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Fluid mechanics - flow through a pipe (bernoulli equation)

  • Thread starter jinro
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Homework Statement


167uh61.gif

water flows steadily with negligible viscious effects through this pipe.

the 4-inch diameter section of the thin walled tubing will collapse if the pressure within is at 6 psi below atmospheric pressure. determine maximum h so that the tube wont collapse.

final answer for h to be in feet.

known data:
p1 = 0, V1 = 0, Z2 = 0, p2 = -6 psi, specific weight of water = 62.4 lb/ft^3, gravity = 32.2ft/s^2


Homework Equations



bernoulli equation
p1/gamma + z1 + V1^2/2*g = p2/gamma + z2 + V2^2/2*g
gamma is specific weight of water, z is height of water, V is velocity of water, g is gravity, p is pressure

to convert from lb/in^2(psi) to lb/ft^2, multiple by 144in^2/ft^2

The Attempt at a Solution


using bernoulli equation and known data, p2 = -6*144 = -864lb/ft^2

with z1 = 4ft
4ft = (-864)/(62.4lb/ft^3)+V2^2/(2*32.2ft/s^2)
-->V2 = 33.90~~ ft/s

from point 1 to 3

p1/gamma + z1 + V1^2/2*g = p3/gamma + z3 + V3^2/2*g
p3 = 0, z3 = -h, V3 = A2/A3*V2 = (D2/D3)^2*V2 = 15.0672 ft/s

4ft = -h + (15.0672ft/s)^2/(2*32.2ft/s^2)
4ft = -h + 3.525
h = -0.4748ft???

a negative value for h would not make sense!

thanks in advance
 

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