Knowledge of bubbly flows is important to the chemical and process industries, where a number of devices operate under bubble-flow conditions in order to attain large interfacial areas for heat and mass transfer. Additionally, gas-liquid bubbly flow phenomena play an important role in many scenarios concerning the safety analysis of nuclear reactor systems. Often, prediction of the phase distribution is of paramount importance for the correct design of the equipment. In bubbly flows, performance of the system is coupled with various hydrodynamic variables, such as bubble size distribution, bubble coalescence and break-up rate and the gas–liquid interfacial area concentration. The mean bubble diameter, serves as a link between the gas volume fraction and interfacial area concentration. An accurate knowledge of local distributions of these three parameters is of great importance to the eventual understanding and modeling of the interfacial transfer processes. Moreover, the local flow conditions of the bubbly flow, both qualitatively and quantitatively can be described. Hence, accurate predictions about such flow variables and the generalized relationships among them are quite necessary to understand the turbulent transport phenomena of two-phase bubbly flow

In recent years, the computational fluid dynamics (CFD) have become a viable technique with such good predictive capabilities. The state-of-the-art of present CFD simulation is to adjust the “mean” bubble size by a trial-and-error procedure by resembling the available experimental data and the value so chosen often is far from reality. Available estimates of the local interfacial area are based on the predicted hold-up and assumed mean bubble size, which introduces error in the calculation. In order to overcome this knowledge of bubble size distribution is essential. The Population balance method is a well known method for tracking the size distribution of the dispersed phase and accounting for the breakage and coalescence effects in bubbly flows. This approach is concerned with maintaining a record of the number of bubbles initially and tracking their evolution in space over time.

In the present work, an attempt has been made to combine the population balance models with the CFD for the case of two phase flow in vertical pipe flow. Commonly practiced break-up and coalescence models have been used for the simulation along with the proposed modifications. The model predictions of local radial distributions of void fraction, Sauter mean diameter, interfacial area concentration, and gas and liquid velocities, are compared with the experimental data of Liu and Bankoff (1993a, b). We discuss the shortcomings of the current models with the possible improvements in light of the extensive comparison.

References:

Liu, T. J., and S. G. Bankoff, Structure of air–water bubbly flow in a vertical pipe. I. Liquid mean velocity and turbulence measurements, Int. J. Heat Mass Trans. 36 (1993) 1049–1060.

Liu, T. J., and S. G. Bankoff, Structure of air–water bubbly flow in a vertical pipe. II. Void fraction, bubble velocity and bubble size distribution, Int. J. Heat Mass Trans. 36 (1993) 1061–1072