3. Results and discussion
3.1. LDPE Sphere
fluidization
As a benchmark, case 1 of the study investigates the fluidization
behavior of LDPE particles with superficial gas velocity from
1.4U mf to 3.4U mf. Figure 5
compares the fluidization behavior of LDPE spherical particles with
varying superficial gas velocity. The image frame with the maximum
expanded bed height has been chosen for each superficial gas velocity
for demonstration purposes (Figure 5a-d). At the lower superficial gas
velocity (1.4 U mf and 2.0U mf ), slug flows are formed due to the
relatively narrow bed configuration, with the maximum expanded bed
height increases significantly due to the increase of superficial gas
velocity from 1.4 U mf and 2.0U mf. Markedly, such a trend ceases with the
further increase of superficial
gas velocity. At 3.0 U mf and 3.4U mf, the maximum expanded bed height drops
significantly compared to 2.0 U mf case, and two
regions with visually different particle void fraction are formed.
Generally similar to a spouted bed, the loosely packed fountain region
is at the top of the bed while a densely packed region forms at the
bottom. To quantify the overall fountain region length, the 500 frames
are overlaid to generate the time-averaged bed images at these
fluidization velocities. The two regions are distinct due to the
grayscale value and the fountain height is measured to be 3.36 cm and
5.02 cm at a superficial gas velocity of 3.0 U mfand 3.4 U mf, respectively. The current transition
from slugging to spouting is similar to the behavior observed in some of
the so-called spout-fluid beds using a central jet together with an
auxiliary flow through a porous or perforated distributor plate at
certain flow conditions33,34.
The velocity field for 2Umf case is shown in Figure 6.
The color encodes the velocity magnitude, Vmag of the
particles. Due to the narrow bed of solids, at t=0s, the bubbles formed
at the distributor plate grow into a flat slug which is clearly
identified by the almost uniform upward velocity region of around 20
cm/s with a size of around 5 cm in length. Above the slugs, the
particles from previously disintegrated slugs fall onto the slug and
have a velocity close to zero. Below the slug, the particles have
relatively low velocities and separate from the slug. At t=0.125s, the
slug continues to move upwards, and in the meanwhile accelerate
particles from previous slugs. At the bottom of the slug, the particles
that cannot stick to the slug start to replenish by raining solids. The
size of the slug remains almost unchanged. At t=0.313s, a large portion
of the slug’s particles starts to fall which spans a length of around 10
cm, whereas the rest of the particles continue to rise and decelerate as
shown in the velocity map. At t=0.556s, all the particles in the slug
start to fall and reach as high as 56 cm/s. The particles at bottom of
the bed start to form another slug with a small upward velocity. At
t=0.763s, the particles continue to fall while another slug continues to
form. In general, the dynamics of the LDPE particles at 2
Umf resembles the typical Geldart type D particles
behavior, even though no complete void space or multiple slugs is formed
presumably due to the relative shallow bed configuration. The rising
rate of the bubble or the slug can be estimated to be around 35 cm/s by
tracking the almost zero velocity band below the slug. This result is in
good agreement with the correlation results proposed by Stewart and
Davidson35,
U b=
0.35(gD t)0.5
where D t is the diameter of the bed, and gis the gravitational constant, which results in a value of 28 cm/s.