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Material and thermal transport in vertical granular flows

Citation

Natarajan, Venkata V. R. (1997) Material and thermal transport in vertical granular flows. Dissertation (Ph.D.), California Institute of Technology. http://resolver.caltech.edu/CaltechETD:etd-01182008-091551

Abstract

The term "granular material flow" is applied in the literature to particulate flows such as the flow of coal down an inclined chute, the discharge of grains from a hopper or the motion of debris in a landslide. In these flows, the material has an overall bulk motion; however, individual particles may collide, roll or slide against each other, and may interact with the bounding surfaces. Hence, the individual particle motions are composed of a mean velocity component and a fluctuating, or random, velocity component. An analogy is drawn between this random motion and the random motion of molecules. As a result, much of the theoretical analysis of these flows has developed from concepts derived from dense-gas kinetic theory. Although this random velocity component is a key property in analytical studies, there have been few attempts to measure its magnitude in experimental studies. In the current work, measurements were made of two components of the average and fluctuating velocities in the flow of granular material in a vertical chute for flows with different particle and boundary properties. The fluctuation velocities were highly anisotropic, with the streamwise components being 2 to 2.5 times the magnitude of the transverse components. Increasing the surface roughness of the particles reduced the fluctuation velocities significantly.

Another area of considerable industrial interest is particle mixing in monodisperse and polydisperse particle flows. Because of the random component of particle motion, the particles can exhibit a diffusive motion similar to that found in gases and liquids. In the second part of this work, local self diffusion coefficients were measured in the granular flow using image processing techniques to track individual particles. The influence of flow shear rates and fluctuation velocities on the self diffusion coefficients was investigated. The self-diffusion coeffecients were found to increase with the shear rate and the fluctuation velocity, with the coefficients in the streamwise direction being an order-of-magnitude higher than those for the transverse direction. The surface roughness of the particles led to a decrease in the self-diffusion coefficients.

The effect of shearing on the convective heat transfer from a heater immersed in a granular flow was investigated experimentally. Comparisons were made with previous experiments and with results obtained for unsheared plug flows. The results indicated that the medium density close to the wall played a critical role in determining the overall heat transfer.

Finally, theoretical solutions, based on a combination of the dense-gas kinetic theory and an empirical friction model, were generated to study and compare experimental and theoretical results for velocity profiles and heat transfer characteristics in vertical, fully developed granular flows. The results indicated good agreement between theoretical and experimentally measured mean velocity proflies but the fluctuation velocity magnitudes were usually underpredicted by the theoretical solutions. There was qualitative agreement between experimental and theoretical results for convective heat transfer.

Item Type:Thesis (Dissertation (Ph.D.))
Degree Grantor:California Institute of Technology
Division:Engineering and Applied Science
Major Option:Mechanical Engineering
Thesis Availability:Restricted to Caltech community only
Research Advisor(s):
  • Hunt, Melany L.
Thesis Committee:
  • Hunt, Melany L. (chair)
  • Brennen, Christopher E.
  • Raichlen, Fredric
  • Brady, John F.
Defense Date:1 October 1996
Record Number:CaltechETD:etd-01182008-091551
Persistent URL:http://resolver.caltech.edu/CaltechETD:etd-01182008-091551
Default Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:221
Collection:CaltechTHESIS
Deposited By: Imported from ETD-db
Deposited On:14 Feb 2008
Last Modified:26 Dec 2012 02:28

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