Wednesday, March 21, 2007

HW #2, P5- Isobaric Expansion of Water - 6 pts

Please post any questions or discussion related to this problem as comments on this message. Feel free to answer other students' questions. I will check the blog M-F and once on the weekend.
Dr. B

13 comments:

chica said...

I have a question on part b, What is the final temperature of the water in the cylinder? I'm not really sure how to approach this....

chica said...

Also, is the critical point needed on the TV diagram? If so, how do I calculate it?

Anonymous said...

For part b, the problem said "heat is added until entire liquid is vaporized so I thought right when all the liquid has moved to the vapor phase, the vapor is completely saturated so i used the values from the saturation table for the final phase and found the corresponding temp. For TV diagram, do we even need to find the critical point?

Dr. B said...

Chica 5:09:
P2 = P1 because it is isobaric, right ? Because the entire liquid is just barely vaporized, you know the final state is sat'd vapor and x2 = 1. So, you can conclude that T2 = Tsat(300kPa) and just look this up in the Sat Pressure table.

Dr. B said...

Chica 5:13:
You just need to make a nice SKETCH of a PV diagram (not a full plot with scales on the axes).
Your diagram should include the critical point and all the regions and curves that you would expect to see on a TV Diagram. Look at the ones in Thermo-CD.

Dr. B said...

jtl610:
Yes, you did the problem correctly. Nice work.

Your TV digram must have the critical point labeled, but since you do not need to put scales (numbers) on the axes, you don't need to know the values of Tcrit and Pcrit or Vcrit.

Anonymous said...

for part c, how do you figure out the enthalpy change from the liquid to the sat liquid?

Dr. B said...

chico:
The tricky part here is determining the enthalpy of the subcooled liquid in state 1 (25degC, 300 kPa). The problem is in the 5 MPa subcooled liquid table you only have values at 20 and 40 degC. So, first, you need to interpolate within the subcooled liquid table to estimate Hhat at 25 degC and 5 MPa. Then you can interpolate again to get Hhat at 25 degC and 300 kPa.

Anonymous said...

Should the TV graph be TV or TVwiggle?

Anonymous said...

When calculating delta(H), how do the units of PV, which I am getting to equal kJ, correspond to the units of delta(U), which are kJ/kg?

Dr. B said...

questioning:
T Vwiggle. This shouldn't be a problem since you will not put scales on your plot.

Dr. B said...

Lance A:
Use P*Vhat when using Uhat and Hhat. This will give you units of kJ/kg for P*Vhat.

cmillns said...

I am having trouble figuring out part A. It requires a double interpolation right? But I cannot figure out how to do this/set it up.