Assessment of Gel Formation in Colloidal Dispersions during Mixing in Turbulent Jets

Onset of gel formation in turbulent jets was studied using a combination of computational fluid dynamics (CFD) and population balance equations (PBE). To describe the interaction between turbulence fluctuations and particle aggregation a micro-mixing model based on presumed probability density function (PDF) was implemented inside the CFD code. Furthermore, effect of the solid phase on the fluid flow was modeled through an effective viscosity of the mixture evaluated from PBE. The results are represented in the parameter space of the primary particle diameter and the solid volume fraction where three distinct regions were identified. In the first region, where aggregation between large primary particles having low solid volume fraction is considered, no gelation was identified before complete mixing of dispersion and coagulant streams. In the second region, corresponding to moderate size of primary particles and moderate solid volume fraction, gelation due to shear aggregation was observed characterized by the runaway of higher-order moments of the particle size distribution (PSD) before viscosity divergence. In the last region,the available space was filled as a result of viscosity increase before higher-order moments of the calculated PSD runaway. Under such conditions the aggregation of small primary particles with high solid volume fraction is solely controlled by the Brownian motion. Results are in good agreement with the experimental data obtained from gelation experiments of concentrated nanoparticle suspensions injected into coagulant solution.