Agglomerate dispersion in cavitating flows

Erosion of apparatus surfaces resulting from cavitation has been for a long time an important technological and research problem in the fields utilizing fluid-handling machinery. This is because collapse of cavitation bubbles creates locally very high stresses. The same mechanism can enhance breakage of agglomerates suspended in the liquid, when the suspension flows through the micro-nozzle. in this paper we investigate experimentally and numerically the flows through the nozzles of diameter between 80 mu m and 200 mu m under pressure difference up to 2400 bar to study deagglomeration effects. Experiments are per-formed for fumed silica Aerosil 200 V agglomerates. The cavitation model is used with the mixture multiphase model and k-epsilon CFD code by FLUENT. The CFD model is implemented with the population balance for agglomerates and rheological equations for aggregated suspension, and applied to simulate the process and interpret experimental data. The model predicts effects of nozzle geometry and pressure difference on deagglomeration. it is shown that both hydrodynamic and cavitation-induced stresses participate to deagglomeration, however, cavitation stresses are higher. Experimental results and model predictions confirm the hypothesis that the deagglomeration process at very high stresses is dominated by the shattering mechanism. It is also shown that the fragmentation number can be used to identify mechanism of deagglomeration. (C) 2008 The Institution of Chemical Engineers. Published by Elsevier B.V. All rights reserved.