[martinrusso3]

Hi !

I am runing a simulation of an oil heather with different layers of insulation, all concentric ones, but one in the middle will be trapped air.
My question would be if this domain should be taken as a fluid or solid, since it wont have any movement.

Boundaries: Inlet; Outlet; OD Outside Temperature; Heat transferr coefficient in the cilindrical inner face of the trapped air and temperature reference (estimated).

Subdomain: heat source in a ceramic band wich in the first colindrical layer of the system.

I need to know the temperature of the fluid at the outlead for an especific amount of energy applied to the ceramic band heather.

The way I found to solve this was to set the trapped air as a solid domain but adding the natural coefficient convection in the surface. But I am afraid the radiation between the ceramic and the housing (trapped air in the middle both) is not being considered.

Any recomendations to ensure that the radiation is being consider ? should I have to add some additional parameter for radiation ?

I could run succesfully the solution, but all the heat is going to the fluid and I would expect to have more losses in the system and I am not sure radiation is being consider automatically. Still not have test to validate the simulation.

any help will be appretiated.

[Gert-Jan]

I would not include it as a volume. Just ignore the volume of this stagnant air layer completely.
Since you have separate volumes around, you will have an interface in between, exactly where the stagnant air is present. At this interface, you can include its thermal resistance by specifying the material (air) thickness and conductivity. Easy as that.

[ghorrocks]

Is this simulation steady state? If so, then why not use a simple thermal resistance model and then this is just a easy hand calculation. No CFD required, and it is easy to include all the layers you like.

[martinrusso3]

Thanks for the replies guys, I will try with the interface thin layer of air.

Regarding the second comment, yes it is a steady state simulation, but the fluid inside is turning reason why my forced convection heat transfer coefficient is not simple to calculate. Since I am runing the CFD i decided to include the insulation.

About radiation, is automatically being consider without adding any control or additional source ?

Thanks!

[Gert-Jan]

Nothing is automatically included. It would be strange if CFX would include things automatically without notifying the user.

You better try to estimate the radiation contribution to the heat transfer by hand and wrap everything up in a higher conductivity. You can even define an equation as function of Temperature for it and use it at the interface.

But this all might end up in fake-accuracy. Best compare it with experiments.....