卵石床系统近壁区改进摩擦相关式的发展
Toward Development of an Improved Friction Correlation for the Near-Wall Region of Pebble Bed Systems
摘要：
利用卵石燃料的核反应堆的发展极大地增加了对改进模拟这些反应堆中发现的填充床的能力的需求。在卵石床中发现的复杂流场使得计算流体动力学（CFD）模拟耗时且昂贵。然而，中间保真度多孔介质模型能够以计算效率高得多的方式近似这些流场。这些模型需要使用闭包来捕捉复杂流动现象的影响，而无需明确地对其进行建模。本研究采用高保真CFD模拟的卵石床，以改善在床的近壁区域的多孔介质模型中使用的阻力封闭获得的数据。具体而言，NekRS，GPU支持的光谱元素CFD代码，用于模拟1,568卵石床在多个雷诺数。该案例被划分为五个同心子域，以提取每个子域中的平均孔隙度、速度和壁面剪切力的径向剖面。在爱达荷国家实验室的Pronghorn多孔介质代码中创建了与高保真模型一致的模型，并选择KTA相关性作为比较的阻力闭合。结果发现，KTA相关高估了近壁区的速度。对两个代码之间的阻力系数的研究表明，KTA相关性低估了最外侧区域的形状因子，而高估了内侧四个区域的形状因子。这项工作中的分析揭示了KTA相关性的近壁区域中的潜在不准确性，并建立了使用高保真度模拟来先验预测更准确的阻力系数的过程，而不是使用手动速度匹配方法。这一过程将允许开发用于多孔介质模型的改进的阻力闭合。
英文摘要：
The development of nuclear reactors that utilize pebble fuel has drastically increased the demand for improving the capabilities to simulate the packed beds found in these reactors. The complex flow fields found in a pebble bed make computational fluid dynamics (CFD) simulations time consuming and costly. Intermediate fidelity porous media models, however, are capable of approximating these flow fields in a much more computationally efficient manner. These models require the use of closures to capture the effects of complex flow phenomena without modeling them explicitly. This research employs data obtained from high-fidelity CFD simulations of a pebble bed to improve the drag closures used in porous media models in the near-wall region of the bed. Specifically, NekRS, a GPU-enabled spectral element CFD code, was used to simulate a bed of 1,568 pebbles at multiple Reynolds numbers. The case was divided into five concentric subdomains to extract radial profiles of the average porosity, velocity, and wall shear in each subdomain. A model consistent with the high-fidelity model was created in Idaho National Laboratory's Pronghorn porous media code and the KTA correlation was chosen as the drag closure of comparison. It was found that the KTA correlation overestimates the velocity in the near-wall region. An investigation of the drag coefficients between the two codes revealed that the KTA correlation underestimated the form factor in the outermost region while overestimating it in the inner four regions. This analysis in this work has revealed the underlying inaccuracy in the near-wall region of the KTA correlation and has set up the process for using high-fidelity simulation to predict more accurate drag coefficients a priori, rather than with a manual velocity-matching approach. This process will allow for the development of an improved drag closure for use in porous media models.
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