规则球床堆熔盐流动压降与对流换热CFD模拟

摘要

球床堆复杂的几何结构导致直接建模进行热工水力模拟非常困难，一般使用多孔介质模型简化处理，但多孔介质已有的压降和对流换热公式在熔盐冷却球床中的有效性仍待验证。本文基于固态燃料熔盐堆建立了6 cm 直径小球的规则球床模型，给定球床进口熔盐流量和球壳发热功率，模拟了球床内的稳态流动与换热，计算了对应的压降和对流换热系数，并分别得到了球床压降、对流换热 Nu 随球床内流动 Re 变化的曲线。对比发现：模拟压降结果与已有公式差异较大，而模拟对流换热 Nu 结果与已有公式的差异相对较小。结合模拟结果和已有的公式，拟合得到了修正的压降和对流换热Nu公式。将修正公式应用于3 cm直径规则球床中，结果表明多孔介质修正模型与直接模拟结果一致。%Background: The computational fluid dynamics (CFD) simulation of thermal-hydraulic characters is essential to ensure the safety in designing and operating a nuclear reactor. To obtain reliable results, a suitable model must be chosen. For the pebble bed reactors, the complex geometries of the core make it difficult to establish a precise model for CFD simulation, especially for the molten salt reactor (MSR)-one of the most hopeful GIV nuclear power system. Purpose: This study is aimed to valid the porous media model used in pebble beds thermal-hydraulic simulation, including the Pressure-Drop formula and the Convection-Heat-Transfer formula. Methods: The fluent model of an orderly-arranged pebble bed was built, and the SST-turbulence model was chosen to simulate the molten salt flow inside the bed. Results:In a large range of fluid Reynolds numbers, the linear pressure drop of the flow was obtained, and also was the Nusselt numbers for convection heat transfer between sphere surface and the salt fluid. Conclusions: The data was fitted to format new formulas. For the pressure drop, simulation result much differs in both of the KTA (Kerntechnischer Ausschuss)-formula and the Ergun-equation. However, the Nusselt number from simulation is more close to the KTA-formula and the Wakao-formula. Modification of the traditional formulas is needed obviously, and new formulas fitted from KTA-formula is much more close to the simulation results, both for the linear pressure drop and the Nusselt number.