Computed tomographic measurements of void fraction and modeling of the flow in bubble columns.

摘要

A complete understanding of the gas-liquid two phase flow in bubble columns is required for the development of reliable models for scale-up of these multiphase reactors. Although several models have been proposed to describe the hydrodynamics, lack of adequate experimental data has hindered meaningful evaluation of model parameters and model predictions. The Computer Automated Radioactive Particle Tracking (CARPT) facility that was implemented earlier has provided interesting results on the recirculation patterns of the liquid phase. The technique has provided quantitative information on liquid velocities and turbulence parameters as well. In addition to these hydrodynamic parameters the measurement of void fraction is also important. To complement the capabilities of CARPT a scanner for {dollar}gamma{dollar}-ray Computed Tomography (CT) was implemented to quantify the local void fraction and its distribution in two phase flow systems. The automated scanner is capable of imaging flows in test sections between 0.025 m and 0.45 m in diameter at different elevations above the distributor. The scanner makes use of the same detectors used in the CARPT facility and with the use of a specially designed moving collimator provides a spatial resolution of about 5 mm. A non-conventional algorithm based on maximum likelihood principles called the E-M algorithm was used for image reconstruction. Long scanning times are required leading to time averaged density profiles. Although the system is only capable of providing time averaged void fraction distributions, it can provide unique information concerning the structure of two phase flow. The system performance was evaluated by identifying the sources of errors in measurement and their bounds.; The scanner was utilized to quantify the local void fraction and its distribution in bubble columns of five diameters (0.10, 0.14, 0.19, 0.26, 0.30 m internal diameter) and at four superficial gas velocities. The effect of various operating parameters such as column diameter, superficial gas velocity, the type of distributor, the static liquid height and some changes in the physical properties of the liquid phase were studied. For the first time a comprehensive characterization of the void fraction in an air water bubble column using a non-invasive technique was achieved.; The experimental data obtained using CARPT and CT under identical operating conditions was used for developing a methodology for scale-up of bubble columns using a one dimensional model for liquid recirculation. Successful scale-up of liquid hydrodynamics 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 non-reproducibility of the flow in different laboratories. 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. Using the present method for prescribing the mixing length scale, model predictions for scale-up compare satisfactorily for the data that was obtained as part of this research. (Abstract shortened by UMI.)