This document discusses various physical factors that influence the flow properties of granular materials. It describes how cohesion, adhesion, friction, and physical interlocking can cause poor flow. It defines internal and external friction, and the coefficients of static and dynamic friction. Higher moisture content and roughness increase the angle of repose, while larger, rounder particle sizes decrease the angle of repose. Mass flow is preferred over funnel flow in silos. Methods to improve flow include reducing electrostatic charges, drying materials to lower moisture content, and using larger outlets to avoid problems like arching and piping.

1. Physical factors Influencing
flow properties
Presented by: Chitesh Kumar
PFE 513
Department of Processing and Food Engineering
Punjab Agricultural University

2. Poor flow due to?
• Cohesiveness
• Adhesion
• Friction
• Physical interlocking
• Angle of repose
etc

3. Friction?
• Friction is a force that resists the
motion of objects that are in
contact with each other
• F = µN,
• Where, F = Force , µ =Coefficient
of friction (static or dynamic)
N = normal load

4. Internal and external friction
• In grain conveying system: Two types of friction
1. External friction/wall friction: External friction is expressed as the wall
friction angle or coefficient of sliding friction.
• The lower the coefficient of sliding friction, less is the slope required for the
flow of material(mass flow).

5. 2. Internal friction: Friction of the kernels or grains against each other.
• Higher angle of internal friction indicates the material is cohesive which
increases with the moisture content.

6. Coefficient of friction
• Coefficient of friction
is given as the tangent
of the angle of the
inclined surface.
• µ=tanθ

7. Types of Coefficient of friction:
1. Static coefficient of friction : predicts the
force at the point in time when motion is
initiated.
2. Dynamic coefficient of friction : predicts the
force required to maintain the motion once it
is initiated
1 > 2
(why?)
Force required to maintain motion is less than or
equal to the force required to initiate motion

8. Rolling friction
• Rolling friction must be overcome to start rolling an object.
• Directly proportional to weight and coefficient of rolling friction and
inversely proportional to the radius of the rolling object
• Fr= 𝜇N/r
• Important in separation process like separating potato and stones.

9. Angle of Repose
• It is the angle between the horizontal and the
inclination of heap.
1. Indicator of flow ability of the product.
2. Lower the angle of repose more easily the
material will flow.
3. More is the cohesiveness larger is the angle of
repose.

10. Factors affecting angle of repose
1. Size
2. Shape
3. Moisture content
4. Orientation of the grain

11. Factors effecting Angle of Repose
Sphericity :- Dia. of largest inscribed circle / dia. Of smallest circumscribed
circle
• Increasing sphericity can decrease the angle of repose
Roundness :- Radius of curvature of the sharpest corners / mean radius of
object
• With increase in roundness angle of repose decreases

12. Particle size:- Coarse particles are more preferred than fines one as they are
less cohesive.
• Removing a portion of fine particle or increasing the coarse particles
improves the flow ability of the material.
• More is the Particle Size less is the Angle of repose.
• Example :- Large rice particle have a much lower angle than that of the fine
grained powder sugar

13. Roughness
• The angle of repose increases with the increase with the roughness particles.
• Particles with very rough surfaces will be more cohesive and have a greater
tendency to interlock than the smooth surface d particles.
Friction caused by roughness to a rolling particle

14. Moisture content
• Moisture content is more the angle of repose is more
Three different varities of wheat
Bani Suwayf 3
Giza 168
Seds 1

15. How to improve flow?
• Reduction of electrostatic charges can improve the flow of granular
materials
• Electrostatic charges can be reduced by reducing the friction to the grains
• Drying of particles can also lead to better flow of the product after reducing
the moisture content
• Drying can reduce the cohesiveness and improve the flow.
• Storing and processing of the product in low humidity conditions.

16. Flow of granular material in silos.
• Type and rate of flow through an orifice depends upon its shape and size.
Types of flow
1. Mass flow
2. Funnel flow

20. Mass flow vs Funnel flow
Mass flow Funnel Flow/pipe flow
Whole silo content moves during discharge Only a portion of silo content moves while rest of
the material remains stationary forming stagnant
zone.
Grain which enters the silo first also comes out
first
While in funnel flow there is last in and first out
type of situation
Problem: Arching Problems: arching, piping, irregular flow or
flooding.
To avoid these problems large outlets should be
provided
Cohesive materials and powders can be stored Where as here course and free flowing materials
are required like oil seeds, grains and pulses

21. Major problems associated with the flows
1. Piping: When bulk solid above the
outlet is flowing out and remaining
bulk solids stays in place known as
dead region.
2. Arching: It is a stable obstruction
that forms over the point of
narrowest cross section of the
vessel.

22. References
• P. Richard, M. Nicodemi, R. Delannay, P. Ribière, D. Bideau, Slow relaxation and
compaction of granular systems, Nat. Mater. 4 (2005) 121–128, https://doi.org/
10.1038/nmat1300.
• B. Kou, Y. Cao, J. Li, C. Xia, Z. Li, H. Dong, A. Zhang, J. Zhang, W. Kob, Y. Wang, Granular
materials flow like complex fluids, Nature (2017) 1–13
• Powder Flow - Powder Metallurgy - Beyond Discovery
• Hamzah M. Beakawi Al-Hashemi, A review on the angle of repose of granular materials,
Article in Powder Technology · May 2018 ,398-411
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