Nusselt number calculation

[MrReds]

Hallo to everybody,

Please, I’d need some clarifications concerning calculation of Nusselt numbers in tubes.
Could you explain me when utilize Dittus-Bolter and when Gnielinski ?
Are this correlations valid for both laminar and turbulent flow ?

Should the fluid be steam, which correlation shall I utilize ?

Could you suggest me any web site or literature reference concerning this issue ?

Thanks in advance !!!

 

[Feric]

Hi there:

Check out the eFunda.com --

http://www.efunda.com

-- web site section dealing with Heat Transfer.

Even though the theoretical and numerical parts may not be 100% applicable to your needs, you will find basic information on how to calculate the Nusselt number.

Thanks,

Gordan Feric, PE
Engineering Software

http://members.aol.com/engware

 

[quark]

Both the correlations deal with turbulent flows inside smooth pipes. Dittus-Boelter deals with fully developed turbulent flow and works for 0.6<Pr<100. Holman comments that Gnielinski is more precise with two equation for various Pr and Re ranges. The second equation can be used for transitional regime for 1.5<Pr<500.

For laminar flow, you can use either a constant Nu (4.364) or Hausen correlation for precise calculation. Sieder&Tate give a simplified correlation but the Nu value becomes negligible if the pipe is too long. I suggest you to refer to Heat Transfer by JP Holman.

When you say steam, you should also consider condensation. Process Heat Transfer by DQ Kern is an excellent and invaluable reference.