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Thermal, steady-flow, table, continuity

  1. Apr 21, 2014 #1
    1. The problem statement, all variables and given/known data

    Steam at 2 MPa and 208°C enters a nozzle with 20m/s. During the expansion process, its enthalpy drops to 2.86 MJ/kg because of the losses encountered.

    a) Determine the exit velocity from the nozzle.
    b) If the mass flow rate is 1kg/s, determine the flow area at the nozzle inlet.

    2. Relevant equations

    Steady-flow Energy Equation: Q-W=m{(hf-hi)+1/2(Uf^2-Ui^2)+g(Zf-Zi)},
    difference between rate of heat and work=difference between rate of energy at inlet and exit

    For Nozzle, heat, work, and PE can be neglected:
    (hi-hf)=1/2(Uf^2-Ui^2)

    Continuity equation: mi=mf=constant, mass flow rate is constant
    m=ρ*A*U, ρ=density, A=flow area, U=fluid velocity

    steam tables: http://enpub.fulton.asu.edu/ece340/pdf/steam_tables.PDF

    3. The attempt at a solution

    a.) checking the steam tables, the enthalpy of sat. liquid is 1236kJ/kg,
    (1236-2.86)=1/2(Uf^2-20*2)
    Uf=53.54m/s, which is not correct. The given answer is around 400m/s.
    I don't know why ;(

    I will try b) after I understand a).
    Please help me with part a!
     
  2. jcsd
  3. Apr 21, 2014 #2
    1MJ = 1000kJ =1,000,000 J

    How did you get the 1236 number?
     
  4. Apr 22, 2014 #3

    haruspex

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    Looks like it came from the 280C row instead of 208C.
    Scrubber, should you be using the sat liquid or sat vapor value? (I genuinely don't know.). Even if you use sat vap at 280, it's still less than 2.86MJ/kg, so it doesn't seem to fit.
     
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