Fluid Dynamics and Erosion in a Fluidized Bed for Energy Production. A Numerical and Experimental Study

Doktorsavhandling, 2004

This thesis presents an experimental and a numerical investigation of fluid dynamics and erosion of heat transfer tubes in fluidized beds for energy conversion. The objective is to obtain recommendations for suitable models that can be used to predict the fluid dynamics and material wastage of heat exchanger tubes in fluidized beds. Two models describing the particle phase rheology are numerically investigated and the results are compared with previous experiments done in a freely bubbling fluidized bed. The comparison shows that the kinetic theory of granular flow gives a significantly better prediction of the bubble flow than the constant particle viscosity model. Simulations with two commercial flow solvers show different results for the same case. Hence, using an 'off the shelf' commercial flow solver does not necessarily give reliable predictions for a fluidized bed. Furthermore, it is shown that the modelling of the interaction between the air feed system and the fluidized bed has a significant influence on the results. The prediction of the flow in a bubbling fluidized bed with an immersed tube bundle shows qualitative agreement with experimental results. The erosion of the tubes is predicted with two different models. The first model is based on the viscous dissipation of kinetic energy and the second on kinetic theory of granular flow. For a tube situated at the mid-height of the bed, the kinetic theory erosion model predicts a double-peak wastage pattern on the bottom part of the tube. This is in accordance with the experimental results. The viscous dissipation model, however, shows only one peak of erosion at the lower part of the tube. By decomposition of the viscous dissipation into a normal and a shearing component it is shown that these have quite different characteristics. The shearing component shows a clear double-peak pattern, while the normal component shows a single-peak pattern at the bottom of the tube. This indicates that the shearing component governs the main part of the wastage pattern and the normal component contributes to only a fraction of the total wastage. From preliminary simulations of a loop seal with fine particles, it is shown that the drag force on the particles exerted by the gas phase is too high in the simulations as compared to the experiments. This difference is believed to be caused by a slight cohesiveness and cluster formation of the particles in the experiments. To account for this in the numerical simulations, the real mean particle diameter is replaced with a fictive mean particle diameter. The simulations using the fictive diameter show that the three-dimensional nature of the loop seal must be taken into account in order to obtain realistic results.