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Thermofluids: Air Flow Measurement (Pitot & Venturi)

  1. Apr 21, 2014 #1
    1. The problem statement, all variables and given/known data
    I did an experiment to measure air flow using a pitot static tube and a venturi meter.
    First attempt: I connected both long and short manometers to the pitot static tube. Then I switch on the fan. The difference in manometer reading is 0.59kPa, which is the pressure. Then I found the volumetric flow rate to be 0.55386.

    Second attempt: I connected both long and short manometers to the venturi meter. Then I switch on the fan. The difference in manometer reading is 1.24kPa, which is the pressure. Then I found the volumetric flow rate to be 0.34575.

    What I don't understand is that, should theoretically the volumetric flow rate of both of them the same? Or it is supposed to be different? Why is it different? Thank you.


    2. Relevant equations
    Pitot tube:
    Q = a*vm
    a = area of duct (duct diameter 0.15m)
    vm = 1.291*sqrt Pv

    Venturi Meter:
    Q = Cd*a2 〖[2∆P/ {ρ(1-〖(a_2/a_1 )〗^2)}]〗^0.5

    Cd= 0.98
    d1= duct diameter 0.15m, a1= duct area
    d2= Throat diameter (0.095) a2= throat area
    ρ= 1.2 kg/m^3


    3. The attempt at a solution
    Pitot-static tube:
    vm = 1.291 √(P_v )
    vm = 1.291 √(0.59×〖10〗^3 )
    vm = 31.36 m2/s

    Q = avm
    Q = π/4×〖0.15〗^2×31.36
    Q = 0.554 m3/s

    Venturi meter:
    Q = Cda2 〖[2∆P/ {ρ(1-〖(a_2/a_1 )〗^2)}]〗^0.5 (m3/s)
    Q = 0.98 × π/4 ×〖0.095〗^2 〖[2×1.23×〖10〗^3/ {1.2(1-〖(〖0.095〗^2/〖0.15〗^2 )〗^2)}]〗^0.5
    Q = 6.943×〖10〗^(-3) 〖[2460/(1.2 × 0.839)]〗^0.5
    Q = 0.3436 m3/s
     
  2. jcsd
  3. Apr 22, 2014 #2
    Can you try the experiment again but move the pitot tube to different positions within the duct cross section?
     
  4. Apr 22, 2014 #3
    Unfortunately, I can't. I am not given the access to the pitot and venturi meter after the lab session was finished.

    I remembered I put it in the middle to obtain max velocity pressure though.
     
  5. Apr 22, 2014 #4
    So should there be a correction factor to account for the fact that the velocity is not uniform along the cross section of the duct? This might explain why the pitot value is higher than the venturi value.
     
  6. Apr 22, 2014 #5
    The truth is I don't know. Do you mean a correction factor for the pitot static tube?
    Does this mean that, theoretically, the value of the flow rate should have been the same?
    Thank you very much.
     
  7. Apr 22, 2014 #6
    Yes, that's what I mean.
     
  8. Nov 4, 2014 #7
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