TRAILING VORTICES AROUND A 45-DEGREES PITCHED-BLADE IMPELLER

The trailing vortex system near impeller blades has been identified as the major-flow mechanism responsible for mixing and dispersion in sti rred vessels, and high turbulence levels in the vortices have an impor tant impact on such phenomena as drop breakup and cell damage in biore actors. Numerical computations of the flows require more detailed info rmation on the velocity characteristics generated by different impelle r designs than is available in the literature. Our study on the mean f low and turbulence structure generated by a pitched-blade turbine with four 45 degrees inclined blades found that single trailing vortex is formed around each turbine blade. The vortex axis spread out radially by less than 0.0015 T and was inclined at 20 degrees to the horizontal plane. The vortices merged into the bulk flow structure at around 135 degrees behind each blade. Periodicity of the mean flow due to the cr ossing of the individual blades and high levels of kinetic energy of t urbulence (k) are contained within a radial distance of around r/T = 0 .23 from the axis and a vertical distance of z/T = 007-0.46 from the b ottom of the vessel. The k levels decay to nearly-uniform and low valu es outside this region. The results are compared with earlier investig ations and their implications for mixing processes and CFD predictions of the flows are discussed. The data identify flow regions accurately where intense turbulence is present and thus give useful indications for the optimization of mixing processes.