BHATTACHARYA, TATHAGATA
(2012)
CONTROLLING MIXING AND SEGREGATION IN TIME PERIODIC GRANULAR FLOWS.
Doctoral Dissertation, University of Pittsburgh.
(Unpublished)
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Abstract
Segregation is a major problem for many solids processing industries. Differences in particle
size or density can lead to flow-induced segregation. In the present work, we employ
the discrete element method (DEM) – one type of particle dynamics (PD) technique – to
investigate the mixing and segregation of granular material in some prototypical solid handling
devices, such as a rotating drum and chute. In DEM, one calculates the trajectories of
individual particles based on Newton’s laws of motion by employing suitable contact force
models and a collision detection algorithm. Recently, it has been suggested that segregation
in particle mixers can be thwarted if the particle flow is inverted at a rate above a critical
forcing frequency. Further, it has been hypothesized that, for a rotating drum, the effectiveness
of this technique can be linked to the probability distribution of the number of times
a particle passes through the flowing layer per rotation of the drum. In the first portion of
this work, various configurations of solid mixers are numerically and experimentally studied
to investigate the conditions for improved mixing in light of these hypotheses.
Besides rotating drums, many studies of granular flow have focused on gravity driven
chute flows owing to its practical importance in granular transportation and to the fact
that the relative simplicity of this type of flow allows for development and testing of new
theories. In this part of the work, we observe the deposition behavior of both mono-sized
and polydisperse dry granular materials in an inclined chute flow. The effects of different
parameters such as chute angle, particle size, falling height and charge amount on the mass fraction distribution of granular materials after deposition are investigated. The simulation
results obtained using DEM are compared with the experimental findings and a high degree
of agreement is observed. Tuning of the underlying contact force parameters allows the
achievement of realistic results and is used as a means of validating the model against
available experimental data. The tuned model is then used to find the critical chute length
for segregation based on the hypothesis that segregation can be thwarted if the particle
flow is inverted at a rate above a critical forcing frequency. The critical frequency, fcrit,
is inversely proportional to the characteristic time of segregation, ts. Mixing is observed
instead of segregation when the chute length L < Uavg*ts, where Uavg denotes the average
stream-wise flow velocity of the particles.
While segregation is often an undesired effect, sometimes separating the components
of a particle mixture is the ultimate goal. Rate-based separation processes hold promise
as both more environmentally benign as well as less energy intensive when compared to
conventional particle separations technologies such as vibrating screens or flotation methods.
This approach is based on differences in the kinetic properties of the components of a mixture,
such as the velocity of migration or diffusivity. In this portion of the work, two examples of
novel rate-based separation devices are demonstrated. The first example involves the study of
the dynamics of gravity-driven particles through an array of obstacles. Both discrete element
(DEM) simulations and experiments are used to augment the understanding of this device.
Dissipative collisions (both between the particles themselves and with the obstacles) give rise
to a diffusive motion of particles perpendicular to the flow direction and the differences in
diffusion lengths are exploited to separate the particles. The second example employs DEM
to analyze a ratchet mechanism where a current of particles can be produced in a direction
perpendicular to the energy input. In this setup, a vibrating saw-toothed base is employed
to induce different mobility for different types of particles. The effect of operating conditions
and design parameters on the separation efficiency are discussed.
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Details
| Item Type: |
University of Pittsburgh ETD
|
| Status: |
Unpublished |
| Creators/Authors: |
| Creators | Email | Pitt Username | ORCID  |
|---|
| BHATTACHARYA, TATHAGATA | TAB62@PITT.EDU | TAB62 | |
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| ETD Committee: |
|
| Date: |
2 February 2012 |
| Date Type: |
Publication |
| Defense Date: |
22 September 2011 |
| Approval Date: |
2 February 2012 |
| Submission Date: |
30 November 2011 |
| Access Restriction: |
No restriction; Release the ETD for access worldwide immediately. |
| Number of Pages: |
207 |
| Institution: |
University of Pittsburgh |
| Schools and Programs: |
Swanson School of Engineering > Chemical Engineering |
| Degree: |
PhD - Doctor of Philosophy |
| Thesis Type: |
Doctoral Dissertation |
| Refereed: |
Yes |
| Uncontrolled Keywords: |
granular flow, particle, mixing, segregation, discrete element method, particle
dynamics, tumbler, chute, periodic flow inversion, collisional flow, rate-based separation,
ratchet, static separator, dissipative particle dynamics, non-spherical droplet. |
| Related URLs: |
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| Additional Information: |
All changes as suggested by Jamie Radocay via email on Sept 22, 2011, have been Incorporated. |
| Date Deposited: |
02 Feb 2012 15:36 |
| Last Modified: |
15 Nov 2016 13:55 |
| URI: |
http://d-scholarship.pitt.edu/id/eprint/10610 |
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