Effect of scale on liquid recirculation in bubble columns

Successful scale-up of liquid hydrodynamics in bubble columns using a one dimensional model requires an adequate closure scheme for the Reynolds shear stress. The existing correlations for the prescription of the eddy viscosity or the mixing length scale are demonstrated to be applicable only for a limited range of conditions and consequently cannot be used for scale-up predictions. A method for estimating the mixing length scale has been explored and an attempt at unifying a wide range of data available in the literature within the purview of the method has been made. The futility of such an attempt is attributed to the nonreproducibility of the flow in different laboratories and the consequent lack of data obtained under identical conditions. It is demonstrated, however, that scale-up based on the mixing length distribution is possible when it is obtained from a consistent set of data for liquid velocity and gas void fraction profiles. Such a data set was obtained in the authors' laboratory using CARPT (Computer Automated Radioactive Particle Tracking) and CT (Computed Tomogaphy). Using the present method for prescribing the mixing length scale, model predictions for scale-up compare satisfactorily for the data that were obtained in the authors' laboratory.