[Richi]

Dear community,

I am simulating a compressible flow in an ejector (two inlets, one outlet) with CO2. When simulating with mass flows at the inlet all results are very accurate to experimental results but pressure at one inlet is impossibly high.

I am not allowed to make data or geometries public but still wanted to ask here.

Mulitple settings were tried (defining mass flows at inlets and avg. static pressure at the outlet, total pressures at the inlets and avg. static pressure at the outlet etc.)

Do you know what could be tried with the setting?

Thank you and best regards

[Gert-Jan]

If you set massflows, than CFX will provide the (total) pressure to obtain this massflow. If it does not match your experiments, this could mean multiple things, like:
- the internal diameter is wrong
- there is a restriction that you overlooked
- you use gas instead of liquid
- did you use ideal gas?
- is the reference pressure correct?
- did you use thermal or total energy, to include compressible effects?
- is temperature/density correct?
- the two streams interact in a strange way that lead to reduction or increase of pressure

[Richi]

Originally Posted by Gert-Jan

If you set massflows, than CFX will provide the (total) pressure to obtain this massflow. If it does not match your experiments, this could mean multiple things, like:
- the internal diameter is wrong
- there is a restriction that you overlooked
- you use gas instead of liquid
- did you use ideal gas?
- is the reference pressure correct?
- did you use thermal or total energy, to include compressible effects?
- is temperature/density correct?
- the two streams interact in a strange way that lead to reduction or increase of pressure

Thank you for your answer Gert-Jan!

- it is not ideal gas and CO2 from peng rob is used
- the reference pressure is correctly set at 1 bar
- total energy is used
- The densities are also incorrect where the massflow is incorrect
- The ejector should lead to a pressure rise at the outlet compared to what comes in at one inlet. The other inlet flow creates the movement through the ejector.
It is a complex system of flows with changes from subsonic to supersonic and back.

[ghorrocks]

And also have a close look at your boundary conditions. The fact that you have tried a few combinations suggests you are not absolutely sure what to use. With two inlets and an outlet you have to get the boundary conditions right - balancing up the flows between the two inlets is likely to be critical, otherwise you will get back flow or some other fundamental flow error.

Keep in mind that small variations between the actual flow and your model may mean that a flow which in the actual device has both inlets flowing the correct direction; now in the simulation that small variation has caused an inlet to flow backwards.

To resolve this you need to think carefully about what is the actual condition driving the flow. It might not be the obvious one. Also, you might need to extend your domain upstream further to somewhere the boundaries are better defined. And in rare cases you need to put a ficticious volume upstream whose job is purely to make the boundary condition act correctly.

This is all general advice as you cannot be specific without seeing the details of what you are modelling.

[Gert-Jan]

Originally Posted by Richi

Thank you for your answer Gert-Jan!

- it is not ideal gas and CO2 from peng rob is used
- the reference pressure is correctly set at 1 bar
- total energy is used
- The densities are also incorrect where the massflow is incorrect
- The ejector should lead to a pressure rise at the outlet compared to what comes in at one inlet. The other inlet flow creates the movement through the ejector.
It is a complex system of flows with changes from subsonic to supersonic and back.

I would start with ideal gas. If that gives reasonable results, then your Boudary conditions and settings are correct. If not, check them carefully.
Then switch to Peng Robinson and see what it brings. If it fails, then Peng is to blame. Or Robinson.