(411e) Steady State CFD for Determining Just Suspended Speed: Theory & Results
A new method for determining the just suspended speed (Njss) in stirred tank reactors was previously developed using steady state CFD simulations. While preliminary results were somewhat empirical based, the method has been improved with more rigorous theoretical backing that is analogously aligned with mean age theory.
Scalar transport equations were solved to track the solid phase independently from the bulk fluid. In this way, Njss was determined based on the mean age of particles in a settling zone along the bottom of the tank. Plotting average age for the solid phase in this region versus impeller speed reveals two distinct magnitudes, with the transition corresponding to Njss. Above Njss, the average age is low due to large convective velocities. Below Njss, the convective scalar transport is negligible, so the average age is several orders of magnitude higher due to the very low rate of diffusive scalar transport.
Simulations have been performed for a range of solids concentrations and various tank geometries. Computational results compare very favorably with published experimental data. This method can be used for determining Njss for systems in which it would be difficult with traditional experimental techniques. It also removes the subjectivity associated with traditional experimental visualization techniques. Most importantly, since the technique employs steady state simulations, it is advantageous over time-dependent simulations, which require much longer computational times due to the multiple phases and transient nature.