DISCRETE CHARACTERIZATION OF COHESION IN GAS-SOLID FLOWS
Fluidization and the transport of solid particles either by gravity
or by pneumatic means are used in a variety of industrial operations,
including fluid catalytic cracking, fluid hydroforming and solid fuel
processes such as coal gasification and liquefaction .
Despite the fact that a sizeable portion of gas-solid flows are cohesive
in nature, the mechanics of cohesive flowing gas-particle systems is still
poorly understood, and manipulation/control of the flow variables is still
largely done on a trial-and-error basis.
Cohesive forces between grains can arise from a variety of sources --
such as liquid bridge (capillary) forces, van der Waals forces, or
electrostatic forces -- and may play a significant role in the
processing of fine and/or moist powders. While recent advances have
been made in our understanding of liquid-induced cohesion in quasi-static
systems at the macroscopic level [2,3],
in general, it is still not possible to directly connect this macroscopic
understanding of cohesion with a microscopic picture of the particle properties
and interaction forces. Moreover, conventional theories on gas-solid flows, make
no attempt to distinguish between these modes of cohesion, despite clear
qualitative differences (lubrication forces in wet systems or electrostatic
repulsion are two good examples).
In this work, we extend the previous work on discrete characterization tools of
wet granular flows , using computations of gas-solid flows, in order to
examine the transition from non-cohesive (dry) to cohesive (wet) behavior
in gas-solid systems. Gas velocity and bridging liquid surface tension are
varied to explore a range of the possible fluidization parameter space and a
characterization criterion based on the the physical picture of liquid-induced
particle-level cohesion is developed for gas-solid flows. Cohesion between wet
particles has been modeled using the concept of liquid bridges. The characterization
tool developed, namely the Granular Capillary Number, is tested by measuring both
the minimum fluidization velocity as well as the mixing rate in fluidized systems.
The systems explored here are traditionally thought to be cohesive but a marked
difference is observed as the Granular Capillary Number changes.
Advisor:Prof. Joseph J. McCarthy; Prof. Robert Enick; Prof. Robert Parker
School:University of Pittsburgh
School Location:USA - Pennsylvania
Source Type:Master's Thesis
Date of Publication:12/02/2002