Numerical simulation of turbulence modulation in Two-Phase flows
Creators: name: Mohanarangam, Krishna
Publisher: name: RMIT University
Year: 2024

description: With the increase of computational power, computational modelling of two-phase flow problems using computational fluid dynamics (CFD) techniques is gradually becoming attractive in the engineering field. The major aim of this thesis is to investigate the Turbulence Modulation (TM) of dilute two phase flows. In order to carry out this approach, an in house research code employing Two-Fluid model, with additional source terms to account for the presence of the dispersed phase in the turbulence equations has been employed. <br><br>Various density regimes of the two-phase flows have been investigated in this thesis, namely the dilute gas-particle flow, liquid-particle flow and also the liquid-air flows. While the density is quite high for the dispersed phase flow for the gas-particle flow, the density ratio is almost the same for the liquid particle flow, while for the air-liquid flow the density is quite high for the carrier phase flow. The study of all these density regimes gives a clear picture of how the carrier phase behaves in the presence of the dispersed phases, which ultimately leads to better design and safety of many two-phase flow equipments. <br><br>For the dilute gas-particle flows, particle-turbulence interaction over a backward-facing step geometry was numerically investigated. Two different particle classes with same Stokes number and varied particle Reynolds number are considered in this study. The turbulence modulation of the carrier phase in the presence of the dispersed particulate phase is simulated and compared against the experimental data. Despite the fact that the two particles used in this study share the same Stokes number their behaviour is found to be considerably different in the turbulent flow field, which basically underlines the fact that the Stokes number alone is not enough to fully describe the behaviour of particles, there by, herein particle Reynolds number is also investigated to fully understand their behaviour. <br><br>A detailed study into the turbulent behaviour of dilute particulate flow under the influence of two carrier phases namely gas and liquid was also been carried out behind a sudden expansion geometry. The major endeavour of the study is to ascertain the response of the particles within the carrier (gas or liquid) phase. The main aim prompting the current study is the density difference between the carrier and the dispersed phase. While the ratio is quite high in terms of the dispersed phase for the gas-particle flows, the ratio is far more less in terms of the liquid-particle flows. <br><br>For the Liquid-Air flows the phenomenon of drag reduction by the injection of micro-bubbles into turbulent boundary layer has been investigated using an Eulerian-Eulerian two-fluid model. Two variants namely the Inhomogeneous and MUSIG (MUltiple-SIze-Group) based on Population balance models are investigated. The simulated results were benchmarked against the experimental findings and also against other numerical studies explaining the various aspects of drag reduction. The under predictions of the MUSIG model at low rates was investigated and reported, their predictions seem to fair better with the decrease of the break-up tendency among the micro-bubbles.<br>