| The effects of scale-up on the hydrodynamics of circulating fluidized beds (CFB) are investigated using a single cold laboratory facility with the ability to recycle fluidization gas mixtures of adjustable kinematic viscosity. Tests are conducted with a plastic grit, glass beads, and steel powder to simulate the high-temperature fluidization in CFB risers of 0.32, 0.46, and 1.0 m diameter, respectively. The hydrodynamic analogy is achieved by matching five dimensionless parameters derived from dimensional analysis.; The hydrodynamic diameter, defined as the particle diameter times particle sphericity replaces the particle diameter appearing in the dimensionless numbers. Study of static pressure profiles recorded in the riser indicates that the riser hydrodynamics is unaffected by variations of the total solid inventory. However, electrostatic forces resulting from particle collisions are shown to affect riser hydrodynamics. These are eliminated from the present experiments by adding an anti-static powder.; Vertical profiles of static pressure are found to scale with bed diameter, while the probability-density-function of pressure fluctuations scales with particle diameter. Once properly non-dimensionalized, these flow variables have similar functional form with analogous combustors of 0.32 m and 0.46 m in diameter.; These observations indicate that the five dimensionless numbers under consideration are sufficient to describe the global hydrodynamics of CFB risers, at least for the analogous diameters above. CFB riser with a larger analogous diameter of 1 m exhibits higher overall dimensionless pressure drop, primarily near the bottom of the riser, larger dimensionless pressure fluctuations, and a greater tendency to choke.; Finally, the onset of choking is found to be well predicted by the correlation of Yang (Powder Technology, 35 (1983) p.143). |
