(108c) Particle Behavior in Taylor Vortex

We report a study on the behavior of solid particles captured in a Taylor-Couette flow in the annulus between two concentric cylinders. The vortex system has a large gap (d/R_o = 1-R_i/R_o = 0.387, where R_i and R_o are the radii of the inner and outer cylinders, respectively) and a low aspect ratio (H/d = 5.17 where H is the height of the inner cylinder). In such a system, end effects are found to be important. We also present preliminary experimental and computational fluid dynamics (CFD) studies on Taylor vortex formation. Six vortices appear in the annulus; and the vortices show no evidence of wavy behavior. Particles are always released in the second vortex from the bottom. We investigate four solid particles with two different density ratios (r_p/r_f = 0.93 and 1.15, where r_p and r_f are particle and fluid densities, respectively) and two different dimensionless diameters (a/d = 0.14 and 0.28, where a is the particle diameter). We employ a high speed camera (250 frames per second) to visualize the spatial and temporal behavior of the particles in the Taylor vortex.

The particles are observed to be captured in the vortex and to follow a toroidal motion. At least approximately, the particles follow the fluid motion. The result is a secondary periodic circular motion around the vortex centre. For instance, at Re = 91 (Taylor vortex regime), a particle with r_p/r_f = 0.93 and a/d = 0.14 completes about two revolutions around the vortex centre for each revolution about the device. The amplitude and period of this secondary motion are found to vary with Reynolds number. When the Reynolds number of the flow is below a critical value (Re_c ~ 40), the particles exhibit a purely circumferential motion, in which they travel near either the outer or inner cylinder without any toroidal behavior. Furthermore, particle image velocimetry (PIV) shows that the vortex formation is almost undisturbed by the presence of particles. A CFD study with particle tracking has also been performed. Preliminary results show that the buoyancy and other forces which act on a particle are balanced by fluid drag.