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The bulk viscosity of suspensions

Citation

Swaroop, Manuj (2010) The bulk viscosity of suspensions. Dissertation (Ph.D.), California Institute of Technology. http://resolver.caltech.edu/CaltechTHESIS:05282010-012507201

Abstract

Particles suspended in a fluid are known to undergo variations in the local concentration in many flow situations; essentially a compression or expansion of the particle phase. The modeling of this behavior on a macroscopic scale requires knowledge of the effective bulk viscosity of the suspension, which has not been studied before. The bulk viscosity of a pure compressible fluid is defined as the constant of proportionality that relates the difference between the mechanical pressure and the thermodynamic pressure to the rate of compression. The bulk viscosity of a suspension is defined analogous to that for a pure fluid as the constant of proportionality relating the deviation of the trace of the macroscopic stress from its equilibrium value to the average rate of compression. The compression flow drives the suspension microstructure out of equilibrium and the thermal motion of the particles tries to restore equilibrium. The Peclet number (Pe), defined as the expansion rate made dimensionless with the Brownian time-scale, governs the departure of the microstructure from equilibrium. The microstructural forcing in compression is monopolar for small Pe resulting in a significantly slower spatial and temporal response of the microstructure compared to shearing or diffusive motion. We have determined the effective suspension bulk viscosity for all concentrations and all rates of compression, accounting for the full thermodynamic and hydrodynamic interactions that particles experience at the micro-scale. Current simulation techniques were enhanced to enable the dynamic simulation of compression flows in a suspension. A 'compression thinning' of the suspension is observed at small rates of compression and there is some 'compression thickening' at large compression rates. The bulk viscosity diverges as the volume fraction nears maximum packing and is in fact larger than the shear viscosity. Existing models for multiphase flows must therefore include the bulk viscosity term to properly simulate variations in particle concentration. An understanding of bulk viscosity effects in suspensions will enable the modeling of certain aggregation and separation behavior and lead to more accurate models for multiphase flows where there are variations in the particle concentration, such as filtration or fluidization.

Item Type:Thesis (Dissertation (Ph.D.))
Subject Keywords:bulk viscosity; suspensions; colloids; colloidal; pressure autocorrelation; stress relaxation; compression; expansion; compressible Stokes flow; Stokesian Dynamics; Brownian Dynamics; dilute; concentrated
Degree Grantor:California Institute of Technology
Division:Chemistry and Chemical Engineering
Major Option:Chemical Engineering
Thesis Availability:Public (worldwide access)
Research Advisor(s):
  • Brady, John F.
Thesis Committee:
  • Brady, John F. (chair)
  • Seinfeld, John H.
  • Wang, Zhen-Gang
  • Hunt, Melany L.
Defense Date:15 April 2010
Author Email:manuj80 (AT) gmail.com
Record Number:CaltechTHESIS:05282010-012507201
Persistent URL:http://resolver.caltech.edu/CaltechTHESIS:05282010-012507201
Default Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:5874
Collection:CaltechTHESIS
Deposited By: Manuj Swaroop
Deposited On:03 Jun 2010 18:51
Last Modified:26 Dec 2012 03:26

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