Learning Forum

[Aayushya Agarwal]

Hello,

I am trying to model a multiphase flow with a n2 gas as a fluid and particle droplets as dense discrete particles with no continuous phase interation. The model has three inlets for the n2 gas (and inlet 1 injects the dense particles). I want to study the steady state distribution of the phases on the oultet at the bottom. When I simulate the multiphase flow, and plot the velocity magnitude of phase 1, I notice that the velocity magnitude does not reach the outlet at the bottom. I would expect the fluid phase to focus onto the outlet at the bottom, and would expect the dense particles to follow. Instead the dense particles begin to diverge out of the constricting point. Any help on the model/model setup would be greatly appreciated

[Rob]

Look at the flow and mesh in region in more detail. Why wouldn't DDPM particles spread out? Is 1cm/s a sensible speed in this system? 

[Aayushya Agarwal]

Thank you for pointing out the DPM speed. We modified the inlet flows so that we are expecting of over 1 m/s flow speed. 

However, when we increase the flow rate, we notice that the flow begins to drift and it doesn't have the symmetry we expect.

 

This seems to cause the dense particles to drift when exiting the nozzle:

 

However, when looking at the scaled residuals, the values seem to converge within an acceptable tolerance:

[Rob]

Can you plot a contour of velocity, and tangential velocity on the same plane as the last image in the first post? You may need to alter the cell zone reference axis, default is for z-axis to be the centre of rotation.  I think you've fallen into the trap of just checking DPM & vectors so have not got enough information to see what's going on. 

[Aayushya Agarwal]

Here is the contour of the velocity magnitude:

 

 

Here is the contour of the tangential velocity:

[Rob]

Oops, is this 2d or 3d?  The root cause is likely to be very similar, and looking at the tangential velocity I suspect 2d? 

[Aayushya Agarwal]

Yes. Sorry I should've mentioned its a 2D model

[Rob]

No worries. Have a look at Coanda Effect and the Pitchfork Bifurcation phenomena.  Your result is probably fine, you've just found some of the more interesting things that happen with fluid mechanics that aren't in most Undergrad courses. 

[Aayushya Agarwal]

Thanks I think that was the problem. I tried an axisymmetric model (to properly model the 3D version of this device) and I don't see the bifurcation

[Rob]

You won't. The "symmetry" part stops it happening. In 3d the jet will wander around, and if it's swirling lots more fun things may happen.  

CFD is a very good way to see lots of very strange fluid flow phenomena that you may miss in experimental studies. However it also means you need a very good understanding of fluid dynamics to ensure what you're reporting is correct.