Investigation of turbulence modulation in solid-liquid suspensions using parallel competing reactions as probes for micro-mixing efficiency

The Bourne and the Villermaux competitive reaction chemistries were applied to study the effects of suspended particles on the yield of an undesired product and hence to infer their effects on local dissipation rates. Two-phase micro-mixing experiments were carried out in a 1 l stirred vessel, agitated by a pitched-blade turbine, using four particle size ranges: 70-100, 250-300, 700-750 and 1000 μm. Experiments were carried out with up to 1.75 vol% particles in the Bourne scheme and 3 vol% in the Villermaux scheme. Both reaction schemes gave qualitatively similar results, although stronger effects of added particles were obtained with the Bourne chemistry. The effect of 700-750 μm particles could not be distinguished from experimental error, but the other size ranges gave increased by-product yields and suppressed the dissipation rates. These results confirmed earlier two-phase PIV observations: smaller particles (70-100 and 250-300 μm) gave maximum suppression at ∼1 vol%. Above this volume fraction, the level of suppression decreased and in some cases turbulence augmentation occurred, indicating that particle concentration, as well as size, is an important factor.