[Paper Review] Radial and axial segregation in horizontal rotating cylinders studied by Magnetic Resonance Imaging (MRI)

Thoa Thi Minh Nguyen, Andrew J. Sederman+2 more|arXiv (Cornell University)|Dec 2, 2006

Fluid Dynamics and Vibration Analysis1 references3 citations

TL;DR

This study uses high-resolution 3D and real-time 2D MRI to investigate radial and axial segregation in horizontal rotating cylinders filled with millet and poppy seeds. It reveals that initial radial segregation results from percolation and stopping of poppy seeds, while axial band formation, traveling, and merging occur at ~3 μm s⁻¹, with core diffusion becoming a dominant mechanism in highly filled cylinders (>75%) and poppy band concentration quantified at 60% by volume for the first time non-invasively.

ABSTRACT

The dynamics of granular materials, mostly radial and axial segregation in horizontal rotating cylinders filled with millet and poppy seeds, is studied by Magnetic Resonance Imaging (MRI). For the first time, full 3D structures and real-time 2D MRI movies showing the progress of segregation over many hours are reported. Data are acquired with sufficiently high temporal and in-plane spatial resolutions (74 ms and 0.94 mm imes 0.94 mm, respectively), giving new insights into the underlying mechanisms. The millet and poppy mixture composition is calibrated based on the signal intensity and is quantified throughout segregation. As for radial segregation, millet and poppy mixture core formation is observed in cylinders of 75% and 82% filling level. The size of the core is calculated and the avalanche layer thickness is therefore determined. 2D MRI movies showing real-time radial segregation suggests that initial radial segregation is due to the stopping and percolation of poppy seeds. Axial segregation is characterized by the formation, traveling and merging of poppy-rich bands. In all cases studied, the formation of poppy-rich bands is observed, after which individual bands start to travel at \sim 3 μm s^{-1} until they are within \sim 3 cm of a stationary band. Adjacent bands then merge into a single, enlarged poppy band as millet seeds move out of the merging region. In both radial and axial segregation, core diffusion is shown to be another mechanism pathway for segregation besides the free surface.

Motivation & Objective

• To understand the mechanisms driving radial and axial segregation in horizontal rotating cylinders using non-invasive imaging.
• To quantify the role of core diffusion versus free surface dynamics in segregation, particularly in highly filled systems.
• To observe and characterize the real-time evolution of axial bands, including traveling and merging processes.
• To provide high-resolution 3D spatial and temporal data for validation of segregation models.
• To calibrate MRI signal intensity for accurate, non-invasive quantification of granular composition throughout segregation.

Proposed method

• Magnetic Resonance Imaging (MRI) with 74 ms temporal and 0.94 mm × 0.94 mm spatial resolution was used to capture real-time 2D and full 3D dynamics of granular segregation.
• 2D FLASH real-time movies were recorded to visualize segregation evolution over hundreds of hours, enabling observation of band formation, traveling, and merging.
• Signal intensity in MRI was calibrated to quantify local concentrations of millet and poppy seeds, enabling non-invasive, spatially resolved measurement of composition.
• The thickness of the avalanche layer was inferred from the size of the core formed in radial segregation at different filling levels (75% and 82%).
• Axial band dynamics were analyzed by tracking band movement and merging events in cylinders of varying length and filling fraction.
• Core diffusion was assessed by observing segregation persistence in cylinders with filling levels above 75%, where a stable core forms beneath the avalanche layer.

Experimental results

Research questions

• RQ1 What is the dominant mechanism for initial radial segregation in rotating cylinders, and how does it differ from core diffusion?
• RQ2 How do axial bands form, travel, and merge over time, and what drives their motion?
• RQ3 What is the quantitative concentration of poppy seeds within axial bands, and does it evolve toward a steady state?
• RQ4 To what extent does core diffusion contribute to segregation in highly filled cylinders (>75%)?
• RQ5 Can MRI provide accurate, non-invasive, and spatially resolved quantification of granular composition during segregation?

Key findings

• Initial radial segregation occurs due to the stopping and percolation of poppy seeds, with smaller particles accumulating near the rotation axis.
• In 75% and 82% filled cylinders, a stable core forms along the rotation axis, with the avalanche layer thickness inferred from core size.
• Axial bands form, travel at a constant speed of approximately 3 μm s⁻¹, and merge when approximately 3 cm apart, driven by lateral migration of millet seeds.
• During merging, millet seeds move outward from the interface region until the poppy concentration in the merged band reaches 60% by volume.
• The ultimate poppy concentration in all bands is consistently 60% by volume, confirmed across multiple intermediate and final bands over 80,000 revolutions (38 hours).
• Core diffusion is shown to be a significant segregation mechanism in cylinders with filling levels above 75%, where the core remains isolated from the avalanche layer and undergoes axial segregation.