Effect of particle shape on the flow of an hourglass

TL;DR

This study investigates how particle shape influences granular flow rate in an hourglass, revealing a nonmonotonic relationship with two optimal aspect ratios that enhance flow compared to spheres.

Contribution

It provides experimental and numerical evidence of the nonmonotonic effect of particle shape on flow rate, highlighting two optimal aspect ratios for improved flow.

Findings

Flow rate peaks at specific aspect ratios around 0.6 and 1.5.

Flow rate decreases for more elongated or flattened particles.

Packing fraction and particle velocity both vary nonmonotonically with shape.

Abstract

The flow rate of a granulate out of a cylindrical container is studied as a function of particle shape for flat and elongated ellipsoids experimentally and numerically. We find a nonmonotonic dependence of the flow rate on the grain aspect ratio a/b. Starting from spheres the flow rate grows and has two maxima around the aspect ratios of a/b = 0.6 (lentil-like ellipsoids) and a/b = 1.5 (rice-like ellipsoids) reaching a flow rate increase of about 15% for lentils compared to spheres. For even more anisometric shapes (a/b = 0.25 and a/b = 4) the flow rate drops. Our results reveal two contributing factors to the nonmonotonic nature of the flow rate: both the packing fraction and the particle velocity through the orifice are nonmonotonic functions of the grain shape. Thus, particles with slightly non-spherical shapes not only form a better packing in the silo but also move faster through the orifice than spheres. We also show that the resistance of the granulate against shearing increases with aspect ratio for both elongated and flat particles, thus change in the effective friction of the granulate due to changing particle shape does not coincide with the trend in the flow rate.