Numerical analysis of hydrodynamics characteristics in a top-covered unbaffled stirred tank

A comprehensive CFD study was conducted to understand the hydrodynamics characteristics in a top-covered unbaffled stirred tank. The turbulent flow in the tank was described by the Reynolds stress turbulence (RST) model, and the impeller rotation was simulated using a single reference frame (SRF) approach. The flow field and power number in the tank stirred by a Rushton turbine were investigated firstly. It is found that circumferential flow is dominant in the unabffled tank, and the flow field can be divided into forced and free vortex zones. A weak two-loop flow pattern forms in the vertical plane, and would evolve into one single-loop when impeller was equipped very close to the bottom of the tank. In general, impeller off-bottom clearance and blade thickness have small effects on flow field in the unbaffled tank. With increasing of rotational speed, the dimensionless liquid velocity rises, especially near the critical radius of two zones. Critical radius is larger for larger impeller, while the variation range of critical radius to impeller diameter ratio is narrow. The effects of operational and geometric parameters on impeller power number (Np) in unbafffled tank are far different from those in baffled tank. In the unbaffled stirred tank, rotational speed, blade thickness and impeller off-bottom clearance have small effects on power number, while impeller diameter has a significant effect on power number. Under the same operating condition, the larger the impeller diameter, the smaller the power number is. Finally, the hydrodynamics characteristics in the tank stirred by four common impellers were investigated and compared.