FIgueroa Amenabar, Isabel Margarita
(2010)
Exploring analogies between granular materials and fluids.
Doctoral Dissertation, University of Pittsburgh.
(Unpublished)
Abstract
The direct application of fluid system approaches to granular materials oftentimes leads to spectacular failures, e.g. two miscible fluids shaken in a container rapidly become an homogeneous mixture, while vibrating a device containing two kinds of powders result in extreme segregation. Nevertheless, much can be learned through analogies between these disparate systems. The approach taken in this work is to focus on adapting concepts from fluid behavior and explore their application in relevant industrial processes involving granular materials---such as mixing/segregation, heat transfer and flowability. We use Particle Dynamics (PD)---a Discrete Element Method---to model transport and flow of granular materials.Adhesion, for example, is commonly found in operations involving fine powders where van der Waals forces start to play an important role. Our PD model, which is capable of simulating dry adhesive interactions, is used to study mixing/segregation. By introducing a characterization tool---the van der Waals Granular Bond Number---the asymptotic mixed/segregated state of the system is analytically predicted. These predictions are most easily summarized by phase-space diagrams which exhibit both mixed and segregated regions. The phase-space diagrams are computationally tested (with PD) by selecting granular materials of different properties so that we explore both mixed and segregated regions. Each of these materials is allowed to reach its asymptotic state in a mixing drum, and this final state is compared to the predictions.Beyond predicting the asymptotic state of a system, the next natural step is to explore the possibility of controlling it. With this purpose, we propose the addition of ``helper" particles that can either promote mixing or segregation. These amphiphilic helper particles---also called Janus particles---act as bridges between the base (non-helper) particles, alternatively promoting mixing in a system that would otherwise segregate (surfactant helpers) or separating a specific kind of particle from a mixture (extractant helpers). Phase space diagrams summarizing these trends are analytically built by comparing the interaction forces in the system: the final state of the system is determined by the interactions that predominate. Again, the predictions are then tested against results obtained by PD simulations of the system including Janus particles, which are further compared to the binary adhesive system that contain no helper particles.The presence of adhesive forces in granular materials can also affect the flowability of a granular material. Glidants---also known as flow aids or conditioners---are frequently added to dry cohesive powders to improve their flow properties and to facilitate their handling. These aids can reduce the inter-particle forces by imposing a physical barrier between host particles but are often identified on a trial-and-error or ad hoc basis. Using our PD model and characterization tools, glidant particles are engineered to improve the flowability properties of a dry cohesive powder.Finally, heat transfer within granular materials is studied by observing the dominant heat transfer mechanisms in the granular bed. Specifically, we examine the conditions necessary to achieve conduction-dominated versus convection-dominated heat transfer and introduce a granular analog of the Peclet number as a means of quantifying the transition. Our experiments consist of PD simulations of rotating tumblers where the granular material is being heated from the walls. When conduction is the dominant mechanism, increasing the mixing rate seems to have a positive impact in the rate of heat transfer; however, under convection-dominated conditions the opposite is often true. In order to further clarify the role of inter-particle mixing, the temperature profiles of the material in the drums are compared to a continuum model for mixing of cohesionless granular materials. Similarities between these two are most likely to be found when convection is the mechanism that dominates the heat transport. Dimensionless numbers are used to correlate the results obtained for multiple systems and surprising degree of similarity is found when compared to analogous fluid correlations.
Share
| Citation/Export: |
|
| Social Networking: |
|
Details
| Item Type: |
University of Pittsburgh ETD
|
| Status: |
Unpublished |
| Creators/Authors: |
|
| ETD Committee: |
|
| Date: |
26 January 2010 |
| Date Type: |
Completion |
| Defense Date: |
22 September 2009 |
| Approval Date: |
26 January 2010 |
| Submission Date: |
5 October 2009 |
| Access Restriction: |
No restriction; Release the ETD for access worldwide immediately. |
| 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: |
adhesion; heat transfer mechanisms; mixing; segregation; flowability; granular materials |
| Other ID: |
http://etd.library.pitt.edu/ETD/available/etd-10052009-090013/, etd-10052009-090013 |
| Date Deposited: |
10 Nov 2011 20:02 |
| Last Modified: |
15 Nov 2016 13:50 |
| URI: |
http://d-scholarship.pitt.edu/id/eprint/9438 |
Metrics
Monthly Views for the past 3 years
Plum Analytics
Actions (login required)
 |
View Item |
![[feed]](/images/feed-icon-32x32.png){kind=icon}