[Paper Review] Mixing Grains with Different Elongation in a Rotating Drums

Claudia Piacenza, Marco Marconati+3 more|arXiv (Cornell University)|Sep 1, 2018

Granular flow and fluidized beds9 references5 citations

TL;DR

This experimental study investigates mixing dynamics of binary granular mixtures with identical volume but varying aspect ratios (elongation) in a rotating drum. Results show that effective mixing occurs only when elongation differences are below a critical threshold; otherwise, strong segregation forms a core enriched with more elongated particles, limiting homogeneity despite prolonged mixing.

ABSTRACT

Mixing grains with different properties is a remarkably challenging process, relevant to many industrial applications. Rotating drums have been used extensively as model systems to study granular media flow and mixing and segregation. Numerous studies considered the mixing of grains with different sizes, but only few studies considered shape and elongation, which have been already identified as important characteristics affecting packing and inducing segregation. In this contribution, the mixing of binary mixtures of grains having the same volume, but different elongations was studied experimentally. The mixing dynamics of a layered granular medium was characterized in a rotating drum, highlighting the impact of the grain shape, of the drum angular speed and of the drum filling ratio. A mixed or segregated state is reached very rapidly, but experiments are continued to verify that the state reached is a steady state. The experiments demonstrated that below a critical difference of elongation, the grains can be mixed effectively. Conversely, when grain elongations are very different, a central core is formed, rich in the more elongated grains. In this case only limited mixing can be achieved. These results can guide the formulation of mixtures of grains with different elongations.

Motivation & Objective

• To understand how grain shape, specifically elongation, influences mixing and segregation in rotating drum systems.
• To investigate the role of drum angular speed and filling ratio in the mixing dynamics of elongated grains.
• To determine the conditions under which binary mixtures of same-volume, different-elongation grains achieve effective mixing versus segregation.
• To identify critical thresholds in elongation difference that trigger irreversible segregation in rotating systems.

Proposed method

• Experiments were conducted using a transparent rotating drum filled with binary mixtures of grains having the same volume but different aspect ratios.
• Grain mixtures were layered at the start of each experiment to simulate initial heterogeneity and monitor evolution over time.
• The drum was rotated at controlled angular speeds, and the filling ratio was systematically varied to assess its influence on mixing behavior.
• Mixing state was monitored visually and quantitatively over time to determine whether a steady state was reached.
• The final spatial distribution of grains was analyzed to classify the outcome as mixed or segregated, particularly focusing on core formation.

Experimental results

Research questions

• RQ1 How does the difference in grain elongation affect the mixing efficiency in a rotating drum?
• RQ2 What role does drum angular speed play in the segregation or homogenization of elongated grains?
• RQ3 How does the drum filling ratio influence the formation of segregated regions in binary granular mixtures?
• RQ4 At what critical elongation difference does effective mixing break down and core segregation dominate?

Key findings

• When the difference in grain elongation is below a critical threshold, the mixture achieves effective mixing with minimal segregation.
• Above this critical elongation difference, a central core rich in more elongated grains forms rapidly, indicating strong segregation.
• Mixing dynamics reach a steady state quickly, but the final state depends critically on the elongation contrast between grains.
• The formation of a core enriched with elongated particles limits the extent of mixing, even after prolonged rotation.
• Drum angular speed and filling ratio modulate the rate of segregation but do not overcome the fundamental influence of elongation mismatch.
• Grains with similar volume but different aspect ratios exhibit distinct migration patterns due to shape-dependent packing and flow dynamics.