Turbulence parameters in a stirred tank

Abstract

tirred tanks are widely used in the chemical indusS try for effecting mixing, but their design has so far been empirical. A theoretical treatment of the flow within a stirred tank is rendered intractable by the inherent randomness and three-dimensionality of the flow and non-linearity of the governing equations of motion. Experimental studies of the large- and small-scale turbulence characteristics in stirred vessels may be expected to lead to formulation of realistic models of the flow. These models can then be used to predict such quantities of engineering interest as the mixing efficiency, pumping capacity, power requirements, etc. It was the objective of this work to measure the turbulence characteristics of the high speed stream issuing from a turbine type impeller in a fully baffled tank. Turbulence parameters were measured using a constant-temperature hot-wire anemometer with air as the fluid within the tank. The hot-wire technique was chosen because of the ease of operation and extremely good frequency response. Use of mr as working fluid Cooper‘” recently showed that the temporal mean velocity distribution in the impeller stream is the same with air and water as the fluid in the tank. We have assumed that the small-scale turbulence characteristics would also be similar at equal Reynolds numbers irrespective of the fluid in the tank. There exists sufficient confirmation in the literature for air and water flows in circular pipes and circular jets, and also grid-generated turbulent flows in wind and water tunnels. Chuang and Cermak“’ have compared their measurements of turbulent intensities, shear stress and energy spectra in the pipe flow of distilled water for a Reynolds number of 5 x lo4 with those of Sandborn‘J) and Laufer“) who studied the flow of air in a pipe at the same Reynolds number. The agreement between the air and water data is remarkable