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 Sarah April 2, 2010 09:12

Conjugate heat transfer: coupled wall temperature

Dear all,

I am fairly new to CFD, I hope the description of my problem is clear.

My model is a room with dimensions 3.9x4.75x2.7m. At opposite sides of the room there is one inlet (average velocity 0.075 m/s) and one oulet (average velocity 0.28 m/s). The air coming into the room has a temperature of 289 K, also used as backflow temperature at the outlet.
I want to model the air flow pattern in the room, and the convective heat transfer at the surfaces. I also want to know the resulting temperature gradients over the surfaces and take into account the influence of the thermal mass of the walls, ceiling and floor. For this, I use shell conduction for all the walls, ceiling and floor with user-defined material and thicknesses of 0.1 to 0.2 m. As an external boundary condition I have to take either constant temperature or constant heat flux (other boundary conditions assume zero wall thickness).

Now I have three questions:

- Is it correct to use shell conduction in this situation? The main references I find where shell conduction is used, are for very thin structures as the fins in heat exchangers, not for heavy walls.

- To choose a temperature boundary condition at the inner side of the shell (so the side away from the flow domain), ideally, I would like to use the wall temperatures calculated by Fluent in the following way, to approximate adjacent identical rooms:
-> to assign the surface temperature of the floor to the inner side of the shell of the ceiling
-> to assign the surface temperature of the ceiling to the inner side of the shell of the floor
-> for the walls, I can simply couple the two sides of the walls
-> one of the walls is facing the exterior environment, so a constant temperature of 289K can be assigned
Do I need a UDF for this, or can I do this in Fluent directly? Is this a sufficient boundary information to solve the flow?

- Is this a steady-state problem? I am including buoyancy in my model, and am having trouble to make it converge. I make a first approximation by reducing the gravity to 0.098 and first order scheme. This simulation runs smoothly to convergence and results are nice symmetric. If, as recommended by the Fluent manual, I continue the calculation with normal gravity, the residuals get stuck and start oscillating around a certain value. I wonder of I may need a transient simulation?

Thanks for any help!

Sarah
PhD-student, KULeuven

 HC code March 18, 2011 13:50

Conjugate heat transfer: coupled wall temperature

Hi Sarah

I just found your question through google search engine. I'm also doing PhD in Conjugate Heat transfer systems.

Thank you..

 Sarah March 24, 2011 07:22

Hi,

I am currently following this approach:
- 2D, in order to reduce calculation time
- transient
- incompressible ideal gas law to take into account the influence of density variations
- solid regions are included in the mesh, instead of using the thin shell conduction

I am focussing on the conjugate heat transfer at ceiling and floor surface, so only here the mass is included in the model. The heat transfer from surface to mass is then coupled. As for the outer boundary condition, se have tested several options: fixed temperature, adiabatic and periodic. For my application, periodic is the most interesting option, where the outer boundaries of floor and ceiling are coupled.
We use a time step of 0.1 s for the first minutes, and continue with a fixed time step of 1 s later. This converges nicely for every time step.

Currently, we have a problem to include radiation in this model. S2S should be the preferred model, but the results are unrealistic. The problem seems to be in the 2D model, so we are testing at the moment with a 3D model, though the calculation time is much higher for this.

If you have questions or comments, feel free.

 HC code April 3, 2011 07:22

Aha Ok.. Are you employing a specific code for this purpose?

 HC code April 3, 2011 07:27

Thank you..

My research area is slightly different, but we may have similarity in setting types of boundary conditions.

I am intending to develop a FORTRAN code for solving 3D CHT problems, regardless to complexity of geometry, by tetrahedral element grids.

I have, so far, developed a 3D heat conduction code in order to internally couple it with a CFD source code. The code was validated against ANSYS and was greatly successful. My boundary conditions at interface are, initially, set as:
1. Continuity of heat flux when solving interface from the heat conduction Q= Q.

2. Continuity of Temperature for the fluid domain, T=T.

I am starting with Steady-state equations.

DO you agree with the type of boundary conditions I am setting?

 satyendra September 14, 2011 01:24

1 Attachment(s)
Quote:
 Originally Posted by Sarah (Post 300864) Hi, I am currently following this approach: - 2D, in order to reduce calculation time - transient - incompressible ideal gas law to take into account the influence of density variations - solid regions are included in the mesh, instead of using the thin shell conduction I am focussing on the conjugate heat transfer at ceiling and floor surface, so only here the mass is included in the model. The heat transfer from surface to mass is then coupled. As for the outer boundary condition, se have tested several options: fixed temperature, adiabatic and periodic. For my application, periodic is the most interesting option, where the outer boundaries of floor and ceiling are coupled. The vertical walls are adiabatic. We use a time step of 0.1 s for the first minutes, and continue with a fixed time step of 1 s later. This converges nicely for every time step. Currently, we have a problem to include radiation in this model. S2S should be the preferred model, but the results are unrealistic. The problem seems to be in the 2D model, so we are testing at the moment with a 3D model, though the calculation time is much higher for this. If you have questions or comments, feel free.
hi sarah,

I want to analyse (2D unsteady transient analysis in fluent) a furnace with one burner inlet and one exhaust oulet, with a mould placed inside. i have given all the properties of solid and fluid regions and coupled all the thermal boundary conditions as required. But the temperature inside the solid mould is not changing even after 500 time steps of 5s each having 20 iterations per time step.

please see the attached pic of the temperature profile.

thanks,
satyendra

 Kristofer. October 21, 2012 18:10

Same problem

Hel(p)lo!

I want to simulate a simplified camera generating heat from inside. I also get the problem: The heat stays in the inner walls and it gets really (unrealistic) hot inside but the temperature on the housing and sun-protector doesnt change

I do mesh all the walls (no thin walls) and have the coupled BC on all walls (except velocity-inlet- pressure outlet - symmetry by the "boarders")

Do you know what Im doing wrong? Should I change the BC´s? Should I setup another radiation model (currently using DO)?

Pleeease help!!!

 ssixr August 12, 2013 22:36

S2S model

Quote:
 Originally Posted by Sarah (Post 300864) Hi, I am currently following this approach: - 2D, in order to reduce calculation time - transient - incompressible ideal gas law to take into account the influence of density variations - solid regions are included in the mesh, instead of using the thin shell conduction I am focussing on the conjugate heat transfer at ceiling and floor surface, so only here the mass is included in the model. The heat transfer from surface to mass is then coupled. As for the outer boundary condition, se have tested several options: fixed temperature, adiabatic and periodic. For my application, periodic is the most interesting option, where the outer boundaries of floor and ceiling are coupled. The vertical walls are adiabatic. We use a time step of 0.1 s for the first minutes, and continue with a fixed time step of 1 s later. This converges nicely for every time step. Currently, we have a problem to include radiation in this model. S2S should be the preferred model, but the results are unrealistic. The problem seems to be in the 2D model, so we are testing at the moment with a 3D model, though the calculation time is much higher for this. If you have questions or comments, feel free.
hi sarah,
i have tried using S2S radiation on coupled walls in a 3D model and got really unrealistic results. In your last msg, you have mentioned about similar condition. Have you any solution?
thanks
sudhir

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