Jacob Larsson & Lisa Lindenbaum presents their master’s thesis

Student: Jacob Larsson & Lisa Lindenbaum
Main Supervisor: Robert Rundqvist (FS Dynamics)
Examiner: Henrik Ström
Opponent: Oskar Tylén

Abstarct

Mixing of liquids occurs in various industrial and laboratory applications and have been extensively studied through experimental and numerical approaches. Computational Fluid Dynamics (CFD) simulations offer a cost-effective alternative to gain insights into the mixing process. This thesis focuses on simulating a stirred tank with a particular emphasis on the impact of vortex presence. Through simulations and experiments, power number, pumping number, and mixing time are determined and analysed. This is done at different operational conditions with and without a vortex. Simulations are done both as steady state and with the free surface fixed and as unsteady with the free surface being solved at operational conditions with a vortex present.
Treating the flow as steady and neglecting the vortex has a minor impact on power number, pumping number, and mixing time, while significantly reducing computational costs. However, the presence of a vortex affects fluid velocity and species transportation, necessitating unsteady simulations with a solved free surface for accurate prediction of transient mixing behaviour. Both steady and transient approaches align well with experiments, but for operational conditions with a vortex, both methods overestimate mixing time by up to 30%.
The results demonstrate that power number, pumping number, and dimensionless mixing time are Reynolds number independent under fully turbulent conditions. While changing water levels impact pumping number minimally, it significantly affects the others. Estimation of mixing time based on pumping number and water volume yields consistent results across various water levels, offering practicality due to the limited variation of the pumping number.