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

Mass flow imbalance in transient state

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

Reply
 
LinkBack Thread Tools Display Modes
Old   June 1, 2009, 01:54
Default Mass flow imbalance in transient state
  #1
Ton
New Member
 
Join Date: Apr 2009
Posts: 7
Rep Power: 9
Ton is on a distinguished road
Hi,

I tested a 2D pipe for transient analysis. BC. is as follows: Inlet, velocity,24*sin(w*t+pi/2),static temperature,900C; Outlet, static pressure,1atm. Solve control is as follows: frenquence,63HZ; total time,5 period;timestep,period/400. The turbulence model is SST.

The problem is as follows:
When i set the wall as adiabatic,the massFlow() @inlet =massFlow()@outlet,but when i set the wall as HTC=200[wm^-2 K^-1],outside temp.=50C, the massflow()@inlet is not equal to massFlow()@outlet and the massFlow()@outlet becomes higher with periods. I can't find the reason.

Don
Ton is offline   Reply With Quote

Old   June 1, 2009, 18:28
Default
  #2
Super Moderator
 
Glenn Horrocks
Join Date: Mar 2009
Location: Sydney, Australia
Posts: 12,638
Rep Power: 98
ghorrocks is a jewel in the roughghorrocks is a jewel in the roughghorrocks is a jewel in the rough
Hi,

Is your simulation adequately converged? Are the fluids compressible?

Glenn Horrocks
ghorrocks is offline   Reply With Quote

Old   June 1, 2009, 20:54
Default
  #3
Ton
New Member
 
Join Date: Apr 2009
Posts: 7
Rep Power: 9
Ton is on a distinguished road
Hi Glenn

The material is helium . Every timestep loops 5. i think the timestep is adequate (one period has 400 timesteps).

Don
Ton is offline   Reply With Quote

Old   June 2, 2009, 01:05
Default
  #4
Super Moderator
 
Glenn Horrocks
Join Date: Mar 2009
Location: Sydney, Australia
Posts: 12,638
Rep Power: 98
ghorrocks is a jewel in the roughghorrocks is a jewel in the roughghorrocks is a jewel in the rough
Hi,

Well in that case the outlet and inlet mass flow rates can be different due to transient effects, compounded by temperature changing the density. I but you have some sort of acoustic waves in the thing.

And unless you have done a sensitivity analysis to show your timestep is adequately small then don't assume so. If acoustic waves are occuring then you have to worry about the compressibility and they requrie much smaller timesteps again.

Glenn Horrocks
ghorrocks is offline   Reply With Quote

Reply

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
Transient vs Steady State Adam CFX 1 April 12, 2007 11:34
mass imbalance ahourri FLUENT 8 October 26, 2004 23:14
Mass imbalance Aly FLUENT 7 October 6, 2004 21:07
OutletBoundary Condition Causes Mass Imbalance Andy Main CFD Forum 0 March 11, 2004 15:05
About the difference between steady and unsteady problems Lisa Main CFD Forum 11 July 5, 2000 14:37


All times are GMT -4. The time now is 11:03.