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[Thermodynamics] Transient Analysis of an air-filled tank

  1. Oct 18, 2006 #1


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    Hey I think I'm missing something fundamental in this problem.

    The problem reads: a 1 m3 tank initially contains air at 300 kPa, 300K. The air slowly escapes until the pressure drops to 100 kPa, via a process where pv1.2=constant (v being specific volume)

    Find the heat transfer for a control volume enclosing the tank, assuming ideal gas behavior with constant specific heats.

    I have determined the specific volumes of the initial and final states and I have looked up enthalpy and internal energy values for initial and final states. This is more of a symbolic question I have so I'll leave those out.

    The energy balance (all for control volume, so I dont have to write cv over and over)

    dU/dt = dQ/dt +(dm/dt)he where he is the enthalpy at the outlet valve.

    Integrating with respect to time from state 1 to 2 would give m[tex]\Delta[/tex]u=Q+(mf-mi)he

    Now this is where I have several questions. 1) It seems like since the enthalpy is varying and not constant that I should have somehow considered that in the integration. I'm just not sure how I would approach the fact that the enthalpy at the outlet is varying over time.

    I considered using the average of the initial and final enthalpies but that didn't yield a correct answer.

  2. jcsd
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