Fluid Mechanics Fundamentals And Applications
Fluid Mechanics Fundamentals And Applications
3rd Edition
ISBN: 9780073380322
Author: Yunus Cengel, John Cimbala
Publisher: MCGRAW-HILL HIGHER EDUCATION
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Chapter 14, Problem 68P
To determine

The net head produced by the pump.

The required brake horsepower.

The angle at which the fluid impinges on the impeller blades a critical parameter in the design of the centrifugal pump.

Expert Solution & Answer
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Answer to Problem 68P

The net head produced by the pump in cm of water column height is 0.294cm.

The required brake horsepower is 13,699W.

When there is a swirl at the inlet, the net head produced by the pump and the required brake horsepower will decrease. Thus, the angle at which the fluid impinges has an important role in the design of the centrifugal pumps.

Explanation of Solution

Given information:

The speed of the pump is 750rpm, the entry angle of the water in the impeller is 7°, the exit angle of the water in the impeller is 35°, the inlet radius is 12cm at which the blade width is 18cm, the outlet radius is 24cm at which the blade width is 16.2cm, the volume flow rate is 0.573m3/s and the efficiency is 100%.

The density of water is 998kg/m3, the density of air is 1.2kg/m3 and the acceleration due to gravity is 9.81m/s2.

Write the expression for the angular velocity of the centrifugal pump in rad/s.

   ω=2πn˙60    ...... (I)

Here, the speed of the pump is n˙.

Write the expression for the normal velocity component at outlet of pump.

   V2,n=V˙2πr2b2    ...... (II)

Here, the volume flow rate is V˙, the outlet radius is r2 and the blade width at exit is b2.

Write the expression for the tangential velocity component at outlet of pump.

   V2,t=V2,ntanα2    ...... (III)

Here, the normal velocity component at outlet of pump is V2,n and the exit angle of the water in the impeller is α2.

Write the expression for the normal velocity component at inlet of pump.

   V1,n=V˙2πr1b1    ...... (IV)

Here, the inlet radius is r1 and the blade width at inlet is b1.

Write the expression for the tangential velocity component at inlet of pump.

   V1,t=V1,ntanα1    ...... (V)

Here, the normal velocity component at inlet of pump is V1,n and the inlet angle of the water in the impeller is α1.

Write the expression for the linear velocity.

   v=r1.ω    ...... (VI)

Write the expression for the total head.

   H=(vg)(r2V2,tr1V1,t)    ...... (VII)

Here, the linear velocity of the centrifugal pump in is v and the acceleration due to gravity is g.

Write the expression for the head in cm of water column.

   Hwater=H(ρ airρ water)    ...... (VIII)

Here, the density of air is ρair, the density of water is ρwater, and the total head is H.

Write the expression for the brake horse power.

   bhp=ρwatergHV˙    ...... (IX)

Calculation:

Calculating the parameters when inlet angle is 7°.

Substitute 750rpm for n˙ in the Equation (I).

   ω=2π( 750rpm)60=4712.38898rpm60=78.54rad/s

Substitute 0.573m3/s for V˙, 24cm for r2 and 16.2cm for b2 in the Equation (II).

   V2,n=0.573 m 3/s2π( 24cm)( 16.2cm)=0.573 m 3/s2π( 388.8 cm 2 ( 1 m 2 10000 cm 2 ))=0.573 m 3/s0.244290m2=2.346m/s

Substitute 2.346m/s for V2,n and 35° for α2 in the Equation (III).

   V2,t=(2.346m/s)tan35°=2.346m/s×0.70020=1.643m/s

Substitute 0.573m3/s for V˙, 12cm for r1 and 18cm for b1 in the Equation (IV).

   V1,n=0.573 m 3/s2π( 12cm)( 18cm)=0.573 m 3/s2π( 216 cm 2 ( 1 m 2 10000 cm 2 ))=0.573 m 3/s1357.168m2=4.22m/s

Substitute 4.22m/s for V1,n and 0° for α1 in the Equation (V).

   V1,t=(4.22m/s)tan7°=4.22m/s×0.12278=0.51813m/s

Substitute 78.54rad/s for ω and 12cm for r1 in the Equation (VI).

   v=12cm×78.54rad/s=12cm( 1m 100cm)×78.54rad/s=0.12m×78.54rad/s=9.4248m/s

Substitute 12cm for r1, 24cm for r2, 0.51813m/s for V1,t, 1.643m/s for V2,t, 9.4248m/s for v and 9.81m/s2 for g in the Equation (VII).

