Computational fluid dynamics modelling of dense particle rheology and freeboard gas velocity fields in a fluidised bed

The demands on the Australian power industry to reduce green house gases have generated research into capturing carbon dioxide. Due to the high moisture content of Victorian brown coal more energy is required to dry coal than Black coal or Anthracite. The consequence of the additional energy to dry the coal is an increase in carbon dioxide emissions. It is for this reason that fluidised bed has become a focus of interest to the Victorian power generators in the form of gasification through integrated gasification combined cycles (IGCC) or combustors. Fluidised bed technology has diverse industrial applications ranging from the gasification of coal in the power industry to chemical reactions for the plastics and oil and gas industries. Benefits of fluidised beds are the excellent heat transfer and mixing as well as containment of the process within a pressurised vessel. The disadvantages are the complex non-linear behaviour of the gas and solids interactions that occur within the process, which are difficult to measure and predict. Computational fluid dynamics (CFD) has the capability to model multiphase flows and can assist in understanding gas-solid fluidised beds by modelling their hydrodynamics. The purpose of this thesis is to advance CFD modelling of fluidised beds by accounting for phenomena that are typically neglected in CFD models. In the past multiphase Eulerian-Eulerian, gas-solid models used the kinetic theory of granular flow to describe the motion of solid within a fluidised bed. While this assumption has clearly shown to be a good approximation for dilute particle system, the kinetic theory of granular flow begins to fall short when particles begin to pack together. An addition to the kinetic theory of granular flow has been developed to include a closure term for the quasi-static stress associated with the long term particle contact at high solid concentrations. Simulations of bubble growth and bed porosity through a two-dimensional fluidised bed are validated against previous quasi static stress model and experimental measurements. Experiments were conducted in a rectangular three-dimensional gas-solid fluidised bed to determine the behaviour of the freeboard gas after a single bubble eruption. The experiments used a particle imaging velocimetry (PIV) measurement technique to visualise and measure the gas flow within the freeboard after a single bubble eruption. The PIV method had the ability to not only measure the velocities of the gas, but also allowed the visualisation of gas vortices within the freeboard. The PIV experiments of the single bubble eruption were then used to validate CFD model of the fluidised bed. Two-dimensional and three-dimensional fluidised bed simulations were conducted using the kinetic theory of granular flow approach including the quasi-static model and turbulence model. The gas vortices were predicted by both two-dimensional and three dimensional fluidised bed simulations. Validation of the simulations with PIV results show good agreement was achieved both qualitatively and quantitatively.