Looking for Nusselt relationship

[packdad]

I was flipping through my old heat transfer text in search of a Nusselt relationship for forced flow along (ie, parallel to) the outside of a tube or cylinder. Most equations are for flow across (ie, perpendicular to) a tube. It occurs to me that the relationships for forced flow across a flat plate may be applicable to the flow-parallel-to-cylinder case. Is this correct?

 

[Gianfranco]

Hi Packdaddy,

Yes, if you have not the specific correlation, it is "always" correct to take into consideration the closer available correlation

Did you take into account the correlation for annular spaces?

Have you decided what is the characteristic dimension: D or L?

Gianfranco

 

[packdad]

The only such thing I could find in my text (Heat Transfer, A Basic Approach, by Ozisik) were relationships for flow through tube bundles with flow across (ie, perpendicular to) the tubes. There are also relationships for flow through ducts of various shapes, though I'm not so sure how well those would apply.

I'm not designing a heat exchanger, by the way -- I'm just trying to determine, in a general sense, the effects of temperature on the overall heat transfer coefficient. That is, if water temperature increases by twenty degrees, for example, does the term UA tend to go up or down as a result.

 

[Gianfranco]

Hi Packdaddy,

For the purpose you said the correlation 8-73a, 8-74, or better 8-78, where you introduce the mu/muw parameter on Table 8-6, are enough to performe the sensitivity analysis around one working condition point (nominal).

D UA
^
|
1 -|
|
|
|
|
-------------------->
0,5 1 1,5
tw(nominal)
pw
other parameters

Regards.

Gianfranco.

 

[packdad]

Gianfranco,

Thanks for the feedback! Eqn. 8-74 is actually the one I had used. I just had a lingering doubt as to whether or not I could apply the flat plate relationship to flow along a tube or cylinder. It seemed to make sense, but then, I often find myself having to tell my wife "Well, it made sense at the time...", so I thought I'd try a second opinion just to cover my behind. Thanks again.