基于概率密度函数的超临界流体强非线性物性湍流传热模型改进研究

Supercritical fluids, due to their highly efficient heat transfer effectiveness, have been extensively concerned to be used in certain advanced energy conversion and power systems. It exists many obvious differences between supercritical fliuds and single phase fluids turbulent heat transfer both in physical mechanism and in engineering applications. Its outstanding characteristic is the presence of highly nonlinear dependency between properties and instantaneous turbulent temperature, which results in that supercritical fluids exhibit a lot of highly nonlinear turbulent heat transfer features. The existing heat transfer models could not accurately reflect these features. In present project, based on a lot of past research experiences, the common nonlinear heat transfer characteristics and the physical mechanisms of supercritical fuids would be investigated by introducing probablity density function from the angle of statistical probability. The highly nonlinear relationship between properties and instantaneous turbulent temperature in time dimension, which was neglected by many previous reasearchers, would be studied intensively. The physically reasonable property averaging techniques and the quantitative relationship between the eigen value of probablity density function and certain specific variables such as the degree of the nonlinear properties, turbulence intensity and heat flux would be investigated in depth. On this basis, the existing empirical correlations and numerical heat transfer models would be improved and verified, which can accurately reflect the nonlinear characteristics of heat transfer from supercritical fluids. The present research provides theoretical basis and technical support for the design and development of advanced power systems which employ supercritical fluids as working fluids.

超临界流体以其突出的传热特性被许多研究人员考虑可用于多种新型的能量转化和先进动力系统中。超临界流体与单相流体湍流传热在微观传热机理和工程实践上都存在明显的差别，其突出的特点是各个物性都与温度存在强烈的非线性函数关系，从而表现出一些特有的强非线性特征。现存的传热模型还不能准确的反映这些特征。本项目基于过去的研究基础，从统计概率角度出发，引入概率密度函数，系统地研究超临界流体共有的非线性传热特征和其内在的物理机制，特别是被很多研究者忽视的物性在时间维度上与瞬时温度的强非线性关系，进一步发展得到物理机制上更加合理的物性时间统计平均方法以及概率密度函数特征值与物性的非线性程度、湍流强度和热流密度等因素的定量关系。在此基础上，对现有经验关系式和数值传热模型进行改进和验证，使其准确反映超临界流体特有的非线性传热特征，为以超临界流体为工质的先进动力系统的设计和开发提供重要的理论基础和技术支持。

超临界流体以其突出的传热特性被许多研究人员考虑可用于多种新型的能量转化和先进动力系统中。超临界流体与单相流体湍流传热在微观传热机理和工程实践上都存在明显的差别，其突出的特点是各个物性都与温度存在强烈的非线性函数关系，从而表现出一些特有的强非线性特征。现存的传热模型还不能准确的反映这些特征。本项目通过理论分析、数值计算和试验验证的方法，从统计概率角度出发，分析了概率密度函数特征值的物理含义，统计概率密度函数特征值与超临界流体物性的非线性程度、湍流强度、热流密度等的定量关系以及物性在时间维度上与瞬时温度的强非线性关系以及由此产生的浮力效应和加速效应的内在机理，深入揭示了超临界流体强非线性物性湍流传热的内在物理机制，发展了基于概率密度函数的强非线性物性时间平均方法，并在此基础上得到了改进的具有一般性的超临界流体强非线性湍流传热模型，该模型从湍流理论的角度改善了现有超临界流体湍流传热的预测精度，新模型预测精度的提高被超临界二氧化碳在高效紧凑式换热器半圆型通道内的湍流传热特性；超临界水在竖直上升带绕丝环状通道内的湍流传热特性和超临界碳氢燃料在圆型高热流密度下的湍流传热特性等不同试验数据证实。同时，新的改进模型可用于超临界二氧化碳新型动力系统、超临界煤油火箭发动机以及第四代超临界水堆的研究开发工作中，为这些新型的动力系统的设计开发提供了理论基础和技术支撑。