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Discrete Modeling of Heat Conduction in Granular Media

Vargas-Escobar, Watson L. (2002) Discrete Modeling of Heat Conduction in Granular Media. Doctoral Dissertation, University of Pittsburgh.

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    Abstract

    This thesis addresses heat conduction in granular systems both under static and slow flow conditions with and without the presence of astagnant interstitial fluid. A novel discrete simulation technique for granular heat transfer, the Thermal Particle Dynamics (TPD) method hasbeen developed. By modeling particle-particle interactions, bed heterogeneities -- e.g., mechanical and thermal -- are directly accountedfor and transient temperature distribution are obtained at the particle level. This technique, based on the Discrete Element Method, not onlysheds light on fundamental issues in heat conduction in particulate systems, but also provides a valuable test-bench for existingcontinuous theories. Computational results, as well as supporting experiments coupled with existing theoretical models are used to probethe validity of the proposed simulation technique. Studies on heat conduction through static beds of particles indicate that stress and contact heterogeneities -- due primarily to theexistence of localized ``chains' of particles which support the majority of an imposed load (stress chains) -- may cause dramaticchanges in the way that heat is transported by conduction. It is found that by matching the microstructure of an experimental system onlyqualitatively, quantitatively accurate estimates of effective properties are possible, without requiring adjustable parameters. Onekey result in this study reveals that an important consideration has been missing from previous granular conduction studies -- the stressdistribution in the particle bed. Extensions of TPD to incorporate the ability to model heat transfer in particulate systems in the presenceof an interstitial fluid indicate that a good qualitative and quantitative agreement between measured and calculated values of theeffective thermal conductivity for a wide variety of materials in the presence of both liquid and/or gas are possible. Simulation results for slow granular flows -- e.g., simple shear cell and a rotating drum -- indicate that in both cases there is anenhancement of the effective thermal conductivity with increase in the shear rate due to enhanced mixing of the particles. These results arein agreement with previous theoretical and experimental investigations. In contrast to the behavior found at high shear rates, where the thermal conductivity is proportional to the shear rate, a complex non-linear relation is found for the effective conductivity in granular flows at low shear rates. This observation has not beenpreviously reported. It is argued that a balance between heat conduction and convection is necessary to explain these observations.


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    Item Type: University of Pittsburgh ETD
    ETD Committee:
    ETD Committee TypeCommittee MemberEmail
    Committee ChairMccarthy, Joseph J.mccarthy@engrng.pitt.edu
    Committee MemberJohnson, J. Karlkarlj@engrng.pitt.edu
    Committee MemberVallejo, Luis E.vallejo@civ.pitt.edu
    Committee MemberParker, Robert S.rparker@engrng.pitt.edu
    Title: Discrete Modeling of Heat Conduction in Granular Media
    Status: Unpublished
    Abstract: This thesis addresses heat conduction in granular systems both under static and slow flow conditions with and without the presence of astagnant interstitial fluid. A novel discrete simulation technique for granular heat transfer, the Thermal Particle Dynamics (TPD) method hasbeen developed. By modeling particle-particle interactions, bed heterogeneities -- e.g., mechanical and thermal -- are directly accountedfor and transient temperature distribution are obtained at the particle level. This technique, based on the Discrete Element Method, not onlysheds light on fundamental issues in heat conduction in particulate systems, but also provides a valuable test-bench for existingcontinuous theories. Computational results, as well as supporting experiments coupled with existing theoretical models are used to probethe validity of the proposed simulation technique. Studies on heat conduction through static beds of particles indicate that stress and contact heterogeneities -- due primarily to theexistence of localized ``chains' of particles which support the majority of an imposed load (stress chains) -- may cause dramaticchanges in the way that heat is transported by conduction. It is found that by matching the microstructure of an experimental system onlyqualitatively, quantitatively accurate estimates of effective properties are possible, without requiring adjustable parameters. Onekey result in this study reveals that an important consideration has been missing from previous granular conduction studies -- the stressdistribution in the particle bed. Extensions of TPD to incorporate the ability to model heat transfer in particulate systems in the presenceof an interstitial fluid indicate that a good qualitative and quantitative agreement between measured and calculated values of theeffective thermal conductivity for a wide variety of materials in the presence of both liquid and/or gas are possible. Simulation results for slow granular flows -- e.g., simple shear cell and a rotating drum -- indicate that in both cases there is anenhancement of the effective thermal conductivity with increase in the shear rate due to enhanced mixing of the particles. These results arein agreement with previous theoretical and experimental investigations. In contrast to the behavior found at high shear rates, where the thermal conductivity is proportional to the shear rate, a complex non-linear relation is found for the effective conductivity in granular flows at low shear rates. This observation has not beenpreviously reported. It is argued that a balance between heat conduction and convection is necessary to explain these observations.
    Date: 18 March 2002
    Date Type: Completion
    Defense Date: 07 March 2002
    Approval Date: 18 March 2002
    Submission Date: 01 February 2002
    Access Restriction: No restriction; Release the ETD for access worldwide immediately.
    Patent pending: No
    Institution: University of Pittsburgh
    Thesis Type: Doctoral Dissertation
    Refereed: Yes
    Degree: PhD - Doctor of Philosophy
    URN: etd-02012002-193942
    Uncontrolled Keywords: Contact conductance; Discrete Modeling; Effective conductivity; Granular media; Heat conduction; Particle Dynamics
    Schools and Programs: Swanson School of Engineering > Chemical Engineering
    Date Deposited: 10 Nov 2011 14:31
    Last Modified: 17 Feb 2012 11:44
    Other ID: http://etd.library.pitt.edu:80/ETD/available/etd-02012002-193942/, etd-02012002-193942

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