(54a) Computation of Minimum Bubbling Velocity for Large Particles

A theory for the origin of bubbles in fluidized beds is described in chapter 6
in Gidaspow's book, Multiphase Flow and Fluidization, Academic Press, 1994. The
theory shows that without the effects of solids pressure, the Ergun drag law
leads to bubble formation for small particle Reynolds numbers and to dispersion
for large Reynolds numbers. We have now discovered by numerical simulation using
the codes in Gidaspow and Jiradilok 2009 book,Computational Techniques, Nova
Science that for large particles there exists a region above minimum
fluidization without bubble formation, similar to that for Geldart group A
particles.

We have visually confirmed the existence of a bubble free region for fluidization of a mixture of 5 mm glass beads and 3.5 mm red glass beads in a  15 cm wide two dimensional glass fluidized bed. Similar to the computation the smaller red beads are near the top of the bed. When the bed width is decreased to about 5 cm, we see distinct slugging. Such behavior is well known and was computed . To obtain non-slugging behavior, the bed width needs to be sufficiently large.

The computations show that in the region between minimum fluidization and minimum bubbling the granular temperatures of the particles are sufficiently large for the dispersion coefficients to be high enough near minimum bubbling velocity. The computed wall to bed heat transfer coefficients are also sufficiently high to allow heating of the bed through the wall. Hence this new flow regime is practical and allows high production rates, due to the high gas velocities that can be achieved with the large particles.