雾速度和降低喷雾密度;③速度滑移对液膜厚度及流场的影响很小,可以忽略。通过分析确定了喷雾冷却中倾斜喷射合理的研究前提:CHF准则,即在保证冷却剂利用率最高的前提下,最大喷雾冲击区域对应最大的CHF值。分析了满足CHF准则时不同锥角的圆锥形和棱锥形喷嘴分别对圆形和方形热源倾斜喷射时喷嘴源点所处的轨迹曲线,以及倾斜喷射时喷雾冲击面积随喷射倾角的变化情况。在分析方形喷嘴倾斜喷射时,将喷嘴的倾斜旋转情况分为轴型旋转和对角线型旋转两类。结果表明,就倾斜喷射喷雾覆盖面积而言,对圆形热源进行喷射和/或倾斜喷射,最好选用圆锥形喷嘴;对方形热源进行喷射时,最好选用合适的方形喷嘴,并且当方形喷嘴倾斜喷射时选择轴型倾斜喷射(倾角不太大时)。结合Visaria等人的实验数据,在满足CHF准则的前提下,从理论上分析了圆锥形喷嘴对方形热源倾斜喷射时喷射倾角对CHF的影响。分析结果表明:喷射倾角对CHF影响不大,CHF随喷射倾角增大而稍有增大。购买并加工制作了实验所需设备或器件(喷嘴、热源、压力泵和各种测量仪器等),搭建了实验平台。应用相关理论,考虑拖曳力的影响,对所建立的实验系统进行了特性评估,包括雾滴的出口速率和直径、雾滴在运动过程中的速率、直径和温度的变化情况等,进而对喷射到热源表面的雾滴特性做出预测。应用所搭建的实验系统,对所选用的三个DANFOSS喷嘴入出口压差与泵压的关系进行了测定,结果表明两者基本呈线性关系;对各喷嘴的喷雾特性进行测定,得出喷嘴总流量与泵压的关系为:喷嘴的总流量约与喷嘴入出口两端压差的开平方成正比;对喷嘴的中心流量密度进行测定,寻找到各喷嘴喷雾相对最均匀的工作压力。应用不同DANFOSS喷嘴对水平热源进行垂直喷射,在变换喷嘴距热源表面高度的前提下,研究最优换热性能出现在何种情况(最优高度),得出H准则:当喷雾形成的液膜的圆形外推区域正好与热源边缘相切时,系统换热性能达到最佳。选择两个喷嘴对方形热源和圆形热源进行倾斜喷射实验,结果认为倾斜喷射的研究重点应该是解决由此引起的表面温度梯度增大问题(并且喷射倾角最好不要太大),而对系统换热性能的影响是次要问题。根据间歇喷射的概念及其研究意义,提出动态喷射的概念,确定了电磁驱动作为驱动喷嘴实现动态喷射的驱动方式。阐述了电磁驱动器件中吸合特性的概念、研究现状及其分析方法。主要针对两种不同结构形式的电磁驱动器件,在磁动势控制模型基础上,分析了它们的准静态、阶跃磁动势和调制阶跃磁动势驱动的吸合特性。更多还原

【Abstract】 With the heat flux of electronic devices becoming higher and higher, it has been an important problem to discharge the heat produced, which could decrease the reliability of the devices. As one of the three promising solutions for cooling of high heat flux applications, spray cooling technology has received much attention and been widely studied in recent years. Much experimental work has been done on spray cooling. However, the experimental results do not always agree with each another, and theoretical understandings on spray cooling are still at their early stage due to the intrinsic complexity of the mechanisms involved.Based on the state of the art of spray cooling, the following topics have been studied:As to a cone-axisymmetric spray cooling systems, the spray area was divided, and the slip-flow boundary condition was introduced. Based on an assumption of the coniform spray, an equivalent continuum model of the average spray density was established. Based on the variable separation method (VSM), the governing equations for the film thickness problem have been solved. A reasonable consistent condition of the momentum equations has been found and solved. Based on the methods ofmathematical physics, series expansion solution of the velocity filed in the film has been derived. A boundary condition at the boundary of the spray area was given and the velocity slip phenomenon has been considered. The undeterminde coefficients were derived, and the functions of the film thickness and the fluid field of the film within the range of the impact area have been established. Simulation results indicate that: 1. if the spray velocity uniformly distributes in the area, the film is even, while if the spray velocity distributes unevenly, the film can be either central-raised or central-cupped; 2. the structure of the liquid film could be approximated as a two-layer liquid layer. The velocity of the liquid in bottom layer is lager than that in the top layer, and there are two methods for a thinner liquid film: increase spray velocity and decrease spray density; 3. the slip-flow boundary condition has little effect on the film thickness and fluid, which could be ignored as a result.A reasonable CHF (critical heat flux) criterion was analyzed and ascertained, which states that the maximum CHF can be achieved when the spray is configured on the condition that the spray impact area inscribes the square test surface. The tracks of nozzles with different cone angles were studied with different inclination angles, based on the CHF criterion aforementioned. When the nozzle is a rectangular one, there are two kinds of inclination modes, namely, the axis-incline and the diagonal-incline modes. Simulation results indicate that, circular nozzles are better if the heated surface is a circular one because of its larger coverage area. Rectangular nozzles are better when the spray cooling surface is a rectangular one, and the axis-incline model is better than the diagonal-incline model when there is an inclination.Based on the data obtained by Visaria et al in their work, a new CHF model was established and new expression of CHF value was derived based on the nozzle track model. The effects of the spray inclination angle on CHF were predicted. Simulation results indicate that CHF increases slightly as the inclination angle increases.An experimental system was constructed, which includes nozzles, a heater, a pump and thermocouples. The characteristics (the droplet speed and diameter) of the system were estimated by using correlative theory considering the effect of the drag force.The relationships of the pressure drop across the three DANFOSS nozzles and the pressure in the outlet of the pump were conducted, linear relations were derived. A separate study was conducted to find the relationships of the total volumetric flow rate Q and the pressure drop across the nozzles ?P. It shows that the experimental total volumetric flow rates are well approximated by a ?P0.5 power law. More studies were conducted to find the special volumetric flow rate at the spray cone centerline, and then get the best working conditions of the three nozzles.Different commercial pressurized full cone DANFOSS nozzles were used to spray onto a 30×30mm2 square copper heated horizontal surface without inclination angle, and the optimal distance between the nozzles and the heated surface were achieved. An optimal heat transfer criterion (called H criterion) was proposed, which means that the optimal heat transfer appears when the region outside the impellent thin spray film inscribes in the square heated surface.Based on the H criterion aforementioned, two DANFOSS nozzles of the three with inclined spraying angles were used to experimentally study the temperature distribution in the heated surface. Distilled water was used to spray on the square surface afore- mentioned and a heated circular copper surface with diameter of 30mm, respectively. The results indicate that the increasing grad of the surface temperature is more important than the decreasing heat transfer capability while there are inclination angles.A new concept called dynamic spray cooling was proposed based on the concept of intermittent spray cooling. Electromagnetic actuated devices were chosen for a dynamic spray cooling system. The pull-in phenomena and its state of the art of the electromagnetic actuated devices were discussed. Based on the classification of the electromagnetic actuated devices, the quasi-static, dynamic, and modulated dynamic pull-in characteristics for parallel-plate and torsional magnetostatic devices with step input magneto motive force were analyzed.更多还原