Baseline Model for Bubbly Flows: Simulation of Monodisperse Flow in Pipes of Different Diameters

Sebastian Kriebitzsch,Roland Rzehak

doi:10.3390/fluids1030029

Abstract

CFD simulations of the multiphase flow in technical equipment are feasible within the framework of interpenetrating continua, the so-called two-fluid modelling. Predictions with multiphase CFD are only possible if a fixed set of closures for the interfacial exchange terms is available that has been validated for a wide range of flow conditions and can therefore reliably be used also for unknown flow problems. To this end, a baseline model, which is applicable for adiabatic bubbly flow, has been specified recently and has been implemented in OpenFOAM. In this work, we compare simulation results obtained using the baseline model with three different sets of experimental data for dispersed gas-liquid pipe flow. Air and water under similar flow conditions have been used in the different experiments, so that the main difference between the experiments is the variation of the pipe diameter from 25 mm to 200 mm. Gas fraction and liquid velocity are reasonably well reproduced, in particular in the bulk of the flow. Discrepancies can be seen in the turbulent kinetic energy, the gas velocity and in the wall peaks of the gas fraction. These can partly be explained by the simplified modelling, but to some extent must be attributed to uncertainty in the experimental data. The need for improved near-wall modelling, turbulence modelling and modelling of the bubble size distribution is highlighted.

Highlights

Many technical processes in industries, such as chemical or electricity, and numerous natural phenomena involve multiphase flow
The system studied by Liu [36] is the vertical up-flow of water and air in a round pipe with inner diameter D = 57.2 mm. They performed experiments designed to show the effects of bubble size by using a special gas injector that allows adjusting the bubble size independent of liquid and gas mass fluxes
A change in the gas fraction profile from wall to core peak with increasing bubble size was observed, as well as turbulence suppression in the pipe center for combinations of high liquid and low gas mass flux, which correspond to the smallest bubble sizes

Summary

Introduction

Many technical processes in industries, such as chemical or electricity, and numerous natural phenomena involve multiphase flow. Multiphase computational fluid dynamics can be a valuable tool for analysis and prediction, as one can get a detailed insight into the local flow field. In this way, CFD potentially could contribute greatly to the optimization of existing and the development of new processes, respectively process equipment. CFD simulations are possible within the Eulerian two-fluid framework of interpenetrating continua In this framework, the details of the flow on the small length scale of the disperse phase are eliminated due to an averaging procedure. The details of the flow on the small length scale of the disperse phase are eliminated due to an averaging procedure This relaxes the requirements on the grid resolution and, facilitates computations on large domains. The interfacial transfer of momentum, mass and heat occurring on the small scales needs to be modelled by suitable closure relations

Results

Discussion

Conclusion