# Plug regime in water fluidized beds in very narrow tubes

## TL;DR

This study investigates granular plug formation and dynamics in water fluidized beds within very narrow tubes through combined experimental and numerical methods, revealing detailed length scales and propagation speeds.

## Contribution

It provides the first combined experimental and numerical analysis of granular plug length and velocity in very narrow water fluidized beds.

## Key findings

- Granular plugs form and propagate with measurable length scales and celerities.
- Experimental and numerical results show good agreement.
- The study advances understanding of plug behavior in narrow tube fluidization.

## Abstract

The dynamics of granular plugs in water fluidized beds in narrow tubes is investigated here both experimentally and numerically. The fluidized beds were formed in a 25.4 mm-ID tube and consisted of alumina beads with 6 mm diameter and specific gravity of 3.69; therefore, the ratio between the tube and grain diameters was 4.23, which is considered a very narrow case. Three different beds were arranged, consisting of 250, 400 and 500 beads, and two water flow rates were imposed, corresponding, respectively, to superficial velocities of 0.137 and 0.164 m/s and to Reynolds numbers based on the tube diameter of 3481 and 4177. Under these conditions, it was possible to observe the formation of granular plugs that propagate with characteristic lengths and celerities. For the experiments, we filmed the fluidized bed with a high-speed camera, and automatically identified and tracked the plugs along images by using numerical scripts. For the numerical part, we performed simulations using a coupled CFD-DEM (computational fluid dynamics - discrete element method) code, together with numerical scripts to identify and track the granular plugs. We obtained, for the first time, the length scales and celerities of the granular plugs in very narrow tubes from experiments and numerical simulations, and the agreement between them is good.