Intermittent gravity-driven flow of grains through narrow pipes

TL;DR

This paper experimentally investigates the formation and dynamics of density waves in gravity-driven granular flows through narrow pipes, combining high-speed imaging, modeling, and stability analysis to understand their behavior.

Contribution

It introduces a combined experimental and theoretical approach to analyze density waves in granular pipe flows, revealing a long-wavelength instability consistent with observations.

Findings

Density waves exhibit specific length scales and celerities.

A linear stability analysis predicts a long-wavelength instability.

Experimental results align with the model's most unstable mode.

Abstract

Grain flows through pipes are frequently found in various settings, such as in pharmaceutical, chemical, petroleum, mining and food industries. In the case of size-constrained gravitational flows, density waves consisting of alternating high- and low-compactness regions may appear. This study investigates experimentally the dynamics of density waves that appear in gravitational flows of fine grains through vertical and slightly inclined pipes. The experimental device consisted of a transparent glass pipe through which different populations of glass spheres flowed driven by gravity. Our experiments were performed under controlled ambient temperature and relative humidity, and the granular flow was filmed with a high-speed camera. Experimental results concerning the length scales and celerities of density waves are presented, together with a one-dimensional model and a linear stability analysis. The analysis exhibits the presence of a long-wavelength instability, with the most unstable mode and a cut-off wavenumber whose values are in agreement with the experimental results.