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Homework Help: Thermodynamics. Find the Mass Flow Rate of the Air and the Exit Area

  1. Feb 13, 2013 #1
    1. The problem statement, all variables and given/known data
    Air expands through a turbine from 8 bar, 960 K to 1 bar, 450 K. The inlet velocity is small compared to the exit velocity of 90 m/s, The turbine operates at a steady state and develops a power output of 2500 kW. Heat transfer between the turbine and it's surroundings and potential energy effects are negligible. Modeling air as an ideal gas, calculate the mass flow rate of air, in kg/s, and the exit area, in m2

    2. Relevant equations

    m' = ρvA

    ρ = p/RT

    du = dQ - dw

    h = u + pv

    1Q2+m'i(hi+v2i/2 + gzi) = m'e(he + ve2/2 + gze)

    p1 = 800,000 Pa
    T1 = 960 K
    v1 = 0m/s

    p2 = 100,000 Pa
    T = 450 K
    v2 = 90 m/s

    3. The attempt at a solution

    Using du = dQ - dw I know that

    dw = -2500 kW

    I also have:

    m'i(hi) = m'e(he + v2e/2)

    To calculate enthalpy

    h = pv

    hi = 2500 kW
    he = 2500 kW + (100,000 Pa)(90 m/s)
    he = 11500000 W

    With this information I should then be able to plug it into the large equation (The 1st law of thermodynamics I believe) and hence get mi

    But when I do this I get mi = 0.2777 kg/s which isn't right. My TA said the answer is round about 4 kg/s.

    Help is appreciated. Thank you
  2. jcsd
  3. Feb 14, 2013 #2
    Re: Thermodynamics. Find the Mass Flow Rate of the Air and the Exit Ar

    Enthalpy is Cp*T.
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