Chapter 6
Conclusions
Nowadays, bubbling gas-solid fluidised beds are employed in a broad range of applications amongst chemical, energy, environmental, and pharmaceutical sectors, where
excellent interphase transport and homogenous mixing are of paramount importance.
Nevertheless, their inherently complex dynamics give rise to difficulties in predicting
spatiotemporal distributions of bubbles, which significantly complicates engineering
design, process control and system scale-up.
Bringing in additional degrees of freedom provides the flexibility to manipulate the
system hydrodynamics. Under certain pulsed flows, gas bubbles rise with an intrinsic
wavelength, independent of bed geometry, and alternate their positions every cycle.
Bubbles self-organise into continuously rising, regular hexagonal array, which is
strikingly different from the chaotic hydrodynamics of fluidised beds operated under
a constant gas flow. This thesis presents a study of this unique fluidisation state of
gas-solid suspensions, in both experimental and computational fronts, to explore
its potentials in engineering design and applications, and understand the underlying
mechanism to its onset and development.
To start the study, Chap. 2 demonstrated the difference of bubbling behaviour
between gas-solid mixtures fluidised under a pulsed flow and a constant flow. In
a quasi-2D geometry, it showed that the use of an oscillating gas is able to overwrite the bubbling frequency, and impose control over bubbles size, number density
and rising velocity of bubbles, to a certain extent, are directly associated with the
frequency, amplitude and offset of the pulsation applied, and therefore can be modulated readily. When pulsed under certain flows conditions, bubbles become structured
and self-organise in the form of a triangle tessellation, which significantly mitigates
interference amongst bubble streams and homogenises bubble motions. In addition to
the controllability, structuring introduces extra effects on tightening the distribution
of bubble size and pitch.
The study in Chap. 3 then addressed the quantification of a patterned bubble flow
by defining a degree-of-order index based on comparing the experimental bubble
arrangement with a modelled triangle tessellation. Using this index, it is able to
clearly distinguish two different regimes in the full operating domain: the structured
© The Author(s), under exclusive license to Springer Nature Switzerland AG 2021
K. Wu, Dynamically Structured Flow in Pulsed Fluidised Beds, Springer Theses,
https://doi.org/10.1007/978-3-030-68752-6_6
143
Conclusions
Nowadays, bubbling gas-solid fluidised beds are employed in a broad range of applications amongst chemical, energy, environmental, and pharmaceutical sectors, where
excellent interphase transport and homogenous mixing are of paramount importance.
Nevertheless, their inherently complex dynamics give rise to difficulties in predicting
spatiotemporal distributions of bubbles, which significantly complicates engineering
design, process control and system scale-up.
Bringing in additional degrees of freedom provides the flexibility to manipulate the
system hydrodynamics. Under certain pulsed flows, gas bubbles rise with an intrinsic
wavelength, independent of bed geometry, and alternate their positions every cycle.
Bubbles self-organise into continuously rising, regular hexagonal array, which is
strikingly different from the chaotic hydrodynamics of fluidised beds operated under
a constant gas flow. This thesis presents a study of this unique fluidisation state of
gas-solid suspensions, in both experimental and computational fronts, to explore
its potentials in engineering design and applications, and understand the underlying
mechanism to its onset and development.
To start the study, Chap. 2 demonstrated the difference of bubbling behaviour
between gas-solid mixtures fluidised under a pulsed flow and a constant flow. In
a quasi-2D geometry, it showed that the use of an oscillating gas is able to overwrite the bubbling frequency, and impose control over bubbles size, number density
and rising velocity of bubbles, to a certain extent, are directly associated with the
frequency, amplitude and offset of the pulsation applied, and therefore can be modulated readily. When pulsed under certain flows conditions, bubbles become structured
and self-organise in the form of a triangle tessellation, which significantly mitigates
interference amongst bubble streams and homogenises bubble motions. In addition to
the controllability, structuring introduces extra effects on tightening the distribution
of bubble size and pitch.
The study in Chap. 3 then addressed the quantification of a patterned bubble flow
by defining a degree-of-order index based on comparing the experimental bubble
arrangement with a modelled triangle tessellation. Using this index, it is able to
clearly distinguish two different regimes in the full operating domain: the structured
© The Author(s), under exclusive license to Springer Nature Switzerland AG 2021
K. Wu, Dynamically Structured Flow in Pulsed Fluidised Beds, Springer Theses,
https://doi.org/10.1007/978-3-030-68752-6_6
143