Simulation of Fuel Mixing in Fluidized Beds Using a Combined Tracking Technique

This paper presents an Eulerian-Eulerian-Lagrangian (E-E-L) numerical method to track a limited number of fuel particles in a bulk of inert particles in a gas-solid fluidized bed. The gas and the inert phases are treated as the interpenetrating continua and resolved within the Eulerian-Eulerian framework, whereas the fuel particles are regarded as a discrete phase. To validate the numerical method, the results are compared with experimental data in the form of preferential positions, velocity vectors and the dispersion coefficient of the fuel particles. It is observed that the proposed numerical technique is able to capture the behavior of fuel particles in fluidized beds

Fluid dynamics of a pressurized fluidized bed: comparison between numerical solutions from two-fluid models and experimental results

A validation of four different two-fluid model closures was carried out to investigate the effect of gas-phase turbulence, drift velocity and three dimensionality on the fluid dynamics of a bubbling fluidized bed. At atmospheric conditions, it is verified that gas-phase turbulence has a negligible effect for the bed material and operating conditions used in the investigation, whereas the validation shows some evidence that the gas-phase turbulence has a significant contribution for higher pressures. The drift velocity shows no noticeable effect on the results at any pressure. A comparison between two- and three-dimensional calculations at atmospheric pressure shows that the three-dimensional effects appear to be considerable

Numerical simulation of fluid dynamics in fluidized beds with horizontal heat exchanger tubes

A numerical code, Gemini, based on the implicit multifield method (IMF) of Harlow and Amsden for Eulerian two-fluid modelling, is used to simulate the fluid dynamics of bubbling fluidized beds, assuming no turbulence in the gas or solid phase. The paper gives a formulation of the equations of motion and empirical closure laws in general curvilinear coordinates for calculation of the fluid dynamics in beds with complex internal geometries. A special discretization method for general curvilinear structured grids with multiblock connectivity is implemented, and two-dimensional non-stationary calculations are performed for a bed with a cross-sectional width of 0.3 m, containing two horizontal heat exchanger tubes. The local visible bubble flow and the gas and particle motion around the tubes are briefly discussed and compared with experimental fluid dynamic results at different pressure levels

Dynamics of fibres in a turbulent flow field – A particle-level simulation technique

A particle-level simulation technique has been developed for modelling the flow of fibres in a turbulent flow field. A single fibre is conceived here as a chain of segments, thus enabling the model fibre to have all the degrees of freedom (translation, rotation, bending and twisting) needed to realistically reproduce the dynamics of real fibres. Equations of motion are solved for each segment, accounting for the interaction forces with the fluid, the contact forces with other fibres and the forces that maintain integrity of the fibre. The motion of the fluid is resolved as a combination of 3D mean flow velocities obtained from a CFD code and fluctuating turbulent velocities derived from the Langevin equation. A case of homogeneous turbulence is treated in this paper. The results obtained show that fibre flocs in air-fibre flows can be created even when attractive forces are not present. In such a case, contacts between fibres, properties of an individual fibre (such as flexibility and equilibrium shapes) and properties of the flow of the carrying fluid are shown to govern the physics behind formation and breaking up of fibre flocs. Highly irregular fibre shapes and stiff fibres lead to strong flocculation. The modelling framework applied in this work aims at making possible a numerical model applicable for designing processes involving transport of fibres by air at industrial scale

Hydrodynamics of a bubbling fluidized bed: influence of pressure and fluidization velocity in terms of drag force

Measurements of the visible bubble flow rate and the through-flow velocity of gas inside bubbles have been carried out in a pressurized fluidized bed. Based on the results, it is demonstrated that the representation of in-bed hydrodynamics in terms of particle drag force facilitates a comparison between the influence of pressure and fluidization velocity. By calculating a "potentially available drag force" corresponding to the different operating conditions, it is shown that most of the in-bed parameters, such as bed expansion, bubble volume fraction, bubble rise velocity and local visible bubble flow rate, fall on single curves when plotted versus this force. Deviations occur due to bubble instability, which is largely a pressure-dependent effect. Some mechanisms that together govern bubble instability and splitting are established: (a) The through-flow velocity of gas through the bubbles decreases considerably as the pressure increases. (b) The fluctuations of the through-flow velocity of gas through the bubbles are of the same order of magnitude at all the operating conditions investigated. (c) A given fluctuation in gas velocity has a higher relative influence on the fluctuation in particle drag force at high pressures than at low pressures. In addition, the overall bed behaviour becomes less stable at high Reynolds numbers, i.e. at high pressures and fluidization velocities. The bubble size at a given location in the bed is determined by a complex balance between bubble splitting and coalescence. Both splitting and coalescence are governed by fluctuations in the particle drag force caused by fluctuations in gas velocity. These velocity fluctuations are largely caused by the gas short-cutting between adjacent bubbles. Due to increased coalescence, the bubble flow is redistributed towards the centre of the bed cross-section with both increasing pressure and fluidization velocity