# Powder properties influence on the local stress profile in a granular column

Duri Agnès, Mandato Sandra, Cuq Bernard et Ruiz Thierry

UMR IATE 1208 CIRAD/INRA/Montpellier SupAgro/Université Montpellier – 2 Place Pierre Viala, 34060 Montpellier cedex 5, France.

Tel. +33 4 99 61 20 60, E-mail: agnes.duri@supagro.inra.fr

Présentation souhaitée : P

Thématique :

3 - Propriétés et caractérisation

In a wet agglomeration process, inside a low shear mixer, the blade displacement allows for homogenizing the pulverized liquid on the powder surface and promoting the growth mechanisms of agglomerates. This mechanical action is efficient if the blade can apply an adapted stress field in the granular media. Low shear mixers with a vertical arm can cyclically mix the grain bed by alternating dynamic states of the particles close to the blade and static states beyond the influence length of the mobile. As a consequence, the blade displacement alternately disturbs the mechanical profile at equilibrium inside the powder bed which strongly affects the flowing powder properties once the powder is in movement. Until recently, the qualification of the static state of a granular media was described by Janssen's model, which under some hypotheses, allows calculating the stress at the bottom of powder column. Recent stress measurements, conducted by some of us, showed that Janssen's profile is different from the local stress profile.

The objective of this work is to experimentally measure the local stress profile inside a semi-confined granular bed with a force sensor immersed in the powder bed. Several reactive powders issued from durum wheat were selected to estimate the physical characteristic impact of the particles (size, angle of rest...). The results show that the shape of the local stress profiles is similar whatever the powder characteristics. The construction of the local profile is discussed according to a structuration of three layers in the powder bed, which seem to be separated by two characteristic lengths.
# Powder property influence on the local force profile in a granular column

Agnès Duri-Bechemilh, Sandra Mandato, Bernard Cuq, Thierry Ruiz

► To cite this version:

Agnès Duri-Bechemilh, Sandra Mandato, Bernard Cuq, Thierry Ruiz. Powder property influence on the local force profile in a granular column. Colloque STPMF - Science et Technologie des Poudres et Matériaux Frittés, Apr 2015, Nancy, France. 2015. [hal-01606453](https://hal.science/hal-01606453)

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https://hal.science/hal-01606453

Submitted on 2 Jun 2020

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# Powder property influence on the local force profile in a granular column

Agnès DURI, Sandra MANDATO, Bernard CUQ, Thierry RUIZ

<figure><img src="image_2.png" /></figure>

## ISSUES

In a mixer equipment, the external mechanical stress induces the mobility of the particles that allows their mixing, segregation or association.

The mobility of the particles are greatly affected by the **initial mechanical static state** of the powder bed.

* How to evaluate the mechanical static state of the powder bed before mixing?

* How the physical properties of the particles influence this mechanical static state?

<figure><img src="image_3.png" /></figure>

<figure><img src="image_4.png" /></figure>

<figure><img src="image_5.png" /></figure>

<figure><img src="image_6.png" /></figure>

## OBJECTIVES

**Short-term:** characterization of the mechanical static state in ensiled granular media of different physical properties (particle size, restitution coefficient, coefficient of friction, cohesion, angle of repose, compactness, history of the bed formation...).

**Long-term:** better understanding the contribution of the initial mechanical static state of the powder bed on the mobility of the particles.

## MATERIALS

**Powders** - 5 wheat-based powders of identical origin but with a different structure.

<table><thead><tr><th>Sample Name</th><th>SEM Image</th><th>d<sub>50</sub> (mm)</th><th>d<sub>90</sub>-d<sub>10</sub> (mm)</th><th>Bed compactness</th><th>Angle of repose (°)</th></tr></thead><tbody><tr><td>Fine semolina</td><td></td><td>0.210</td><td>0.321</td><td>0.416</td><td>32.2</td></tr><tr><td>Semolina</td><td></td><td>0.298</td><td>0.434</td><td>0.446</td><td>31.4</td></tr><tr><td>Fine couscous</td><td></td><td>0.676</td><td>0.645</td><td>0.578</td><td>12.1</td></tr><tr><td>Medium couscous</td><td></td><td>0.953</td><td>0.517</td><td>0.573</td><td>9.8</td></tr><tr><td>Large couscous</td><td></td><td>1.095</td><td>0.406</td><td>0.578</td><td>7.6</td></tr></tbody></table>

## METHOD

**Approach** - Transposition from an industrial mixing device (with a 3D blade motion) to an **unidirectional mixing device**.

<figure><img src="image_7.png" /></figure>

The mechanical static state is achieved by measuring the **local vertical force** (in triplicate) perceived by the blade (**immobile**) in different **z** positions in the powder bed.

## RESULTS

### Local - Global profiles

<figure><img src="image_8.png" /></figure>

### Local profiles - Physical properties of the particles

<figure><img src="image_9.png" /></figure>

## CONCLUSIONS

The study of the mechanical static state indicates:

* An **inhomogeneous lateral repartition** of the **mechanical static state** of the powder bed.

* A **superficial layer** of about **50 mm** with **low shear level** and **independent** from the **physical properties** of the particles.

* A **median layer** of about **70 mm** with **higher friction level** for semolina than for couscous.

* A **bottom layer** of about **20 mm** for which the **bottom wall effect** is **independent** from the **physical properties** of the particles.

## PERSPECTIVES

* **PIV analysis** will be performed to characterize the **particle motion** under the action of the blade motion in the three zones of the bed.

* Define **sizing criteria** of powder mixers from the **characteristic lengths**.