Size, Spatial Distribution and Velocity of Bubbles and the Fluid Dynamics of the continuous Phase in Stirred Aerated Vessels

Aerated stirred vessels are widely used in the chemical and process industry. In these vessels the interfacial area between the two phases and the gas hold-up characterize the efficiency of the gas-liquid contact. Whereas the interfacial area determines the heat and mass transfer rate, the gas hold-up determines the residence time. For these reasons, improving the knowledge concerning the bubble characteristics, namely their sizes, spatial coordinates and number is of primary importance. These parameters depend on the operating conditions, such as the volumetric flow rate of the gas, the type and speed of the impeller, the injector type and its geometry and the physical properties of the liquid phase. From the viewpoint of the technical application the development of empirical correlations is of major interest. Most often, these equations show good agreement when they are compared with the database which they are based on, but they often fail to predict the gas hold-up for the other operating conditions. In particular, difficulties have been reported for extremely low gas hold-up and low gas flow rate, which are of major interest for the validation of numerical two-phase codes.

In order to provide a wide range of experimental results, the entire volume of a stirred aerated vessel has been recorded by means of the non-intrusive short time holography. The optically reconstructed 3D distribution of the interfaces between both phases from the hologram is saved as a series of 2D images representing slices of the entire vessel at different depth positions. These slices are generated by an intensified CCD camera with a small depth of focus moving in the depth direction with respect to the hologram during the reconstruction (Fig. 1).

For the fully automated processing of the images, for the determination of the bubble characteristics and the numerical reconstruction of the 3D phase distribution in the vessel, powerful imaging routines have been developed, which do no require manual control of the data-handling. In additional, post-processing steps, the gas hold-up, the interfacial area, the bubble size distribution and the bubble spacing are calculated. The flow chart of the image processing steps is presented in the Fig. 2.

The applied optical measurement technique in conjunction with new developed image processing software allowed 3D numerical reconstruction of the bubbles distribution (Fig. 3) and calculation of bubbles characteristics in the whole investigated volume at once (click on the image to see rotating object).