建筑储热围护结构固液相变与多种传热机制耦合问题研究

The coupled heat transfer characteristics of thermal environment inside building with phase change material (PCM) wallboards and the nonlinear analysis will be performed by experimental study and numerical simulation in this research project. The PCM products which are accessible for the building industry are very beneficial for the humans and as well as for the energy conservation. The coupled heat transfer problem consists of solid-liquid phase change, heat conduction, natural convection and radiation in one physical model. The purpose of this theory research is to contribute to fill the void on studies done on this subject and to enhance the knowledge already acquired in this field. The research show important practical importance and academic value. The work in this project about the subject is focused on the following issues: (1) to develop numerical methods for coupled heat transfer physical model and to enhance the knowledge of nonlinear flow and heat transfer systems; and (2) to establish a set of (four) comprehensive and rigorous approach for making a distinction between numerical oscillation and physical oscillation and to improve the reliability of numerical simulation for nonlinear systems; and (3) to detect chaos (the irregular and unpredictable time evolution of the nonlinear systems has been called chaos) by the power spectrum and to identify the route(s) to chaos and illustrate the dynamical response of the flow with the change of the control parameter; and (4) to study the nonlinear characteristics of the coupled thermal system by means of phase portraits, phase space reconstruction, correlation dimension, Kolmogorov entropy and Lyapunov exponents; and (5) to reveal the inherent laws of the physical problem based on experimental and numerical results and to illustrate the mechanism of the flow pattern evolution and transition. The work in this project will be beneficial to thermal control applications in this particular field and provide the technical support for thermal design in building with phase change material (PCM) wallboards.

本课题以建筑节能技术中具有相变储热围护结构的热环境控制为背景，对抽象出的耦合了固液相变的导热、辐射与自然对流等多种传热机制问题及其非线性特性进行数值模拟和实验研究。此项研究既是工程实际所需，又有扩展和丰富对流传热基础理论的学术价值。研究内容包括：发展多种传热机制耦合的流动和换热问题数值模拟方法，推进对流换热问题非线性特征的深层次描述；探究运用多角度的综合验证方法，辨识数值解振荡的物理真实性；利用谱估计提取随机时间序列的混沌信息，揭示其包含的物理意义，刻画其从层流到混沌之路；采用相图、重构相空间、关联维数、Kolmogorov熵和最大Lyapunov指数等工具明晰混沌解的吸引子特征；结合关键数据的实验验证，获得耦合问题复杂换热的基本规律，阐明流态形成机理和流态转换机制，为相关建筑节能技术的有效利用和热环境控制提供技术支持。

本课题研究具有相变储热围护结构的建筑中，固液相变和导热、辐射与自然对流等多种传热机制的耦合换热特性，及该复杂动力系统所涉及到的非线性问题研究。课题首先以加装相变储热材料（Phase Change Material, PCM）地板结构的建筑热环境控制为背景，在和实验结果比对的基础上，采用Fluent 软件和自编程序相结合的数值方法，对抽象出的底部加装PCM 相变材料的低温热水辐射采暖的二维方腔模型进行数值模拟，该模型考虑变物性计算，并包含相变区以及导热、固体壁面间辐射与自然对流的耦合换热。研究了PCM相变材料的相变温度和导热系数对空气腔内温度的影响，拟合出了相变温度和空气腔内温度的变化关系式，研究结果也表明，提高相变材料的导热系数可使采暖系统的热惰性减弱，即加快单位时间内腔内空气的温升。此外，在对模拟结果进行分析的基础上，提出了一个新的地板蓄热结构。课题还研究了不同填充材料时的方腔内空气流动的非稳定特性。研究结果表明在蓄放热过程中，混凝土作为填充层材料时，流动一直保持稳定的2-涡结构，但PCM为填充层材料时，腔内的温度场和流场在部分时段中出现了流动的不稳定性，出现了和底部加热Rayleigh-Bénard对流类似的解的分岔现象。此外，本课题分别采用所建立的LBM和Fluent+自编程序的数值方法，求解了另两个经典非稳定问题，以求通过算例横向验证算法的可靠性。一是圆内开缝圆环自然对流换热不稳定性问题进行数值模拟；二是从非线性问题解的分岔的角度对于结构对称的锅炉炉膛中的流动和传热进行了数值求解和分析；并和实测数据相比较，验证了该数值方法用于非稳定性计算的可靠性。最后课题采用格子Boltzmann方法针对固液相变问题进行数值模拟，很好模拟出了相变界面的变化过程。