An exploration of Granular Material

Abstract

Granular material is ubiquitous in our everyday lives. For example, in nature the dynamics of granular materials determine how sand dunes are formed and how avalanches and rockslides propagate. Many products we consume are made of granular materials, e.g. tablets in the pharmaceutical industry or instant coffee in the food industry. Although granular materials are omnipresent and have been studied since decades, there still remains much to be learned about their properties and dynamics. For example, the mixing of granular materials requires high energy inputs, hence, an improved understanding of its mixing behaviour would lead to enormous benefits in energy reduction. Chemical products are often produced from granular materials in a chemical reactor which often is a fluidized bed. Fluidized beds channel gas through a bed of particles, and can operate in multiple regimes, depending on the velocity of the gas flow. When operated above the minimum fluidization velocity, the particles are fluidized and have liquid like properties. It is possible to further agitate the bed with vibration. In this dissertation we have observed for the first time a new family of gravitational instabilities in mixtures of granular materials when agitated by both vertical vibration and upward gas flow. These instabilities include a Rayleigh-Taylor (R-T)-like instability in which lighter grains rise through a layer of heavier grains forming fingers and granular bubbles. This R-T-like instability was found to arise from the upward-directed drag force that is increased locally by gas channeling and downward-directed contact forces. Therefore, the physical mechanism behind the formation of such fingers is distinctly different to that in liquids. The same gas channeling mechanism was found to lead to further gravitational instabilities, i.e. the rising of granular bubbles and the splitting of granular droplets during their sinking. The sinking and splitting of granular droplets is examined in more detail. Granular droplets of smaller, but denser particles in a bed of larger and lighter particles is probed both numerically and experimentally. It is observed that a sinking granular droplet performs a series of binary splits resembling the fragmentation of a liquid droplet falling in a miscible fluid, although a different physical mechanism is responsible for the split. Particle-image-velocimetry (PIV) and numerical simulation show that the droplet of high-density particles forms an immobilized zone directly below it. This immobile zone obstructs the downwards motion of the droplet and forces the droplet to spread and ultimately to split. The resulting daughter droplets sink at inclined trajectories around the immobilized zone until they split themselves. The occurrence of consecutive splitting events is explained by the creation and relaxation of the immobilized zone underneath the droplet fragments. A key requirement for the observation of R-T instabilities is the agitation of the granular bed by both vibration and gas-fluidization. We have explored to what extent the addition of mechanical vibration can reduce the minimum fluidization velocity (Umf). A series of experiments show that vibration reduces both Umf and the minimum bubbling velocity (Umb) in both Geldart Group B and D particles. Umf is reduced more than Umb, allowing a bubble-free fluidization state when the superficial gas velocity is between Umf and Umb. This is a key requirement for the observation of R-T-like instabilities in granular media. Umf and Umb both decrease when the vibration frequency and amplitude are increased. The particle density does not seem to affect the influence of vibration on Umf and Umb, while on the other hand particle size affects the influence of vibration on Umf and Umb non-monotonically. Next the dissertation studies the mixing in rotating cylinders, a key industrial process. Particle image velocimetry was performed on the free surface of the bed of particles within a rotating cylinder that is operated in the avalanching regime. The velocities of the particles determined by PIV are used to examine the validity of existing avalanche models. The movement of bed particles (at the surface) is shown to depend on their location on the surface of the bed at the start of the avalance with particles located near the center of the bed travelling the farthest. The distance travelled by particles during an avalanche decreases at an increasing rate the farther the particle is from the center. The start of an avalanche can be linked to a single initiation point, that may even be located on the bottom half of the bed. An avalanche quickly propagates through the entire free surface, with 90 % of the surface in motion within 257 ms (20 % of the duration of an avalanche event). An updated geometric model is proposed which better describes the motion on the surface during avalanching of the granular bed. Experimental mixing measurements confirm the higher accuracy of the newly proposed avalanching model when compared to exisiting ones.