Friday, April 14, 2006
HW #5 - P #10 - Filling a Balloon with Helium - 10 pts
A balloon initially contains 65 m3 of helium gas at atmospheric conditions of 100 kPa and 22oC. The balloon is connected by a valve to a large reservoir that supplies helium gas at 150 kPa and 25oC. Now, the valve is opened and helium is alowed to enter the balloon until pressure equilibrium with the helium at the supply line is reached. The material of the balloon is such that its volume increases linearly with pressure. If no heat transfer takes place during this process, determine the final temperature of the helium in the balloon.
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21 comments:
This one's got me stuck. I came up with an equation for T2 using the IG EOS and the uniform state energy balance. I've found msys1, and wb no problems.
Using the HPP, I came up with U^sys2 = (C^v)delta T ~ 9kJ/kg.
Similarly, H^in=(C^p)delta T ~ 16kJ/kg
Unfortunatly, these # don't give me the right answer. I'm sure my dervied equn is right (dim analysis is correct). I'm not calculating H^in and U^sys2 correctly, am I?
Oh, wait, U^sys is a function of the final temp: U^sys=Cv(Tfinal-Tref) isn't it?
I'm confused. How do you find Hin? it says to use a hpp from the reference state to the inlet?
hey graham i may not have done it right but i did get the 60 or whatever it was i don't have it here with me. it looks like you have used the cp*delta t for hin and that gives delta h instead of hin i think. that may be where your numbers are getting messed up.
hope this helps if dr. b isn't around
Delta H = H in though
it seems weird that your saying delta t is the temp of incoming gast - temp of intial when the final isn't the temp of the incoming gas therefore your not really getting the delta H of the gas because and i don't think that its hin but i don't really know i'm just trying to be helpful.
i found h^ by using U^ p*delta v and got the right awnser maybe give it a try
Everyone in the chat uh couldnt actually calculate this, we were getting like T2=251.6 K. we were using m2u2=minH^-Wb. m2=7040/T2
u2= 3.11T2-919.6 min=m2-10.6 h^=15.6 Wb=4062 help?
graham... i am also getting -21.52*C....
Graham 1:15 PM
Because the P increases 50%, V also increases by 50%.
Yes, U1 = Uref = 0 and H1 = P1 V1. But why do you want to know H1 ? To determine Hin perhaps ?
Yes, to find U2, use U2-U1=int{Cv} from T1 to T2.
To find Hin, use Hin-H1=int{Cp} from T1 to Tin.
The IG assumption is required because we are using the IDEAL GAS heat capacity data from NIST !
I hope this helps.
steve 4:36 PM
I do not know how you calculated Usys2, so I could not say why Usys1 is wrong, but it is.
I calculated Hin^ = Cp (Tin - Tref) ~ 630 kJ/kg. Either your Cp or your deltaT is not correct. I am not sure what you did here.
Yes, Usys2 is a function of T2.
I hope this helps.
Anon 9:32 PM
U1 = Uref = 0 and H1 = P1 V1.
To find Hin, use Hin-H1=int{Cp} from T1 to Tin.
Graham 10:05 PM
I guess 2:43 is the morning ...
I am not sure what Tballoon means. Is this T1 (initial T of He in the balloon) ?
Here is my idea:
Eq 2: M2 U2^ =-Wb + Min*Hin
Since U1^ = 0 if you use the reference state that I suggested.
I think I found the problem and I think Brendan has too.
Hin= Href + Cp(Tin-Tref)
Uref = 0, but Href = Uref + Pref Vref.
Href is NOT zero. I got Href > 600 kJ/kg !
I hope this helps.
marc 10:35 PM
I am not sure what you mean here.
Please see my last post to Graham.
Brendan various times
Thank you for your help. You may or may not know why, but you evaluated Hin correctly. Good work.
Anon 12:24 AM
The problem is most likely Hin. The correct value is about 630 kJ/kg for reasons I explained in my recent reply to Graham.
do you look up Pref and Tref up from NIST when U = 0 then?
Anon 12:24 PM
In the hints, I suggested you use the initial condition of the HE in the balloon as the reference state. I still think that is the best choice. At this state, U^ = 0 kJ/kg.
may ı ask a question ı found the answer 333.67 k . am ı right ?
It has been a very long time, but I got 60.6 degC or 333.75 K.
Yeah
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