Lagrangian trajectory model for turbulent swirling flow in an annular cell: comparison with residence time distribution measurements
Résumé
Knowledge of the flow field can be very useful for reactor design and optimization. Data concerning the displacement and trajectories of reactive elements are of primary interest for a better understanding of process running. Results can be deduced from computational fluid dynamics or experimental measurements. However, numerical simulation is difficult to achieve for complex flow such as swirling decaying flow, and trajectories need to be calculated on the basis of velocity field measurements. This problem can be simplified by using a Lagrangian formulation, in which case the only major difficulty is to express the influence of turbulence on calculated trajectories. Velocity fluctuations can be considered as pure random functions or related to turbulence correlations. These two methods were used to calculate trajectories of elementary fluid particles in a swirling decaying flow on the basis of hydrodynamic characteristics obtained by particle image velocimetry (PIV) studies. Trajectory dispersion was then compared with experimental results obtained by PIV measurements of the instantaneous flow field. This comparison shows the great dependence of velocity fluctuation (and thus of trajectories) on spatial correlations. Finally, the correct mathematical formulation of velocity fluctuation was checked by using the trajectory calculation algorithm to determine residence time distribution (RTD). A comparison with experimental RTD confirmed the efficiency of the method for determining trajectories in swirling decaying flow.
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