   H=( 9.4248m/s 9.81m/ s 2 ){(24cm)(1.643m/s)(12cm)(0.51813m/s)}=(0.96073s){( 24cm( 1m 100cm ))( 1.643m/s )( 12cm)( 1m 100cm )( 0.51813m/s )}=(0.96073s){(0.24m)(1.643m/s)(0.12m)(0.51813m/s)}=2.442m

Substitute 2.442m for H

   998kg/m3 for ρwater and 1.2kg/m3 for ρair in the Equation (VIII).

   Hwater=2.442m( 1.2 kg/ m 3 998 kg/ m 3 )=2.442m×(1.202404× 10 3)=2.93627×103m( 100cm 1m)=0.294cm

Substitute 2.442m for H

   998kg/m3 for ρwater

   9.81m/s2 for g and 0.573m3/s for V˙ in the Equation (IX).

   bhp=(998kg/ m 3)(9.81m/ s 2)(2.442m)(0.573 m 3/s)=13,699kgm2/s3( 1W 1 kg m 2 / s 3 )=13,699W

Calculating the parameters when inlet angle is 0°.

Substitute 750rpm for n˙ in the Equation (I).

   ω=2π( 750rpm)60=4712.38898rpm60=78.54rad/s

Substitute 0.573m3/s for V˙, 24cm for r2 and 16.2cm for b2 in the Equation (II).

   V2,n=0.573 m 3/s2π( 24cm)( 16.2cm)=0.573 m 3/s2π( 388.8 cm 2 ( 1 m 2 10000 cm 2 ))=0.573 m 3/s0.244290m2=2.346m/s

Substitute 2.346m/s for V2,n and 35° for α2 in the Equation (III).

   V2,t=(2.346m/s)tan35°=2.346m/s×0.70020=1.643m/s

Substitute 0.573m3/s for V˙, 12cm for r1 and 18cm for b1 in the Equation (IV).

   V1,n=0.573 m 3/s2π( 12cm)( 18cm)=0.573 m 3/s2π( 216 cm 2 ( 1 m 2 10000 cm 2 ))=0.573 m 3/s1357.168m2=4.22m/s

Substitute 4.22m/s for V1,n and 0° for α1 in the Equation (V).

   V1,t=(4.22m/s)tan0°=4.22m/s×0=0m/s

Substitute 78.54rad/s for ω and 12cm for r1 in the Equation (VI).

   v=12cm×78.54rad/s=12cm( 1m 100cm)×78.54rad/s=0.12m×78.54rad/s=9.4248m/s

Substitute 12cm for r1, 24cm for r2, 0m/s for V1,t, 1.643m/s for V2,t, 9.4248m/s for v and 9.81m/s2 for g in the Equation (VII).

   H=( 9.4248m/s 9.81m/ s 2 ){(24cm)(1.643m/s)(12cm)(0m/s)}=(0.96073s){(24cm( 1m 100cm ))(1.643m/s)}=(0.96073s){(0.24m)(1.643m/s)}=3.157m

Substitute 3.157m for H

   998kg/m3 for ρwater and 1.2kg/m3 for ρair in the Equation (VIII).

   Hwater=3.157m( 1.2 kg/ m 3 998 kg/ m 3 )=3.157m×(1.202404× 10 3)=3.8×103m( 100cm 1m)=0.38cm

Substitute 3.157m for H

   998kg/m3 for ρwater

   9.81m/s2 for g and 0.573m3/s for V˙ in the Equation (IX).

   bhp=(998kg/ m 3)(9.81m/ s 2)(3.157m)(0.573 m 3/s)=17710.4082kgm2/s3( 1W 1 kg m 2 / s 3 )=17710.4082W( 1kW 1000W)=17.710kW

Conclusion:

The net head produced by the pump in cm of water column height is 0.294cm.

The required brake horsepower is 13,699W.

When there is a swirl at the inlet, the net head produced by the pump and the required brake horsepower will decrease. Thus, the angle at which the fluid impinges has an important role in the design of the centrifugal pumps.

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Chapter 14 Solutions

Fluid Mechanics Fundamentals And Applications

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