[adilarvind]

I am simulating an axial flow turbine with velocity inlet and pressure outlet BC. The simulation is giving good result with respect to torque and pressure drop matching with the experimental results. I have also checked with different mesh counts for grid independence study. The mass flow from inlet and outlet is also matching.

However the massflow rate is not matching with experimental or analytical result ( i.e A*V* density) and as a result the efficiency is not matching with the experimental results. The difference is in the order of 30%.

Any clue in this regard will be very helpful.

[ghorrocks]

Aren't you defining the mass flow rate at your inlet boundary? Does this mean your inlet boundary condition is wrong?

[adilarvind]

I am defining the velocity at the inlet, i.e normal velocity to the inlet. The mass flow rate should be calculated from CFD.

[ghorrocks]

1) How are you calculating the mass flow rate?

2) Is the flow incompressible or compressible?

3) Steady state or transient?

4) If the mass flow rate is wrong then can you adjust the inlet boundary velocity to get the correct mass flow rate?

[adilarvind]

1. I am calculating mass flow from the function calculator in cfx post: massflow()@inlet
2. The flow is incompressible
3. i am using SST k-omega model for steady state
4. I cant change the inlet velocity because that is my inlet BC, i have to find out the torque and efficiency for particular inlet flow velocity

[ghorrocks]

Then it appears your desired mass flow rate is inconsistent with your inlet velocity.

Or are you saying the mass flow() calculation is wrong?

[PeMo]

Just to ensure: You are using periodic BC, are you multiplying the calculated massflow with the number of segments?

[adilarvind]

Yes i am multiplying the torque and massflow with the number of segments. I am checking the physics again and re-doing the simulation; i will get back after getting the result.

Thanks for your suggestions.

[turbo]

I see your model is an axial rotor with a lot extended up/downstreams cut in the axial direction which means the relative frame of reference only. How do you know the inlet velocity BC value flowing normal to the boundary?

[highorder_cfd]

Are you sure that your simulation is converged? Have you monitored the mass flow rate to see if it is still changing?
Also, have you tried to run the full model, just to understand if it is the periodic BC making mess?

[adilarvind]

Originally Posted by highorder_cfd

Are you sure that your simulation is converged? Have you monitored the mass flow rate to see if it is still changing?
Also, have you tried to run the full model, just to understand if it is the periodic BC making mess?

I have monitored the mass flow rate and it is not changing after certain iterations.

No i have not tried running the full model, i will check it. Thanks for the suggestion @highorder_cfd

[adilarvind]

Originally Posted by turbo

I see your model is an axial rotor with a lot extended up/downstreams cut in the axial direction which means the relative frame of reference only. How do you know the inlet velocity BC value flowing normal to the boundary?

From the physical model i know that inlet velocity value is normal to the inlet boundary. Is that what you mean @turbo?

[ghorrocks]

So if you know the inlet velocity and that it is perpendicular to the inlet boundary, then the mass flow rate = density*area*normal velocity at the inlet, so you know the mass flow rate without doing any CFD. If this mass flow rate does not match your desired mass flow rate then you have a problem with the inlet boundary condition being inconsistent with the desired mass flow rate, don't you?

If I have missed something please explain it to me, I will be keen to hear it.

[adilarvind]

Thank you @ghorrocks and @highorder_cfd for your inputs.

The mass flow rate calculated using the formula is matching with the mass flow rate calculated from cfd. I guess my problem is with the mesh or the domain, as a result the efficiency is not matching with the experimental result.

[highorder_cfd]

By the way, could you post here a pic of your mesh? It might be too coarse where you are prescribing the Boundary Condition.

Originally Posted by adilarvind

Thank you @ghorrocks and @highorder_cfd for your inputs.

The mass flow rate calculated using the formula is matching with the mass flow rate calculated from cfd. I guess my problem is with the mesh or the domain, as a result the efficiency is not matching with the experimental result.