The critical sticking velocity of non-spherical graphite particles: A numerical study and validation

For high temperature gas-cooled reactors (HTGR), the accumulation of the micro-sized graphite dust in the primary loop is a major concern during a potential accident such as the water ingress and loss of coolant accidents. The critical sticking velocity and the restitution coefficient are important for estimating the deposition rate of graphite particles. However, as far as the authors know, the experimental data of the graphite particle–wall collision is limited. Besides, the traditional theoretical models based on approximately spherical particles are not applicable to disk-like graphite particles. In this study, we use the finite element method (FEM) to simulate the particle–wall collision process at a microcosmic scale. The surface adhesion is determined by the van der Waals force between the two contact surfaces. The viscoelastic damping behavior of the material is described by the Maxwell standard model. At lower incident velocities, the effect of the surface adhesion becomes more important, which not only directly contributes to the energy loss by the adhesion work but also indirectly contributes by increasing the strain rate near the interface. For spherical particles, the method shows a good agreement with the experimental data. Then the FEM model is extended to study wall collisions of micro-sized dike-like graphite particles and both the restitution coefficient curve and the critical sticking velocity are obtained. The results show that the critical sticking velocity increases with the decreases of the aspect ratio and the particle size. A correlation for predicting the critical sticking velocity of disk-like graphite particles is proposed. Our work could provide a realistic particle–wall model for predicting the graphite particle deposition rate using Eulerian-Lagrangian CFD method.