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Bret Cahill
science forum Guru

Joined: 05 May 2005
Posts: 480

Posted: Mon Jun 19, 2006 5:35 am    Post subject: Hydraulic Diameter for Moody, Fanning, Darcy

For concentric circular annular flow hydraulic dia = D-d.

Can you get away with just substituting the hydraulic dia for a pipe
dia. in simple pipe flow friction factor charts?

Bret Cahill
Ed Ruf (REPLY to E-MAIL1
science forum beginner

Joined: 03 Dec 2005
Posts: 46

Posted: Mon Jun 19, 2006 9:25 am    Post subject: Re: Hydraulic Diameter for Moody, Fanning, Darcy

On 18 Jun 2006 22:35:34 -0700, in sci.engr.mech "Bret Cahill"
<BretCahill@aol.com> wrote:

 Quote: For concentric circular annular flow hydraulic dia = D-d. Can you get away with just substituting the hydraulic dia for a pipe dia. in simple pipe flow friction factor charts?

This is a trick question, right? By definition the hydraulic diameter of a
circular pipe is it's diameter.
--
Ed Ruf (Usenet2@EdwardG.Ruf.com)
Bret Cahill
science forum Guru

Joined: 05 May 2005
Posts: 480

Posted: Mon Jun 19, 2006 11:09 am    Post subject: Re: Hydraulic Diameter for Moody, Fanning, Darcy

 Quote: Can you get away with just substituting the hydraulic dia for a pipe dia. in simple pipe flow friction factor charts? This is a trick question, right? By definition the hydraulic diameter of a circular pipe is it's diameter.

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

Bret Cahill
N:dlzc D:aol T:com (dlzc)
science forum Guru

Joined: 25 Mar 2005
Posts: 2835

Posted: Mon Jun 19, 2006 1:15 pm    Post subject: Re: Hydraulic Diameter for Moody, Fanning, Darcy

Dear Bret Cahill:

"Bret Cahill" <BretCahill@aol.com> wrote in message
 Quote: For concentric circular annular flow hydraulic dia = D-d. Can you get away with just substituting the hydraulic dia for a pipe dia. in simple pipe flow friction factor charts?

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

How big is d_outer_dia / D_inner_dia?

David A. Smith
Ed Ruf
science forum beginner

Joined: 19 Jun 2005
Posts: 40

Posted: Mon Jun 19, 2006 2:44 pm    Post subject: Re: Hydraulic Diameter for Moody, Fanning, Darcy

On 19 Jun 2006 04:09:13 -0700, in sci.engr.mech "Bret Cahill"
<BretCahill@aol.com> wrote:

 Quote: Can you get away with just substituting the hydraulic dia for a pipe dia. in simple pipe flow friction factor charts? This is a trick question, right? By definition the hydraulic diameter of a circular pipe is it's diameter. The graph is for ordinary pipe flow but I want to use the chart for annular flow.

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.
Paul Skoczylas
science forum beginner

Joined: 29 Apr 2005
Posts: 20

Posted: Mon Jun 19, 2006 3:36 pm    Post subject: Re: Hydraulic Diameter for Moody, Fanning, Darcy

"Bret Cahill" <BretCahill@aol.com> wrote in message
 Quote: For concentric circular annular flow hydraulic dia = D-d. Can you get away with just substituting the hydraulic dia for a pipe dia. in simple pipe flow friction factor charts?

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
Ed Ruf (REPLY to E-MAIL1
science forum beginner

Joined: 03 Dec 2005
Posts: 46

Posted: Mon Jun 19, 2006 10:18 pm    Post subject: Re: Hydraulic Diameter for Moody, Fanning, Darcy

On Mon, 19 Jun 2006 15:36:54 GMT, in sci.engr.mech "Paul Skoczylas"
<pauls@DELETE.cfertech.com> wrote:

 Quote: "Bret Cahill" wrote in message news:1150695333.948786.23740@r2g2000cwb.googlegroups.com... For concentric circular annular flow hydraulic dia = D-d. Can you get away with just substituting the hydraulic dia for a pipe dia. in simple pipe flow friction factor charts? 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).

I don't understand all the standard Moody plots are f vs Re.
--
Ed Ruf (Usenet2@EdwardG.Ruf.com)
N:dlzc D:aol T:com (dlzc)
science forum Guru

Joined: 25 Mar 2005
Posts: 2835

Posted: Tue Jun 20, 2006 1:56 am    Post subject: Re: Hydraulic Diameter for Moody, Fanning, Darcy

 Quote: Dear Bret Cahill: "Bret Cahill" wrote in message news:1150695333.948786.23740@r2g2000cwb.googlegroups.com... For concentric circular annular flow hydraulic dia = D-d.

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

 Quote: Can you get away with just substituting the hydraulic dia for a pipe dia. in simple pipe flow friction factor charts? If the supporting documentation for those charts says yes, sure. How big is d_outer_dia / D_inner_dia?

