The effect of inertia on the dispersion of particles in the flow around a two-dimensional flat plate

Abstract

Wind tunnel experiments and a visualization procedure are described with the purpose of investigating the effects of inertia on the dispersion of particles in flows around a bluff body. Four different grits of narrowly graded aluminum oxide optical powders were used to generate the test aerosol, which was released from a point source upstream the obstacle. The resultant aerosols covered the range of median particle aerodynamic diameter from 14 to 47 μm . Measurements were taken for the residence time of the particles in the near-wake region of a long flat plate positioned perpendicularly to the free-stream after the aerosol injection had been suddenly interrupted. The experiments covered air velocity and particle size ranges corresponding to Reynolds’ number ( Re) ranging from 1400 to 10,200 and Stokes’ number ( St) from 0.02 to 1.8. Results for the dimensionless residence time H (defined as H= τU/ D, where τ is the time constant for the concentration decay for particles in the near wake, and U and D are the free-stream velocity and characteristic dimension of the bluff obstacle, respectively) for the larger particles varied between about 5 and 13, while the H-values for small inertialess particles (obtained in previous experiments; Gomes, Vincent, and Pui (Aerosol Sci. Tech. 26 (1997) 269) had varied between about 7 and 10. This is in accordance with the general finding that the apparent diffusivity of the large particles may, depending on the flow and particle characteristics, be either smaller or larger than the diffusivity of the fluid, as has been proposed by several authors in recent years. An important result from the visualization of the particle trajectories was the identification of organized structures, or “particle vortices”. These appear as a consequence of centrifugal inertial effects acting on the large particles which are responsible for concentrating them in the outer regions of the large coherent eddy structures while keeping the center practically clear. The same phenomenon has also been discussed for several free shear flow configurations by Crowe (Proceedings of the 11th Annual Meeting of the American Association for Aerosol Research (AAAR), Plenary Lecture, San Francisco, 1992), Tang et al. (Phys. Fluids A 4 (1992) 2244) and Yang, Crowe, Chung, and Troutt (Int. J. Multiphase Flow 26 (2000) 1583). Most notably, it was predicted numerically by Chein and Chung (Chemical Engineering Science 43 (1988) 1621) for particles in the near wake of the same two-dimensional flat plate configuration studied here.