2025 Theses Doctoral

In Silico Simulation Studies of Structured Gas-Fluidized Granular Flows

Omidi, Javad

Granular materials that are fluidized by gas flowing up and external mechanical forces show a wide range of multiphase behaviors, including bubble formation, surface-wave dynamics, and particle mixing. While vibrated beds and structured patterns in gas-fluidized beds have been studied separately, the synergistic effects of various external forces are not yet fully comprehended, especially concerning the multiaxial vibration and temporally modulated gas injection in influencing pattern formation and transport.

This dissertation employs high-fidelity two-fluid model (TFM) and computational fluid dynamics – discrete element method (CFD–DEM) simulations to investigate three interrelated mechanisms: the generation of Faraday waves under combined vibration, the induction of structured bubbling by oscillatory gas flow, and the effect of horizontal vibration on beds that are already displaying structured bubbling. The goal of these studies is to come up with general rules for designing ordered flow structures that improve particle mixing while keeping the bubbling structure predictable.

The initial section examines Faraday waves in gas-fluidized beds subjected to concurrent vertical and horizontal vibrations, utilizing validated TFM simulations. This study shows that adding horizontal vibration keeps wave formation and greatly improves horizontal mixing. Vertical vibration frequency lowers the wavelength and wave height, while horizontal frequency has a weaker effect on wave shape. The gas flow rate also changes how waves behave by making the wavelength longer and the amplitude lower. To describe wave properties and mixing performance across a wide range of operating conditions, dimensionless correlations and regime maps are created. It is additionally shown that horizontal vibration alone cannot produce surface waves, and that phase offsets between vertical and horizontal forcing results in only negligible alterations in wave dynamics.

The second part looks at structured bubbling that happens when gas is injected with oscillations, using distributors that are split into slices that can be modulated independently. TFM simulations show that phase offsets between slices make a wider range of bubble-lattice configurations than the usual triangular pattern. Localized particle solidification beneath bubbles creates these structures. By changing the number of slices and the phase offset, it is possible to find conditions that improve horizontal mixing compared to both unstructured bubbling and structured bubbling at the same overall gas-flow rate. The best regime happens when the number of slices is equal to the natural number of bubbles per row and a phase shift of π/2 is used.

In the third part, CFD–DEM simulations measure how horizontal vibration changes bubbling and mixing in beds that are already bubbling in a structured way. Vibration strengths close to one and vibration-to-gas-oscillation frequency ratios of 3 to 5 greatly improve horizontal and vertical mixing while keeping the bubble-lattice order. Stronger vibrations or lower ratios mess up structuring, while weaker vibrations or higher ratios have little effect. This gives us useful design rules for adding horizontal vibration to structured-bubbling systems.

The last chapter brings together the ideas from all three parts, points out some promising future directions, and shows how they can be used more broadly to control flow behavior in fluidized systems. These studies together show how using both vibration and structured gas injection can be adjusted to control wave dynamics, bubble structure, and particle mixing in gas-fluidized beds. This gives us a single computational framework for designing reactors that can predict how they will work.