球形容器内纳米复合相变材料凝固传热过程的定量研究

发布时间：2018-09-17 14:10

【摘要】：热能作为生产生活中最普遍且可以直接利用的能源形式之一,常常伴随着各种能源转换和利用过程。热能存储可以有效解决能源在时间和空间上的供求不匹配,因此成为了提高能源利用效率的关键手段。相变储热是利用相变材料在物态变化的过程中,吸收或释放大量潜热而进行的,具有储能密度高、吸放热过程中温度几乎不变等优点。常用的有机相变材料虽然具有较大的比热容和相变潜热,但是其导热系数低、换热性能差等缺点成为了制约实际应用的主要因素。为了提高潜热储能系统的换热效率,在相变材料中添加具有高导热系数的纳米颗粒,是一种常见的强化传热手段。在实际应用中,球形容器常被用于堆积床式潜热储能系统的储热单元。为了研究堆积床式的潜热储能系统中能量的吸收和释放,研究单个球形容器内的熔化和凝固换热过程则显得尤其重要。因此,本文针对相变材料向内凝固过程难以实现可视化的问题,提出了一种基于体积收缩定量研究球形容器内相变材料凝固换热过程的实验方法,同时通过制备不同质量分数的纳米复合相变材料,研究了高导热系数的纳米颗粒对相变材料的强化传热影响。本文以十四醇(熔点37℃)为相变材料,采用"两步法"制备了石墨烯纳米片质量分数分别为0%、0.5%、1.0%、3.0%的复合相变材料,并对纳米复合相变材料的热物性,包括导热系数、相变焓、相变温度、比热容、密度以及粘度等进行了测试。本文提出的间接实验方法,通过在球形容器内部插入下降管,将凝固过程中体积收缩产生的空隙转化为下降管内液态相变材料高度的变化,从而进一步计算相变材料的瞬态液相质量分数。实验结果显示,三组平行试验的温度曲线最大实验偏差百分比约为3.7%,下降高度的相对偏差不超过6.0%,说明实验重复性良好。同时,将实验测试的的总下降高度与基于固液相密度差的理论高度进行对比,相对偏差在10%-15%;将液相质量分数曲线拟合成基于FoSte的无量纲关联式,考虑到材料、几何尺寸以及工况的不同,实验曲线与前人的理论曲线趋势一致,说明实验具有良好的可靠性。本文采用体积收缩法,还进一步研究了纳米颗粒对球形容器内相变材料凝固换热过程的影响。研究结果表明,球形容器内纳米复合相变材料的总凝固时间随着纳米颗粒质量分数的增加而减少,这是因为添加纳米颗粒增强了复合相变材料的导热系数。此外,三种复合材料的液相质量分数曲线存在交叉点,这可能是因为液体相变材料的粘度随着质量分数的添加而大幅度提高,导致相变材料流动性下降,延缓了下降管内液体高度的变化。因此,更高质量分数样品的下降高度在实验初期存在时间滞后,导致初始凝固速率被低估。对实验数据进行拟合,得出了液相质量分数和Nu基于FoSte的无量纲关联式,对工程应用具有一定的指导意义。
[Abstract]:Thermal energy, as one of the most common and direct energy forms in production and life, is often accompanied by various energy conversion and utilization processes. Thermal energy storage can effectively solve the mismatch of energy supply and demand in time and space, so it has become a key means to improve energy efficiency. In the process of state change, a large amount of latent heat is absorbed or released, which has the advantages of high energy storage density and almost constant temperature in the process of heat absorption and release. In order to improve the heat transfer efficiency of latent heat storage system, adding nanoparticles with high thermal conductivity into the phase change material is a common means to enhance heat transfer. Therefore, in view of the difficulty in visualizing the inward solidification process of PCM, an experimental method based on volume shrinkage is proposed to quantitatively study the solidification heat transfer process of PCM in a spherical vessel, and different mass is prepared. The effect of nano-particles with high thermal conductivity on the heat transfer enhancement of phase change materials was studied. The composite phase change materials with 0%, 0.5%, 1.0% and 3.0% graphene Nano-sheets were prepared by two-step method using tetradecanol (melting point 37 C) as phase change materials. Physical properties, including thermal conductivity, phase change enthalpy, phase change temperature, specific heat capacity, density and viscosity, were measured. The indirect experimental method proposed in this paper is to convert the void caused by volume shrinkage into the change of the height of liquid phase change material in the falling tube by inserting a falling tube into the spherical vessel. The experimental results show that the maximum deviation percentage of the temperature curves is about 3.7%, and the relative deviation of the drop height is less than 6.0%. The experimental repeatability is good. For the deviation between 10% and 15%, the liquid phase mass fraction curve is fitted into a dimensionless FoSte-based correlation. Considering the different materials, geometric sizes and working conditions, the experimental curves are consistent with the previous theoretical curves, indicating that the experiment has good reliability. The results show that the total solidification time of nanocomposite phase change materials in spherical vessels decreases with the increase of nano-particle mass fraction, which is attributed to the enhancement of thermal conductivity of composite phase change materials by adding nano-particles. At the crossing point, this may be because the viscosity of the liquid phase change material increases greatly with the addition of the mass fraction, resulting in the decrease of the flowability of the phase change material and delaying the change of the liquid height in the descending tube. Fitting the experimental data, the dimensionless correlation between the liquid mass fraction and Nu based on FoSte is obtained, which has a certain guiding significance for engineering application.
【学位授予单位】：浙江大学
【学位级别】：硕士
【学位授予年份】：2017
【分类号】：TB34

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