[CellZone]

Hi,

I have a simulation with a Box (walls) , filled with air (which is heated). And in this box, there is an additional solid bullet.

When I set the initial condition for temperature for the bullet to 280 Kelvin, and an adiabatic boundary condition, the temperature of the bullet does not rise.

But which boundary condition (thermal specification) do I have to choose if the temperature of the bullet should rise (passive) as well with the heated air?

I can choose in StarCCM:

Heat Flux -- > bullet is not heated by itself
Heat Source -- > bullet is not heated by itself
Temperature -- > i do not want a fix temperature of the bullet
Convection -- > yes there is convection but I do not want to predefine the htc
Environmet -- > same as for convection

I am looking for a boundary condition where the bullet's temperature is rising due to the surrounding air.

Can you follow my problem?

Best regards
Cellzone

[THUCT]

I think you should set the temperature of bullet surface as same as the temperature of surrounding gas, which means there is not a temperature jump across the surface.
This means the T values on the boundary surface(cell face) that could be calculated using nodes values of either solid zone or fluid zone are same.
If you use the adiabatic BC, there will not be heat flux across the surface, which means the grad(T) on the both side of surface are zero.
If you use Fluent, the BC is "coupled". But I did not use this software you used. Sorry for this and my poor English.

[CellZone]

"I think you should set the temperature of bullet surface as same as the temperature of surrounding gas, which means there is not a temperature jump across the surface."

But this would only be reasonable for the initial condition correct. But then the bullet is heated by the air through convection as well.

So I can not set a fix temperature condition. ??

[THUCT]

No matter how long the fluid flows, the temperature is continuous, except for a very rarefied gas which would lead to a temperature jump near the wall. The bullet will be heated by the heat flux flowing into solid zone. But the convection is only the behavior of fluid. Although the convection of fluid has a relationship with the heat transfer of solid, but this need to be established through the coupling thermal conditions. This condition could be temperature, heat flux or the convection. These different BCs all mean the T relationship.

Just think about that no matter how strong or weak the convection is, the temperature on the both sides being equal is a very natural and sensible answer. If the bullet is heated or cooled by a fluid with same temperature, the convection or fluid flow will have no effect on the T distribution with solid zone because the fluid will not change the surface temperature of solid zone, there will not be any grad(T) which is the only way of heat transfer for solid zone if the radiation is neglected.

If you set a fixed T, the fluid zone and solid zone will calculated individually. I think for different fluid inlet T or velocity, the T field with solid will not change if the fixed T are same in different case and only the T field in fluid zone will change.

I don't know whether my understanding is right or wrong, but you could check it by simple geometry and mesh.

[FMDenaro]

Originally Posted by CellZone

Hi,

I have a simulation with a Box (walls) , filled with air (which is heated). And in this box, there is an additional solid bullet.

When I set the initial condition for temperature for the bullet to 280 Kelvin, and an adiabatic boundary condition, the temperature of the bullet does not rise.

But which boundary condition (thermal specification) do I have to choose if the temperature of the bullet should rise (passive) as well with the heated air?

I can choose in StarCCM:

Heat Flux -- > bullet is not heated by itself
Heat Source -- > bullet is not heated by itself
Temperature -- > i do not want a fix temperature of the bullet
Convection -- > yes there is convection but I do not want to predefine the htc
Environmet -- > same as for convection

I am looking for a boundary condition where the bullet's temperature is rising due to the surrounding air.

Can you follow my problem?

Best regards
Cellzone

Adiabatic condition means that the normal derivative of the temperature (related to the heat flux) is zero. Therefore, the wall can have both an increasing and a decreasing of temperature depending on the internal fluid temperature.

[amod_kumar]

How is air getting heated? Can you share temperature profile at few cross-sections?

[CellZone]

Adiabatic condition means that the normal derivative of the temperature (related to the heat flux) is zero. Therefore, the wall can have both an increasing and a decreasing of temperature depending on the internal fluid temperature.

So when my node at the wall does not see a heat flux, how does it come, that temperature can rise?

By the way: the right boundary condition should be just an interface between the solid bullet and the air.

[FMDenaro]

Originally Posted by CellZone

So when my node at the wall does not see a heat flux, how does it come, that temperature can rise?

By the way: the right boundary condition should be just an interface between the solid bullet and the air.

dT/dn=0 at a wall means that T_wall adapts itself to the internal condition. If you have in the interior that the temperature of the fluid increases, the temperature at the wall must increase to fulfill the adiabatic condition.