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Homework Help: Need help with a tricky thermodynamics question involving isentropic efficiency

  1. May 22, 2012 #1
    1. The problem statement, all variables and given/known data
    An engine operates on an air cycle consisting of the following non-flow
    processes in succession:
    1-2 isentropic compression from an initial state of 80 kPa, 27°C and the
    volume of 0.071 m
    through the volume compression ratio of 17.75;
    2-3 reversible heat addition at constant volume;
    3-4 reversible heat addition at constant pressure, during which the
    temperatureincreases from 1150°C to 1595°C ;
    4-5 adiabatic irreversible expansion with an isentropic efficiency of 80%;
    5-1 reversible constant pressure cooling which restores the air to its initial

    2. Relevant equations
    a) Calculate for the cycle:
    i) the total heat supplied;
    ii) the heat rejected;
    iii) the thermal efficiency.
    b) Determine the change in entropy during:
    i) the compression process 1-2;
    ii) the expansion process 4-5;
    iii) the constant pressure cooling process 5-1.
    Explain why the entropy changes in each case.

    3. The attempt at a solution
    I already find out the heat supplied equaling to 52.01 kj. Currently stuck on heat rejected. I tried to use Qin-Qout=Wnet to find the heat rejected, but couldn't find the work done from 5 to 1. The mass of the air in the system was calculated to be 0.066kg. Please help with the heat rejection part as the rest of the questions will be simple after that. Thanks.
  2. jcsd
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