(550g) Shear-Thinning Mixing With Three-Coaxial Pitched-Blade Impellers
AIChE Annual Meeting
Thursday, November 8, 2007 - 11:00am to 11:25am
The problem of mixing in shear-thinning fluids is widely explored using a variety of different impeller configurations. In this case study, a stirred tank with three co-axial pitched-blade impellers is simulated using ORCA? CFD and validated with experiments. The initial simulations are run for a fluid with a consistency index m = 40 Pa?sn and a power-law index n = 0.2 with impeller rotation rates of 50, 150, and 300 RPM. These studies show a fast top-bottom transport along the central shaft of the impeller and slow progression along the walls of the stirred tank. Mixing for the 300 RPM case appears to occur with 5 minutes (300 seconds). Studies are also performed altering the characteristics of the fluid: reducing the shear-thinning nature (m = 40 Pa?sn and n = 0.5) and reducing the consistency of the fluid (m = 0.1 Pa?sn and n = 0.2). In the n = 0.5 cases, qualitative speed of mixing was improved; however, a bifurcation occurred in the behavior of the fluid with the lower consistency index (m = 0.1 Pa?sn). In this case, the simulations show that the higher RPM cases (150 and 300 RPM) do not have the top-to-bottom transport that is the feature of the 50 RPM reduced consistency case (and all the cases with m = 40 Pa?sn). The top-to-bottom shaft transport and slow wall transport were validated using a similar three impeller stirred tank and a Carbopol solution with consistency index m = 5.0 Pa?sn and n = 0.2. Beyond simple validation, the dye studies performed in the experimental tank show a compartmentalization of flow into moving segregated regions. The case of the segregated bifurcation has not been observed in the experimental system at this time.