填充泡沫金属的相变蓄热单元管热性能研究

Thermal Performance Research on Unit Tube for Phase Change Heat Storage Embedded with Metal Foam

作者: 专业:化工过程机械 导师:崔海亭 年度:2011 学位:硕士  院校: 河北科技大学

Keywords

phase change thermal storage unit tube, FLUENT software, metal foam, enhance heat transfer, numerical simulation

        吸热蓄热器是空间站太阳能热动力发电系统(Space Solar Dynamic Power System——SSDPS)的关键部件,其采用的蓄热方式是高温相变蓄热。高温相变蓄热大都选用氟盐作为相变材料(Phase Change Material——PCM),由于固液密度不同,PCM在相变过程中会产生空穴,这严重影响了蓄热容器的传热性能及结构可靠性。为改善氟盐PCM带来的不良影响,本文采用在PCM中填充泡沫金属的措施,泡沫金属以其高导热、质轻、高比热等特点作为填充物,成为强化相变蓄热的有效手段,泡沫骨架在强化PCM导热的同时还能分散空穴分布,有效降低因空穴集中而产生的恶化PCM传热的影响。在航天、航空等领域有着广阔的应用前景。本文以太阳能热动力发电系统地面模拟试验中采用的吸热蓄热器内部的相变蓄热单元管为研究对象,选择氟盐80.5LiF-19.5CaF2作为蓄热材料,钴基合金Haynes188作为PCM容器及工质导管材料,并采用孔隙率为95%的多孔泡沫镍来研究单元管内部的强化传热情况。主要研究内容如下:(1)基于焓法,建立了相变蓄热单元管的数学模型,并考虑了由于空穴对相变蓄热带来的影响。利用FLUENT软件,通过写入用户自定义函数(UDF)设置周期性热流边界条件,对PCM容器的最高壁温、工质气体出口温度及PCM的熔化份额在5个轨道周期内随时间的变化值进行了数值计算,并且与一些文献进行了对比,证明了本文数学模型的正确性。(2)通过对填充与未填充泡沫金属的两种蓄热单元管的数值计算结果的比较和理论分析,说明采用泡沫金属确实能够在一定程度上强化单元管的蓄、放热效果。模拟结果表明传热温差越大,蓄放热效率越高,泡沫金属的添加不但加快了蓄放热速率,而且使整个蓄热容器里的温度分布更加均匀。(3)为保证PCM的利用率及泡沫金属的强化效果,必须合理选择工质的进口温度、流量及外壁热流。数值计算结果较好的预测了蓄热单元管的吸、放热性能,该热分析模型对空间站太阳能吸热—储热器的总体设计具有一定的借鉴作用。
    Heat Receiver is the key component of the space solar dynamic power systems (Space Solar Dynamic Power System——SSDPS), it adopts the method of high temperature phase change thermal storage, in which the molten salt is usually selected as PCM. However, solid density of PCM is different from its liquid density, which will cause cavity in the phase change process. The cavity will seriously affect the heat transfer performance and structure reliability of heat storage container. Therefore, PCM is encapsulated in many containers and metal foam is embedded in PCM to reduce the adverse effects of PCM. Metal foam, with its high thermal conductivity, light weight, high heat capacity and etc, is selected as a filling with PCM, and is becoming an effective way of strengthening the heat storage. Foam skeleton not only can enhance the PCM thermal conductivity, but also can disperse the cavity distribution and reduce the bad effects caused by the cavity concentration effectively. It will have broad application prospects in the field of aerospace, aviation and so on.The paper takes the phase change thermal storage unit tube used in Solar Dynamic Power System ground test as the research object. The molten salt LiF-CaF2 is selected as thermal storage material in heat exchange tube, Co-based alloys Haynes188 is selected as material of PCM container and working fluid tube, and the porous nickel foam of 95% porosity is selected to strengthen heat transfer. The following topics are studied:(1)A numerical model of the single-tube phase change heat storage system is developed, based on enthalpy method, and cavity problem is also considered. The change of maximum wall temperature of PCM canisters, gas outer temperature and liquid PCM fraction with time in 5 orbital periods have been numerically calculated by FLUENT, and the periodic heat flux boundary condition is set by writing user-defined function (UDF). In order to prove the correctness of the mathematical model of this paper, numerical results are compared with the result in some literature and the trends are in close agreement.(2) The results indicate that metal foam dose could enhance the heat storage effect of the unit tube by a certain extent. Numerical results show that the greater heat transfer temperature difference will result in higher heat efficiency. At the same time, metal foam not only increases the heat release rate, but also distributes the temperature of the heat storage containers.(3) Working fluid inlet temperature, flow and wall heat flux must be selected reasonably to ensure the utilization of PCM and effect of heat transfer enhancement using metal foam. The numerical result predicts the thermal performance of the unit tube well, it proves that the thermal analysis model has a certain reference to the overall design of space station solar heat receiver.
        

填充泡沫金属的相变蓄热单元管热性能研究

摘要4-5
Abstract5-6
第1章 绪论9-16
    1.1 课题研究背景和现实意义9
    1.2 国内外相变蓄热及强化的研究进展9-13
        1.2.1 加入多孔介质10-11
        1.2.2 在相变材料中分散高导热颗粒11-12
        1.2.3 安置金属结构12
        1.2.4 添加具有高导热率、低密度的填充材料12-13
    1.3 国内外对相变蓄热系统进行数值模拟的研究13-14
    1.4 存在的不足及待深入研究的问题14
    1.5 本课题的来源及主要研究内容14-16
第2章 相变蓄热理论及数值求解16-23
    2.1 相变传热的特点16
    2.2 相变传热问题的数学模型16-18
        2.2.1 温度法模型16-17
        2.2.2 焓法模型17-18
    2.3 相变传热的数值求解18-19
        2.3.1 数值求解方法18
        2.3.2 数值求解步骤18-19
    2.4 利用FLUENT 模拟相变问题的基本理论19-22
        2.4.1 FLUENT 简介19-20
        2.4.2 FLUENT 求解相变问题的融化/凝固模型20-22
    2.5 本章小结22-23
第3章 相变蓄热单元管的结构及选材23-29
    3.1 相变蓄热单元管的应用及结构23-24
        3.1.1 相变蓄热单元管的应用23-24
        3.1.2 相变蓄热单元管结构24
    3.2 相变材料及封装容器材料的选择24-25
    3.3 多孔泡沫金属基的选择25-27
        3.3.1 泡沫金属的结构特征25-26
        3.3.2 泡沫金属基体的选择26-27
    3.4 相变材料和基体的化学相容性研究27-28
    3.5 本章小结28-29
第4章 物理及数学模型的建立和处理29-40
    4.1 物理模型的建立29-30
    4.2 单元管数学模型分析30-31
    4.3 空穴传热模型31-32
    4.4 利用FLUENT 计算过程中参数的设置32-39
        4.4.1 模型的建立及网格划分32-34
        4.4.2 初始和边界条件34-35
        4.4.3 FLUENT 参数设置35-39
    4.5 本章小结39-40
第5章 数值模拟计算结果及分析40-58
    5.1 工质的不同进口温度的影响40-45
    5.2 工质的不同进口流量的影响45-49
    5.3 外壁不同热流的影响49-53
    5.4 温度场及液相率分布云图53-57
    5.5 本章小结57-58
第6章 试验方案设计58-62
    6.1 试验用试件58
    6.2 试验系统58-60
    6.3 试验方法60
    6.4 本章小结60-62
结论62-64
附录64-70
参考文献70-73
攻读硕士学位期间所发表的论文及科研成果73-74
致谢74
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