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Homework Help: Fluid dynamics continuity equation

  1. Aug 3, 2016 #1
    1. The problem statement, all variables and given/known data
    Through a (cylindrical) heating tube, warm air has to flow with a velocity of 3m/s in order to heat a rectangular spacewhich is 12 m long, 10 m wide and 2.5 m high. How large the radius of the tube should be if the air in the room get refreshed every 15 minutes ? Assume that the air-density in the tube and in the room are equal.

    2. Relevant equations
    Continuity equation
    A1 x v1 = A2 x v2

    3. The attempt at a solution
    Flowrate = Volume/ t V/t = A x v
    Flowrate = 12x10x2.5 = 300 m3 / 900 s = 0.33
    0.33 = A2 x 3m/s
    A2= 0.111 m^2
    So to find the radius you have to use area of cilinder
    A=pi x 2x (r)^2
    0.111= pi x 2x (r)^2
    r= 0.132 m why do i get this answer ?? . Its wrong .answer has to be 0.1881
  2. jcsd
  3. Aug 3, 2016 #2
    What is the equation for the area of a circle?
  4. Aug 3, 2016 #3
    A= pi x (r)^2
    Now its correct
    Thank you
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