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-   -   Convection with Shell Conduction (http://www.cfd-online.com/Forums/fluent/49815-convection-shell-conduction.html)

Corentin November 17, 2008 08:35

Convection with Shell Conduction
 
Hi!

I have a few questions about the shell conduction. I am calculating the air temperature in a room. I am doing unsteady calculations and I would like to consider the heat mass of the walls. I am using shell conduction. There is convection on both sides of the wall but how can I deal with that? In Fluent, in the convection panel, I can put only ONE convection coefficient. I don't understand how this works. I cannot specify the convection on each side? What if the coefficients are different? All in all, what coefficient Fluent is expecting? The inner one (close to fluid)? The outer one (external)?

Thanks!

JSM November 17, 2008 23:38

Re: Convection with Shell Conduction
 
Hi Corentin,

The heat transfer coefficient you are applying in the wall is only for external surface. For internal wall, fluent will automatically calculate the heat transfer coefficient depends upon temperature difference.

Hope it will help

with regards, JSM

Corentin November 18, 2008 01:04

Re: Convection with Shell Conduction
 
Ok, thank you. So, I don't have to worry about the internal convection coefficient. I just have to compute the external one.

Now, I want to calculate this external coefficient through a UDF.

It should be this :

h=1.31*abs(T_wall_ext-T_ext)^0.33

T_ext is the external temperature of the environment, not modeled.

I am using the WALL_TEMP_INNER(f,t) function to get T_wall_ext. It seems strange to use the "INNER" word to obtain the outer (external) wall temperature, but it seems to be the thing to do.

Am I correct?

Regards, Corentin.

JSM November 21, 2008 04:58

Re: Convection with Shell Conduction
 
Hi Corentin,

If you want to calculate the heat transfer coefficient by using some function, UDF is the only way.

From your last reply, I understand that the thickness of the wall is not modeled for simulation. If wall thickness is not modeled, then there is no inner wall and outer wall concept. Fluent takes this as zero thickness wall. You can take the temperature value at this wall for heat transfer coefficient calculation in UDF.

If you think the thickness really affect the heat transfer coefficient, the thickness of the wall also should be modeled.

Hope it will help U

With regards, JSM

Corentin November 21, 2008 05:58

Re: Convection with Shell Conduction
 
Hi!

I am using shell conduction with NON-zero thickness (because I want to consider the effect of the wall mass inertia on the temperature in unsteady calculations).

That is why i have TWO wall temperatures : interior (contact with modeled fluid) and exterior (contact with outside air, not modeled, free stream temperature).

I am using a UDF to compute the exterior convection coefficient which is :

h = 1.31 * abs [ ( T_wall_ext - T_ext ) ]

That is why I need to get the value of T_wall_ext.

To do so, I am using the WALL_TEMP_INNER(f,t) function.

The Fluent Guide is not very clear on this point, and I think this is strange because the exterior wall temperature should logically be named OUTER but from what I understood I should type WALL_TEMP_INNER instead of WALL_TEMP_OUTER to get the exterior wall temperature (isn't that strange?).

Anyway, could you confirm that WALL_TEMP_INNER will give me the EXTERIOR wall temperature?

About the INTERIOR convection coefficient, you told me that Fluent is dealing with it, so I don't have to worry about that.

Is it still the case?

Anyway, with shell conduction activated and non-zero thickness, Fluent only ask for ONE convection coefficient. Not two.

So I assume Fluent is expecting the exterior one (wich I compute with a UDF) and I guess Fluent is automatically taking care of the interior one.

Am I correct?

Thanks a lot :)


JSM November 24, 2008 00:17

Re: Convection with Shell Conduction
 
Hi,

Yes. You are correct. If you are modeling the thickness of wall, convection coefficient is applied on the exterior wall.

With shell ocnduction, the names given in Fluent is different. Outer wall means that it is modeled in preprocessing. Inner wall means that it is the imaginary wall with the thickness you specified. This wall is in contact with atmosphere. So take the inner wall temperature for convection heat transfer coefficient UDF.

with regards, JSM

Corentin November 24, 2008 02:13

Re: Convection with Shell Conduction
 
Ok, thank you very much JSM, you were of a great help :)

arjun3020 January 30, 2012 13:12

Hi,
Could you please let me know that up to what thikness i can use shell conduction.?

i have model with only surface for flow analysis, if i want to know the heat transfer from model using shell conduction without modeling the thickness,
actual model have 5 mm thickness.
please help.


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