Inlet boundary conditions

Mattia

Dear all,

good evening.

My name is Mattia and I am a PhD student at the Delft University of Technology (The Netherlands).

I am currently working in the turbomachinery field and I am focused on the analysis of a centrifugal compressor used in a micro gas turbine. The actual configuration has a vaneless diffuser and my idea is to enhance its performance by designing a vaned diffuser.

Anyway, let's talk about CFD.

I want to make a "rough estimate" of the flow field inside the vaneless diffuser and I am using CFX as CFD software.

I am simulating only one portion of the diffuser and only the diffuser itself. I have a 3D mesh.

Based on same papers I read, the best boundary conditions to use are pressure and temperature at the inlet (plus flow direction) and mass flow outlet. Thus, this is what I am trying to do and here the problems come out.

Of course I know the mass flow and I calculated the inlet conditions at the diffuser inlet (=impeller outlet) by means of velocity triangles (no experimental data available): it means I know the total pressure and temperature, plus absolute velocity (and its components) and the flow angles.

The biggest problem I have concerns the flow direction: how can I set it properly? Since I know the angles from the velocity triangles, I shouldn't have any trouble, but on the contrary I do have problems. Can you please help me with that?

Moreover, I read in the "help manual" that the total pressure to set as inlet b.c. is the RELATIVE TOTAL PRESSURE: does this mean that I have to subtract, from the calculated value, the reference pressure (=1 atm)?

I can provide you more details if needed.

Thank you very much!

Kind regards, Mattia.