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-   -   Implications for heat transfer when outlet pressure boundary condition used (https://www.cfd-online.com/Forums/cfx/63000-implications-heat-transfer-when-outlet-pressure-boundary-condition-used.html)

 Blob March 25, 2009 11:00

Implications for heat transfer when outlet pressure boundary condition used

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!

 ghorrocks March 25, 2009 17:43

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

 Blob March 26, 2009 07:22

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

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