反应堆中近壁面棒状颗粒气动曳力研究

Aerodynamic Drag of Rod-shaped Particles Near-wall in Nuclear Reactor

  • 摘要: 先进反应堆放射性气溶胶运动行为对反应堆安全分析具有重要意义。本文为探究近壁面区域流体对颗粒的气动曳力特性,使用数值模拟方法对近壁面区非球形棒状颗粒气动曳力的增强程度和影响因素进行了研究。研究结果表明:近壁面气动曳力增强由剪切流不均匀的速度分布和偏移的压力分布产生。随着颗粒靠近壁面和颗粒来流角的增加,气动曳力增强效应更加明显。针对非球形棒状颗粒本身的形貌特征和近壁面剪切流产生的曳力增强,使用最小二乘法拟合得到了不同颗粒近壁面气动曳力增强系数的表达式,可为先进反应堆中近壁面区域颗粒运动特性的研究提供基础。

     

    Abstract: Aerosols in advanced reactors are considered the primary carriers of radioactive nuclides, and the motion behavior of aerosol particles in the near-wall region of reactors is of significant importance for reactor safety analysis. For example, for a high-temperature gas-cooled reactor (HTGR), the graphite dust generated due to the operation of spherical fuel elements may move in the primary circuit under the drag action of the coolant, which will result in deposition, resuspension and then affects the reactor performance. The aerodynamic drag produced by the interaction between fluid and particles during the near-wall motion of aerosols was studied, and the computational fluid dynamics method for near-wall drag numerical calculation was employed in this paper. In order to construct the model for non-spherical particles, several spherical particles (3-5) were combined into non-spherical particles according to the rod-shaped dust particle samples obtained from samples in high-temperature gas-cooled reactor. Based on the geometric model of the particle, the Realizable k-ε turbulence model was used to simulate the Reynolds stress of the mainstream fluid, and the enhanced wall treatment model was used to calculate the turbulent viscosity in the near-wall and around particle region. The three-dimensional incompressible steady SIMPLE algorithm was employed for the computations. The results show that the drag force exerted by the airflow on the particles can be divided into two components, namely, the pressure difference drag force and the shear drag force. Compared to the drag force on the particle under uniform inflow in an infinite space, the near-wall particles withstand a greater aerodynamic drag (enhancement factor) due to non-uniform velocity distribution, offset pressure distribution, and asymmetrical flow vortices. As particles get closer to the wall, the effect of aerodynamic drag enhancement becomes more pronounced. Moreover, the enhancement effect of aerodynamic drag on the near-wall particles becomes more evident as the non-spherical particles elongate and the flow-facing area (angle of attack) increases. Finally, considering the shape of non-spherical rod-shaped particles and the drag enhancement due to near-wall shear flows, the least squares method was applied to fit the expression using an exponential format for the enhancement factor of near-wall aerodynamic drag for different particles. It shows that the difference of drag enhancement factors at the same δ (dimensionless particle-wall distance) among different particles in the near-wall region usually does not exceed 10%. Thus, in most cases, the enhancement factor of spherical particles can be used to estimate non-spherical particles, roughly. This study provides a basis for investigating particle motion characteristics in the near-wall region of advanced reactors.

     

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