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Defining ‘Length’ and ‘Area’ fields in ‘Reference Values’ section

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Old   September 20, 2018, 08:40
Post Defining ‘Length’ and ‘Area’ fields in ‘Reference Values’ section
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Mohammad
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Hello

I’m modeling a 3D shell and helically-coiled tube heat exchanger in Fluent. Simulation is done and anything related to a CFD simulation is OK. Now I want to do postprocessing and get some reports. I need to know Nusselt number inside the helically coiled tube. I read Fluent Help about ‘Reference Values’ and I know that reference values in this section are only used for postprocessing. (calculating parameters like some coefficients, Reynolds number, Nusselt number and so on.) I also know that I can select reference values for a particular zone from the ‘Compute From’ drop-down list, but the Area and Length fields need to be filled manually. Now the problem is, when I try to get the Nusselt number for the coiled- tube zone from the reports, I don’t know if the Area and Length for this zone is from the tube wall and straightened area and length or it’s something else. When I set this area and length from the straightened state of the tube, the Nusselt number seems to be wrong.(around 300)
My second question is, a friend said, for calculating Nusselt number for a zone, you need to select all the items related to that zone below the Surfaces field, namely: interior, inlet and outlet and the surface of that zone.
Is this right ?

Thanks,
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File Type: jpg Reference Values.jpg (43.6 KB, 7 views)
File Type: jpg Nu number.jpg (71.3 KB, 6 views)

Last edited by Mh-Kasayi; September 20, 2018 at 10:11.
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Old   September 20, 2018, 13:32
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Lucky
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Nusselt number is a surface phenomenon. Just select the surfaces you want the Nusselt number on. There's really no such thing as Nusselt number of a zone because surface convection happens on surfaces... Although we like to say that my pipe or my heat exchanger has an overall heat teat transfer rate and Nusselt number of such and such, we really mean the surfaces.

You don't need to use the drop down menu to calculate the reference values, just enter them manually. The compute from is just a convenience feature, it is not a substitute for not knowing what your reference values ought to be. So whatever numbers you end up seeing when you use the compute-from feature, those had better be the correct reference values! If not, then you should change them to the correct values!

As for what should be the reference values? These values are in the definition of your variables. For example, there is a reference length in Nusselt number and also a reference temperature in the heat transfer coefficient from which the Nusselt number is derived. If you say my heat transfer coefficient is such and such, and my Nusselt number is such and such, it implies you have already defined the reference temperature and reference length.


I.e. if you think the Nusselt number of 300 is wrong, what is the correct value? Whatever this correct value is, what reference length is used in that definition of Nusselt number? And what reference temperature was used in that definition of heat transfer coefficient. There is not only one definition of Nusselt number (i.e. you can choose any reference length). You need to use the same reference length (and hence talk about the same Nusselt number) when you make comparisons.
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Old   September 20, 2018, 17:10
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Quote:
Originally Posted by LuckyTran View Post
Nusselt number is a surface phenomenon. Just select the surfaces you want the Nusselt number on. There's really no such thing as Nusselt number of a zone because surface convection happens on surfaces... Although we like to say that my pipe or my heat exchanger has an overall heat teat transfer rate and Nusselt number of such and such, we really mean the surfaces.

You don't need to use the drop down menu to calculate the reference values, just enter them manually. The compute from is just a convenience feature, it is not a substitute for not knowing what your reference values ought to be. So whatever numbers you end up seeing when you use the compute-from feature, those had better be the correct reference values! If not, then you should change them to the correct values!

As for what should be the reference values? These values are in the definition of your variables. For example, there is a reference length in Nusselt number and also a reference temperature in the heat transfer coefficient from which the Nusselt number is derived. If you say my heat transfer coefficient is such and such, and my Nusselt number is such and such, it implies you have already defined the reference temperature and reference length.


I.e. if you think the Nusselt number of 300 is wrong, what is the correct value? Whatever this correct value is, what reference length is used in that definition of Nusselt number? And what reference temperature was used in that definition of heat transfer coefficient. There is not only one definition of Nusselt number (i.e. you can choose any reference length). You need to use the same reference length (and hence talk about the same Nusselt number) when you make comparisons.
Thanks for spending time to write my answer
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area and length fields, helically-coiled tube, nusselt number, reference -values

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