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 11, Problem 64P
To determine

The updraft velocity of air motion.

Expert Solution & Answer
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Answer to Problem 64P

The updraft velocity of air motion is 0.6177m/s.

Explanation of Solution

Given information:

The diameter of the dust particle is 0.1mm, the density id 2.1g/cm3, the pressure of the air is 1atm, and the temperature of the air is 25°C.

Write the expression for the weight of the body.

   W=FD+FB …… (I)

Here, the drag force is FD and the buoyant force is FB.

Write the expression for weight of the body in terms of density.

   W=ρDgV …… (II)

Here, the density of dust particle is ρD, volume of the dust particle is V, and the gravitational acceleration is g.

Write the expression for the drag force as per stokes law.

   FD=3πμVD …… (III)

Here, the velocity of the dust particle is V, the viscosity of air is μ, and the diameter of dust particle is D.

Write the expression for the buoyant force.

   FB=ρagV …… (IV)

Here, the density of air is ρa.

Write the expression for the volume of the dust particle.

   V=πD36 …… (V)

Calculation:

Substitute 0.1mm for D in Equation (V).

   V=π( 0.1mm)36=π( 0.1mm( 1m 1000mm ))36=π( 0.0001m)36=5.23×1013m3

Substitute 9.81m/s2 for g, 2.1g/cm3 for ρd and 5.23×1013m3 for V in Equation (II).

   W=(2.1g/cm3)(9.81m/s2)(5.23×1013m3)=(2.1g/cm3( 10 3 kg/ m 3 1g/ cm 3 ))(9.81m/s2)(5.23×1013m3)=(2.1×103kg/m3)(9.81m/s2)(5.23×1013m3)=1.077×108kgm/s2

   W=1.077×108kgm/s2(1N1kgm/ s 2)=1.077×108N

Refer to Table A-9, “Properties of air” to obtain the values of ρa as 1.184kg/m3 and μ as 1.849×105kg/ms at temperature 25°C.

Substitute 9.81m/s2 for g, 1.184kg/m3 for ρa and 5.23×1013m3 for V in Equation (IV).

   FB=(1.184kg/m3)(9.81m/s2)(5.23×1013m3)=6.074×1012kgm/s2(1N1kgm/ s 2)=6.074×1012N

Substitute 6.074×1012N for FB, 1.077×108N for W, 3πμVD for FD in Equation (I)

   1.077×108N=3πμVD+6.074×1012N3πμVD=(1.077×108N)(6.074×1012N)V=1.0764×108N3πμD …… (VI)

Substitute 1.849×105kg/ms for μ and 0.1mm for D in Equation (VI).

   V=1.0764×108N3π(1.849× 10 5kg/ms)(0.1mm)=1.0764×108N( 1 kgm/ s 2 N)3π(1.849× 10 5kg/ms)(0.1mm( 1m 1000mm ))=1.0764×108kgm/s23π(1.849× 10 5kg/ms)(0.0001m)=0.6177m/s

Conclusion:

The updraft velocity of air motion is 0.6177m/s.

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