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Discrete Element Method Collision Model

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Old   December 23, 2016, 03:31
Arrow Discrete Element Method Collision Model
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sfn
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Alexander
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Hello

I'm trying to model multiphase flow (water - air - mineral particles) in spiral separator using VOF model and Discrete Phase model + DEM collision model. I got problem modeling particles behavior. The problem is that there is a difference in results of physical experiment and CFD experiment. In my geometry there are 4 outlets and yield of different particles in these outlets isn't agreed with results of physical experiment.
My set up:
1) VOF dispersed implicit model (all others setting are default here)
2) Transition k-kl-omega model
3) Discrete phase model: unsteady particles tracking, no interaction with continuous phase, implicit tracking, no accuracy control (all others setting are default here)
4) DEM collision model: Spring Collision Law (k=1, max velocity =1 m/s; all others setting are default here)
5) There is one cell zone: operating pressure=101325pa, gravity on, operating density=1.225kg/m3
6) Boundaries: one velocity inlet, 4 pressure outlets, all others are walls
7) All solution controls are default, time step size 0.01 s
I've tried a lot of different setting and these provide the best results.
In my opinion the problem is DEM collision model because when this model is turned off i get good results agreed with real life experiments except the fact that collisions aren't considered so i get results that doesn't describe real process.
In DEM collision model i can only change Edge Scale Factor (i think default value is good for me, i've tryed to change it but i didn't get good results), Maximum Particle Velocity (in my case it was set to 1m/s, because water velocity is about 0.5 m/s and i don't think that particles will move faster), Contact Force Laws (i think the problem is here).
I've tried different laws with different coefficients in both normal and tangential directions (spring, spring-dashpot, hertzian, hertzian-dashpot, friction, rolling friction) and could not obtain accordance with real process. I decided to set only normal laws because tangential laws didn't affect particles motion in my case. The problem here is coefficient value.
In spring collision law i have to set K value. ANSYS provides the formula to calculate it (16.15.1 chapter of user manual). I need to provide parcel diameter, particle mass density, relative velocity between two colliding particles, fraction of the diameter for allowable overlap. But all these values are varying in wide range excepting allowable overlap. So i got range of k values from 0.1 to 100, what should i use? If i set big value of k particles are jumping from the walls, if i set small value particles are stuck. The best results i got with k=1.
In spring dashpot collision law i have to set coefficient of restitution additionally. Still couldn't obtain good results here.
In hertzian, hertzian-dashpot law i need to set Young’s Moduli and their Poisson’s ratios. I couldn't set the values providing good results again.

So my question to you how to set up DEM collision model properly, what laws and coefficients should i use?
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Old   January 17, 2017, 11:22
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any thoughts?
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Old   March 2, 2017, 00:41
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David Li
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Hi sfn. Did you make any progress these days. Now I'm trying to simulate the particles interaction in the flow field. The case is a very simple 2D case. I chose the DEM model. Unfortunately I found that the particles didn't collide each other even if I chose different laws. Did you meet this situation before? Looking forward to your reply. Thanks.
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Old   March 9, 2017, 03:44
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its hard to say without full information about your case. i may recommend you to check "dpm collision partners" in injection setting and dpm setting.
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Old   March 11, 2017, 08:08
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I'm trying to model multi-phase flow (water - oil) in corrugated plat separator using multi-phase eulerian model and dence Discrete Phase model + stochastic collision +coalescence model. I got problem modeling oil droplet behavior.
My set up:
1) multi-phase eulerian model active dense Discrete Phase model (all others setting are default here)
2) Transition k-kl-epsilon model
3) Discrete phase model: unsteady particles tracking, interaction with continuous phase, implicit tracking, no accuracy control (all others setting are default here)
4) check in physical model stochastic collision +coalescence model all others setting are default here)
5) There is one cell zone: operating pressure=101325pa, gravity on, operating density=1100kg/m3
6) Boundaries: one velocity inlet, 1 pressure outlets, all others are walls
7) All solution controls are default, time step size 1 s
i have message reversed flow pressure outlet
So my question to you how to set up oil- water gravity separation problem properly, what models should i use?
pleas help me
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Old   May 27, 2018, 10:25
Smile Oil and water separation
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Hi .actually i am already modeling a two phase oil and water separator. I found that beside eulerien modle you should use PBM (population balabce model) approach to. For this you have to enable population model at ansys . Actually i do not know exactly but i am working on it. If you have find a solution and get to answer please help me to. I need it very much. My time is less and i have not finished it . Thank you

Last edited by Shahla; May 27, 2018 at 11:44.
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