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Thermodynamics problem 1st law

  1. Sep 5, 2016 #1
    1. The problem statement, all variables and given/known data

    Air flows steadily into a well isnulated piping junction through the two pipes and is heated by an electric resistor at uknown rate befoe exiting through the pipe. Pressure renaubs approcimetaly constant
    p=0.1 MPa, first inlet: volumetric flow rate = 10m^3/s, t1= 20 C, 2nd inlet: volumetric flow rate = 30m^3/s, t2= 30C. At the outlet: t3= 55C, cross section area = 2m^2. Assume the effects of kinetic and potential energy are negligible and air behaves as a perfect gas iwth Cp= 1,0kJ/(kgK). Find the : mass flow rate at exit, heat rate of electric heater and exite velocity of air.
    2. Relevant equations


    3. The attempt at a solution:
    m1+m2=m3
    Q-W= m1*ht1 + m2*ht2 - m3*ht3
    W=0
    Q=m1ht1+m2ht2-(m1+m2)ht3
    m=A*ro*velocity
     
  2. jcsd
  3. Sep 5, 2016 #2
    This looks OK so far, except for the signs of the right hand side...they are all opposite to what they should be. Please show some more detail.
     
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