## Fluids

Topics relate to Fluent, CFX, Turbogrid and more

#### Fluent, Setup for Two phase CO2+H2O

Subscriber

I have a 2D pipe, 2" by 60" inches. my BCs are pressure., with an inlet of 500.1 psi and an outlet of 500 psi. (0.1 pressure drop)

gravity direction is negative y. (multiphase model is set to default Eulerian). The primary phase is CO2, secondary is set to be H20, with a default diameter.

My problem is the following:

if I have two phases entering the pipe on the left side, from fluid mechanics perspective I expect, water (as a denser phase) to accumulate in the bottom of the pipe, and due to pressure change, flow towards the outlet. However, in my case, Water droplets chaotically flow through a pipe which is not realistic. I have attached phase velocity and pressure figures.
What I am doing wrong in setting up the model?

• Rob
Ansys Employee

If the mixture of phases takes around 0.25s to reach the mid-point (about a metre at 4 m/s) and the droplet diameter is 10microns (default) how far would that droplet fall under gravity? Use Stokes Law.

Subscriber

I increase the size to 100 microns and reduced the pressure drop from 1 psi to 0.1 (inlet 500.1, and outlet 500).

same picture

• Rob
Ansys Employee

And the speed?

Subscriber

It also reduced the fact that no matter how slow I make the flow, water particles or drops are still within the flow. They do not go to the bottom because of gravity. Can you try to check this out on your PC?

It works perfectly fine when my pressure inlet has only one fluid coming in (water); in this case, as soon as water enters the pipe, it falls to the bottom part due to gravity.

I think I figured out what the problem was. I checked the velocities of CO2 and H20; they are equal. Why are they equal? CO2 has much more density; what should I fix to make it work? I know that with mass inlet I can give different velocities to both phases, but I am interested in how exactly to do this with pressure inlet.

• Rob
Ansys Employee

Can you post images of the multiphase panel setup and operating conditions?

Subscriber

• Rob
Ansys Employee

Looks fairly sensible. How is the convergence?

Subscriber

This is my Convergence, I also have attached the velocity profile of Co2 and H2O. Overnight I was able to tune the problem a little bit, my water droplet go down due to gravity, but my velocities for both phases are almost the same.

• Rob
Ansys Employee

Why would the velocity be different?

Subscriber

Because, for the same pressure drop, gases move much faster than fluids. they have very different densities.

Co2 density is 1.73 kg.m3, while h20- density is close to 1000kg/m3.

• Rob
Ansys Employee

Based on (0.5 rho v^2) yes, you're right. But that doesn't account for drag effects.