Local hydrodynamics investigation within a dynamic filtration unit under laminar flow

A dynamic filtration module, called a Rotating and Vibrating Filtration (R.V.F.) module, was designed and dedicated to the treatment of highly viscous fluid, such as fermentation broth or liquid food. To this end, an experimental study was undertaken, using a laminar flow regime with a viscous Newtonian model fluid in a dynamic filtration module in order to quantify the effect of local hydrodynamics on filtration. Instantaneous velocity fields can be measured and analyzed within an R.V.F. by using Particle Image Velocimetry (P.I.V.). In this study, we applied P.I.V. to study the laminar local hydrodynamics in 3 different slices within the 3 mm gap between the membrane and the impeller and 3 vertical slices at different radial positions, with rotation speeds from 0 to 10 Hz. Radial and vertical profiles of tangential velocity were then plotted. Proper Orthogonal Decomposition (P.O.D.) was applied to the P.I.V. data to discriminate between mean flow and fluctuating velocities induced by the periodic motion of the impeller. Thus, viscous shear stress profiles were deduced in terms of both mean shear stress profile and root mean squared (r.m.s.) fluctuating shear stress profile; wall values were then deduced. With this approach, we were able to quantify the distribution of viscous shear stress at the wall (membrane), in terms of mean value and r.m.s. contribution. Dynamic filtration efficiency was thus enlightened by local hydrodynamics.