Learning Forum

[z.sang]

Hello everyone, I am using CFD-DEM coupling method by Rocky and Fluent in 2023R2 to simulate the . Generally, there should be liquid spreading and penetration phenomena which can wet the particles. However, what I can observe from the numerical results is only the droplet impact which resulted in a hole in the center of the powder bed, but the droplet just kept spherical or elliptic.

In ANSYS FLuent, I used Eulerian model for air, liquid and particle phases and set air as the primary phase. I set a cell region filled with the liquid which falls vertically with the velocity of 1m/s. Here are some important parameters :
Droplet diameter: 72 µm, liquid density: 1028kg/m3, liquid viscosity: 0.0028 kg/m*s, liquid surface tension: 0.015 N/m
In ANSYS ROCKY I first generated the powder and used module to consider drag force and pressure gradient force. Here are some important parameters :
Particle size distribution:10-35 µm, density of particles: 7890 kg/m3.

I have tried many adjustment of a lot of paramters but I can not reproduce the liquid wetting phenomenon.
Could anyone please give my some advice? In which direction should I try? Parameter adjustment ?Model selection? Or CFD-DEM coupling setup problem?

Thank you in advance!

[Rob]

Have a very careful look at what the various models do, and how they interact. 

I'd use DEM to calculate building up the particle bed. But even with the new Rocky SPH model(s) you'd struggle to handle wetting at the resolution you're wanting: that may need Fluent's VOF model which doesn't work with the Rocky DEM particles. There's also the level of mesh refinement needed to capture the bed AND particles at 10microns AND their roughness AND the film thickness AND gaps. 

This is not an easy simulation so please review the project goals relative to how much time (and training) you have. 

[z.sang]

Thank you very much for your reply. :)

From you advice, it seems the mesh refinement is really important. Actually, because I am running two-way fluent coupling simulation which requires mesh size larger than particle size, I set the mesh size to 20 µm. For the current feasibility study, I have to consider the efficiency. However, in future I would definitely consider the mesh refinement.

Fow now, it is more likely that droplet smashed into the powder bed instead of flowing into the powder bed due to the capillary force. I am really confused.

 

[Rob]

Again, please review the model limitations and behaviour. Particles bigger than a cell can be considered, but you'll not see any capiliary effects. For that you may need the particles to be fixed and modelled in detail: ie not using Rocky when looking at the droplet/film part. 

[z.sang]

Thank you very much for your explanation. I am not sure whether I got the point. The situation that particles are larger than the mesh cell is applied for unresolved coupling, in which the interaction along the particle surface is not determined. Therefore, I cannot reproduce the capillary effect by unresolved coupling.

I have considered your advice that I fixed the particles. It is really helpful.

I wonder if the semi-resolved coupling would help, becasue two-way coupling is really important to my project. 

[Rob]

Are the particles moving? Or can you consider them as fixed in place? For capiliary effects you need to resolve the particle in the CFD mesh, but that also means you're not going to capture any motion easily. Hence thinking very carefully what you want to model. 

[z.sang]

Yes, the particles are moving, because what I want to see is how liquid droplet leads to the particle rearrangment after liquid spreading and penetration. Therefore, it will be tough if I fix the particles. I will consider your advice. It is very helpful to discuss with you. Thanks a lot.

[Rob]

That's going to be more difficult: modelling vastly different length scales always is.