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Old   March 30, 2017, 08:07
Default CFX centrifugal compressor
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prashanth
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I am new to CFX.
I am working on centrifugal compressor for small turbojet engines.
I created the geometry using bladegen, meshed it in turbogrid(only one blade with periodic boundary). 1.5 million cells.
geometry has three domains: 1.Inlet 2. Impeller 3. outlet
inlet and outlet are stationary and impeller is rotating at 43250 RPM.
interfaces are connected using frozen rotor.
It is a steady state problem. Total energy including viscous dissipation is used. ideal gas.
Inlet boundary conditions are Stagnation pressure and Stagnation temperature.
Outlet - mass flow or static pressure.
I need to plot performance of compressor for different mass flow rates.
From basic calculations the design exit pressure is 400 kpa. so, I varied exit pressure from (200 kpa-380 kpa) from results it is observed that mass flow is almost constant around(5.2 kg/sec) . After 380 kpa solution is not converging below 10^-3 and mass flow is observed to be very low(0.75 kg/sec).
I tried with mass outlet boundary condition(4.2-5 kg/sec) - but this is having converging issues. I cannot find what is problem with mass flow boundary condition at exit.

In Solver control advanced options,compressible control and High speed numerics is switched on.

please help me with this problem i am working on this for past 1 month.
thanks in advance

Last edited by prashanth_kasarla; April 2, 2017 at 04:42.
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Old   April 4, 2017, 09:28
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For compressible turbomachinery problems it is indeed recommended to define the static pressure instead of mass flow at the outlet. In easy words it gives the solver more flexibility and stability instead of explicitly defining the mass flow. Because normally the mass flow is a result (and that's why computed) of the pressure gradient between inlet and outlet.

Is there a reason why you only computed the choke region? Because you only decreased the outlet static pressure (operating point 400 kpa, you computed 200-380 kpa)., This should increase the mass flow. If you increase the static pressure you go into stall conditions.

Are you sure your design point outlet static pressure is correct and is related to the rotational speed your are using? Have you any experiential data to know the stall and choke outlet static pressure?

Please take note that there are CFD tools out there, which are much more suitable for turbomachinery applications, for example NUMECA's AutoGrid5, a multi-block structured meshing tool for turbo machines of all kind. In the related solver FINE/Turbo you can set up performance curve computations with with many operating points with just a few clicks. Finally, needless to say that you are way faster with a structured solver like FINE/Turbo, instead of an unstructured one like CFX.
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centrifugal compressor, cfx & air content, compressible flow, turbomachinery mesh cfx

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