极端与复杂物性对近壁区湍流标量场影响研究

The coolant of Generation IV reactors involves liquid metal with extreme low Prandtl number, gas with variable property or supercritical fluids with extreme large Prandtl number and huge property variations. Investigation of turbulent scalar field near the wall region with extreme and complicated property effects are important to the design and safety of advanced nuclear power system. In the current investigation, turbulent convection with extreme low and with extreme large Prandtl number are respectively simulated by DNS (Direct Numerical Simulation). Relationship between the coherent structure and turbulent heat flux is created by analyzing the characteristics of fluctuated momentum and scalar field. Based on the critical differences between the momentum and scalar field, new turbulent Prandtl number models are created. The scalar field of turbulent convection with extreme and constant properties are divided into different layers by length scale analysis. Turbulent convection with variable-property-gas and with supercritical fluids are also simulated by DNS. Property variation effects on coherent structure and turbulent heat flux are analyzed. Variations of time averaged statistics kinds like turbulent kinetic energy and turbulent kinetic energy dissipation rate are analyzed to modify the coefficients in the conventional RANS (Reynolds Averaged Navier–Stokes) turbulence models. Both the momentum and scalar fields are divided into layers by deriving the effective length scale for variable property convection.

第四代核能系统的冷却剂涉及到普朗特数极小的液态金属、变物性气体和普朗特数极大且物性变化剧烈的超临界流体。极端和复杂物性条件下近壁区湍流标量场研究对于先进反应堆的设计和安全具有重要意义。本项目采用DNS方法模拟极小和极大普朗特数流体的湍流换热过程。通过分析动量与标量场脉动特征，建立极端常物性流体的湍流相干结构与湍流热流密度关联模型。确定影响湍流动量与标量场分布不同的关键参数，建立湍流普朗特数模型。结合尺度分析建立标量场分层模型。采用DNS方法模拟变物性气体和超临界流体的换热过程。分析物性变化对湍流相干结构和湍流热流密度的影响。通过分析湍动能、湍动能耗散率等时均变量的改变，修正传统RANS模型中经验系数，推导有效尺度建立变物性动量场与标量场分层模型。

第四代核能系统冷却剂涉及到普朗特数极小的液态金属、变物性气体和普朗特数极大且物性变化剧烈的超临界流体。极端和复杂物性条件下近壁区湍流标量场研究对于先进反应堆的设计和安全具有重要意义。本项目采用DNS方法模拟了极小、极大普朗特数以及变物性流体的湍流换热过程，分析了流体物性对近壁区湍流动量和热量扩散过程的影响。结合DNS数据，给出了液态金属湍流换热过程中的无量纲温度分布幂律，并在此基础上给出了液态金属湍流换热系数公式。结合湍流普朗特数模型，采用指数函数表征Pr>1时近壁区的湍流热扩散系数。推导出应用范围较为广泛(1<Pr/Sc<10000)的无量纲温度(浓度)分布公式和努塞尔数(舍伍德数)公式。通过分析气体物性变化对壁面黏性尺度以及物性变化对无量纲速度分布中线性区和对数律区分界点的影响，推导出了变物性气体湍流换热过程中的无量纲速度分布公式和阻力计算公式。通过对比近壁区湍流热扩散系数和分子热扩散系数的大小，给出了新的常物性气体湍流换热过程中的无量纲温度分布公式和湍流换热系数公式。通过分析物性变化对导热主导区和对数律区分界点的影响，推导出了变物性气体湍流换热过程中的无量纲温度分布公式，并在此基础上给出了换热系数公式。区别于传统的温度比物性修正方法，所推导公式对不同种类的气体均使用。本项目所推导公式均与实验数据或DNS数据进行了对比，验证了本项目所提公式的准确性。结合本项目所推导出的无量纲速度和温度分布公式，可对现有RANS模型中的壁面函数进行修正以提高RANS模型模拟极端普朗特数流体和变物性流体换热过程的准确性和经济性。