I am trying to track small particles in CFX. When the particle Reynolds number is between 0.1 and 1000 it is recommended to use the Schiller drag model. When modeling small particles (less than 10 micron) the particle slip velocity must be taken into account, and a correction factor based on the particle diameter called the Cunningham correction factor is used to modify the particle drag.

Is the Cunningham slip model accounted for automatically for small particles in CFX, it is not mentioned in the help? If not how can it be implemented?

Any help on this one would be great Thanks

Hi,

The particle slip velocity is what the particle Re number is calculated from! What you are talking about is already in CFX as that is how it calculates the forces on the particles. I have no idea what the Cunningham model is but I also don't see why you would need it from your description as particle slip velocity is the most basic concept in multiphase modelling.

Glenn Horrocks

Glenn thank you responding to my post. I should have been more specific about the problem. When the particle diameters become small (less than 10 microns in air) the fluid can no longer be considered continuous. The drag coefficient on each particle, which is calculated using standard correlations (Schiller Neauman), must be divided by the Cunningham correction factor. The correction factor is greater than 1 which means that the effective particle drag coefficient goes down. This reduction in particle drag was the "particle slip" to which I referred in my previous post.

So to implement this correction factor I need to divide the locally calculated particle drag by a scalar quantity ( I am only modelling a single particle size) Cd/Cc. I can't see how one can do it in Pre, so is there another way?

Thanks again

John

Hi John,

The cunningham correction factor is not used in CFX but if you use the "Particle Transport Drag Coefficient" in the Momentum Transfer tab in Pre you can specify any drag model that you need using CEL relations. First try to redo the Schiller Nauman relation, verify it with the CFX implementation and then correct it with the Cunningham factor. The Schiller Nauman will be the hardest part because it is an implicit relation.

Bart

Bart, Thanks for responding to my post, that sounds like a good plan, I'll give it a go

Thanks again

John