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Thermodynamics: h vs u and Quality? 
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#1
May1609, 04:28 AM

P: 145

Hi,
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? Thanks a lost Mechanical engineering student 


#2
May1609, 05:56 PM

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P: 2,907

... is off just a bit. The correct equation is: Y = Y_{f} + X Y_{fg} Where Y_{f} is the fluid property at 100% saturated liquid and Y_{fg} 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 where dH = (U_{in} + pV_{in})  (U_{out} + pV_{out}) 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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