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

CFX overpredicting turbine massflow

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

Reply
 
LinkBack Thread Tools Display Modes
Old   January 28, 2010, 13:26
Default CFX overpredicting turbine massflow
  #1
Member
 
Sherif Kadry
Join Date: May 2009
Posts: 38
Rep Power: 8
sherifkadry is on a distinguished road
I have been modeling a 2 stage axial turbine, and basing my Total Pressure inlet boundary condition and static pressure exit condition based on my experimental results. The turbulence model used is SST, and its a steady state case that's being run. I notice that at all shaft speeds the massflow is overpredicted by 5%. However, total-to-static efficiencies are within 0.5%. What could be the reason for the overprediction? Here is an image of my
model:
http://i615.photobucket.com/albums/t...Screenshot.png

Any expert opinions would be helpful thanks.
sherifkadry is offline   Reply With Quote

Old   January 31, 2010, 16:40
Default
  #2
Senior Member
 
Attesz's Avatar
 
Attesz
Join Date: Mar 2009
Posts: 355
Rep Power: 8
Attesz is an unknown quantity at this point
5% is not too much I think. It can caused by the inaccuracy of the measurement too. You can find it out by measuring an other quantity. Or doing some mesh sensitivity check can be helpful..
Attesz is offline   Reply With Quote

Old   January 31, 2010, 17:04
Default
  #3
Super Moderator
 
Glenn Horrocks
Join Date: Mar 2009
Location: Sydney, Australia
Posts: 10,824
Rep Power: 85
ghorrocks has a spectacular aura aboutghorrocks has a spectacular aura aboutghorrocks has a spectacular aura about
In general, to get the simulation accurate within that last 5% will require very careful and detailed refinement. You will need to do mesh, convergence and timestep sensitivity studies to a much tighter tolerance than you usually would. You will also have to consider the accuracy of your physics to additional precision - for instance does surface roughness contribute? Do you know your gas composition accurately? Are you using real gas models? Is SST suitable for this level of detail? You obviously have a good handle on most of these things if you are within 5% but to get even more accurate you will need to reconsider.
ghorrocks is offline   Reply With Quote

Old   February 1, 2010, 04:15
Default
  #4
Member
 
Sherif Kadry
Join Date: May 2009
Posts: 38
Rep Power: 8
sherifkadry is on a distinguished road
attesz, ghorrocks, thanks for your replies. Ghorrocks you banged on an important point here. When I ran the cfd simulations I ignored the effect of humidity, being that I ran my experiments in Texas (very humid), I think humidity plays an important role, now, I need to figure out how to incorporate humidity, which reduces the density and could explain the 5% difference. Anyone have any ideas on how best to incorporate humidity into CFX, should I run a water-vapour+air ideal gas mixture?
sherifkadry is offline   Reply With Quote

Reply

Tags
cfx massflow turbo

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
Two stage axial turbine in CFX sherifkadry CFX 14 January 25, 2010 05:16
Tesla Turbine analysis in CFX sivarama1 CFX 2 September 5, 2009 20:37
CFX application to radial inflow gas turbine Chetan Mistry CFX 5 June 2, 2007 13:13
re: Massflow reset in CFX 10 Wooster CFX 5 March 16, 2007 21:08
Help CFX Steam turbine Yogesh CFX 2 April 26, 2006 01:38


All times are GMT -4. The time now is 20:13.