dc.contributor.advisor | Sun, Jin | en |
dc.contributor.advisor | Ooi, Jin | en |
dc.contributor.author | Gupta, Prashant | en |
dc.date.accessioned | 2015-06-22T10:07:16Z | |
dc.date.available | 2015-06-22T10:07:16Z | |
dc.date.issued | 2015-06-29 | |
dc.identifier.uri | http://hdl.handle.net/1842/10449 | |
dc.description.abstract | Fluidization of solid particles using gas flow is an important process in chemical and
pharmaceutical industries. The dynamics of fluidisation are intricately related to particle
scale physics. Fluid-particle interactions dominate gas-solid fluidization behaviour
for particles with average size and density greater than 10-4 m and 103 kg/m3, respectively,
classified as Geldart B and D particles. Inter-particle forces, such as cohesion,
play an increasingly important role in the fluidization dynamics of smaller particles,
which are classified as Geldart A and C. In particular, interesting fluidization regimes
have been noticed for weakly cohesive Geldart A particles, exhibiting a window of uniform
fluidization before the onset of bubbling behaviour. Despite widespread industrial
interests, the fundamental understanding of the mechanisms that underlie these fluidization
regimes is poor. The present study aims to improve the understanding of
fluidization dynamics of Geldart A regimes using numerical simulations.
A DEM-CFD model was employed to capture the widely separated spatial and temporal
scales associated with fluidization behaviour. The model couples the locally averaged
Navier-Stokes equation for fluid with a discrete description of the particles. The
methodology and its computer implementation are verified and validated to assess the
extent of fluidization physics that it is able to capture. Verification cases check the implementation
of the inter-phase momentum transfer term, drag model implementation
and pressure-velocity coupling. The test cases are employed in order to cover a wide
range of flow conditions. Robust validation tests for complex fluidization phenomena
such as bubbling, spouting and bidisperse beds have been conducted to assess the predictive
capabilities of the DEM-CFD solver. The simulation results for time and spatially
averaged fluidziation behaviour are compared to experimental measurements obtained
from the literature, and are shown to have capture fluidization physics qualitatively.
Robust features of bubbling fluidization, such as minimum fluidization velocity, frequency
of pressure drop fluctuations, segregation rates and solid circulation patterns
were captured. Furthermore, the DEM-CFD model is critically assessed in terms of
model conceptualization and parameter estimation, including those for drag closures,
particle-wall boundary conditions, bed height and particle shape effects. The validation
studies establish modelling best-practice guidelines and the level of discrepancy against
the analytical solutions or experimental measurements.
Having developed the model and established its predictive capability, it is used to probe
the hydrodynamics of weakly cohesive particles. Cohesive interactions are captured by
employing a pair-wise van derWaals force model. The cohesive strength of the granular
bed is quantified by the ratio of the maximum van der Waals force to the particle
gravitational force, defined as the granular Bond number. The Bond number of the bed
is increased systematically from 0-10 to examine the role of cohesion in the fluidization
behaviour of fine powders while keeping the particle size and density constant across
all the simulations. The idea was to segregate the hydrodynamics associated with size
and density of the particles from the inter-particle interactions. The size and density
of the particles are carefully chosen at a scale where inter-particle forces are present
but minimal [Seville et al., 2000]. The Geldart A fluidization behaviour is captured
for granular beds with Bond numbers ranging from 1 to 3. Many robust features of
Geldart A fluidization, such as pressure drop overshoot, delay in the onset of bubbling,
macroscopic Umf predictions and uniform bed expansion are captured in the DEM-CFD
framework. The expanded bed was characterized according to criteria that the particles
are highly immobile in this regime and the expanded porosity is related to inlet velocity
by Richardson–Zaki correlations.
Sudden jumps in the magnitudes of global granular temperature were found near the
regime transitions. This observation was used an indicator of the onset of bubbling
and quantification of minimum bubbling velocity (Umb). The window of the expanded
bed regime (quantified as Umb - Umf) was shown to be an increasing function of cohesive
strength of the bed. Furthermore, the stability of the expanded bed was probed
by studying the response of the expanded bed to sudden inertial and voidage shocks.
A kinematic wave, generated as a response to the voidage shock, was shown to slow
down with increasing cohesion and decreasing hydrodynamic forces. Furthermore,
predictions of Umb by DEM-CFD simulations for weakly cohesive beds were compared
against empirical correlations by Valverde [2013] with an excellent match. Stress analysis
of the expanded bed revealed the presence of tensile stresses. As the inlet velocity
is increased beyond the minimum fluidization velocity, a longitudinal shift of these negative
stresses is observed until they reach the top of the bed. Negative stresses were
seen at the bed surface at the onset of bubbling. The role of cohesion stresses in the
formation of expanded bed and suppression of bubbling was highlighted.
Finally, the microstructure of the expanded bed was probed at different local micro and
mescoscopic length scales. Evidence of clustering, agglomeration and cavities were
presented in the expanded bed. Expanded bed expansion was shown to have mesostructural
inhomogeneities present, which is contrary to the belief of homogeneous
expansion. | en |
dc.contributor.sponsor | other | en |
dc.language.iso | en | |
dc.publisher | The University of Edinburgh | en |
dc.relation.hasversion | Gupta, P., Sun, J. & Ooi, J.Y. DEM-CFD simulation of a dense fluidized bed: wall roughness and particle size effects. To be submitted for to Powder Technology, September 2014 | en |
dc.relation.hasversion | Gupta, P., Sun, J. & Ooi, J.Y. Validation and Verification of an open source DEM-CFD code. World Congress of Particle Technology 7, Beijing, China, May 2014 | en |
dc.relation.hasversion | Gupta, P., Sun, J. & Ooi, J.Y. Micro-structural analysis of homogeneous fluidization of Geldart A particles using DEM-CFD. World Congress of Particle Technology 7, Beijing, China, May 2014 | en |
dc.relation.hasversion | Gupta, P., Sun, J. & Ooi, J.Y. Effect of hydrodynamic interactions on the segregation rate in bidisperse gas–solid fluidised bed and validation studies. 7th International Conference for Conveying and Handling of Particulate Solids– CHoPS, Friedrichshafen, Germany, September 2012 | en |
dc.relation.hasversion | Gupta, P., Sun, J. & Ooi, J.Y. Study of drag force models in simulation of bidisperse gas–solid fluidised beds using a CFD–DEM approach. The 8th European Congress of Chemical Engineering, Berlin, Germany, September 2011 | en |
dc.subject | fluidization | en |
dc.subject | DEM-CFD framework | en |
dc.title | Verification and validation of a DEM-CFD model and multiscale modelling of cohesive fluidization regimes | en |
dc.type | Thesis or Dissertation | en |
dc.type.qualificationlevel | Doctoral | en |
dc.type.qualificationname | PhD Doctor of Philosophy | en |