3.1.3 Cavity under different gas superficial velocities
The cavity appears in the upper of the gas upstream. Its size grows when
the gas superficial velocity increases(Fig 3 and Fig.7).
The cavity formation is analyzed by the particle normal forces inx and z directions. In the x direction, the
gas-solid force pushes the particles away from the gas upstream of the
bed. The particle normal force in x direction in the gas upstream
when cavity appears. Many researches distinguish the appearance of the
cavity by this method. In the z direction, the gas flows upward
near the gas upstream in the feed influence zone. The gas-solid force
stops the particles from moving down. In this paper, can also be taken
as the signal of the cavity occurrence.
(a) Particle normal force in x direction
The particle normal force in x direction is taken to be zero when
cavity appears. Under the same gas superficial velocity, the gas-solid
force seems to be unvaried. As the bed width of left/right area in Type
RB is smaller than Type RA, the particles have low movement in axial
direction in Type RB.
As the particle normal force in x direction is hardly measured in
experiment, it is mainly discussed by theoretical method, which is
described in Appendix . In type RA, the cavity is firstly
assumed to occur before the pinning. The pressure drop is 437 Pa by
computation when in equation (A13) of Appendix , corresponding
to 0.15 m/s. In this condition, the pinning appears according to
equation (2). The assumption is invalid and then the cavity appears
after the pinning. In equation (A14) of Appendix , the critical
pressure drop is 630 Pa, corresponding to 0.23 m/s.
In the same way, the critical pressure drop can also be determined in
type RB. Through calculation, the pinning happens before the cavity. The
cavity appears when pressure drop is 660+660 Pa (left and right beds),
corresponding to 0.39 m/s.
Under the same conditions, the critical gas superficial velocity of
cavity is improved from about 0.23 to 0.39 m/s. The experimental results
is 0.29 to 0.39 m/s, which has the same trends with the calculation
results. The cavity is eliminated by the gas-solid baffles in type RB in
terms of the particle normal force in x direction, especially
when the baffles located at x /L =0.5.
(b) Particle normal force in z direction
One signal of the cavity occurrence is the particle normal force in z
direction becomes zero at the ends of the gas upstream Johnson net.
The gas axial velocity distribution can be computed by CPP method30, which is explained in Appendix . Indicated
by Fig.8, near the gas upstream Johnson net x /L =0, the gas
axial velocity has a maximum value. It increases with the growing of the
gas superficial velocity. Meanwhile, the gas axial velocity decreases
greatly when the gas-solid baffles are used. It denotes the baffles
restrain the gas axial velocity in the feed influence zone and then the
cavity is eliminated.
In detail, the particle normal force in z direction is mainly
influenced by the gas-solid drag, the particle gravity and the
particle-wall frictional forces. The equivalent particle acceleration is
introduced to considering the active forces: gas-solid drag, the
particle gravity. Only when the , (equation (5)), the cavity may occurs.
The equivalent particle acceleration is computed by equation (6). The
coefficient C 2 is introduced to consider the
difference between the average and local gas velocity, the experimental
error. The computed critical gas superficial velocity of cavity improved
from about 0.29 to 0.52 m/s when C 2=2.56. Through
calculation, the baffles have greatest effects on cavity when placed atx /L =0.5.
(5)
, (6)