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Thermodynamics adiabatic diffuser question

  1. Feb 19, 2015 #1
    An adiabatic diffuser is employed to decrease the velocity of an airstream from 220 to 30 m/s. The air enters the diffuser at a rate of 8 kg/s with a temperature of 300c and a pressure of 100kpa. Determine the exit area of the diffuser if the exit pressure is 125kpa.

    Answer is 0.37 m^2

    Guys im really lost with this one.. I used equations pv=mrt, m=p'v'a, v=v'a
    Note (v' is velocity as in> m/s and p' is density) just to prevent confusion

    What i only got are the initial values v1, a1, p'1..
     
  2. jcsd
  3. Feb 19, 2015 #2
    I cant get a2(exit area) have any ideas guys?
     
  4. Feb 20, 2015 #3

    billy_joule

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    What does adiabatic mean? How can you use this information to relate the inlet and outlet properties?
     
  5. Feb 21, 2015 #4
    i for got to add the equation for conservation of energy on open system
    q=∑(exi
    adiabatic mean
    adiabatic means heat transfer is zero (Q=0)

    i forgot to add the equation for open systems
    Q= ΔH + ΔPE + ΔKE + W --- in this equation Q, W, & PE is equal to zero..
    and
    ΔH = ΔU + Δ(PV)

    this equations might be usable.. my question is how do i use these equations to determine exit area
     
  6. Feb 21, 2015 #5

    billy_joule

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    You can find the outlet temp. via conservation of energy. Then you can find outlet density as you have temp. & pressure.
    Then use the mass flow equation you gave in your first post to solve for area.
     
  7. Feb 21, 2015 #6

    billy_joule

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    I must add that there are probably other, potentially simpler ways to solve the problem.
     
  8. Feb 21, 2015 #7
    conservation of energy equation? i dunno how to get the final temperature.. can you be more specific with equation for conservation of energy?
    i only know this equation for conservation of energy for open system : Q= ΔH + ΔPE + ΔKE + W
    i can' simplify tthe equation that i gave to input the temperature in the equation
     
    Last edited: Feb 21, 2015
  9. Feb 21, 2015 #8

    billy_joule

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    You have one unknown in that equation so you should be able to solve it.
    If you have no idea how to proceed it's generally best to review your text book and/or lecture notes. We can help with specific problems but a forum is a poor format to learn course material from.
     
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