Simulation of bubbly flows: Comparison between direct quadrature method of moments (DQMOM) and method of classes (CM)

In typical bubbly flow applications, bubbles can break or coalesce due to bubble–bubble and bubble–fluid interactions in presence of turbulence. Under this assumption, a fixed bubble size model might not be suitable for predicting correct multiphase flow behaviour in the gas–liquid system. For example, breakage and coalescence events produce very different bubble size distribution and then affects the interfacial interactions between the phases as heat and mass transfer and exchange forces, for example the drag and lift forces. In the present work, a rectangular bubble column and stirred tank reactor are modelled using an open-source computational fluid dynamics CFD package OpenFOAM. A population balance equation is introduced in the mathematical model to account the effects of bubble size distribution taking account the effect of coalescence and break-up phenomena on the hydrodynamic behaviour of multiphase flow. For solving the population balance equation an efficient numerical technique is integrated to enable the simulation of more complex flows that are encountered in industrial applications. Furthermore, the direct quadrature method of moments (DQMOM) and the method of classes (CM) are implemented and compared using an open source CFD package (OpenFOAM). An Eulerian–Eulerian approach with a standard k–εk–ε model of turbulence is used. The momentum exchange between the bubbles and the continuous phase is taken into account with drag, lift and virtual mass forces. The predicted results are compared with measured data available in the scientific literature; they show that the gas volume fraction, velocity profiles and local bubble size are in good agreements.