Hydraulic Diameter for Moody, Fanning, Darcy

This is a trick question, right? By definition the hydraulic diameter of a circular pipe is it's diameter.

-- Ed Ruf ( snipped-for-privacy@EdwardG.Ruf.com)

The graph is for ordinary pipe flow but I want to use the chart for annular flow.

Bret Cahill

Dear Bret Cahill:

If the supporting documentation for those charts says yes, sure.

How big is d_outer_dia / D_inner_dia?

David A. Smith

That is the beauty of the concept of the hydraulic diameter, that is it's specific application and usefulness.

If you are looking at very thin annuli, then you might care to take a look at a modification to the hydraulic diameter as applied to high aspect ratio channels by Jones.

An Improvement in the Calculation of Turbulent Friction in Rectangular Ducts, by O.C. Jones Jr. Journal of Fluids Engineering, Transactions of the ASME, June 1976, pp173-181.

That depends on the chart.

Does the chart have flow rate or velocity as an input? If it's flow rate, then your answer is a resounding NO. If it's velocity, then you might be okay... But even then, you have to make sure you're in the right flow regime (turbulent vs. laminar).

-Paul

I don't understand all the standard Moody plots are f vs Re.

-- Ed Ruf ( snipped-for-privacy@EdwardG.Ruf.com)

Shouldn't that be "wetted area / wetted perimeter"? Which would be: (D^2 - d^2) / (4 * (D + d))

David A. Smith

Don't know about "wetted area" but I believe the "4" is either in the definition or implicit to one of the graphs.

As to your other question I'm looking for the biggest annulus that still has a high pressure drop.

I'm thinking about a new kind of "roughness" that REALLY increases pressure drop.

Bret Cahill

Now do some simple factoring and eliminate like terms. An exercise left to the students, see problem 6 solution.

Good point. I assumed he was using one of those charts found in piping handbooks which do all the intermediate work for you (giving you pressure loss vs flow rate), rather than a true Moody chart.

If you're using a real Moody chart (f vs Re), then calculate Re using the hydraulic diameter, and use the line for the relative roughness corresponding to absolute roughness divided by hydraulic diameter. That should be good enough.

On the other hand, if you're using one of those simplified charts, or a simplifed equation (e.g. Hazen-Williams), then if it uses flow rate as an input, you canNOT use it for anything other than a circular cross section (unless it specifically says so). If it uses velocity as an input, it MIGHT be okay.

-Paul

"N:dlzc D:aol T:com (dlzc)" wrote in message news:6BIlg.33999$AB3.21792@fed1read02...

No. It should be 4*area/perimeter. And when you include that 4 (to cancel the 4 you put in the denominator), and do the algebra, you get D-d.

-Paul

Hmmm, I didn't pick up the problem with the factor of 4.

One other thought. The civil engineers who deal with open channel flow have another definition of the hydraulic radius which is not equivalent to this definition.

-- Ed Ruf ( snipped-for-privacy@EdwardG.Ruf.com)

Dear Ed Ruf and Paul Skoczylas:

"Ed Ruf (REPLY to E-MAIL IN SIG!)" wrote in message news: snipped-for-privacy@4ax.com...

Thanks to you both for catching (all of) that.

David A. Smith

Yes, but still VERY important.

-- Ed Ruf ( snipped-for-privacy@EdwardG.Ruf.com)