Modelling of dispersed bubble and droplet flow at high phase fractions

Abstract

The present paper describes an Eulerian two-fluid model for the prediction of dispersed two-phase (gas/liquid and liquid/liquid) flow at high volume fractions of the dispersed phase. The model is based on the standard Eulerian approaches for modelling two-phase flow that have hitherto been limited in validity to dilute systems. An extension to high phase fractions is made here and this involves two aspects. First, the closure models for inter-phase forces (namely drag and lift) are modified to account for the high concentration of the dispersed phase. Second, a turbulence model based on the k– ε equations for the mixture of the two phases is formulated. This turbulence model is suitable for computations at all phase fraction values and reduces to the equivalent single phase model in the extremes when only one or other of the phases is present. The model uses a response function to link the turbulent fluctuations of the continuous and dispersed phases. The variation of this response function with phase fraction is determined from experimental evidence made available recently. The overall model is applied to the prediction of air/water bubble flow in a pipe with a sudden enlargement where phase fractions can reach 25% and for which experimental data exist. The calculations show that marked improvement in the quality of the predictions, as compared to measurements, is obtained over the available model for dilute systems.