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

compressible vs incompressible flow

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

Reply
 
LinkBack Thread Tools Search this Thread Display Modes
Old   March 15, 2017, 08:58
Default compressible vs incompressible flow
  #1
Senior Member
 
Nick
Join Date: Nov 2010
Posts: 126
Rep Power: 15
Nick R is on a distinguished road
Hey guys,

I have two fundamental questions in fluid mechanics which I'm hoping to resolve here. Let's say we have a pipe in which the flow is fully developed and adiabatic. If I write the energy equation for the control volume that sits within the pipe and touches the inner surface, I end up with hinlet=houtlet. where h is the enthalpy. Now if I assume incompressibility, then since p is dropping and v is constant u(internal energy) should go up. This means that temperature would increase, right? However, if I assume compressible flow then hinlet =houtlet yields cpTinlet=CpToutlet. Therefore temperature wouldn't change! I wonder what really happens, say if we have fully-developed gas flow thru a pipe?

My second question is, in many textbooks in Engineering heat transfer they use h=cpT for incompressible flow. But isn't this incorrect? I mean with incompressible flow in a piper the pressure drops so how could this equation apply? As in the previous example applying this would result in constant temeprature for the incompressible flow whereas with water flow in a pipe I'd expect a slight temeprature rise.
Nick R is offline   Reply With Quote

Old   March 15, 2017, 11:29
Default
  #2
Senior Member
 
Filippo Maria Denaro
Join Date: Jul 2010
Posts: 6,768
Rep Power: 71
FMDenaro has a spectacular aura aboutFMDenaro has a spectacular aura aboutFMDenaro has a spectacular aura about
Concerning the incompressible flow case (you have the exact solution), pressure has no thermodynamic meaning. If you would use the state equation you would have simply p=constant.

As concern to the compressible case, you could use the results of quasi one-dimensional flows with presence of friction (Fanno flow).
FMDenaro is offline   Reply With Quote

Reply

Thread Tools Search this Thread
Search this Thread:

Advanced Search
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 Off
Pingbacks are On
Refbacks are On


Similar Threads
Thread Thread Starter Forum Replies Last Post
Compressible viscous flow newbie: when NOT to assume incompressible, non-gas EOS? Srh Main CFD Forum 0 April 19, 2013 02:12
Cavitation in compressible flow soni7007 CFX 3 March 29, 2013 12:06
starting compressible flow from incompressible flow results mihaipruna OpenFOAM Running, Solving & CFD 2 December 3, 2012 10:02
Incompressible, Unsteady Cylinder Flow startingcfd Main CFD Forum 1 March 15, 2011 01:12
Incompressible and compressible flow boris FLUENT 0 February 20, 2002 03:08


All times are GMT -4. The time now is 09:47.