Learning Forum

[Abhishek Shingala]

I am try to simulate a steady state centrifugal compressor and my rotating speed is such that at the tip mach number reaches upto 1.2. When I run simulation using only volute, impeller and inlet pipe it runs fine, but when i add ported shroud or backspace, after feq iterations I get high mach number locally at blade tip and linear solver fails.

Does anyone know best practice, how to start such simulations?

simulating using first order, laminar, with low speed, low mass flow rate?

[Federico]

Are you using moving reference frame formulation? It might be worth ramping up the speed of the rotation to help with solution stabilization. 

10.2. Flow in Single Moving Reference Frames (SRF) (ansys.com)

[Abhishek Shingala]

Thanks for the answer.

Yes I am using MRF, where volute and ported shroud are statinoary, and backspace, impeller and inlet region are rotating, which I connected with mixing plane interface. I tried different physical time step upto low as 2e-6 sec. but it is not working. on the other hand if I dont include backspace then it works, so now I am initialising 'the complete model' with converge 'without backspace model' and this time I am using local time scale value of 3, as it allows different physical time in different region depending on the speed of the flow. I will update if it works.

By the way I use this speed ramping method and it works for this 'without backspace model' but not for 'with backspace model', but in this study i used fixed physical time step.

[Federico]

Is this steady or transient case? You mention physical time step.

If the interactions between stationary and rotating components are significant, I would recommend switching to sliding-mesh model. Although, you may still want to get a stable solution with MRF as your initial solution.