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-   -   Question on heat transfer coefficient!!! (http://www.cfd-online.com/Forums/fluent/36636-question-heat-transfer-coefficient.html)

Benny May 11, 2005 23:59

Question on heat transfer coefficient!!!
 
Dear All,

Now I have a question on heat transfer The problem concerned is simple: there is a straight circular duct, water flows through the duct, and heat flux loads on the duct's wall. What I want to know is heat transfer coefficient. But I found the results of the calculation are not correct at all comparing with results obtained from the formula: h=Nu*conduct/d, where h is heat transfer coefficient, Nu is the Nusselt number, conduct is thermal conductivity and d is the diameter of the duct. In the Fluent help document, I found that Surface Heat Transfer Coef. is defined by h=q/(T_wall-T_ref), where q is the heat flux, T_wall is the wall temperature and T_ref is the reference temperature which means the average temperature of the fluid. The default value of the T_ref is 288.16K. But I think maybe in different conditions the T_ref is different, so I change it to the average temperature of the fluid. My question is: Is that right to do so and could I get the correct heat transfer coefficient by doing so?

Best regards!

Benny

Thomas May 12, 2005 15:49

Re: Question on heat transfer coefficient!!!
 
Yes, you are right by doing so. Actually you should set this T_ref to the fluid temperature close to the wall, where you want to know the htc. Or, you could also use the htc in your turbulence modell.

Benny May 12, 2005 21:52

Re: Question on heat transfer coefficient!!!
 
Dear Thomas,

Thank you very much for your answer. But how could I get the average fluid temperature close to the wall? For I want to know the htc distribution on the whole surface of the wall. And what does this mean: Or, you could also use the htc in your turbulence modell? Thank you again for your time!

Best wished!

Benny

Balaji May 16, 2005 02:23

Re: Question on heat transfer coefficient!!!
 
hi,

the htc is between wall temperature and bulk temperature. Bulk tempereature can be calculated using mass weighted average of temperature.I think if you use this you will get it.

Balaji

Benny May 16, 2005 08:41

Re: Question on heat transfer coefficient!!!
 
Dear Balaji,

I want to know what the bulk temperature is? I can get it as your advices, bu where to change its value? I appreciate your help if you could give me some advices in details.

Benny

Balaji May 20, 2005 00:19

Re: Question on heat transfer coefficient!!!
 
hi,

Bulk temperature we can calculate by Mass weighted average Temperature. This we can get from surface integrals. I don't know whether we can use this value directly to calculate the HTC.One thing is we can write a udf to calculate Bulk Temp and wall Temp at given locations and get HTC.

Balaji

seirov May 28, 2005 20:09

Re: Question on heat transfer coefficient!!!
 
Hi!

I am studying a similar problem, but with a external flow. I am not a english spoken person, and I don't know what is the meaning of bulk temperature, but in my case, the reference temperature is the inflow temperature (temperature defined in the velocity inlet boundary conditions) and not the average temperature, as you say. And I think internal flow is the same.

Good luck!

Mahesh June 7, 2005 09:25

Re: Question on heat transfer coefficient!!!
 
Then if you are calculating HTC based on Inlet Temperature, give the same in Reference panel and using Contours--> wall Fluxes, u can plot/get the HTC.


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