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Aalborg Universitet

Prediction of Particle Agglomeration and Deposition by Reduced Particle Stiffness Discrete Element Simulations

Hærvig, Jakob; Sørensen, Kim; Condra, Thomas Joseph

Publication date:

2017

Link to publication from Aalborg University

Citation for published version (APA):

Hærvig, J., Sørensen, K., & Condra, T. J. (2017). Prediction of Particle Agglomeration and Deposition by Reduced Particle Stiffness Discrete Element Simulations. Poster præsenteret ved PhD Research Day 2017, Aalborg, Danmark.

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Prediction of Particle Agglomeration and Deposition by Reduced Particle Stiffness

Discrete Element Simulations

Jakob Hærvig, Kim Sørensen, Thomas J. Condra

Dept. of Energy Technology, Aalborg University, Denmark

e-mail: jah@et.aau.dk, tel.: +45 22508131

Introduction

I The collisions of small particles (dp < 10 µm) in dry air are typically dominated by van der Waals attractive forces.

I Discrete element method (DEM) simulations combined with the analytical JKR adhesive model [1] is a promising mechanistic- based approach to accurately predict agglomeration and deposition of micron- sized particles [2].

I The general applicability and relevance in scientific fields ranging from particle fouling prediction [3] to early stages of planet formation in outer space [4] make the JKR model increasingly popular [5].

I However, resolving collisions of micron-sizes particles typically requires time step sizes in the order of nano seconds (10−9 s).

Purpose of study

I To provide a criterion on how to introduce softer particles with the same adhesive behaviour in DEM.

I Using softer particles (lower Young’s modulus) allows for an increased time step size and thereby lower computational cost.

Main conclusions

I Simulation time can be reduced several orders of magnitude by reducing Young’s modulus from E to Emod while modifying the surface energy density γ as:

γmod = γ

Emod

E

2/5

(1)

I Simulations that would take years can now be done in hours or days.

Overview of modified model

I Figure gives an overview of the force-overlap relation of the JKR model and modified model based on eq. (1):

0 δn,0 δn,0,mod

(8/9)Fc(8/9)F c,mod0

Normal overlap δn NormalforceFn=F spring+F jkr

Original JKR model

Modified model using (1)

I Where Fc = 3πγR is the critical pull-off force to separate particles and δn,0 = 3π2γ2R/E21/3

is the equilibrium normal overlap. Graphical overview of DEM forces:

F damp,t

Fspring,n

v

Fjkr,n F damp,n

F spring,t δn

Agglomerate formed by van der Waals attractive forces

Validation cases

Initial position

Final possible states

(e1) (e2)

vj

vi vi,f

vj,f vi,f = 0

vj,f = 0

Initial position

Final possible states

v Fext

(d) (e2)

F ext ω

Fext (e3)

ω = 0

(e1) v Fext ψ

Collaborating partners

Coupling to turbulent flow

I Coupling DEM to large eddy simulations (LES) is a promising approach to investigate particle agglomeration and deposition mechanisms in details.

I Information is passed between fluid and particle phase by momentum exchange terms. Fluid drag takes local particle volume fraction into account.

Velocity magnitude u/Ub

0 0.2 0.4 0.6 0.8 1.0 1.2 1.4

I Example of pipe flow at Reb = 10, 000 in a

periodic domain with length L/D = 4.

References

[1] K.L. Johnson, K. Kendall and A.D. Roberts, Surface energy and the contact of elastic solids, Proc. R. Soc. Lond. A., 324:301- 313, 1971

doi: 10.1098/rspa.1971.0141

[2] E.J.R. Parteli, J. Schmidt, C. Blümel, K. Wirth, W. Peukert, T.

Pöschel, Attractive particle interaction forces and packing den- sity of fine glass powders, Nature Scientific Reports, 4:6227, 2014doi: 10.1038/srep06227

[3] C. Henry, J.-P. Minier and G. Lefèvre, Towards a description of particulate fouling: From single particle deposition to clog- ging, Advances in Colloid and Interface Science, 185-186:34- 76, 2012

doi: 10.1016/j.cis.2012.10.001

[4] A. Chokshi, A.G.G.M Tielens and D. Hollenbach, Dust Coagu- lation, The Astrophysical Journal, 407:806-819, 1993

doi: 10.1086/172562

[5] E. Barthel, Adhesive elastic contacts – JKR and more, Journal of Physics D: Applied Physics, 41:163001, 2008

doi: 10.1088/0022-3727/41/16/163001

Outlook

I Particulate fouling in heat exchanger pipes is a major problem with high associated costs. The particulate fouling process can be decomposed up into the following sub-processes:

Particle

agglomeration

Particle deposition

Re-entrainment Particle

collision

Agglomerate brake-up

Agglomerate re-entrainment Agglomerate

deposition

I Full-scale simulation using OpenFOAM and LIGGGHTS shows early stages of particulate fouling. Larger agglomerates are being formed in the centre of the pipe:

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