CFD modeling of gas-fluidized beds with a bimodal particle mixture

A computational fluid dynamics model was developed for gas-solid fluidized beds containing a mixture of two particle species. To calculate stresses of the solid phase, the kinetic theory of granular flow was extended to consi der a binary mixture of smooth, nearly elastic, spheres. The developed mode l was simulated to demonstrate key features of binary mixture fluidization. Bed expansion with a binary mixture of different size panicles, but with i dentical densities, was much higher than that of a system consisting of mon o-sized particles of the same mean size as the bimodal mixture. Minimum flu idization velocity for the binary particle system was significantly lowered . The mixing behavior- of the binary mixture of particles, characterized by the mixing index, increased with increasing superficial gas velocity. For a binary mixture of particles of larger size with lower density and smaller site with higher- density larger; lighter particles segregated to the top of the fluid bed while smaller, heavier particles segregated to the bottom. With increasing fluidization velocity, this segregation pattern reversed a nd "inversion occurred. The drag and gravity force difference between small , heavy particles and large, light particles was dominant at low gas veloci ties. With an increase in gas velocity, however the gradients in granular t emperature and pressure became dominant terms in the equations for the rela tive force and thus velocity between two different particle species.