7 replies to this topic

[BiggidyBo]

Hi all,

Is there a Nusselt method that takes into consideration the friction of the pipe for turbulent flow? If so what is the equation?
I assume it incorporates Darcy's friction formula (and the moody chart)

I have calculated the NU, to find my coefficient with the Dittus Boelter method, but this is for smooth pipes.


Any ideas?


Thanks

[Art Montemayor]

Biggidy:

I have not heard of a Nusselt “method”. There is a Nusselt Number – and it is prominent in heat transfer, not in fluid mechanics (or fluid flow). However, there is some relationship when you have simultaneous heat transfer and fluid flow – as in a heat exchanger. Is that what you mean? Please explain in detail.

The Darcy equation, on the other hand is strictly a fluid flow relationship that leads to a predicted pressure drop. It employs the Moody Chart (for identifying the required friction factor). We now almost always employ equations such as the Churchill, Chen, etc., to calculate the friction factor directly.

You mention the “Dittus Boelter method” and “smooth pipes”. One has practically nothing to do with the other. The Dittus Boelter equation is used to calculate a heat transfer film coefficient while it has nothing to do with whether a pipe is smooth or not. Are you getting mixed up with all these “buzz words”?

[BiggidyBo]

Hi Art,

Thanks for your help.

I think I am getting my knickers in a twist over these buzz words.

I just want to confirm that to get the Convective heat transfer coefficient:

I will need to use Reynolds and Prandtl

and these two will give me Nusselt! which I can rearrange to find my answer?

Is this correct?


Thanks again

[Art Montemayor]

Biggidy:

Why do you resist giving us what I requested: an explanation in detail?

Convective heat transfer is what the Nusselt Number is all about, and that is a complex and specific process. There are multiple cases of convective heat transfer examples: forced and natural, laminar and turbulent flows, single phase and mixed phase, horizontal and vertical, flat, spherical, and cylindrical, etc., etc. Just exactly what is it that you are trying to do? In engineering applications, convective heat transfer is an empirical application because of its complexity and you should be specific about what it is that you are trying to do. If it is a theoretical exercise, then so state it. It will save everyone here a lot of time and bother.

In the meantime, you can find out a lot about how the Nusselt number is calculated at such web sites as:

http://en.wikipedia..../Nusselt_number
http://www.engineeri...sfer-d_430.html
http://www.ehow.com/...elt-number.html

and many more…..

[sheiko]

BiggidyBo,

In addition to what Art has already pointed out and after you read all the material made available to you, I'd suggest you have a look at the attached spreadsheets in order to double-check your calculations.

Attached Files

•  53.xlsx   303.96KB   54 downloads
•  01.xls   84KB   27 downloads

Edited by sheiko, 19 February 2011 - 01:51 PM.

[sheiko]

Some more spreadsheets. Even if the OP asked about turbulent flow, I have added a spreadsheet to be used for laminar flow so that you can compare.

Attached Files

•  Excel_template_Forced_Convection_H_T_Coeff_Turb_Pipe_Flow_SI_units.xls   182KB   37 downloads
•  Excel_template_Forced_Convection_H_T_Coeff_Laminar_Pipe_Flow_SI_units.xls   172.5KB   30 downloads

Edited by sheiko, 17 February 2011 - 08:30 AM.

[BiggidyBo]

Thank you so much guys...your help is much appreciated!

[pradnya]

Hello Everyone,

I'm trying to calculate the full heat transfer through multiple layer insulated - two phase pipe in particular with fluid as water. Can someone help me with the calculation sheet..?