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

Implications for heat transfer when outlet pressure boundary condition used

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

Reply
 
LinkBack Thread Tools Display Modes
Old   March 25, 2009, 11:00
Default Implications for heat transfer when outlet pressure boundary condition used
  #1
New Member
 
Join Date: Mar 2009
Posts: 8
Rep Power: 8
Blob is on a distinguished road
Hello!

I am simulating a ventilated and air-conditioned room with an inlet, oulet, and a heat source using incompressible flow and Boussinesq approximation for bouyancy (transient run). For the inlet, I specify the velocity and temperature, and for the outlet, zero pressure. However, it is unclear to me what the implications are for heat transfers occuring at the boundaries, especially at the outlet.

For the inlet, it's Dirichlet boundary condition since I fix the temperature of the incoming air, but also there is energy inflow due to advection. The manual says Q_advect = m_dot*h_tot. Is this then entered as a source term S_E at the boundary nodes in the energy equation?

For the outlet, what is the boundary condition for the energy equation when zero pressure is assigned? Is it zero flux?? And the energy outflow due to advenction is a time-varying quantity since the temperature at the outlet is changing until steady state is reached. Also, what is the boundary condition for the Navier-Stokes equation in this case? Since I'm using incompressible flow, what's coming in has to go out so is the outlet velocity simply determined by the inlet volumetric flow?

I would greatly appreciate anyone's insight, and please correct me if I am interpreting something wrong. Thanks!
Blob is offline   Reply With Quote

Old   March 25, 2009, 17:43
Default
  #2
Super Moderator
 
Glenn Horrocks
Join Date: Mar 2009
Location: Sydney, Australia
Posts: 10,959
Rep Power: 85
ghorrocks has a spectacular aura aboutghorrocks has a spectacular aura aboutghorrocks has a spectacular aura about
Hi,

There is no source term at the inlet. The flow coming in is just assigned to have the temperature you specify, just as for a normal Dirichlet boundary. The heat flow equation in the manual is used to calculate the heat entering the domain to calculate the system heat imbalance.

At the outlet the temperature is that of the fluid leaving the domain. Yes, for an incompressible flow the mass flow rate in equals the mass flow rate out to the accuracy of convergence. But this is applied as a pressure boundary by default (unless you selected another option) so the velocity can vary across the outlet if it wants to.

Glenn Horrocks
ghorrocks is offline   Reply With Quote

Old   March 26, 2009, 07:22
Default
  #3
New Member
 
Join Date: Mar 2009
Posts: 8
Rep Power: 8
Blob is on a distinguished road
Hi Glenn,

Thanks for your comments! Very good to know that the heat flow is just for the heat imbalance calculation and doesn't enter the energy equation.

-blob
Blob is offline   Reply With Quote

Reply

Tags
boundary condition, heat transfer, hvac, pressure

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
Pressure boundary condition C-H Kuo Main CFD Forum 10 August 26, 2015 07:45
ATTENTION! Reliability problems in CFX 5.7 Joseph CFX 14 April 20, 2010 15:45
Concentric tube heat exchanger (Air-Water) Young CFX 5 October 6, 2008 23:17
Question about pressure inlet boundary condition. Alina FLUENT 1 November 30, 2007 08:39
Outlet boundary condition CN FLUENT 6 May 22, 2005 09:37


All times are GMT -4. The time now is 13:07.