78
C. Neugebauer et al.
mean Sauter diameter of the mixture, i.e. fluidization velocities relate to multiples of
the minimum fluidization velocity of the mixture. All experiments were performed at
ambient temperature with non-heated fluidization gas. For under-flow experiments
the weir gap height was set to 20 mm, in over-flow experiments the weir was set to
height of 25 cm. In all experiments the left chamber was filled to a static bed height
of 12.5 cm, the right chamber to a height of 10 cm.
In the over-flow weir scenario, both chambers were filled with particles until the
static bed heights were achieved. The mass flow controller for the fluidization gas
was then switched on. Following the start-up period of the mass flow controller
(which can result in a small amount of particle exchange between the chambers), the
high-speed recording was started and images of particle motion were taken for the
next 5 s, resulting in 5000 images.
All batch experiments to study the recirculation at under-flow weirs were performed in the following way: (1) Filling both chambers of particles to a certain bed
height (gap closed by a lid); (2) start of fluidization in both chambers via setting
reference value for gas mass flow controller; (3) after start-up of fluidization sudden
removal of lid (opening the gap) and start of image acquisition with the high-speed
camera system.
An example sequence of images, showing every 500th frame, is shown in Fig. 7.
These sequences form the basis for image analysis, particle tracking and the determination of the exchange rates between the chambers.
3.4 Results
The presentation and discussion of the results is structured as follows: First a measure for quantification of internal particle circulation is introduced. Then the results
obtained from PTV measurements and Voronoi-tracking are presented for 1.8 mm
particles at different fluidization velocities. Hereby results for under-flow weirs are
compared to results obtained for over-flow weirs. Following is the presentation of
results for 3 mm particles and a comparison with results for 1.8 mm particles, characterizing the influence of particle size on internal circulation. Finally, results for
internal circulation for mixtures of 1.8 and 3 mm particles of different mass fractions
are discussed to show the influence of bi-(poly-)disperse particles on transport at
over-flow and under-flow weirs.
In order to quantify the internal recirculation R, we use the concept as introduced
by Charlou et al. [28] in their study on residence time behavior in paddle dryers.
They related the internal circulation as the ratio of particles moving ‘backwards’,
B (against the dominant transport direction) to the net value of particles moving
‘forwards’, F:
R(t) =
B(t)
F(t) − B(t)
(12)
C. Neugebauer et al.
mean Sauter diameter of the mixture, i.e. fluidization velocities relate to multiples of
the minimum fluidization velocity of the mixture. All experiments were performed at
ambient temperature with non-heated fluidization gas. For under-flow experiments
the weir gap height was set to 20 mm, in over-flow experiments the weir was set to
height of 25 cm. In all experiments the left chamber was filled to a static bed height
of 12.5 cm, the right chamber to a height of 10 cm.
In the over-flow weir scenario, both chambers were filled with particles until the
static bed heights were achieved. The mass flow controller for the fluidization gas
was then switched on. Following the start-up period of the mass flow controller
(which can result in a small amount of particle exchange between the chambers), the
high-speed recording was started and images of particle motion were taken for the
next 5 s, resulting in 5000 images.
All batch experiments to study the recirculation at under-flow weirs were performed in the following way: (1) Filling both chambers of particles to a certain bed
height (gap closed by a lid); (2) start of fluidization in both chambers via setting
reference value for gas mass flow controller; (3) after start-up of fluidization sudden
removal of lid (opening the gap) and start of image acquisition with the high-speed
camera system.
An example sequence of images, showing every 500th frame, is shown in Fig. 7.
These sequences form the basis for image analysis, particle tracking and the determination of the exchange rates between the chambers.
3.4 Results
The presentation and discussion of the results is structured as follows: First a measure for quantification of internal particle circulation is introduced. Then the results
obtained from PTV measurements and Voronoi-tracking are presented for 1.8 mm
particles at different fluidization velocities. Hereby results for under-flow weirs are
compared to results obtained for over-flow weirs. Following is the presentation of
results for 3 mm particles and a comparison with results for 1.8 mm particles, characterizing the influence of particle size on internal circulation. Finally, results for
internal circulation for mixtures of 1.8 and 3 mm particles of different mass fractions
are discussed to show the influence of bi-(poly-)disperse particles on transport at
over-flow and under-flow weirs.
In order to quantify the internal recirculation R, we use the concept as introduced
by Charlou et al. [28] in their study on residence time behavior in paddle dryers.
They related the internal circulation as the ratio of particles moving ‘backwards’,
B (against the dominant transport direction) to the net value of particles moving
‘forwards’, F:
R(t) =
B(t)
F(t) − B(t)
(12)