Particle and bubble movements around tubes immersed in fluidized beds - a numerical study

A numerical study was conducted based on the discrete element method (DEM) to analyze the behavior of particles and bubbles in fluidized-bed combustors with immersed tubes. Effects of fluidization velocity and tube arrangement were investigated in terms of bed voidage, particle-tube impact velocity, impact angle and the number of impacts around a tube for the case of a two-dimensional bed. Calculated particle and bubble flow patterns around a tube were in good agreement with previous experimental findings. Particle-tube impacts were found to be concentrated mainly during the initial period of bubble wake attack. The maximal impact velocities were observed on those parts of the surface of a single tube corresponding to 60 degrees are deflection with respect to the bottom of the tube. The maximal tube erosion rates are predicted for those regions. As for staggered and inline tube banks, quite different particle impacts and bubble behaviors were observed. Bubbles were easily elongated and tended to pass vertically through the lane between the inline tube columns. The distributions of average particle impact velocity and impact angle around a tube were generally asymmetrical. This is attributed to local bubble-passage habit. Validations were performed using a two-dimensional fluidized bed. The experimental results compared well with the DEM simulation results. (C) 1999 Elsevier Science Ltd. All rights reserved.