【作者】 侯燕；

【导师】 淮秀兰； 陶毓伽；

【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， 2014， 博士

【摘要】 随着各行业对冷却技术要求的不断提高,喷雾冷却应运而生。在过去30多年的研究中,国内外学者将大量精力放在单喷嘴喷雾冷却影响因素的实验研究上。近几年,由于多喷嘴喷雾冷却散热更均匀,散热热流密度更高,围绕多喷嘴的实验研究逐渐增加。已有的关于多喷嘴喷雾冷却的研究,以喷嘴数目和喷射参数对雾化特性和换热效果影响的实验研究居多,对多喷嘴喷雾冷却过程中液滴喷射与壁面换热相结合的整体理论研究较少,对喷雾冷却强化换热机理的认识不明确。为此,本文以多喷嘴喷雾冷却为研究对象,首先采用商业软件对液滴喷射和壁面换热整体过程进行了整体数值模拟,系统研究了不同喷嘴数目和工况下的雾化特性和壁面换热效果,得到了壁面过热度、进口压力、进口流量、喷嘴高度、喷嘴数目等对壁面散热热流密度大小和分布的影响规律；揭示了不同工况下多喷嘴喷雾冷却雾化特性,为多喷嘴喷雾冷却优化设计和强化换热机理的研究提供了可靠数据。模拟结果表明最优喷射高度为喷雾直接冲击区与被加热表面外切时对应的值,随着喷嘴数目的增加,壁面平均热流密度增加,然而增加幅度减小。模拟工况下,最优喷嘴数目为8。喷雾冷却过程中会在壁面上形成一层薄液膜层,薄液膜内复杂流动和换热过程直接影响了最终换热效果。为了对多喷嘴喷雾冷却壁面薄液膜内的复杂流动和换热过程进行详细研究,本文采用高速摄影仪和显微放大镜相结合,系统拍摄观察了多喷嘴喷雾冷却热壁面液膜层的形成和流动,利用图片灰度值矩阵定量分析了不同工况下液膜形成过程、液膜厚度和波动特性,分析了单个气泡周期内气泡形变对当地液膜厚度的影响。实验结果表明随着韦伯数的增加,无量纲平均液膜厚度总体上呈现减小的趋势,薄液膜表面波长和波幅也呈减小的趋势。随着表面温度的增加,无量纲液膜厚度先迅速增加,然后缓慢变化,最后又呈增加的趋势,壁面薄液膜表面波长和波幅随壁面温度的增加没有明显的变化。气泡生长和上升过程的当地液膜厚度比气泡形成和聚集过程更厚。首次发现不同壁面温度下,薄液膜形成过程中当地液膜厚度的平均值存在峰值点。同时本文还采用数值方法研究了薄液膜内的流场和温度场分布,建立了气液两相流数学模型,采用VOF方法捕捉气-液界面,研究了不同液滴初始状态下薄液膜内气泡的形变过程和流动换热特点,分析了连续过冷液滴非正对撞击薄液膜强化换热机理和液滴各参数对气泡形变的影响规律,得到了不同情况下薄液膜内热边界层和气泡的变化规律。结果表明过冷液滴的撞击使壁面附近温差变大,壁面上薄液膜厚度和热边界层大大减小从而强化了对流换热；同时液滴的非正对连续撞击使气泡提前脱离壁面,减小了壁面干烧面积,强化了换热。另外,学者们并不满足于多喷嘴喷雾冷却目前的散热能力,正在寻求有效的方法强化换热能力。将壁面微结构和多喷嘴喷雾冷却相结合就是一种新方法。本文实验研究了多种尺寸和形状的微结构表面下,多喷嘴喷雾冷却换热效果和壁面温度分布均匀性,得到了不同工况下各微结构表面的换热特性曲线、壁面换热系数和温度分布,并与平板固体壁面实验结果进行了对比,揭示微结构表面强化换热的机理,在此基础上提出了表征微结构表面强化换热的无量纲数。更多还原

【Abstract】 With the continuous improvement of cooling technology requirements of the industry, the spray cooling emerges as the times require. In the past30years, domestic and foreign scholars focus more on the experimental study on the influence factors of single nozzle spray cooling. In recent years, due to the more uniform temperature distribution and higher heat flux, experimental study on multi nozzle spray cooling increases. The experimental studies on the effects of the nozzle number and injection parameters on the atomization and heat transfer characteristics are a lot, but the overall theoretical research combined jet and wall heat transfer is less, and the understanding of heat transfer enhancement is not clear.This paper studied atomization and heat transfer characteristics with different nozzle numbers and working conditions using commercial software, obtained the effects of the wall superheat, inlet pressure, inlet flow rate, nozzle height, nozzle number on the heat flux, reveals the atomization characteristics of multi nozzle spray cooling under different conditions, and provides reliable data for optimization of multi nozzle spray cooling and the study of heat transfer enhancement mechanism. The simulation result shows that the optimal injection height corresponding to the value when the spray directly impact zone cuts the heated surface. With the increasing of the nozzle number, averaged heat flux increases, but the increase in reducing. In this paper, the optimal nozzle number is eight.Spray cooling process will form a layer of thin film on the wall surface, thin liquid film in the complex flow and heat transfer process directly affect the final effect of heat transfer. In order to obtain the detailed liquid flow and heat transfer in thin liquid film. This paper uses a high-speed camera and microscope image capturing system to observe the formation and flow of multi nozzle spray cooling film layer, uses image gray value matrix to conduct quantitative analysis of the forming process, studies the film thickness and the fluctuation characteristics of liquid film under different conditions, analyzes the influence of bubble deformation on the local film thickness. The experimental results showed that with the increase of the Weber number, the dimensionless averaged film thickness has decreased, the surface wavelength and amplitude was decreased. With the increase of the surface temperature, the dimensionless film thickness increased rapidly at first, and then slowly change, and finally increased, thin film surface wavelength and amplitude had no obvious change. The local film thickness in bubble growth and rising process is thicker than that in the bubble formation and accumulation process. First discovered in different temperature, there is a local film thickness peak in the film forming process. At the same time, this paper studies the thin liquid film flow field and temperature field by numerical method. The mathematical model of gas-liquid two-phase flow, using the VOF method to capture the gas liquid interface, was used to study the deformation of bubbles and heat transfer characteristics of thin film. We analysed the heat transfer enhancement mechanism of the continuous subcooled droplets impacting on thin film and the influence of droplet parameters on bubble deformation, obtained the variation law of thin film thermal boundary layer and the bubble deformation under different conditions. The results show that the droplet impact makes the thicknesses of thin liquid film and thermal boundary layer are greatly reduced,the subcooled droplets reduces the temperature difference nearby the wall, both of which enhance convection. The initial droplet position and continuous impact makes the bubble detached from the wall ealier, and decreases the dry area of the wall.In addition, people are not satisfied with the spray cooling capacity at present, are seeking effective methods to enhance the spray cooling heat transfer. Combined the multi nozzle spray cooling and micro-structured surface is a new type of heat transfer enhancement technique. In order to study this method, with smooth surface as reference, this paper investigated the heat transfer enhancement and temperature distribution uniformity of micro-sturctrued surface with different sizes and shapes experimentally, obtained the heat transfer characteristic curves of the micro-structured surfaces, heat transfer coefficient and the temperature distribution under different conditions. A dimensionless number was proposed to characterize the heat transfer enhancement of micro-structured surfaces.更多还原