Mixing in large scale storage tanks: Studies on the boundary layer thickness at the base of storage tanks agitated with side-entry propellers

Abstract

Studies of the thickness of the viscous sub-layer and its distribution around the base of a tank fitted with a sideentry propeller has been studied by simulating the process on a 1.2m diameter tank. An electro-chemical technique has been employed to study the thickness of the viscous sublayer and the effect of impeller speed, impeller diameter, shaft angle, liquid depth, and physical properties of the solution on this thickness. Also a hot-film anemometer was used to check and compare the results obtained by the electro-chemical technique and it was found that both were in agreement. However, the results obtained by the electrochemical technique gives an indication of the effec of the diffusivity and the thickness of the viscous sub-layer was related to both Reynolds number and Schmidt number. A mathematical model has been developed to describe the velocity distribution along the impeller jet and to predict the thickness of the viscous sub-layer along the jet. The results obtained from this model agree with those obtained by experiment. Studies on minimizing the thickness of the viscous sublayer showed that the presence of one or more small baffles on the base of thetank help to lift the sediments into the bulk of the solution. Finally, full-scale measurements were carried-out on 79m diameter tank to provide data for scale-up.