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Find Mass Flow Rate and the Exit Flow Area

  1. Feb 6, 2013 #1
    1. The problem statement, all variables and given/known data
    Air enters aone-inlet, one-exit contreol volume at 6 bar, 500K and 30m/s through a flow area of 28cm3. At the exit the pressure is 3 bar, the temperature is 456.5 K and the velocity is 300 m/s. The air behaves as an ideal gas. For steady state operation determine

    a) the mass flow rate, in kg/s
    b) the exit flow area, in cm3

    2. Relevant equations

    m' = dm/dt = ρvA

    3. The attempt at a solution

    I have the area of the cross section, A, and the velocity at both ends, but I'm not sure about the ρ.

    Can I use:

    ρ = p/RT to find the air density?
  2. jcsd
  3. Feb 6, 2013 #2
    I've managed to find the mass flow rate, using the density equation given above, I found the air density and then plugged this number into m' =ρvA

    m' = (4.1803 kg/m3)(30 m/s)(28cm2)

    m' = 0.351 kg/s

    Does the mass flow rate stay constant? Can I rework these equations to find the Exit flow area?
  4. Feb 6, 2013 #3


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    Mass is a a conserved quantity. In steady state, the mass within the volume is neither increasing nor decreasing, so the mass flow rate in must equal the mass flow rate out.
  5. Feb 6, 2013 #4


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    Areas are usually measured in square cm, rather than cubic cm.
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