Using computational fluid dynamics modeling and ultrasonic Doppler velocimetry to study pulp suspension mixing

In this study, the flow field of a cylindrical pulp mixing chest equipped with a side-entering impeller was modeled using commercial computational fluid dynamics (CFD) software (Fluent) with the rheology of the pulp suspension approximated using the Herschel-Bulkley model. To validate the model, CFD results for the power and velocity field were compared to experimental data. Ultrasonic Doppler velocimetry (UDV), a noninvasive fluid flow measurement technique for opaque systems, was used to measure pulp suspension velocity. In order to calculate the mixing time, an unsteady state solver in Fluent was applied to monitor the tracer species concentration as a function of time in the tank. The validated CFD model provided useful information regarding the mixing time and the formation of the cavern around the impeller in the mixing of pulp suspension. The size of cavern predicted by the CFD model was in good agreement with that calculated using Solomon's model.