含多孔骨架储能材料的相变传热特性研究

发布时间：2018-12-09 21:24

【摘要】：在能源危机的时代背景下,作为传统能源的化石燃料面临不可再生的消耗,人们开始考虑新能源的开发与利用,建筑节能已成为广泛关注的话题,相变储能技术应运而生。通过在室内安装相变储能材料的蓄热设备,或在墙壁、门窗和地板中加入相变储能材料,利用其相变过程储存与释放热量的特性有效降低建筑物室内温度波动,提高舒适性。其中固液相变储能材料的应用受到广泛重视,对固液相变过程特性的研究成为建筑节能领域的重要方向。本文主要以石蜡为研究介质,采用实验研究和数值模拟方法相结合的方法,深入研究了石蜡相变的过程特性和换热机理。首先通过DSC(差示扫描量热法)和Hotdisk技术测试石蜡的相变温度、相变潜热值、导热系数和比热容等物性参数;以此为基础,搭建了固液相变实验台,实验研究含多孔骨架石蜡的融化过程及其特性,通过CCD相机观测相变过程,重点关注相界面的形成和迁移;通过在多孔骨架中埋设布置微型热电偶来定量测量其温度分布,研究在不同的加热功率下石蜡的相变过程特性。实验结果表明,石蜡在蓄热过程中等温线在边界处和固液相界面处最为密集,温度梯度大。沿热流方向温度逐渐降低,随着蓄热过程的进行,最终趋于一致。沿高度方向上温度变化趋势大致相同,随着高度增加温度增大,随着蓄热过程的进行最终趋于平稳。增加热流密度可以加快蓄热材料的相变传热过程,但相变响应时间并不与其成正比。蓄热材料的物性会影响相变换热的过程,比热容影响其液化前的升温速率,比热容越小,升温速率越大;相变潜热影响融化阶段的升温速率,潜热愈大,潜热阶段温度变化愈平稳。为了与实验结果相互验证并得到本实验目前无法测得的相关结论,如速度场分布与液相率的变化等,进一步研究蓄热材料的相变换热机理,本文建立了数值模拟计算模型,采用了金属铝和尼龙两种骨架材料,研究了在蓄热材料中不同物性的多孔骨架对其传热和流动的影响。结果表明含金属铝骨架的石蜡导热性能良好,温度和速度变化最为均匀,腔体内加入金属铝骨架使得相变材料在各方向上温度变化更缓和,骨架的存在强化了导热,减弱了自然对流换热;相变过程的流线分布在腔体边界处和固液相界面处最密集;含尼孔骨架的腔体石蜡固液相界面由于尼龙材料的低导热率而呈现阶梯状,单介质石蜡呈现为一光滑曲线,含金属骨架的石蜡相界面近似为一条倾斜的直线。通过模拟结果与实验结果的相互对比,验证结果吻合良好。
[Abstract]:Under the background of energy crisis, fossil fuels, as traditional energy sources, are faced with non-renewable consumption. People begin to consider the development and utilization of new energy sources. Energy saving in buildings has become a topic of widespread concern, and phase change energy storage technology has emerged as the times require. By installing thermal storage equipment of phase change energy storage material indoors, or adding phase change energy storage material to walls, doors, windows and floors, the characteristics of heat storage and heat release during phase transformation process are used to effectively reduce indoor temperature fluctuation and improve comfort of buildings. Among them, the application of solid-liquid phase change energy storage materials has been paid more and more attention, and the study of the characteristics of solid-liquid phase change process has become an important direction in the field of building energy conservation. In this paper, the process characteristics and heat transfer mechanism of paraffin phase transition are studied by using the method of experiment and numerical simulation. First, the phase change temperature, latent heat value, thermal conductivity and specific heat capacity of paraffin wax were measured by DSC (differential scanning calorimetry) and Hotdisk technique. On this basis, a solid-liquid phase transition experimental platform was built, the melting process and characteristics of paraffin with porous skeleton were studied experimentally. The phase transition process was observed by CCD camera, focusing on the formation and migration of phase interface. The temperature distribution of micro-thermocouple embedded in the porous skeleton was measured quantitatively and the phase transition characteristics of paraffin under different heating power were studied. The experimental results show that the isotherm of paraffin wax is the most dense at the boundary and the solid-liquid interface during the process of heat storage, and the temperature gradient is large. The temperature decreases gradually along the direction of heat flux, and eventually tends to be consistent with the process of heat storage. The variation trend of temperature along the direction of height is approximately the same, and the temperature increases with the increase of height, and finally becomes stable with the process of heat storage. Increasing heat flux can accelerate the phase change heat transfer process of regenerative materials, but the phase transition response time is not directly proportional to it. The physical properties of heat storage materials will affect the process of phase change heat transfer, and the specific heat capacity will affect the heating rate before liquefaction. The smaller the specific heat capacity is, the greater the heating rate is. The latent heat of phase change affects the heating rate of melting stage. The bigger the latent heat is, the more stable the change of temperature in latent heat stage is. In order to verify the experimental results and obtain some conclusions, such as the distribution of velocity field and the change of liquid phase rate, the mechanism of phase change heat transfer of heat storage materials is further studied. A numerical simulation model is established in this paper. The effects of porous frameworks with different physical properties on heat transfer and flow in thermal storage materials were studied by using two kinds of framework materials, aluminum and nylon. The results show that the thermal conductivity of paraffin wax containing metal aluminum skeleton is good, and the temperature and velocity change is the most uniform. The temperature change of phase change material is more moderate with the addition of metal aluminum skeleton in the cavity, and the thermal conductivity is strengthened by the existence of framework. The natural convection heat transfer is weakened; The streamline distribution of the phase transition process is most dense at the boundary of the cavity and the solid-liquid interface. Because of the low thermal conductivity of nylon material, the solid liquid phase interface of cavity paraffin with nylon pore skeleton presents a ladder shape, the paraffin in single medium presents a smooth curve, and the interface of paraffin phase with metal skeleton is approximately an inclined straight line. The comparison between the simulation results and the experimental results shows that the results are in good agreement with each other.
【学位授予单位】：山东建筑大学
【学位级别】：硕士
【学位授予年份】：2017
【分类号】：TU59;TU201.5

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