Fluid Mechanics
Fluid Mechanics
8th Edition
ISBN: 9780073398273
Author: Frank M. White
Publisher: McGraw-Hill Education
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Chapter 6, Problem 6.118P
To determine

Overall pressure drop p1p2 in lbf/in2.

Expert Solution & Answer
Check Mark

Answer to Problem 6.118P

p1p2=149.327lbf/in2

Explanation of Solution

Given information:

All pipes are concrete, and roughness is equal to 0.04in

The fluid is water at 20°C

Q=20ft3/s

Fluid Mechanics, Chapter 6, Problem 6.118P

In parallel pipe system the pressure drop is same in each pipe.

ΔhAB=Δhb=Δhc=Δhd

Total flow is sum of individual flows,

Q=Q1+Q2+Q3

The velocity V of the flow is equal to,

V=QA Where, A is the cross sectional area

The Reynolds’s number is defined as,

Re=DVν

D - Diameter of the pipe

V - Velocity of the flow

ν - Kinematic viscosity

Roughness ratio is equal to,

Roughnessratio=D

- Roughness

The friction factor can be determined by,

1f1/21.8log6.9Re+ /d 3.7 1.11

The total head loss hf can be defined as,

hf=fLDV22g

L - Length of the pipe

Calculation:

For pipe a,

Find the velocity of the flow in pie a,

V=QA=20ft3/sπ 6 12 ft 2=25.464ft/s

Determine the Reynolds’s number,

Assume the kinematic viscosity of water at 1atm and 20°C as

Re=DVν= 12 12 ft 25.464ft/s1.09× 10 5ft2/s=2.34×104

The roughness ratio is equal to,

Roughnessratio=D=0.0412=3.33×103

To determine the friction factor,

1f 1/21.8log 6.9 R e + /d 3.7 1.11b24ac1.8log 6.9 2.34× 10 4 + 3.33× 10 3 3.7 1.11f0.03114

To determine the head loss in pipe a,

hf=fLD V 2 2g h fa=0.03114 1000ft 12 12 ft 25.464ft/s 2 2 32.2ft/ s 2 =313.534ft

For pipes b,c&d

hfb=hfc=hfd

To find velocity of the flow in each pipes,

Vb=QbAb=Qbπ 4 12 ft 2=2.865QbVc=QcAc=Qcπ 6 12 ft 2=1.273QcVd=QdAd=Qdπ 7.5 12 ft 2=0.815Qd

By equating the head losses in each pipe,

fb1500ft8 12ft 2.865 Q b 22 32.2ft/ s 2 =fc800ft 12 12ft 1.273 Q c 22 32.2ft/ s 2 =fd1200ft 15 12ft 0.815 Q d 22 32.2ft/ s 2 1

If, fb=fc=fd

Qb=0.264QcQd=1.425Qc

Therefore,

Qb+Qc+Qd=0.264Qc+Qc+1.425Qc=20ft3/sQc=7.438ft3/s

By substituting,

Qb=1.964ft3/sQd=10.599ft3/s

To find relevant Reynolds’s number,

R eb=DVν= 8 12 ft 2.865 1.964f t 3 /s1.09× 10 5ft2/s=3.442×105 R ec=DVν= 12 12 ft 1.273 7.438f t 3 /s1.09× 10 5ft2/s=8.687×105 R ed=DVν= 15 12 ft 0.815 10.599f t 3 /s1.09× 10 5ft2/s=9.906×105

The relevant roughness ratio will be,

Db=0.048=5×103 Dc=0.0412=3.33×103 Dd=0.0415=2.667×103

The relevant friction factor will be,

1fb 1/21.8log 6.9 R e + /d 3.7 1.111.8log 6.9 3.442× 10 5 + 5× 10 3 3.7 1.110.030681fc 1/21.8log 6.9 R e + /d 3.7 1.111.8log 6.9 8.687× 10 5 + 3.33× 10 3 3.7 1.110.031141fd 1/21.8log 6.9 R e + /d 3.7 1.111.8log 6.9 9.906× 10 5 + 2.667× 10 3 3.7 1.110.0255

By substituting these values in equation (1),

Qb=0.267QcQd=1.574Qc

Therefore,

Qb+Qc+Qd=0.267Qc+Qc+1.574Qc=20ft3/sQc=7.0398ft3/s

To find the head loss across pipe c,

h fc=0.03114 800ft 12 12 ft 1.273×7.0398f t 3 /s 2 2 32.2ft/ s 2 =31.067ft

Therefore the total head loss will be,

h ftotal= h fa+ h fc=313.534ft+31.067ft=344.601ft

To find pressure drop between point 1 and point 2

Consider the specific weight of the water as 62.4lbf/ft3

p1p2=γ h ftotal=62.4lbf/ft3344.601f=21503.102lbf/ft2=149.327lbf/in2

Conclusion:

The pressure drop p1p2 is equal to 149.327lbf/in2.

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