Title: An efficient model for the breakage of agglomerates by wall impact applied to Euler-Lagrange LES predictions
Authors: Khalifa, Ali Ahmad  
Breuer, Michael  
Language: eng
Keywords: Particle-laden flows;Modeling and simulation;Wall impact;Breakage of agglomerate;Hard-sphere model;DEM
Issue Date: Mar-2021
Publisher: Elsevier
Document Type: Article
Source: International Journal of Multiphase Flow 142 (2021) 103625
Journal / Series / Working Paper (HSU): International Journal of Multiphase Flow
Volume: 142
Issue: 103625
Publisher Place: Amsterdam
The present study completes the development of a model for predicting
the effect of wall impacts on agglomerates in turbulent flows. Relying
on an Euler-Lagrange hard-sphere approach this physical phenomenon is
described in an efficient manner allowing practically relevant
multiphase flow simulations at high mass loadings. In a recent study
\citep{khalifa2020data} conditions for the onset of breakage and the
resulting \added[id=2]{fragment} size distribution were derived. In
the present
investigation a data-driven description of the post-breakage kinetics
of the fragments is developed based on extensive DEM simulations
taking a variety of impact conditions (impact velocity, impact angle,
agglomerate size) into account. The description relates the
velocity vectors of the fragments after breakage to three parameters:
The reflection angle, the spreading angle and a velocity ratio of the
magnitude of the fragment velocity to the impact velocity of the
agglomerate. Relying on the DEM results Weibull distribution functions
are used to describe the parameters of the wall-impact model. The
shape and scale parameters of the Weibull distributions are found to
mainly depend on the impact angle of the agglomerate. Consequently,
relationships between the shape and the scale parameters and the
impact angle are established for each of the three parameters based on
a fourth-order regression. This allows to determine the velocity
vectors of the fragments randomly based on the corresponding Weibull
distributions of the reflection angle, the spreading angle and the
fragment velocity ratio.

The devised model is evaluated in a turbulent duct flow at five
Reynolds numbers and three agglomerate strengths given by powders
consisting of primary particles of different size. The analysis first
concentrates on the pure wall-impact breakage but then also includes
agglomerate breakup due to turbulence, drag forces and rotation
allowing to determine the shares of the different physical
phenomena. It is found that with increasing Stokes number the
wall-impact breakage occurs less effectively due to the reduced
responsiveness of the agglomerates to the secondary flow motions in
the duct. However, in the very high range of St$^+$ other mechanisms
such as the turbophoresis and the lift force augment the breakage at
walls. Comparing the contributions of the different breakage mechanism
reveals that the wall impact is dominant at the lowest Reynolds
numbers, whereas the drag stress prevails at high Re.
Organization Units (connected with the publication): Strömungsmechanik 
Publisher DOI: 10.1016/j.ijmultiphaseflow.2021.103625
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