An Experimental Research On Fluid Structure Interaction In A Gas Liquid Stirred Tank

The unsteady fluctuation of the flow field in a gas-sparged stirred tank leads to the asymmetry of fluid loads on the impeller. At the same time, when gas flow impacts directly on the shaft and impeller, the shaft will vibrate and shake. These factors cause the shaft to deflect and move laterally, which in turn induces further nonuniform and unstable flows around it and leads to more imbalanced and unstable load on the impeller. This behavior is the mechanism of the complex Fluid Structure Interactions (FSI) in gas liquid stirred tanks. The FSI will exert alternating bending moment and torque on the shaft end, which can easily cause fatigue failure on the shaft.The bending moment and torque on the shaft end of a series of typical axial flow and radial flow impellers are measured by a moment sensor under different gas flow rate and rotating speed. The influence of gas flow rate and rotating speed on the FSI in a gas liquid stirred tank is researched, including the influence on the mean torque, torque amplitude distribution, the mean bending moment, bending moment amplitude distribution, bending moment fluctuation, bending moment frequency characteristics and the combined torque. Two typical groups of impellers are choosen with one group consists of axial flow impellers and the other consists of radial flow impellers. Axial flow impellers include one up-pumping Pitched Blade Turbine (PBT Up) and one down-pumping Pitch Blade Turbine (PBT Down). Radial flow impellers include four kinds of Dish Turbine with different blade curvatures (RT, CD, HEDT and PDT).Research shows that the bending moment amplitude distribution of both axial flow impellers and radial flow impellers can be well described by Weibull distribution. The torque amplitude distribution, however, show a trend to transform from near Normal distribution to typical bimodal distribution as the gas flow rate grows. The change regularities of the mean torque and the mean bending moment are influenced by the gas carrier way of the impeller. Both the mean torque and the mean bending moment on the shaft of PBT Up decrease smoothly as the gas flow rate grows. The relative mean torque on the shaft of PBT Down presents a trend of three stages including a slow decline, a rapid decline and a slow rise as the gas flow rate grows and exists a minimal value. The trend of the relative mean bending moment on the shaft of PBT Down over gas flow rate also includes three stages, changing from rise to decline and then to rise. For all radial flow impellers in this research, the relative mean torque on the shaft presents an overall downward trend. As the gas flow rate increases, the relative mean bending moment on the shaft of RT, CD and PDT firstly rises and then decreases and finally rises, manifesting a three-stage trend. The relative mean bending moment on the shaft of HEDT increases rapidly at the beginning and then turns to rise slowly. Analyses of PSD of the bending moment on the shaft of all axial and radial impellers indicate that the speed frequency and the macro low frequency are evident in the frequencies of the bending moment fluctuation. For radial flow impellers, the proportion of macroscopic low frequency increases with the increase of gas flow rate, and the smaller the blade curvature is, the more easily the macro instability in the stirred tank will be affected.