> 4000 turbulent and water/t… · equation” by well estimating of “ fricti factr , f “ ;...
TRANSCRIPT
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Lect.No.2 Asst.Prof.Dr. Jaafar S. Maatooq
1st Semester Flow Dynamics in Closed Conduit (Pipe Flow) 1 of 14
The flow in closed conduit ( flow in pipe ) is differ from this occur in open
channel where the flow in pipe is at a pressure (does not have a free surface ) .
The flow in pipe can be demonstrated such as :-
- Laminar flow ,
- Transitional flow ,
- Turbulent flow .
To distinction between the above features , the well known “ Reynold,s
Number” can be used , according to experiments that given by “ Osborn
Reynold in 19th
century “ .
1-Reynold’s Experiment
In 1883, Osborne Reynolds demonstrated that there are two distinctly
different types of flow by injecting a very thin stream of colored fluid having
the same density of water into a large transparent tube through which water is
flowing. And from the feature of streaming this dye fluid , Reynold give a
number can be considered as a boundary between flow faces , this number is a
function of , flow velocity , fluid density , pipe diameter , and fluid viscosity ,
where ;
R= f (V , ρ , υ (or μ ) , D ) …………………….. (1)
and then , R=VDρ
μ or R=
VD
υ ;
R= Reynolds No.,
μ = dynamic viscosity ,
υ = kinematic viscosity .
See Figure(1) , below for Reynold”s experiments ;
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Lect.No.2 Asst.Prof.Dr. Jaafar S. Maatooq
1st Semester Flow Dynamics in Closed Conduit (Pipe Flow) 2 of 14
Fig.(1) : Experiments shows the flow state as demonstrated by Reynolds
Observations (dye) Reynolds
Number, Re
Flow
Classification
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Lect.No.2 Asst.Prof.Dr. Jaafar S. Maatooq
1st Semester Flow Dynamics in Closed Conduit (Pipe Flow) 3 of 14
Transitional/
Turbulent
> 4000
Turbulent
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Lect.No.2 Asst.Prof.Dr. Jaafar S. Maatooq
1st Semester Flow Dynamics in Closed Conduit (Pipe Flow) 4 of 14
2-Viscous (Real) Flow in Conduits ,Head Loss in Pipes from
Friction ( Major Losses)
The head loss between two points in a circular pipe carrying a fluid under
pressure can be found by ; hf=Δp/γ
Where: ∆p = p1 − p2 , and can be measured by using piezometer tubes.
The velocity of the flow can be found by using a Pitot tube. The reading of
the Pitot tube is the total head = pressure head + velocity head
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Lect.No.2 Asst.Prof.Dr. Jaafar S. Maatooq
1st Semester Flow Dynamics in Closed Conduit (Pipe Flow) 5 of 14
The total “ friction head loss “ (hL) , can be calculated using “ Darcy
Equation” by well estimating of “ friction factor , f “ ; where :-
Also the “ friction head loss“ (hL) , can be calculated by using Hazen William
Equation , where ;
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Lect.No.2 Asst.Prof.Dr. Jaafar S. Maatooq
1st Semester Flow Dynamics in Closed Conduit (Pipe Flow) 6 of 14
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Lect.No.2 Asst.Prof.Dr. Jaafar S. Maatooq
1st Semester Flow Dynamics in Closed Conduit (Pipe Flow) 7 of 14
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Lect.No.2 Asst.Prof.Dr. Jaafar S. Maatooq
1st Semester Flow Dynamics in Closed Conduit (Pipe Flow) 8 of 14
3-Head Loss versus Discharge
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Lect.No.2 Asst.Prof.Dr. Jaafar S. Maatooq
1st Semester Flow Dynamics in Closed Conduit (Pipe Flow) 9 of 14
The friction factor of “Darcy Equation” can be estimated , using “ Moody
Diagram”
as shown in Fig.(2) , below ;
Fig.(2): Friction Factor estimation as presented by Moody
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Lect.No.2 Asst.Prof.Dr. Jaafar S. Maatooq
1st Semester Flow Dynamics in Closed Conduit (Pipe Flow) 10 of 14
4-Method to Determine Darcy-Weisbach friction factor ( f )
PIPE FLOWS
Laminar (R < 2,000) Turbulent (R > 4,000)
f = 64/R
Smooth Transitional Wholly Rough
(δv > e) (0.071e ≤ δv ≤ e) (δv < 0.071e)
Turbulent (Smooth):
Prandtle ………..
