Particle transport, interaction and agglomeration for nuclear reactor and nuclear waste flow applications

The work presented explores the dynamics of multiphase turbulent flows, with particular emphasis on micro-scale particle-fluid and particle-particle interactions and their effects such as turbulence modulation, and particle collisions and agglomeration. A range of simulation techniques are used to elucidate the dynamics that underpin the fine-scale motions of varying particle-fluid density ratio flows, with particle behaviours ranging from tracer-like to inertial, momentum-driven. To study the bulk behaviour of high-concentration dispersions, a single-phase turbulent channel flow at a shear Reynolds number of 180 is obtained and investigated using a spectral element-based direct numerical simulation (DNS) code. To model large quantities of dispersed solids, a Lagrangian particle tracker (LPT) is developed, with both simulation tools validated against recent DNS-based results, with strong agreement shown. The LPT is used primarily to study bulk flow mechanisms such as dispersion, interaction and agglomeration for both tracer-like and inertial particles. Phenomenon not explained in the literature is elucidated using a range of analytical techniques. In particular, the existence of near-wall increased particle streamwise velocity fluctuations is explained through the generation of understanding on particle migration mechanisms using turbulence classification techniques. Two-way coupling and particle-particle collisions are explored which relate the dynamics of the particulate phase to the turbulent structures experienced within the channel flow. Finally, an immersed boundary method is implemented to perform fully-resolved binary particle collision simulations in different scales of turbulence. Inter-surfacial forces such as van der Waals attraction and electric double layer repulsion are explicitly calculated to study interaction and agglomeration events on the particle scale. Results indicate that particles are more likely to aggregate and remain bound in regions of low turbulence, provided they collide. Investigations are also performed surrounding the variation of chemical and mechanical properties, with the coefficient of restitution exhibiting the greatest effect on the resulting dynamics.