【作者】 张震；

【导师】 姜培学；

【作者基本信息】 清华大学 ， 动力工程及工程热物理， 2013， 博士

【摘要】 近年来电子系统、高能激光系统和空间卫星等设备因功率的增加对散热能力的要求越来越高。喷雾冷却以其散热能力强、温度均匀性好、启动系统可靠和稳定时间长备受关注，并已经在汽车行业、功率逆变器等设备中得到了应用。本文首先通过实验对比了连续和间歇这两种不同形式的喷雾冷却的换热效果，得出两种喷雾形式的液滴参数相近，若热流密度很高，连续喷雾的换热性能较高，优于间歇喷雾，若热流密度较低，间隙喷雾在无沸腾区对工质的利用更有效，优于连续喷雾，并分析了喷射周期、喷射占空比和喷停时间对间歇喷雾换热性能的影响。随后实验研究了光滑表面、纳米表面、微米表面以及微纳米组合表面上喷雾冷却的换热情况，推导出二维导热问题的理论解，观测到不同热流密度下的换热图像，结合Particle Master Shadow系统测量的液滴参数，分析了喷雾冷却的换热规律。研究发现，在薄膜区和部分蒸干区，微米、纳米表面及组合表面的换热效果均优于光滑表面；对应于一种喷雾，存在最优微结构尺寸，使得微结构表面增大的换热面积在换热中能够被充分利用，换热效果最优；在微结构上表面增加碳纳米管膜对于微结构尺寸小于液滴直径的表面更有效，且对于有微纳结构的硅表面，在沸腾区当功率在很大范围内增大时，表面温度增加很小，且温度的波动很小。由于喷雾冷却的换热效果与液滴参数有关，且液滴多为微米量级，为了从微观上分析液滴特性对喷雾冷却换热的影响，应用VOF模型，使用静态接触角、实验动态接触角、Kistler模型和Blake模型四种方法来处理接触角，模拟了单个微米液滴冲击平板的流动情况。计算发现采用Blake模型得到的结果与已有文献的实验结果符合最好，指出在计算中根据铺展速度直接计算出并作为条件赋值的角度并不一定等于视在接触角，也分析了液滴冲击过程中铺展直径、接触角的变化，液滴受力情况及液滴参数对铺展的影响。随之采用三维模型，使用动态接触角方法模拟了单颗微米液滴冲击光滑表面、与液滴直径尺寸相当的微米级矩形凹槽和方柱结构表面的流动，分析了液滴的受力及表面不同位置的润湿情况，发现方柱结构更利于液滴对其底表面的润湿。更多还原

【Abstract】 Numerous industrial applications require the removal of high heat fluxes, such aselectronic systems, high-power lasers, energy weapons and aerospace satellite. Spraycooling is one of the best solutions due to its high heat dissipating capability with lowcoolant mass fluxes at low wall superheats, precise temperature control, low cost andreliable long-term stability. Spray cooling has been used in hybrid vehicle electronics,power inverter modules, laser surgery and so on.Continual and intermittent spray cooling heat transfer experiments on a flat surfacewere performed and the spray droplet parameters were similar. The results show that ifthe heat flux is quite high, the continual spray cooling is better than the intermittentspray cooling since it has a higher heat transfer capability and the effective heat transfercoefficient is equivalent to the intermittent spray cooling in the boiling region. If theheat flux is not high, however, intermittent spray cooling should be used since it is moreeffective than continual spray cooling in the non-boiling region. The effects of the spraycycle, duty ratio and the spray or non-spray time were studied to make more efficientuse of the coolant in intermittent spray cooling.The spray characteristics and the differences between spray cooling on smooth andenhanced silicon surfaces with micro-, nano-and hybrid structures was investigated.The shadowgraph technique was used to measure the spray droplet parameters andpictures were got by a camera during the heat transfer process. The two-dimensionalheat conduction analytical equations were derived by local thermal non-equilibriummodel. The spray cooling experiments show that the heat transfer for the enhancedsurfaces was much greater in the thin film and partial dryout regions than for the smoothsurface. There is an optimal groove depth corresponding to a given droplet parameter,groove width and stud size for the liquid to completely take advantage of the areaenlarged by the micro-structures and enhance the heat transfer. Adding carbon nanotubes on the top surfaces of the micro-structured surfaces was more effective as themicro-structure sizes were smaller than the droplet diameter. The wall temperaturedifference and the temperature fluctuations were both small for a wide range of powers.The spray cooling heat transfer is mainly influenced by the droplet parameters. A numerical simulation on the micrometer-sized droplet impinging on a flat, unheatedsurface has been investigated based on the spray cooling technology, using the VOFmodel with static contact angle, experimental dynamic contact angle, Kistler model andBlake model for dynamic contact angles. The results show that the simulation resultswith the Blake model agree well with the experimental results and the contact anglecalculated by the spreading velocity and set as the boundary condition in simulationmight not be equal to the apparent contact angle. The development of the spreadvelocity and the the contact angle was described and the droplet parameter effects wereanalyzed by changing the droplet diameters and the velocities.A micrometer-sized droplet impinging on enhanced surfaces with micro-grooves ormicro-studs whose size were the same order with the droplet was simulated bythree-dimensional model using dynamic contact angle. The surface area wetted by fluidat various positions was calculated and the results show that the surfaces withmicro-studs were superior for the bottom surface wetted by the droplet.更多还原