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Analysis and validation of space averaged drag model for numerical simulations of gas-solid flows in fluidized beds

Shah, Srujal (2012-12-13)

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isbn 9789522653437.pdf (2.708Mb)
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Väitöskirja

Shah, Srujal
13.12.2012
Lappeenranta University of Technology

Acta Universitatis Lappeenrantaensis

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Julkaisun pysyvä osoite on
https://urn.fi/URN:ISBN:978-952-265-3437

Tiivistelmä

This thesis presents an approach for formulating and validating a space averaged drag
model for coarse mesh simulations of gas-solid flows in fluidized beds using the two-fluid
model.
Proper modeling for fluid dynamics is central in understanding any industrial multiphase
flow. The gas-solid flows in fluidized beds are heterogeneous and usually simulated with
the Eulerian description of phases. Such a description requires the usage of fine meshes
and small time steps for the proper prediction of its hydrodynamics. Such constraint on
the mesh and time step size results in a large number of control volumes and long computational
times which are unaffordable for simulations of large scale fluidized beds.
If proper closure models are not included, coarse mesh simulations for fluidized beds do
not give reasonable results. The coarse mesh simulation fails to resolve the mesoscale
structures and results in uniform solids concentration profiles. For a circulating fluidized
bed riser, such predicted profiles result in a higher drag force between the gas and solid
phase and also overestimated solids mass flux at the outlet. Thus, there is a need to formulate
the closure correlations which can accurately predict the hydrodynamics using coarse
meshes. This thesis uses the space averaging modeling approach in the formulation of
closure models for coarse mesh simulations of the gas-solid flow in fluidized beds using
Geldart group B particles.
In the analysis of formulating the closure correlation for space averaged drag model, the
main parameters for the modeling were found to be the averaging size, solid volume fraction,
and distance from the wall. The closure model for the gas-solid drag force was
formulated and validated for coarse mesh simulations of the riser, which showed the verification
of this modeling approach. Coarse mesh simulations using the corrected drag
model resulted in lowered values of solids mass flux. Such an approach is a promising
tool in the formulation of appropriate closure models which can be used in coarse mesh
simulations of large scale fluidized beds.
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  • Väitöskirjat [1113]
LUT-yliopisto
PL 20
53851 Lappeenranta
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