Thermal Radiation between Surfaces

[feavnik]

Hi All,

I'm doing some FEA on heat transfer by radiation between surfaces of solid bodies(using ANSYS). A hollow cylinder in "VACUUM" is subjected to uniform heat fulx on it's outer surface. Two water cooled jackets, one on the outside and the other inside maintained at 26C are used to absorb the radiated heat. The model looks like 3 concetric hollow cylinders.

The problem that I'm runnin into is with the space temperature, i.e. the temperature between the cylinders. I guess the space/gap temperature depends on the temperatures of the cylinder surfaces. But, I need to specify the space/gap temperature in order to solve the problem. How can I pick a particular temperature for this gap? It is vacuum and nothing is really maintaining this temperature. I randomly used different numbers for this gap tamperature and the cylinder temperatures are different for different gap temperatures. Therefore, I concluded that the gap temperature is important to obtain sensible results.

can you guys please comment on this.

All the suggestions are appreciated

Thanks,
FEAVNIK

 

[rb1957]

personally, i think you've answered your own question ...

i think the heat capacity of the coolant is dependent on the temperature in the middle of the cyclinder (amongst a host of other parameters). if the coolant can remove the radiant heat, then the temperature inside the cyclinder will be constant. if the coolant can't the the temperature in the cyclinder will increase; why wouldn't it run away ?

the problem would be trickier if you had a control valve on the quantity of coolant, so that if you detect a temperature increase inside the cyclinder, then the heat capacity of the coolant is increased (higher flow?) trying to stabilise the situation.

 

[corus]

Radiation doesn't involve the temperature of the space gap, only the temperature of the two surfaces and their emissivities. The only way it might is if the space was filled with a gas and the surfaces radiated to that, and/or heat loss was through forced convection to that medium.

corus

 

[Transient1]

Hi FEAVNIK,

I agree with corus. In fact, there is no temperature in a vacuum.

http://www.physlink.com/education/AskExperts/ae127.cfm

There is no participating medium in a vacuum. There is only the radiating and recieving surfaces. You would, of course see variation in the results if you include a temperature in the vacuum space. That is not reality and the results you are seeing are false.

 

[feavnik]

rb1957, Corus and transient1, thanks for your replies. I understand it better now

 

[rb1957]

so how did you resolve your problem ?

you got replies about 180deg apart. i agree that there is no temperature in a vaccuum, as there's nothing to absorb the heat. so is it a software "glitch" ?

you've got heat input and heat output ... how are these kept in equilibrium ?

 

[corus]

I imagine that what the softwaer is asking for is the ambient temperature outside. Unless you have an enclosed cavity or the two surfaces are assumed to be so close together that they only view each other then you also need to know the ambient temperature. This isn't the temperature between the two bodies but the temperaure outside of the two bodies to which each will radiate to as a black body.

corus