the particle concentration, Dh is the hydraulic diameter, μ is the
friction coefficient and j is the Janssen's coefficient. In Eq. (3) the
second term on the right side of the equation is positive when the
particles are being fluidized.
The compressive yield stress σs, is expected to increase monotonically
with an increase in the particle concentration, ϕ. Thus, Jackson
[28,37] proposed the following expression:
σs =
F
ϕ−ϕmin
ϕmax−ϕ
if ϕmin
≤ϕ≤ϕmax
0 if ϕbϕmin
8><
>:
ð4Þ
where F is a constant that can be adjusted experimentally and ϕmin
and ϕmax are the minimum and maximum particle concentration,
respectively. For the defluidization branch, Eq. (3) can be restructured
as follows
∂ϕ
∂z
=
−ρsgug
vt
ϕ
ð1−ϕÞn− 4
Dh
μjσs + ϕρsg
∂σs
∂ϕ
; ð5Þ
where ∂σs/∂ϕ can be obtained from Eq. (4). Eq. (5) can be integrated
numerically to obtain the voidage distribution along the height of the
bed. The integration starts at z=0 (free surface of the bed), where
σs=0 (ϕ=ϕmin) and finishes at z=h (the bottom of the bed), where
the following condition must be fulfilled:
Aρs∫h
0 ϕðzÞdz = m; ð6Þ
where m is the mass of particles in the bed.