Large Eddy Simulation of Liquid Particle Deposition in a Turbulent 90° Bend Flow
Pp. 69-94 (26)
Abdallah Sofiane Berrouk
LES was performed to study Aerosol deposition in a turbulent 90° bend flow with tubular crosssection. Numerical predictions were compared to the experimental observations of Pui et al. and the DNS-like work of Breuer et al. Due to the complexity of the turbulent flow in curved pipe characterized by curved streamlines and zones of recirculation and the lack of comparison studies of the same flow parameters, a good deal of care has been taken to ensure that the carrier phase is accurately simulated. Every effort was made to adapt the mesh to the dynamical features of the flow and boundary conditions were set such that the inlet and outlet conditions would not influence the turbulent flow in the bend.
The numerical predictions of the secondary flow and streamlines in the symmetry plan and in cross sections at different angle of deflection showed a good agreement with the DNS-like work of Breuer et al. A-posteriori estimation of the filtered-out kinetic energy demonstrated that the present LES is adequate according to the LES index of quality developed by Celik et al.
For the dispersed phase, a stochastic model that accounts for inertial particle transport by SGS motion was used. It was anticipated that such modeling should be crucial owing to the very small-Stokes-number particles tracked. An estimation of the time scale of the SGS fluctuations that are discarded by the filtering operation in LES showed clearly that particles with Stokes number smaller than 0.25 do sense the SGS turbulent fluctuations.
Numerical results concerning the deposition efficiency of inertial particles with Stokes number that range between 0.005 and 1.5 demonstrate the ability of the stochastic modeling to reproduce with good accuracy the SGS effects on small-Stokes-number particles. As it was expected the use of the filtered velocity field only to track particles with Stokes number smaller than 0.3 has proven inaccurate. The complete formulation of the stochastic model showed its superiority compared to the standard formulation. The latter was showed to produce an incorrect level of SGS turbulence.
It was shown clearly that the deposition efficiency of small inertial particles can be predicted with a very good accuracy in the framework of LES using a coarse numerical description. To achieve that, the effect the SGS motion has on inertial particle transport needs to be taken into account. The Langevin-type stochastic diffusion process has proven very adequate in this regard.
Bend flows, Large eddy simulation, particle-laden flow, turbulence, Lagrangian description, aerosols, stochastic process, Dean number, deposition, helical vortices
Chemical Engineering Department Petroleum Institute, Abu Dhabi United Arab Emirates.