44
2 Bubbling Properties in Pulsed Fluidised Beds
Table 2.3 Scenarios of
oscillatory flows U 0 = A·U mf
+ B·[1 + sin(2πft)]
investigated
Symbol
Parameter
Value
f
Frequency (Hz)
3, 4, 5, 6, 7, 8, 9, 10
B
Amplitude (cm/s)
5, 7, 9, 11
A
Offset/U mf (–)
0.5, 0.75, 1, 1.25
Table 2.4 Selected
referenced gas flows U 0 =
A·U mf + B·[1 + sin(2πft)]
Abbreviation f (Hz)
A (–)
B (cm/s)
F4–F8
4, 5, 6, 7, 8 0.5
5
B5–B11
5
0.5
5, 7, 9, 11
A1–A4
5
0.5, 0.75, 1, 1.25 5
The selected offsets are normalised with the minimum fluidised velocity U mf , and
set to different values 0.5 (half-fluidisation), 0.75 (sub-fluidisation), 1 (minimumfluidisation) and 1.25 (full-fluidisation), respectively. Overall, the selected values for
those flow parameters are summarised in Table 2.3. The response of bubble properties to the variations in pulse conditions is fairly nonlinear, so each parameter is
examined with four carefully selected values at least. For the purpose of comparison,
particles are also fluidised at a corresponding constant flow rate U 0 = A·U mf + B.
Therefore, the overall volumetric flow of gas entering the system is the same as the
pulsed flow used.
For convenience, references are chosen from the investigated pulsed flows above
as the representative cases to analyse the characteristics of the flow patterns. They
are given the abbreviations as shown in Table 2.4. For example, F4 stands for the
system pulsed under a frequency of 4 Hz, an amplitude of 5 cm/s and an offset ratio
of 0.5.
2.3.2.1 Bubble Formation in Pulsed Beds
Different from steady flow fluidisation, at which bubbles continuously form, bubbles
only nucleate periodically under pulsed flows. It can be detected that the injected
pulse initially induces a channel-like slug at the surface of the distributor plate, and
it grows as the oscillating gas flow rate increases, as shown in Fig. 2.7a–d. After the
gas velocity reaches its maximum, the solids start to contract, and the horizontal slug
begins to collapse. The slug firstly forms tips (Fig. 2.7b), then it converges towards the
nearby tips (Fig. 2.7c), and eventually split into bubbles. When the system creates a
structured flow, the entire process repeats with bubbles shifting their nucleation sites
alternatively. On the other hand, in the corresponding steady flow fluidised bed, no
slug is observed at the bottom. Small bubbles form initially, and then rapidly coalesce
after detaching from the plate, as shown in Fig. 2.7e–h.
Previous experimental studies were conducted by Regelink [11] at Delft University of Technology. The author investigated bubble flows using mainly Geldart B
2 Bubbling Properties in Pulsed Fluidised Beds
Table 2.3 Scenarios of
oscillatory flows U 0 = A·U mf
+ B·[1 + sin(2πft)]
investigated
Symbol
Parameter
Value
f
Frequency (Hz)
3, 4, 5, 6, 7, 8, 9, 10
B
Amplitude (cm/s)
5, 7, 9, 11
A
Offset/U mf (–)
0.5, 0.75, 1, 1.25
Table 2.4 Selected
referenced gas flows U 0 =
A·U mf + B·[1 + sin(2πft)]
Abbreviation f (Hz)
A (–)
B (cm/s)
F4–F8
4, 5, 6, 7, 8 0.5
5
B5–B11
5
0.5
5, 7, 9, 11
A1–A4
5
0.5, 0.75, 1, 1.25 5
The selected offsets are normalised with the minimum fluidised velocity U mf , and
set to different values 0.5 (half-fluidisation), 0.75 (sub-fluidisation), 1 (minimumfluidisation) and 1.25 (full-fluidisation), respectively. Overall, the selected values for
those flow parameters are summarised in Table 2.3. The response of bubble properties to the variations in pulse conditions is fairly nonlinear, so each parameter is
examined with four carefully selected values at least. For the purpose of comparison,
particles are also fluidised at a corresponding constant flow rate U 0 = A·U mf + B.
Therefore, the overall volumetric flow of gas entering the system is the same as the
pulsed flow used.
For convenience, references are chosen from the investigated pulsed flows above
as the representative cases to analyse the characteristics of the flow patterns. They
are given the abbreviations as shown in Table 2.4. For example, F4 stands for the
system pulsed under a frequency of 4 Hz, an amplitude of 5 cm/s and an offset ratio
of 0.5.
2.3.2.1 Bubble Formation in Pulsed Beds
Different from steady flow fluidisation, at which bubbles continuously form, bubbles
only nucleate periodically under pulsed flows. It can be detected that the injected
pulse initially induces a channel-like slug at the surface of the distributor plate, and
it grows as the oscillating gas flow rate increases, as shown in Fig. 2.7a–d. After the
gas velocity reaches its maximum, the solids start to contract, and the horizontal slug
begins to collapse. The slug firstly forms tips (Fig. 2.7b), then it converges towards the
nearby tips (Fig. 2.7c), and eventually split into bubbles. When the system creates a
structured flow, the entire process repeats with bubbles shifting their nucleation sites
alternatively. On the other hand, in the corresponding steady flow fluidised bed, no
slug is observed at the bottom. Small bubbles form initially, and then rapidly coalesce
after detaching from the plate, as shown in Fig. 2.7e–h.
Previous experimental studies were conducted by Regelink [11] at Delft University of Technology. The author investigated bubble flows using mainly Geldart B