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-   -   free stream temperature (http://www.cfd-online.com/Forums/fluent/101501-free-stream-temperature.html)

vaibhavraikhare May 5, 2012 10:00

free stream temperature
 
What does Free stream temperature and external radiation temperature meant in boundary condition--> Thermal.

LuckyTran May 5, 2012 14:30

Quote:

Originally Posted by vaibhavraikhare (Post 359473)
What does Free stream temperature and external radiation temperature meant in boundary condition--> Thermal.

Freestream temeprature is exactly that, freestream temperature and so is external radiation temperature, it's the temperature at which the external radiation occurs.

It should be self explanatory if you know anything about heat transfer. Did you even bother to read the Fluent help file? If you don't know those quantities perhaps you shouldn't be using Fluent?

vaibhavraikhare May 15, 2012 03:03

@LuckyTran: Ohhkkk.. That means you should have read the whole FLUENT user manual. So tell me the equation to calculate the external radiation temperature and how can we define the value of both???
And for your knowledge i would like to tell you that the free stream temp and external radiation temperature don't have any physical significance in Heat transfer..Both are defined by FLUENT itself..

LuckyTran May 15, 2012 10:41

Quote:

Originally Posted by vaibhavraikhare (Post 361107)
@LuckyTran: Ohhkkk.. That means you should have read the whole FLUENT user manual. So tell me the equation to calculate the external radiation temperature and how can we define the value of both???
And for your knowledge i would like to tell you that the free stream temp and external radiation temperature don't have any physical significance in Heat transfer..Both are defined by FLUENT itself..

Both temperatures have been in use long before the first version of Fluent. Fluent did not make up these terms, they have been around and in use by heat transfer practicioners before then. And no, I don't need to read the entire Fluent help file, actually I don't need to read the Fluent help file at all to know that the the concept of free-stream velocity and free-stream temperature have been around a long time.

And for your knowledge, I would like to tell you that, for both conduction and convection, without a free-stream temperature there is no heat transfer as there is no heat. Therefore, the free-stream temperature and external radiation temperature both have a physical significance in Heat Transfer and have a direct influence on it. Heat transfer occurs via conduction, convection, and radiation. Just because it doesn't play a direct or obvious role in radiation does not mean that it has no physical significance.

I will now tell you the equation to calculate the freestream temperature and external radiation temperature as you requested. Here they are: ( ) there. They do not have any equations to them as they are just temperatures of specific configurations.

LuckyTran May 15, 2012 10:59

Because I am a nice guy I did spend time reading the Fluent help file just for you.

In section 7.3.13.3 Thermal Boundary Conditions at Walls (different sections occur in different versions of the manual, it is 7.3.14 in some versions but they do not differ by a whole lot)
you will find the discussion on what each term means, Fluent mentions external radiation temperature here

7.3.13.11 Fluent explains that the external radiation temperature is the temperature of the radiation source or sink on the exterior of the domain, defined by you. defined here means in the sense of applied boundary conditions, i.e. imposed. That does not mean that you get to make up a new bizarre definition of external radiation temperature, only that it is specified

There is a similar corresponding discussion for inlets in 14.3.7. For inlets, in addition to the incoming bulk temperature of the flow, you must also set the temperature of the external surface.

vaibhavraikhare May 16, 2012 04:51

Hi LuckyTran.
Sorry for being so rude. Actually i got so frustrated as every person to whom i asked gave several answers and none of it helped me. If you can help me.? I am applying radiation model on a box but always getting negative heat transfer rate at absorber plate which should be positive. :confused:

wangyun April 16, 2014 03:36

Quote:

Originally Posted by LuckyTran (Post 361232)
Because I am a nice guy I did spend time reading the Fluent help file just for you.

In section 7.3.13.3 Thermal Boundary Conditions at Walls (different sections occur in different versions of the manual, it is 7.3.14 in some versions but they do not differ by a whole lot)
you will find the discussion on what each term means, Fluent mentions external radiation temperature here

7.3.13.11 Fluent explains that the external radiation temperature is the temperature of the radiation source or sink on the exterior of the domain, defined by you. defined here means in the sense of applied boundary conditions, i.e. imposed. That does not mean that you get to make up a new bizarre definition of external radiation temperature, only that it is specified

There is a similar corresponding discussion for inlets in 14.3.7. For inlets, in addition to the incoming bulk temperature of the flow, you must also set the temperature of the external surface.

hi,
can you sent the fluent help to me if you have time? i don't have a clear idea for the free steam temperature and external radiation temperature.
I will be very grateful to you. thank you.
my email:1203596116@qq.com
wang yun:)

Abhya October 17, 2014 06:50

Quote:

Originally Posted by LuckyTran (Post 361224)
Both temperatures have been in use long before the first version of Fluent. Fluent did not make up these terms, they have been around and in use by heat transfer practicioners before then. And no, I don't need to read the entire Fluent help file, actually I don't need to read the Fluent help file at all to know that the the concept of free-stream velocity and free-stream temperature have been around a long time.

And for your knowledge, I would like to tell you that, for both conduction and convection, without a free-stream temperature there is no heat transfer as there is no heat. Therefore, the free-stream temperature and external radiation temperature both have a physical significance in Heat Transfer and have a direct influence on it. Heat transfer occurs via conduction, convection, and radiation. Just because it doesn't play a direct or obvious role in radiation does not mean that it has no physical significance.

I will now tell you the equation to calculate the freestream temperature and external radiation temperature as you requested. Here they are: ( ) there. They do not have any equations to them as they are just temperatures of specific configurations.

Hi LuckyTran
Dude I have seriously never heard such a term prior to reading it in FLUENT
I have been trying to find out how FLUENT processes the External emissivity and the external radiation temperature to calculate the heat transfer to the enviornment
I have goggled beyond my patience limit and still I don't find any reference to how fluent calculates radiation heat transfer from these inputs nor do I find any document under the name External Radiation, Even the FLUENT theory guide simply says this about it 4 lines not mentioning anything about the exact method

Although it seems obvious it uses the t^4 (Stefan-Bolztman's Law) Have u know any document which explicitly says that? So I void verifying the same thru simulations


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