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Ratio of Flow Rates.

by Ekh
Tags: flow, rates, ratio
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Jan15-10, 06:08 PM
P: 5
1. The problem statement, all variables and given/known data
Two tubes carry the same incompressible fluid with viscosity 1.5 Pl. They have lengths L1 = 6 and L2 = 22 m and diameters d1 = 1.2 and d2 = 4.5 cm. What is the ratio of their flow rates F1/F2?

2. Relevant equations
Poiseuille's law: 8nLI/(pi*r^4)
while n is viscosity
L is the length
R is radius

3. The attempt at a solution
F1= (8*1.5*6*I) / (pi*(1.2/2)^4)
F2= (8*1.5*22*I) / (pi*(4.5/2)^4)

The ratio F1/F2 is:
(8*1.5*6*I) / (pi*(1.2/2)^4)* (pi*(4.5/2)^4)/ (8*1.5*22*I)

F1/F2= 46.296*1.1649= 53.932.

And this is wrong, can u help??
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Jan15-10, 06:22 PM
HW Helper
ideasrule's Avatar
P: 2,323
You need to write out your steps more clearly. 8nLI/(pi*r^4) is not Poiseuille's equation; Poiseuille equation is ΔP=8nLI/(pi*r^4), where I represents flow rate. You now have 2 equations:


If you assume the two ΔP's are the same and divide one equation by the other, you'll see your mistake.
Jan15-10, 06:32 PM
P: 5
Thank you alot :)

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