This HW covers CB chapters 1 & 3 or LT chapters 1 & 2.

The homework consists of 15 problems for a total of 71 pts.

Please begin your question with the problem number you are asking about.

Cengel & Boles: Ch 1: 7E(3pts), 11E(4pts), 37E(2pts), 41E(3pts), 78(6pts)

WB-1 (4pts) , WB-2 (5pts)

Cengel & Boles: Ch 3: 26(4pts), 29E(4pts), 37E(5pts), 53(6pts), 66(4pts),

83(6pts) - Assume that the air in the tire behaves as an ideal gas, but then check the validity of this assumption.

WB-3 (10pts) , WB-4 (5pts)

## 21 comments:

How do you do #26, and is the spring in equilibrium to start with on #37E?

-Ross

On 1-78 from the book, when it says to determine the the pressure difference between the two pipelines, I just wanted to clarify what it meant, as in is the first pipeline w/ fresh water and mercury, then the second the one with the seawater?

Thanks.

3.26 - Ross, 12:20pm

In each row, you know the values of two intensive variables. You can use the thermodynamic data tables for water in your book to help you evaluate all of the other (unknown) properties. The 1st row is just like the problem we did in class today.

3.37E - Anon, 12:20 pm

It turns out that it does not matter whether the spring is exerting any force on the back of the piston in the initial state or not. The CHANGE in the pressure inside the cylinder only depends on the CHANGE in the length of the spring. But I understand that it is easier to grasp the problem if you ASSUME the spring exerts zero force on the back of the piston in the initial state. FYI: Because P1 > Patm, if the spring exerts no force in state 1, there must be a weight on the back of the piston to keep it from flying out of the cylinder.

1.78 - Anon, 1:03 pm

Yes, you are correct. You need to determine Psw - Pfw.

On 3-37E I know the initial and final pressures, the initial temperature and the initial and final volumes. What relationship do I use to find T2 since the Ideal Gas EOS doesn't seem to work here?

How do you find enthalpy on #37E and how do we check the ideal gas assumption on #83

#1 what is a lbmol? is it lb/mol or lb*mol?

what is a lbmol? Is it lb/mol or lb*mol or what?

Anon, 1:51 PM, 3.37E

To find T2, you need to do a triple-interpolation, just like we did in in the example problem inclass on Thursday.

I interpolated between 40 and 50 psia at 60 and 80 degF to get Vhat values at P2 at 60 and 80 degF. Then, I interpolated a 3rd time, between 60 and 80 degF, to get T2 that corresponds to the P2 and V2 values I had already found.

I hope this is helpful!

Anon 3:23 pm, 3.37E

Once you know T2, you need to do a double-interpolation on the superheated vapor tables to evaluate H2. Why ? Because there is no table in the book that exactly matches P2. (I got 40psia<P2<50psia). Also, none of the V2 values in the 40psia or 50psia table match the value of V2 that calculated. The double interpolation is just like the one I explain in LT.com. Here is the link to that explanation:

http://www.learnthermo.com/T1-tutorial/ch02/lesson-C/pg21.php

I hope this helps!

Jayson 1:55 pm, WB-1

You always use molecular weight in units of g/gmole, right ? Well, it is also true that the units of molecular weight can be lbm/lbmol. So, let's use water with MW 18 g/gmole or 18 lbm/lbmole. So, if we have 45 g of water, n = 45g/(18g/gmole) = 2.5 gmole. Similarly, if we have 45 lbm of water, n = 45 lbm / (18 lbm/lbmole) = 2.5 lbmole. If you need some more help with this idea, check out LearnThermo.com:

http://www.learnthermo.com/T1-tutorial/ch01/lesson-B/pg04.php

I hope this helps !

here's a yahoo answer on that lbmole thing too

http://answers.yahoo.com/question/index?qid=20090208133045AAnijuX

On WB-2, one of the online hw problems, there is a given pressure 1.4 bar and it says it is a gage pressure of tank A, but it is inside tank B. Does that mean that the gage pressure given is being affected by the pressure of tank B?

WB-2, Jayson, 6:06 pm

Good question!

Yes, the reading on the gauge IS affected by the pressure in tank B.

Remember that a gauge reads the difference between the pressure inside the line or vessel it is connected to and the pressure surrounding the outside of the gauge ! So, here, PAgauge = PA - PB where PA and PB are the absolute pressures inside tanks A & B respectively.

I hope this helps!

Where should I begin on #37E?

3.37E Anon, 2:33pm

I assume your trouble is in prob 37E in ch3.

I began by using the R-134a tables to determin the specific volume and total volume in states 1 and 2. I used this info with the spring constant and dimensions of the piston to determine P2. This led to a triple-interpolation to find T2 followed by a double interpolation to get H2.

I hope this roadmap helps !

how do i find the enthalpy for the third line of #3-29? since T is less than the sat temp for 70 PSIA, it is a subcooled liquid right? do i just use the value of h_f?

Is there some way to check your answers? Or is that what the in-class-homework day is for?

3.29E, Part C, Anon 4:35pm

Excellent question!

You are absolutely correct. The R-134a in part (d) is a subcooled liquid and CB did NOT include a subcooled liquid table for R-134a.

I had solved this problem using my own LT tables that include a subcooled liquid table for R-134a.

Your solution is a very good one. In the absence of better data, go ahead and assume the liquid is incompressible. This means that Hsubliq(10degF) = Hsatliq(10 degF). This assumption most definitely does NOT mean that Hsubliq(10degF&70psia) = Hsatliq(10 degF).Hsatliq(70 psia) ! H IS a fxn of T, even for an incompressible liquid. Use the saturation tables to convince yourself that I am telling you the truth AND that the difference is not a minor one.

Answers, Jayson, 4:48 pm

The book gives you some answers.

I gave answers for the WB problems right on the HW assignment.

We can definitely check answers in the problem session.

I could post more answers on future HWs if you think it would be helpful.

But let's just leave things as they are on this 1st HW.

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