CFD Online Logo CFD Online URL
www.cfd-online.com
[Sponsors]
Home > Forums > Software User Forums > ANSYS > CFX

Getting characteristic curve of centrifugal compressor

Register Blogs Members List Search Today's Posts Mark Forums Read

Reply
 
LinkBack Thread Tools Display Modes
Old   November 25, 2017, 05:29
Default Getting characteristic curve of centrifugal compressor
  #1
New Member
 
Vasudevan K
Join Date: Oct 2015
Posts: 13
Rep Power: 5
CFDvasu11 is on a distinguished road
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?
CFDvasu11 is offline   Reply With Quote

Old   November 25, 2017, 14:55
Default
  #2
Senior Member
 
DarylMusashi's Avatar
 
Holger Dietrich
Join Date: Apr 2011
Location: Germany
Posts: 174
Rep Power: 9
DarylMusashi is on a distinguished road
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)?
Attached Images
File Type: png Surge.png (23.7 KB, 19 views)
DarylMusashi is offline   Reply With Quote

Old   November 29, 2017, 23:25
Default
  #3
New Member
 
Vasudevan K
Join Date: Oct 2015
Posts: 13
Rep Power: 5
CFDvasu11 is on a distinguished road
Quote:
Originally Posted by DarylMusashi View Post
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
Attached Images
File Type: jpg Characteristic curve.JPG (72.2 KB, 9 views)
CFDvasu11 is offline   Reply With Quote

Old   November 30, 2017, 13:13
Default
  #4
Senior Member
 
DarylMusashi's Avatar
 
Holger Dietrich
Join Date: Apr 2011
Location: Germany
Posts: 174
Rep Power: 9
DarylMusashi is on a distinguished road
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.
DarylMusashi is offline   Reply With Quote

Old   November 30, 2017, 22:00
Default
  #5
New Member
 
Vasudevan K
Join Date: Oct 2015
Posts: 13
Rep Power: 5
CFDvasu11 is on a distinguished road
Quote:
Originally Posted by DarylMusashi View Post
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.
Attached Images
File Type: jpg Pr and N.JPG (22.9 KB, 6 views)
CFDvasu11 is offline   Reply With Quote

Old   November 30, 2017, 22:05
Default
  #6
Super Moderator
 
Glenn Horrocks
Join Date: Mar 2009
Location: Sydney, Australia
Posts: 15,055
Rep Power: 115
ghorrocks is a jewel in the roughghorrocks is a jewel in the roughghorrocks is a jewel in the roughghorrocks is a jewel in the rough
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.
ghorrocks is offline   Reply With Quote

Old   November 30, 2017, 22:21
Default
  #7
New Member
 
Vasudevan K
Join Date: Oct 2015
Posts: 13
Rep Power: 5
CFDvasu11 is on a distinguished road
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?
CFDvasu11 is offline   Reply With Quote

Old   November 30, 2017, 22:44
Default
  #8
Super Moderator
 
Glenn Horrocks
Join Date: Mar 2009
Location: Sydney, Australia
Posts: 15,055
Rep Power: 115
ghorrocks is a jewel in the roughghorrocks is a jewel in the roughghorrocks is a jewel in the roughghorrocks is a jewel in the rough
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.
ghorrocks is offline   Reply With Quote

Old   December 1, 2017, 01:06
Default
  #9
New Member
 
Vasudevan K
Join Date: Oct 2015
Posts: 13
Rep Power: 5
CFDvasu11 is on a distinguished road
Quote:
Originally Posted by ghorrocks View Post
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.
CFDvasu11 is offline   Reply With Quote

Old   December 1, 2017, 04:25
Default
  #10
Super Moderator
 
Glenn Horrocks
Join Date: Mar 2009
Location: Sydney, Australia
Posts: 15,055
Rep Power: 115
ghorrocks is a jewel in the roughghorrocks is a jewel in the roughghorrocks is a jewel in the roughghorrocks is a jewel in the rough
Have you read the CFX documentation on the best practises guide? There are a few which relate to turbomachinery and getting performance curves.
ghorrocks is offline   Reply With Quote

Reply

Tags
centrifugal compressor, performance curves

Thread Tools
Display Modes

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off
Trackbacks are On
Pingbacks are On
Refbacks are On


Similar Threads
Thread Thread Starter Forum Replies Last Post
[gmsh] Vertex numbering is dense KateEisenhower Open Source Meshers: Gmsh, Netgen, CGNS, ... 7 August 3, 2015 10:49
Centrifugal compressor mass flow error Attesz CFX 18 May 27, 2012 10:17
CENTRIFUGAL PUMP, characteristic curve and radial forces curve tareqkh FLUENT 0 February 25, 2012 15:53
CENTRIFUGAL PUMP, characteristic curve Ivanrips CFX 4 September 14, 2010 10:29
CFX4.3 -build analysis form Chie Min CFX 5 July 12, 2001 23:19


All times are GMT -4. The time now is 06:56.