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Homework Help: Thermodynamics Question Regarding assuming Irreversibilities

  1. Apr 19, 2017 #1
    1. The problem statement, all variables and given/known data
    There exists a tank filled with air with a given volume, temperature, and pressure. The tank exists in a room at a given temperature and pressure.
    That is:
    For the tank: P=1MPa, T=700k, V=1m^3
    Outside: T=295K, P=100kPa

    2. Relevant Equations

    \psi 2-\psi 1=(1-T0/Tb)*1Q2-[W=P0*(V2-V1)-T0


    3. Attempt at solution

    Will the transfer happen slowly enough that it is safe to assume zero reversabilities? Meaning sigma is zero? In this case I am a little confused between the definitions of internal and external reversibilities.

    With the assumptions that work and entropy production are zero.

    \psi 2-\psi 1=(1-T0/Tb)*1Q2

    Then we can get Q from first law with the assumption that the tank will eventually reach the environmental temperatures. So we can get u2 and u1 from the tables at the tank conditions and the environment conditions.

    (210.49-512.33)Kj/kg*k=-301.84 Kj/kg*k=Q/m

    So (multiplying (Q/m)*m)
    \psi2-\psi1=(1-700k/295k)*(-301.84 Kj/kg*k)*(171.823)
    =71,201.8302 Kj*K

    That is a very large number. Is that correct?
    Last edited: Apr 19, 2017
  2. jcsd
  3. Apr 19, 2017 #2
    I'm confused about the problem statement and your approach to the problem. First regarding the problem statement:

    1. Are you assuming that the tank is not insulated, so that the room air temperature and the tank air temperatures are the same in the final state?
    2. Are you assuming that the pressures inside the tank and outside the tank are equal in the final state/
    3. Are you assuming that the room is sealed and isolated from the environment, and that the initial mass of air in the room is much larger than the initial mass of air in the tank?

    Let's temporarily wait on discussing reversibilities, internal and external reversibilities, and sigma for the moment. I would like to first like to focus on establishing the initial and final states.

    Is that OK with you?
  4. Apr 19, 2017 #3
    Great thank you. I am assuming 1 and 3. I did not take into account 2 because I only used the pressure inside of the tank to calculate the mass in the tank. Since volume stays constant in the rigid tank (another assumption) the P0 term goes to zero. So no I did not assume #2 since I did not think I needed to.

    Thanks for your time,
  5. Apr 19, 2017 #4
    Still confused. Are you saying that the final pressure in the tank does not equilibrate with the pressure in the room? Do you shut the valve before the two pressures equilibrate?
  6. Apr 19, 2017 #5
    My exact problem statement is:

    I am allowed reasonable assumptions. So I guess I would be assuming that pressures would also equilibrate. Sorry for the confusion!
    Thanks for your time.
  7. Apr 19, 2017 #6
    OK. So, in the final state, the temperature and pressure of all the air that was originally inside the tank (some of this air remains in the thank and some is outside in the final state) are 295 K and 1 bar, correct?

    Are you trying to determine the entropy change of just the air that was originally inside the tank, or the change in entropy for the combination of the air inside the tank and the room air?
  8. Apr 19, 2017 #7
    A one cubic meter tank containts a substance at 700k and 1Mpa. The tank is in a room at 295k and 100kPa. I am asked to determine the exergy in kJ. So I would imagine it would be the exergy in the tank.
  9. Apr 19, 2017 #8
    I don't know too much about exergy, but can't it be calculated from the straightforward equation:
  10. Apr 19, 2017 #9
    Yes it can be. I am using the net change form for a closed system. My question is: is it okay to assume that entropy production is zero, and there are no reversibilities because the system cools down slowly to the environmental temperature?
  11. Apr 19, 2017 #10
    Why don't you try it both ways, and compare the results?
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