Laser-Doppler measurements of turbulent-flow parameters: comparison between baffled and unbaffled stirred tanks

摘要

A detailed experimental investigation of the flow field in turbulent regime induced by a Rushton impeller inside a closed flat-bottomed cylindrical stirred vessel is presented. The flow throughout the vessel was measured by means of a laser-Doppler anemometer (LDA). Firstly, data were acquired in the vessel equipped with four radial baffles; afterwards, baffles were removed and the new flow pattern was analysed and compared to the previous one. The effect of baffles was also quantitatively pointed out by comparing flow numbers, circulation and renewal times of the two configurations. Finally, an attempt was made to assess the degree of turbulence anisotropy inside the stirred vessel, introducing a parameter that takes into account the three fluctuating velocities. For the baffled vessel, this parameter indicates turbulence anisotropy near the impeller, whereas the turbulence can be considered isotropic in the fluid bulk. In contrast, turbulence was found to turn away from isotropy inside the unbaffled vessel even in the bulk regions. This result probably explains the failure of CFD tools in predicting unbaffled stirred vessels when isotropic turbulence models as the k-ε are employed.