【作者】 陶毓伽；

【导师】 淮秀兰；

【作者基本信息】 中国科学院研究生院（工程热物理研究所） ， 工程热物理， 2010， 博士

【摘要】 大功率固体激光器在工业、农业、国防军事及现代高新技术等众多领域具有广阔应用前景。随着功率水平不断提高及装置尺寸日益小型化,激光器热效应成为制约其输出功率与性能进一步提高的严重障碍,是技术发展的主要瓶颈之一。迫切需要在有限空间内及时消除因功率耗散所转化的废热,有效解决散热冷却与温度控制问题。本文围绕大功率固体激光器热管理,通过实验研究与数值模拟相结合的方法,分别对激光工作物质外部高效雾化喷射冷却与内部超常热量传递过程进行了系统深入研究,为有效解决大功率固体激光器的散热冷却问题提供重要实验与理论指导。针对激光工作物质外部高效散热冷却,建立了双喷嘴无沸腾雾化喷射冷却可视化实验观测平台,采用先进高倍数显微放大高速摄影系统对雾化喷射过程进行了详细可视化观测,详细研究了各因素对冷却效果的影响规律。实验结果表明,雾化喷射冷却在低表面温度下具有很好的换热能力,可以满足大功率固体激光器及电子元器件类低表面温度、高热流密度的冷却需要；换热过程中单相对流传热和薄液膜蒸发起主导作用,雾化液滴直径约为500μm。。喷射流量与距离、冷却工质进口温度等是影响换热冷却效果的主要因素,喷射流量增大可明显改善换热效果,提高换热系数；喷射距离增大,换热系数先增大后减小,存在最佳喷射距离使冷却效果最佳；随冷却工质进口温度降低,热表面温度下降,换热系数增加；提出在工质中添加适当表面活性剂来进一步提高换热能力与降低热表面温度,实验发现表面活性剂浓度对雾化喷射冷却换热效果有显著影响；在大量实验研究基础上,得出实验关联式；揭示了无沸腾雾化喷射冷却的特殊流动与传热机理,得出提高大功率固体激光器及电子元器件类散热冷却效果与消除／减小热效应的有效措施与方法。建立了雾化喷射过程三维几何模型,采用基于欧拉—拉格朗日法的离散相颗粒模型对喷雾腔内流场与温度场进行了详细的数值模拟研究,揭示了其复杂流动与雾化过程特性。结果表明,雾化压力是实现雾化的主要因素；雾化后液滴粒径及其在热表面区域内分布较为均匀,液滴碰壁速度从喷雾中心到边缘逐渐降低；随质量流量的增大,液滴碰壁平均速度增大,相应热表面上的液膜层最大厚度随之减小；当喷射高度变化时,液膜层厚度变化不规则,热表面温度分布趋向均匀,当液膜层厚度最小时相应热表面温度最低。因此在确定具体的喷射距离时,要综合考虑热表面温度及其分布均匀性。建立了热壁面薄液膜层内两相流动与换热物理数学模型,综合考虑了气液界面相变、流体重力、表面张力和粘性等因素的影响,对薄液膜层上方有无液滴坠落两种情况下液膜层内气泡运动、相界面变化及壁面换热效果进行了详细数值模拟,并对液滴下降速度、液滴直径与初始位置、多液滴碰撞对液膜层内流动与换热的影响进行了详细模拟分析,揭示了液滴与薄液膜层相互作用机制以及薄液膜层内复杂多相流动与冷却换热过程。结果表明,液滴对液膜层碰撞既可加快液膜层内气泡变形速率,又会引起液膜层内二次核化,显著增强热壁面的换热效果；随着液滴下降初始速度增大,壁面换热系数峰值显著增加且逐渐向左侧移动；存在最佳液滴初始直径,使壁面换热效果最优；液滴初始位置分布对壁面换热效果也有重要影响,当液滴碰撞中心点发生在气泡易于变形处时壁面换热系数最大；多液滴碰撞的壁面换热效果明显优于单液滴碰撞,壁面换热系数分布根据液滴数和碰撞中心点出现不同的波峰。因此,可以通过控制雾化后的液滴参数合理设计整个喷雾冷却系统,实现其在低壁面过热度下达到最好的冷却效果以及冷却均匀。针对大功率固体激光器工作物质内部热量传递过程,考虑了连续与脉冲固体激光器实际工作中的超常规传热现象,首次建立了激光工作物质内部传热的一维非稳态非傅立叶导热模型,采用有限差分法进行了详细数值模拟计算,揭示了极端条件下工作物质的超常传热机理及不同泵浦功率密度、脉冲宽度及表面换热系数下工作物质的温度场分布,并与经典的傅立叶模型计算结果进行了对比分析。结果表明,当泵浦功率密度大于104W/m2时,在泵浦瞬间应考虑激光工作物质的非傅立叶效应。随着泵浦功率的增大,非傅立叶效应明显增强；随脉冲宽度的增加,非傅立叶效应明显减弱。另一方面,增大激光工作物质尺寸可使泵浦功率密度下降,也有利于降低泵浦面温度。热松弛时间对非傅立叶效应的影响较大,随热松弛时间增大,非傅立叶效应增强。该工作为进一步深入研究奠定了重要理论基础与新的研究思路和方法,同时拓宽了非傅立叶导热定律的应用范围。更多还原

