[CFDvasu11]

Hi

I am trying to get the performance curve (Pressure ratio and Efficiency vs non-dimensional mass flow rate) of a centrifugal compressor. I've been given the pressure ratio, mass flow rate and efficiency at operating point for validation. But How do I generate the curve. I use 101.325 (total pressure) at inlet. What should I use as my outlet condition so that changing it will give me the data points for characteristic curve?

[DarylMusashi]

Dear Vasu,

a stable boundary condition is Total Pressure at the Inlet and Static Pressure at the Outlet. Beginning from the design point you can successively compute the operating points in choke direction (right end at the compressor map). You do this by reducing the Static Pressure at the Outlet. Typically, the pressure ratio will drop very fast from point to point when you are in the maximum choke region.

To evaluate the stall point you can begin to slightly increase the Static Pressure at the Outlet (on basis of your design point data). But in general, when you are in stall region, you will encounter the problem that little changes in the Static Pressure at the Outlet will significantly influence the corresponding mass flow. The reason is, that the compressor curve in stall region (near the surge line) is nearly a horizontal line. Therefore, in stall region it is a good practice to switch to another boundary condition: Mass Flow at Outlet. This gives you a more precise method to compute your operating points here.

Have you thought about using dedicated turbomachinery meshing (AutoGrid) and computation tools (FINE/Turbo) from NUMECA for your project(s)?

[CFDvasu11]

Originally Posted by DarylMusashi

Dear Vasu,

a stable boundary condition is Total Pressure at the Inlet and Static Pressure at the Outlet. Beginning from the design point you can successively compute the operating points in choke direction (right end at the compressor map). You do this by reducing the Static Pressure at the Outlet. Typically, the pressure ratio will drop very fast from point to point when you are in the maximum choke region.

To evaluate the stall point you can begin to slightly increase the Static Pressure at the Outlet (on basis of your design point data). But in general, when you are in stall region, you will encounter the problem that little changes in the Static Pressure at the Outlet will significantly influence the corresponding mass flow. The reason is, that the compressor curve in stall region (near the surge line) is nearly a horizontal line. Therefore, in stall region it is a good practice to switch to another boundary condition: Mass Flow at Outlet. This gives you a more precise method to compute your operating points here.

Have you thought about using dedicated turbomachinery meshing (AutoGrid) and computation tools (FINE/Turbo) from NUMECA for your project(s)?

Thank you for the reply. I have used the condition that you have mentioned, but my mass flow rate is hardly reducing as my static pressure at outlet increases. The pressure ratio keeps increasing and there is no sign of the pressure ratio flattening near surge point.
Also from arithmetic calculations and the designers of the compressor I was to told to expect the surge point at static pressure of 300 to 320 kPa.
I have attached the mass flow rate, pressure ratio and temperature ratio data in this reply.
I am using SST for turbulence, High Resolution for advection and turbulence numerics, Total energy for heat transfer and air as ideal gas with 101.325 kPa and 288 K as inlet conditions

[DarylMusashi]

Well, why don't you increase the static pressure until the solver blows up or at least shows an oscillating behavior in the residuals. Assume this point as stall point. The you can, as mentioned, switch to a mass flow boundary condition to evaluate the other points between design and stall point.

[CFDvasu11]

Originally Posted by DarylMusashi

Well, why don't you increase the static pressure until the solver blows up or at least shows an oscillating behavior in the residuals. Assume this point as stall point. The you can, as mentioned, switch to a mass flow boundary condition to evaluate the other points between design and stall point.

I had changed the material from 'air ideal gas' to 'air at 25 C' and now the mass flow rate is varying from surge to choke which was not the case earlier. But the pressure ratio is looking like an upward parabola while it should be the opposite. The efficiency curve is looking alright.

[ghorrocks]

Be aware that when you changed to "Air at 25C" you activated a material model which has constant density. You will not have ideal gas compressibility effects modelled with material model.

[CFDvasu11]

Thank you for letting me know. But How will compressibility affect me in analyzing the compressor? Is that the reason for the shape of the the pressure ratio curve that I've obtained?

[ghorrocks]

An incompressible fluid will not choke as choking is a compressible fluid thing. Also it appears your pressure ratio is around 2.0, so there is a density ratio of around 2 across your device. An incompressible fluid model will just assume everything is the same density.

It appears unlikely an incompressible fluid model is going to be useful in your case.

[CFDvasu11]

Originally Posted by ghorrocks

An incompressible fluid will not choke as choking is a compressible fluid thing. Also it appears your pressure ratio is around 2.0, so there is a density ratio of around 2 across your device. An incompressible fluid model will just assume everything is the same density.

It appears unlikely an incompressible fluid model is going to be useful in your case.

Thank you for the reply, It might have been the reason why my mass flow rate barely changed when I altered my outlet static pressure starting from 90 kPa. Now I'm seeing considerable change in massflow rate and my efficiency curve seems to be right. The pressure ratio though, is not flattening near surge point.

[ghorrocks]

Have you read the CFX documentation on the best practises guide? There are a few which relate to turbomachinery and getting performance curves.