|May16-09, 04:28 AM||#1|
Thermodynamics: h vs u and Quality?
I am trying to understand the solution to a quiz I had, and am having trouble.
The question was:
Y = Yliq + (Quality)*Ysat.vapor
so why dont they use it in this scenario?
Second, why do they use internal energy and not enthalpy?
--a lost Mechanical engineering student
|May16-09, 05:56 PM||#2|
... is off just a bit. The correct equation is: Y = Yf + X Yfg
Where Yf is the fluid property at 100% saturated liquid
Yfg is the difference between the fluid property at 100% saturated gas and 100% saturated liquid.
Regarding why not enthalpy, if you rewrite the first law it helps to clear this up. Normally, we write:
dU = dQ + dW
But for any control volume with flow in and out, we can also rewrite it:
dU = dQ + dW + dH
dH = (Uin + pVin) - (Uout + pVout)
Note that gas flowing into or out of a control volume does work pV so we say H = U + pV
(See ref: http://web.mit.edu/16.unified/www/FA.../chapter_4.htm )
So if we apply this to the system with two separated tanks that has a control surface around both tanks, we find no heat or work crossing the control surface (dQ=0 and dW=0), nor do we find any flow across the control surface, so dH = 0. All we're left with is dU = 0 (or U1 + U2 = U3).
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