√
√
for R > 4000 ….. (2)
Blasisus ………..
for 3000 < R < 100000 … (3)
Turbulent ( Transitional) :
Colebrook ……..
√ -
√ ……………… (4)
Turbulent ( Wholly Rough ):
Von- Karamen …
√
………………………. (5)
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Lect.No.2 Asst.Prof.Dr. Jaafar S. Maatooq
1st Semester Flow Dynamics in Closed Conduit (Pipe Flow) 11 of 14
5-Direct Calculation of Flow Velocity
Combining the “Darcy” and “Colebrook” equations yield’s the explicit
equation for average flow velocity in pipe :-
- √
√ ………………….. (6)
Where S=hf / L and ( ν ) is a kinematic viscosity
When using Eq.4 (Colebrook equation) and due to the implicit form of this
formula for “f” , it can be use the following formula to find a friction factor
which presented by “Moody” :-
................ (7)
Eq.7 can be used just with :-
R ranged between 4000 – 10000000
e/D up to 0.01
and from the above limitations the accuracy of “f” resulting is within (+/- 5% ) .
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Lect.No.2 Asst.Prof.Dr. Jaafar S. Maatooq
1st Semester Flow Dynamics in Closed Conduit (Pipe Flow) 12 of 14
6-Types of Water flow Problems
In design and analysis of pipe systems that involve the use of the “Moody
Diagram” or “Colebrook formula” , it is usually found a three types of
problems in practice . In all these problems the fluid type and roughness of
pipe must be specified . The classification of the three problem can be shown
in the following ;
Problem Type Given Find
1 L , D , Q hL
2 L , D , hL Q
3 L , hL , Q D
The solution of the above problems can be cleared as the following steps
1- The solution of problems of the first type is by using directly the “Moody
Chart” .
2- The solution of problems of the second type obtained by:-
*Assume fully turbulent flow region (high Reynold’s number) , for a given
roughness of pipe .
*From this assumption find “friction factor” .
*By using Darcy formula the flow rate can be obtained .
*The friction factor can be corrected using Moody diagram or Colebrook
equation and the above process is repeated until the solution converges .
3- The solution of problems of the third type will be :-
*Start calculation by assume a pipe diameter .
*The head loss is calculated by this assumption is then compared to the
given head loss .
*The calculation repeated with another pipe diameter until solution obtained.
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Lect.No.2 Asst.Prof.Dr. Jaafar S. Maatooq
1st Semester Flow Dynamics in Closed Conduit (Pipe Flow) 13 of 14
Swamee and Jain in 1976 suggested the following explicit relations to avoid
iteration . The results from this relation are within 2% with the results
obtained by using Moody chart ;
- ….. ……. (8)
It is valid just for :-
10-6
< e/D < 10-2
3000 < R < 3x108
-
…… (9)
It valid for R > 2000
….(10)
It is valid just for 10-6
< e/D < 10-2
& 5000 < R < 3x108
7-Simplified Equations to Calculate Head Losses in Commercial Pipes
The new empirical equations used for head losses calculation in most
commercial pipes that may be used in practice were submitted by Ibrahim
Can by using in direct solution of head losses without need to use Colebrook
Equation or Moody diagram . These proposed formulas are listed in Table
below :-
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Lect.No.2 Asst.Prof.Dr. Jaafar S. Maatooq
1st Semester Flow Dynamics in Closed Conduit (Pipe Flow) 14 of 14
Empirical Equation Pipe Type
=0.000934
1.818
D4.821 PVC ( e=0.0015mm )
=0.00103
1.882
D4.963 Steel (e=0.05mm )
=0.00112
1.929
D5.08 Asphalted Cast Iron (e=0.12mm)
=0.00141
1.974
D5.205 Concrete (e=0.5mm)
Note that the above equations used with SI-units where ; Q in (m3/s) , L in
(m) , D in (m) and hL in (m) . The maximum error from the above formulas
compared with measured in practice not exceed +/- 2% .