Blow-Pipe with a nozzle

[Anonymous]

Hello,

i have a problem with a CFD-Simulation. The settings are in the picture.

Boundary conditions

Inlet Blow-pipe: 235m/s; 2,35bar

Outlet Blow-pipe: 2,35bar

Outlet nozzle: nozzle inside 2,5bar and 275g/s

The pressure of 2,5bar is vaible, but the mass flow of 275g/s isn't possible. I have the half of it.

I used the boundary condition 'Unknown' but this doesn't work better.

Can somebody help me?

Kind regards,

Kevin Ölscher

Attachment CFZ File; picture with Boundary conditions

[Jon.Wilde]

Hi,

Thanks for sharing this. Could I point you to the page here on compressible flow. I think it might be really useful.

If this is a compressible analysis (it looks like it might be!), we really need Total Pressures at the inlets if you can. Also Total Temperatures might be useful, unless you are certain that the Total Temperature in the Solve window is correct.

I would also recommend using a Mass Flow rate where you have a known condition like your flow speed here.

Then try an unknown condition at the outlet also.

I see that you are using filters here too. They are not so suitable for compressible models as CFD averages out local velocity and pressure over their area. You can get away with it if the flow is incompressible by this point, but I would suggest testing without them first to see if they are having a negative impact.

Hope this helps.

Thanks,

Jon

[Anonymous]

Hello and thanks for the proposals,

sry for my late answer. I tryed your proposal but it didn't work better.

The Total Pressure at the inlets generated an undersized mass flow of 530 g/s.

If I set mass flow at the blow-pipe inlet, than the pressure inside the nozzle is 5,34 bar. That is too high.

My simulation needs the compressible flow for the nozzle. It is possible compressible and incompressible flow to realise in one simulation?

Kind regards, Kevin Ölscher

[Jon.Wilde]

Hi,

We only turn on 'compressibility' if there are regions in the model that are compressible. This is the correct approach.

Did you also assign Total Temperatures to the inlets?

Thanks,

Jon

[Anonymous]

Hello,

ok than I have a compressible flow, because the fluid flow through the nozzle.

Yes, I have Total Pressure at the inside.

Which boundary condition can I put together?

Mass flow, Total Pressure and Total Temperatur at one inlet?

I think that I make a mistake somewhere

Kind Regards

Kevin

[Jon.Wilde]

Hi,

Did you read through the link I shared in my first post? I think this will really help to answer your question. If not, please let me know.

Kind regards,

Jon

[Anonymous]

Hello,

yeah I read the paige on Autodesk Help. I tryed it but the results wasn't satisfactory.

By set the mass flow at the inlet, the mass flow inside the nozzle was the desired size but the pressure was to high.

If I set Total pressure and Total Temperatur at the inlet, the Mass flow inside the nozzle was to low and the pressure at the inlet wasn't the pressure I set.

I need 1,9 kg/s at the inlet and in the nozzle 0,5 kg/s and 2,5 bar.

The simulation needs 5,4 bar for 0,5 kg/s inside the nozzle. This results I get every time.

I havn't heat transfer so I set the Total Temperatur at the solve window. I think that is right.

I have another question. Is it right that I make a static Simulation? Is this a mistake?

Kind regards

Kevin

[Jon.Wilde]

Hi Kevin,

I am not sure I cam keeping up with the numbers. You have two inlets that feed into the nozzle right? Remember that as you have a half model, the mass flow rate here will also be halved compared to reality but the pressure would be the same as a full model.

The mass flow rate from the nozzle would be the sum of these two values right?

Then you have a second inlet that allow flow to run past the nozzle, although in the analysis I have here, there is a low pressure behind the nozzle than in front of it - which I am not sure about.

By static simulation, do you mean steady state? Yes, this is fine. Transient is only for capturing a simulation where variables change over time. Even then it is good to get a steady state up and running first.

Kind regards,

Jon

[Anonymous]

Hello,

The mass flow in my simulation is to be split up. Some mass flow through the nozzle and the rest at the outlet of the blow pipe.

I think I have one inlet and two outlets.

Yes, I know about it that the mass flow is half by a half model and the pressure is the same as a full model.

The mass flow rate is always right. That isn't a problem. I always have convergence, too.

The second inlet outside the nozzle and the blow pipe is in favor for backpressure. I also need a little mass flow against the shot of the nozzle.

Kind regards

Kevin

[Jon.Wilde]

OK, sounds good! Did you have any other questions?

[Anonymous]

Hello,

yes, I have one question. If I give the wall roughness a value, what would be change in the simulation like in the blow pipe for example?

Thanks for answering the past questions.

Kind regards

Kevin

[Jon.Wilde · Accepted solution]

This would likely increase the pressure drop, as it adds friction to the walls.

[Anonymous]

Hello,

okay, thats sound plausible. Thanks for answering my questions.

Kind regards

Kevin