【Abstract】 The high-power solid-state lasers have a good prospect of applications in many fields such as the industry, the agriculture, the national defense and military, and the modern high technology. With the development of the laser output and the packages being more compact in size, the thermal effects become the major barriers obstructing the technological development in high-power solid-state laser and the traditional cooling methods are reaching their limits. In this dissertation, the high-effective spray cooling method outside the laser medium and the non-classical heat transfer in the laser medium were studied systematically and deeply. Both experiments and numerical simulations were carried out, which provides important experimental and theoretical instructions for resolving the problem of thermal energy removal of the high-power solid-state lasers.In order to understand the non-boiling heat transfer behavior of the spray cooling, an experimental setup with two full cone spray nozzles was established using pure water as the working fluid. A microscopic lens system in conjunction with an advanced high-speed camera was used to observe the spray process. The influences of the liquid flux, the spacing between the nozzles and the heated surface and the inlet temperature of the liquid on the cooling effect were investigated. It is found that the non-boiling spray cooling method can remove high heat flux from surfaces while maintaining low surface temperature, which satisfies the cooling requirements of the high-power solid-state lasers and the electronic components. The flow and heat transfer mechanism is believed to consist of convective heat transfer and direct evaporation from the surface of the liquid film. The droplet diameter is about 500μm. It is concluded that increasing the liquid flux increases the overall heat transfer coefficient and enhances the cooling effect distinctly. When increasing the space between the nozzles and the heated surface, the heat transfer coefficient increases first and then reduces. So there exists an optimal spacing in the experiment. With the reducing of the inlet temperature of the liquid, the surface temperature decreases and the heat transfer coefficient increases. The thermal performance can be improved further by adding proper surfactant. The concentration of surfactant plays an important role in spray cooling. A heat transfer correlation about Nu was developed on the basis of large amounts of experiments. The effective method to promote heat transfer and reducing the thermal stress is proposed.A three-dimensional geometric model was developed and the numerical simulations have been carried out to investigate the flow of the liquid pressure atomization and spray process using the Discrete Phase Model which follows the Euler-Lagrange approach combined with the Wall-Film boundary conditions. The complicated flow and atomization process was discovered. It shows that the spray pressure is the main factor to realize the atomization. After atomization, the droplet is basically uniform in the diameter and was also distributed on the heated surface uniformly. The droplet collision velocity reduces from the center to the edge. Increasing the liquid mass flux can increase the average droplet collision velocity, but the corresponding maximum film thickness on the heated surface declines. The film thickness changed irregularly with the increase of the spray distance, however, the temperature distribution tends to be uniform and the surface temperature happened to be the lowest where the corresponding film thickness was the least. Hence, both the surface temperature and its uniformity should be considered when confirming the best spray distance.A mathematical model was developed to investigate the details of two-phase flow and heat transfer in a thin liquid film. The model considers the effects of phase change between vapor and liquid, gravity, surface tension and viscosity. The dynamics of bubble growth in the film, the movement of the interface between two fluids and the surface heat transfer characteristics were numerically simulated for two cases:(1) when a liquid droplet impacts a thin liquid film with vapor bubble growing and (2) when the vapor bubble grows and merges with the vapor layer above the liquid film without droplet impacting. The influences of droplet falling velocity, droplet diameter and initial position, the multi-droplets impact on the flow and heat transfer were discussed. The interaction mechanism between the droplet and the thin film, the complicated multiphase flow and heat transfer characteristics were revealed. It is found that the droplet impact can improve the surface heat transfer notably because it can quicken the distortion speed of bubble, and leads to the secondary nuclei in the film. The peak value of surface heat transfer coefficient increases with the increase of the droplet velocity and meanwhile the position of the peak value moves left gradually. The heat transfer coefficient does not increase linearly with the increase of the diameter. The initial position of the droplet plays an important role in the heat transfer. When the droplet impacts the right side of the bubble where the collision leads more acute disturbing to the thin liquid film and promotes the bubble distortion speed, the surface heat transfer coefficient is largest. The heat transfer due to multi-droplets impact exceeds the case of single droplet impact significantly when the diameter and the falling speed are same. The distinct peak value of the surface heat transfer coefficient would appear according to the difference of the droplet number and the impact center. The surface heat transfer coefficient will tend to be uniform if the number of the droplets is enough. The above conclusions provide the theoretical basis on designing the spray cooling system by controlling the droplet parameters, which can realize the best cooling effect under low surface superheat.Considering the non-classical heat transfer phenomena in the process of series or impulse solid-state lasers, a one-dimensional unsteady non-Fourier heat conduction model with non-uniform heat source was developed to simulate the transient heat transfer in the laser medium under extreme conditions for the first time. The temperature fields were numerically analyzed using the finite difference method combined with the TDMA algorithm for different pump power densities, pulse durations, thermal relaxation time and cooling intensities, respectively. The calculated results are compared with those predicted by the classical Fourier heat conduction law. The results indicate that the non-classical heat transfer phenomenon of laser medium should be considered at the moment of pumping when the pump power density is more than 104W/m2. The larger the pump power density, the more apparent the non-Fourier effects. The longer the pulse duration, the less significant the non-Fourier effects in the laser medium. At the same time, increasing the size of the laser medium can reduce the pump power density, which also is helpful to debase the maximum temperature. The thermal relaxation time is a crucial factor to determine whether the non-classical heat transfer behavior is significant or not. The non-Fourier effects will be improved as the increase of the thermal relaxation time. So it is necessary to further confirm this value. This study provides an important theoretical basis and a new research method. Also, it expands the applications of the Non-Fourier heat transfer law.更多还原