58
2 Bubbling Properties in Pulsed Fluidised Beds
emergence of dynamic order structures depresses interference amongst bubbles
and promotes more precision on system control. The experiments demonstrate the
increased uniformity in bubble size and separation when bubbles are structured in
triangular lattices. In structured flows, the population of size and separation are
greatly restricted to a much smaller dispersion in comparison to those in constant
flow fluidisation. In this context, the dominant bubble size and separation become
sufficiently representative to describe the entire population.
However, the bulk rising velocity of bubbles is seemingly independent of structuring and pulsation. After detaching from the distributor, the bubbles in pulsed beds
undergo a defluidisation period while rising to a higher level. When the following
pulse is injected, the acceleration in the gas velocity quickly pushes the bubbles
to leave the domain. Therefore, one reason is attributed to the insufficiently high
static bed loading, where the initial stage of bubbles dominates the population. For
pulsed beds, since the characteristic bubbling frequency is overwritten by the pulse
frequency, the axial separation of bubbles, therefore, can be simply estimated as the
product of reciprocal of pulse frequency and the bubble rising velocity. Moreover, it
is observed that the dominant rising velocity is almost unchanged in the experiments
using a 10 cm deep loading, so the axial separation of bubbles is dependent on the
pulse frequency applied. As a result, it is reasonable to expect that a preferable pattern
frequency range exists for structured flow formation.
The appearance of patterns is easily recognised and distinguished visually from
chaotic flows. According to the snapshots of the experimental flows, the formation
of structures only emerges at moderate pulsed flow rates, which fluctuate around the
minimum fluidisation states. A too-low flow rate is insufficient to generate recognisable bubbles, while a too-high velocity, such as a large A or B, also quickly disturbs
the patterns via frequent bubble coalescence and breakup. The experiments have also
shown that the self-organisation of bubbles emerges at different levels of regularity.
Such a transition between structured and unstructured state is not first-order. Therefore, robust quantification of structuring is required to identify properly the structure
and transition regimes in the parametric domain, and pinpoint the optimal oscillatory
flows for inducing highly structured systems.
2.4 Conclusions
This chapter demonstrates a comparative study of the impacts of pulsed flows on
the bubbling behaviour in a quasi-2D, pulsed, gas-solid bubbling fluidised bed. The
experiments results have shown that pulsating the gas flow is a plausible way to
modulate the bubble dynamics. By manipulating the oscillatory flow in a controlled
manner, the size, rising velocity and number of bubbles become tuneable, to a certain
extent, in comparison to the conventional fluidised beds. In addition, the formation of
a regular bubble pattern occurs under moderate rates of pulsed flows. When operating
the system with a structured flow, one can effectively suppress gas channelling and
induce extra control precision over the spatial distribution and nucleation process of
2 Bubbling Properties in Pulsed Fluidised Beds
emergence of dynamic order structures depresses interference amongst bubbles
and promotes more precision on system control. The experiments demonstrate the
increased uniformity in bubble size and separation when bubbles are structured in
triangular lattices. In structured flows, the population of size and separation are
greatly restricted to a much smaller dispersion in comparison to those in constant
flow fluidisation. In this context, the dominant bubble size and separation become
sufficiently representative to describe the entire population.
However, the bulk rising velocity of bubbles is seemingly independent of structuring and pulsation. After detaching from the distributor, the bubbles in pulsed beds
undergo a defluidisation period while rising to a higher level. When the following
pulse is injected, the acceleration in the gas velocity quickly pushes the bubbles
to leave the domain. Therefore, one reason is attributed to the insufficiently high
static bed loading, where the initial stage of bubbles dominates the population. For
pulsed beds, since the characteristic bubbling frequency is overwritten by the pulse
frequency, the axial separation of bubbles, therefore, can be simply estimated as the
product of reciprocal of pulse frequency and the bubble rising velocity. Moreover, it
is observed that the dominant rising velocity is almost unchanged in the experiments
using a 10 cm deep loading, so the axial separation of bubbles is dependent on the
pulse frequency applied. As a result, it is reasonable to expect that a preferable pattern
frequency range exists for structured flow formation.
The appearance of patterns is easily recognised and distinguished visually from
chaotic flows. According to the snapshots of the experimental flows, the formation
of structures only emerges at moderate pulsed flow rates, which fluctuate around the
minimum fluidisation states. A too-low flow rate is insufficient to generate recognisable bubbles, while a too-high velocity, such as a large A or B, also quickly disturbs
the patterns via frequent bubble coalescence and breakup. The experiments have also
shown that the self-organisation of bubbles emerges at different levels of regularity.
Such a transition between structured and unstructured state is not first-order. Therefore, robust quantification of structuring is required to identify properly the structure
and transition regimes in the parametric domain, and pinpoint the optimal oscillatory
flows for inducing highly structured systems.
2.4 Conclusions
This chapter demonstrates a comparative study of the impacts of pulsed flows on
the bubbling behaviour in a quasi-2D, pulsed, gas-solid bubbling fluidised bed. The
experiments results have shown that pulsating the gas flow is a plausible way to
modulate the bubble dynamics. By manipulating the oscillatory flow in a controlled
manner, the size, rising velocity and number of bubbles become tuneable, to a certain
extent, in comparison to the conventional fluidised beds. In addition, the formation of
a regular bubble pattern occurs under moderate rates of pulsed flows. When operating
the system with a structured flow, one can effectively suppress gas channelling and
induce extra control precision over the spatial distribution and nucleation process of