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Homework Help: Thermodynamics Problem

  1. Jun 23, 2006 #1
    A piston-cylinder device initially contains 10 kg. of water at a temperature of 85 C. The cylinder has a section area of 0.12 meter square and the piston has no mass. A linear spring (spring constant 120,000 N/m) is touching the piston as shown but exerts no force on it initially. Then heat is slowly added to the water in the cylinder, determine: (assume atmospheric pressure to be 100 kPa)

    a) How much heat is needed to start to form vapor in the cylinder?

    b) How much heat is needed to raise the piston 0.2 meter above its original position.

    c) Show the process on a T-v diagram with respects to saturation lines.

    For answer a I got 614.4 KJ from m ( u2-u1) the change in the internal energies multiplied by the mass using u2 at 100 kPa at the sat temp. and u1 from Tsat @ 85 C since it can be reasonably substituted for a compressed liquid.

    For answer b I got 4.8 KJ

    V1= 10kg* .001032 (Vf@85 C)
    V2= .01032+.12*.2

    One way i did it was find the specific volumes then finding the total volumes and drawing a P-V Diagram and looking at the area under the curve, then checking it by calculating the total work done.

    300+100 * 1/2 * (.03432-.01032) =4.8kj

    checking using work methods ( 1/2kx^2 + pressure*area*distance)
    1/2 * 120 * .2^2 + 100*.2*.12=4.8kj

    C) did a check of specific volumes to confirm that the final temp was indeed the saturation temperature. then drew the T-V diagram. 133.52 C is my final temp that i calculated.

    vf@300kpa vg@300kpa
    .001073<.0034232<.60582 --> is the saturated temperature

    Could someone please confirm these results? :)

  2. jcsd
  3. Jul 3, 2006 #2
    yes you r right(to all i know)(to very sure but in he rite trak).....but 1atm is 101.345kpa....doesnt dat cause a significant variation in you ans?
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