Experimental Study Of Macro-Instabilities In A Stirred Tank

Stirred tanks are widely used in chemical processing and allied industries. Mixing process is difficult to measure and quantify, and scaling up is not reliable due to the lack of fundamental knowledge of mixing mechanisms. Numerous experimental and theoretical studies have been carried out to characterize the time -averaged flow fields in stirred tanks. In reality, however, the fluid flow is three-dimensional and transient, with a set of hierarchically organized unstable eddy and vortices motions differing in pattern alteration, but also a significant influence on mixing, heat and mass transfer. The understanding of spatio-temporal characteristics of the fluid flow in stirred tank is of great practical importance.In the present investigation, to character how the flow instabilities influence the mechanical equipments in an agitated tanks, impeller torque sensor and wall-pressure were measured in a fully baffled flat-bottomed cylindrical stirred tank (800mm diameter, filled with the liquid to the height equal to its diameter) . In order to predict the occurrence of MI in industrial stirred tanks, characteristic MI occurrence in these devices were derived for the three types of stirrers used in this work (Rushton and 45 Pitched blade turbine for D/T=0.56 and D/T=0.37). Water was used as working fluid for three impeller speeds: 30rpm, 60rpm and 90rpm. Off-bottom clearance was varied(C=T/4, T/3 and T/2).The discovery of MI in stirred tanks has been described based on the study ofturbulent mixing by means of torque and pressure sensors technique. Time-resolved torque and wall-pressure were measured and the frequency content of the flow field reflected by the time recordings was analysed with spectrum analysis respectively. The operating conditions (stirrer type, impeller rotational speed , stirrer diameter and...