Granular flows are ubiquitous both in natural and industrial processes such as pharmaceuticals production, mining and food–processing. Within a gas-solid fluidized bed, fundamental phenomena such as heat and mass transfer are impacted by particle convection, mixing and segregation due to rising gas bubbles. The free-bubbling regime is promising in enhancing the gas-solid contact but its mathematically chaotic bubble motion make system design challenging. Furthermore, a better understanding of rheological characteristics of these gas-solid flows is pertinent to developing accurate mathematical descriptions of granular flows. Despite their decades of usage and significance in all walks of technology, gas-solid fluidized beds remain poorly understood.
In the first section of this dissertation, we investigate the combined effects of vibration and gas flow on a binary gas-solid fluidized bed. At resonant conditions, this external excitation generates a periodic, triangular, equisized, structured array of rising bubbles, reducing the chaos. Through a combination of experiments, Computational Fluid Dynamics-Discrete Element Method (CFD-DEM), and Multi Fluid Model (MFM) simulations, we demonstrate that the structured bubbling facilitates particle mixing whereas the gas flow alone (no vibration conditions) results in smaller, unstructured bubbles that promote segregation. The CFD-DEM simulations accurately capture the bubble structuring and somewhat, qualitatively and quantitively, match with the optically imaged experimental data. These investigations provide valuable insights into the dynamics of particle mixing and segregation under complex fluidization conditions. The MFM simulations failed to predict the mixing observed, indicating a need for further refinement of these models.
In the second section of this dissertation, we explore the complex rheological behavior of the dry, dense particulate flows using the Discrete Element Method (DEM) simulations. In these simulations, we choose 3−D Couette cell geometry to visualize Granular Taylor–Vortex flow for various particle diameters and densities. The simulations examined the effects of varying particle volume fractions, from 0.45 to 0.60, under different shear rates, revealing a distinct rheological transition from shear-thickening to Newtonian and finally to shear-thinning behavior. The formation and nature of these vortices were compared with those observed in continuum simulations of Newtonian and shear-thinning fluids, to elucidate the unique aspects of granular flow dynamics.
The third section of this thesis investigated the rheology of granular particles subjected to 1−D shear combined with sinusoidal vibration, aiming to understand the effects on pressure, shear stress, and coordination number across a range of shear rates and particle fractions. A novel vibrational regime was observed at low shear rates, below a critical threshold, characterized by a rate-independent pressure intermediate between the inertial and quasi-static regimes. These findings provide significant insights while advancing our understanding of granular material dynamics.
| Identifer | oai:union.ndltd.org:columbia.edu/oai:academiccommons.columbia.edu:10.7916/6751-a902 |
| Date | January 2024 |
| Creators | Sanghishetty, Jagan Mohan |
| Source Sets | Columbia University |
| Language | English |
| Detected Language | English |
| Type | Theses |
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