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Flow of air in a pipe

  1. Aug 25, 2012 #1
    1. The problem statement, all variables and given/known data
    air flows in a 4" pipe. at point A a 2[bar] pressure was measured and the temperature was 22°C. the velocity V1was 5[m/sec].
    At point B, down the stream, a 1.2[bar] pressure was measured and the temperature was 32°C.
    What is the velocity V2at point B.

    2. Relevant equations
    The continuity equation:
    [itex]\gamma[/itex]1A1V1=[itex]\gamma[/itex]2A2V2
    Where [itex]\gamma[/itex]=specific gravity, A=section area.

    The gas law:
    pv=RT
    Where: p=absolute pressure
    [itex]v=\frac{1}{\rho}[/itex], [itex]\rho[/itex]=specific mass


    3. The attempt at a solution
    The specific gravity at point A:
    [tex]\gamma_1=10\frac{2E5}{287\cdot 295}=23.6[N/m^3][/tex]

    The specific gravity at point B:
    [tex]\gamma_2=10\frac{1.2E5}{287\cdot 305}=13.7[N/m^3][/tex]


    The continuity equation, without the area, since it's the same on both points:
    [tex]23.6\cdot 5=13.7\cdot V_2 \Rightarrow V_2=8.6[/tex]
    The answer should be 7.05.
     
  2. jcsd
  3. Aug 25, 2012 #2

    TSny

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    Are the measured pressures "gauge pressures"?
     
  4. Aug 25, 2012 #3
    the problem statement is exactly as written in the book.
    Maybe they should be, but that's the books mistake.
     
  5. Aug 25, 2012 #4

    TSny

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    Well, measurements of pressure are often gauge pressures. See what you get for the answer if you assume they are gauge pressures :smile:
     
  6. Aug 26, 2012 #5
    yes, thank you, it solved. bye
     
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