[论文解读] Direct Numerical Simulation of high Prandtl number fluids and supercritical carbon dioxide canonical flows using the spectral element method

Tri Q. Nguyen, Elia Merzari+1 more|arXiv (Cornell University)|Mar 13, 2022

Heat transfer and supercritical fluids被引用 5

一句话总结

本研究采用谱元直接数值模拟（DNS）方法，借助Nek5000和NekRS求解器，研究高普朗特数流体及在混合对流条件下超临界二氧化碳（sCO2）的传热特性。主要发现表明，在sCO2上升流动中，由于物性变化和湍动能（TKE）降低，导致传热恶化；而在下降流动中，TKE增加导致传热增强。该研究为改进先进核反应堆设计中简化模型的性能提供了高保真度数据库。

ABSTRACT

The design of advanced nuclear reactors (Gen IV) involves an array of challenging fluid-flow issues that affect safety and performance. Currently, these problems are addressed in an ad-hoc manner at varying scales which are time-consuming and expensive. The creation of a high-resolution heat transfer numerical database has the potential to help develop to accurate and inexpensively reduced resolution heat transfer models. Such models can help address industrial-driven issues associated with the heat transfer behavior of advanced reactors. The models can be developed using the multiscale hierarchy developed as part of the recently DOE-funded center of excellence for thermal-fluids applications in nuclear energy. Ultimately this can lead to fast-running reliable models, thus accelerating the deployment of advanced reactors. In this paper, we performed a series of Direct Numerical Simulation using the spectral element codes Nek5000 and NekRS to investigate heat transfer in mixed convection conditions. First, we investigate the heat transfer of the flow in heated parallel plates for high Prandtl number fluids. The calculated database will eventually be used to evaluate existing heat transfer correlations and some modifications will be proposed for cases where no satisfactory choice is available. We have also investigated the heated transfer alteration phenomena in a straight heated tube for supercritical carbon dioxide (sCO2). The low-Mach-number approximation is used to decouple thermal and dynamic pressure, as pressure drop is negligible in this problem. The properties of sCO2 are calculated using multi-region polynomials. We observed that the heat transfer deterioration occurred in combination with the property changes of sCO2 and the depreciation of turbulence kinetic energy (TKE) for upward flow. Whereas, in downward flow, the heat transfer is enhanced thanks to the increase of TKE.

研究动机与目标

• 生成高分辨率数值数据库，用于高普朗特数流体和超临界二氧化碳（sCO2）的传热研究，以支持开发精确且低成本的简化模型。
• 解决在反应堆工况下sCO2缺乏可靠传热关联式的问题，特别是在混合对流条件下的关联式缺失。
• 研究sCO2流动中物性变化与湍流动力学之间的相互作用。
• 利用DNS数据评估并优化现有传热关联式，以提升第四代反应堆应用中的预测能力。

提出的方法

• 采用Nek5000和NekRS求解器，基于谱元法实现不可压缩流体的高保真度直接数值模拟（DNS）。
• 应用低马赫数近似以解耦热力与动压，该方法在所研究构型中因压力损失可忽略而有效。
• 采用多区域多项式模型，精确计算不同温度和压力区域下超临界CO2的热物理性质。
• 模拟两种典型流动：加热平行板间的混合对流（高普朗特数流体）和直加热管中的流动（sCO2）。
• 追踪湍动能（TKE）的演化，以建立流动不稳定性与传热性能之间的关联。
• 采用多尺度框架，与美国能源部核能热流应用卓越中心保持一致，确保工业相关性。

实验结果

研究问题

• RQ1 超临界CO2中的物性变化如何影响混合对流流动中的传热性能？
• RQ2 湍动能（TKE）在sCO2流动中传热恶化或增强过程中发挥何种作用？
• RQ3 流动方向（上升与下降）如何影响sCO2中浮升力、湍流与传热之间的相互作用？
• RQ4 现有传热关联式在高普朗特数和超临界状态下对sCO2行为的预测失败程度如何？
• RQ5 DNS生成的数据能否用于开发适用于先进反应堆系统的改进型简化传热模型？

主要发现

• sCO2上升流动中传热恶化与显著的物性变化及湍动能（TKE）同步降低密切相关。
• 在sCO2下降流动中，观察到传热增强，归因于TKE增加，表明湍流混合增强。
• DNS结果表明，浮升效应在下降流动中更为显著，促进湍流并改善传热性能。
• 本研究识别出现有sCO2传热关联式在混合对流及高普朗特数条件下的局限性。
• 生成的高分辨率数据库为开发精确、计算快速的简化模型提供了基础，适用于先进反应堆设计。
• 谱元法成功以高空间和时间分辨率捕捉了sCO2和高普朗特数流体中的复杂热流体动力学行为。