David A. Smith
Bret Cahill
science forum Guru

Joined: 05 May 2005
Posts: 480

Posted: Tue Jun 20, 2006 4:47 am    Post subject: Re: Hydraulic Diameter for Moody, Fanning, Darcy

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

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
Ed Ruf (REPLY to E-MAIL1
science forum beginner

Joined: 03 Dec 2005
Posts: 46

Posted: Tue Jun 20, 2006 9:13 am    Post subject: Re: Hydraulic Diameter for Moody, Fanning, Darcy

On Mon, 19 Jun 2006 18:56:25 -0700, in sci.engr.mech "N:dlzc D:aol T:com
\(dlzc\)" <N: dlzc1 D:cox T:net@nospam.com> wrote:

 Quote: Dear Bret Cahill: "Bret Cahill" wrote in message news:1150695333.948786.23740@r2g2000cwb.googlegroups.com... For concentric circular annular flow hydraulic dia = D-d. Shouldn't that be "wetted area / wetted perimeter"? Which would be: (D^2 - d^2) / (4 * (D + d))

Now do some simple factoring and eliminate like terms. An exercise left to
the students, see problem 6 solution.
http://www.chem.mtu.edu/~fmorriso/cm310/Homework_4_2003.html
http://www.chem.mtu.edu/%7Efmorriso/cm310/HW4_plm6_soln.pdf
--
Ed Ruf (Usenet2@EdwardG.Ruf.com)
Paul Skoczylas
science forum beginner

Joined: 29 Apr 2005
Posts: 20

Posted: Tue Jun 20, 2006 3:23 pm    Post subject: Re: Hydraulic Diameter for Moody, Fanning, Darcy

"Ed Ruf (REPLY to E-MAIL IN SIG!)" <egruf_usenet2@cox.net> wrote in message
 Quote: On Mon, 19 Jun 2006 15:36:54 GMT, in sci.engr.mech "Paul Skoczylas" pauls@DELETE.cfertech.com> wrote: "Bret Cahill" wrote in message news:1150695333.948786.23740@r2g2000cwb.googlegroups.com... For concentric circular annular flow hydraulic dia = D-d. Can you get away with just substituting the hydraulic dia for a pipe dia. in simple pipe flow friction factor charts? 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). I don't understand all the standard Moody plots are f vs Re. --

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
Paul Skoczylas
science forum beginner

Joined: 29 Apr 2005
Posts: 20

Posted: Tue Jun 20, 2006 3:25 pm    Post subject: Re: Hydraulic Diameter for Moody, Fanning, Darcy

"N:dlzc D:aol T:com (dlzc)" <N: dlzc1 D:cox T:net@nospam.com> wrote in
 Quote: Dear Bret Cahill: "Bret Cahill" wrote in message news:1150695333.948786.23740@r2g2000cwb.googlegroups.com... For concentric circular annular flow hydraulic dia = D-d. Shouldn't that be "wetted area / wetted perimeter"? Which would be: (D^2 - d^2) / (4 * (D + d))

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
Ed Ruf (REPLY to E-MAIL1
science forum beginner

Joined: 03 Dec 2005
Posts: 46

Posted: Tue Jun 20, 2006 10:01 pm    Post subject: Re: Hydraulic Diameter for Moody, Fanning, Darcy

On Tue, 20 Jun 2006 15:25:33 GMT, in sci.engr.mech "Paul Skoczylas"
<pauls@DELETE.cfertech.com> wrote:

 Quote: "N:dlzc D:aol T:com (dlzc)" wrote in message news:6BIlg.33999\$AB3.21792@fed1read02... Dear Bret Cahill: "Bret Cahill" wrote in message news:1150695333.948786.23740@r2g2000cwb.googlegroups.com... For concentric circular annular flow hydraulic dia = D-d. Shouldn't that be "wetted area / wetted perimeter"? Which would be: (D^2 - d^2) / (4 * (D + d)) 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.

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 (Usenet2@EdwardG.Ruf.com)
N:dlzc D:aol T:com (dlzc)
science forum Guru

Joined: 25 Mar 2005
Posts: 2835

Posted: Wed Jun 21, 2006 12:39 am    Post subject: Re: Hydraulic Diameter for Moody, Fanning, Darcy

Dear Ed Ruf and Paul Skoczylas:

"Ed Ruf (REPLY to E-MAIL IN SIG!)" <egruf_usenet2@cox.net> wrote
in message news:0srg92l2uq0b05afbhdjij8uh6ml5pm88u@4ax.com...
 Quote: On Tue, 20 Jun 2006 15:25:33 GMT, in sci.engr.mech "Paul Skoczylas" pauls@DELETE.cfertech.com> wrote: "N:dlzc D:aol T:com (dlzc)" wrote in message news:1150695333.948786.23740@r2g2000cwb.googlegroups.com... For concentric circular annular flow hydraulic dia = D-d. Shouldn't that be "wetted area / wetted perimeter"? Which would be: (D^2 - d^2) / (4 * (D + d)) 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. Hmmm, I didn't pick up the problem with the factor of 4.

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

 Quote: 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.

http://en.wikipedia.org/wiki/Open_channel_flow
the only difference between HD and HR is the factor of 4.

David A. Smith
Ed Ruf (REPLY to E-MAIL1
science forum beginner

Joined: 03 Dec 2005
Posts: 46

Posted: Wed Jun 21, 2006 9:20 am    Post subject: Re: Hydraulic Diameter for Moody, Fanning, Darcy

On Tue, 20 Jun 2006 17:39:46 -0700, in sci.engr.mech "N:dlzc D:aol T:com
\(dlzc\)" <N: dlzc1 D:cox T:net@nospam.com> wrote:

 Quote: http://en.wikipedia.org/wiki/Open_channel_flow the only difference between HD and HR is the factor of 4.

Yes, but still VERY important.
--
Ed Ruf (Usenet2@EdwardG.Ruf.com)

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