24
1 Introduction
Both the experimental and numerical results indicate that the formation of patterns
in shallow layers is a consequence of system hydrodynamics. As discussed, de Martín
et al. [30] demonstrated that the onset of a gas-driven pattern can be described by p
and f /f n , independent of the particle shape and the apparent solid friction. However,
for vibro-driven patterns, many researchers have associated the solids friction with the
selection of the pattern wavelength and configuration [13, 69, 80, 95], showing that
the solid friction is responsible for the saturation of instability growth. Therefore,
more pronounced effects of solid friction should be expected in dense and deep
granular systems.
1.6 Origin of Bubble Patterns in Pulsed Fluidised Beds
The study of surface patterns in shallow granular systems opens a new gateway
toward creating macroscopic structured flows in the systems of interest, for applied
processing engineering and the intensification of gas-solid operations. By creating
predictable flow structures, one could control tightly the transport rates and degree
of mixing via the application of an external actuator. Therefore, a broad range
of applications, such as mixers, coaters, dryers, and reactors, can be designed
accordingly.
In the context of fluidisation, vibro-fluidised beds have been used extensively to
improve the powder flowability and the solids mixing, both alone [27, 71, 73] or in
combination with the pulsation in gas flows [52]. However, the experimental work
demonstrated the surface structures degenerate quickly when the thickness of the
layer increases, limited up to approximately 40 particle diameters in Geldart A to D
particles [78], whereas a range from 3 to 30 particle diameters in Geldart B particles
[105].
One potential cause for the collapse of the pattern is due to the limitation of
energy propagation. The energy injected via the particle-bottom plate via mechanical
impact limits the source of energy to a plane boundary (e.g., the shaken plate in a
vibrated bed). As a result, energy is gradually dissipated through the inelastic particle
collision within the bed and cannot transmit to the particles dispositioned at a higher
level. For this reason, intensively used vibro-assisted fluidisation can be regarded as
a good approach to facilitate the mixing of powder and homogenise particle flow,
but it is difficult to impose recursive macroscopic flow patterns. Alternatively, via
oscillating the gas flow, one introduces extra mechanical energy via the solid-fluid
interaction, in which the fluid, as an energy source, is distributed across the entire
volume and exchange momentum simultaneously with every individual particle. The
dissipation of energy through interparticle inelastic collisions still defines the length
scale in the response of single particle, but since the entire system is excited, one can
create a better coupling between the dynamic of the perturbation, in this case, the
dynamic drag force, and the collective response of the solid motion. For that reason,
the surface waves in a pulsed shallow layer can persist to a height of approximately
40 particle diameters for Geldart B particles, above which gas bubbles begin to form.
1 Introduction
Both the experimental and numerical results indicate that the formation of patterns
in shallow layers is a consequence of system hydrodynamics. As discussed, de Martín
et al. [30] demonstrated that the onset of a gas-driven pattern can be described by p
and f /f n , independent of the particle shape and the apparent solid friction. However,
for vibro-driven patterns, many researchers have associated the solids friction with the
selection of the pattern wavelength and configuration [13, 69, 80, 95], showing that
the solid friction is responsible for the saturation of instability growth. Therefore,
more pronounced effects of solid friction should be expected in dense and deep
granular systems.
1.6 Origin of Bubble Patterns in Pulsed Fluidised Beds
The study of surface patterns in shallow granular systems opens a new gateway
toward creating macroscopic structured flows in the systems of interest, for applied
processing engineering and the intensification of gas-solid operations. By creating
predictable flow structures, one could control tightly the transport rates and degree
of mixing via the application of an external actuator. Therefore, a broad range
of applications, such as mixers, coaters, dryers, and reactors, can be designed
accordingly.
In the context of fluidisation, vibro-fluidised beds have been used extensively to
improve the powder flowability and the solids mixing, both alone [27, 71, 73] or in
combination with the pulsation in gas flows [52]. However, the experimental work
demonstrated the surface structures degenerate quickly when the thickness of the
layer increases, limited up to approximately 40 particle diameters in Geldart A to D
particles [78], whereas a range from 3 to 30 particle diameters in Geldart B particles
[105].
One potential cause for the collapse of the pattern is due to the limitation of
energy propagation. The energy injected via the particle-bottom plate via mechanical
impact limits the source of energy to a plane boundary (e.g., the shaken plate in a
vibrated bed). As a result, energy is gradually dissipated through the inelastic particle
collision within the bed and cannot transmit to the particles dispositioned at a higher
level. For this reason, intensively used vibro-assisted fluidisation can be regarded as
a good approach to facilitate the mixing of powder and homogenise particle flow,
but it is difficult to impose recursive macroscopic flow patterns. Alternatively, via
oscillating the gas flow, one introduces extra mechanical energy via the solid-fluid
interaction, in which the fluid, as an energy source, is distributed across the entire
volume and exchange momentum simultaneously with every individual particle. The
dissipation of energy through interparticle inelastic collisions still defines the length
scale in the response of single particle, but since the entire system is excited, one can
create a better coupling between the dynamic of the perturbation, in this case, the
dynamic drag force, and the collective response of the solid motion. For that reason,
the surface waves in a pulsed shallow layer can persist to a height of approximately
40 particle diameters for Geldart B particles, above which gas bubbles begin to form.