ME300Lecture19

ME300Lecture19 - Lecture 19: Evaluating Property Changes...

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19.1 Lecture 19: Evaluating Property Changes for Real Fluids Clapeyron equation for liquid-vapor phase change Considering the 3 rd Maxwell relation During a liquid-vapor phase-change process, p sat = f(T sat ) During this process, p = constant Then we obtain Clapeyron equation gf g f g f sat sat gg g f g f ff 21 g f 2 1 sat sat ss dp dp s s (v v ) or dT dT v v dh Tds vdp dh h h T(s s ) hh dp dp or dT T(v v ) dT v ) Tv sp vT
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19.2 Continue Evaluating Property Changes for Real Fluids Clapeyron equation for solid-vapor phase change At low v g >> v f , use idea gas low for v g = RT/p That is Clapeyron-Clausius equation g f g f 2 gf sat fg 2 sat sat fg fg 2 1 1 2 sat sat h h p(h h ) dp dT T(v v ) RT or h dp dT pR T When h does not change much with T h p 11 ln p R T T
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19.3 Example 19-1 Given: (1) The Clapeyron equation, (2) refrigerant R-134a, and (3) T = 20 o C Find: » estimate the value of the enthalpy of vaporization » compare it with the tabulated value
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19.4 Continue Example 19-1
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19.5 Continue Evaluating Property Changes for Real Fluids Internal energy changes for single phase fluid v v T T vT v v u u u du dT dv c dT dv (1) T v v also ss ds dT dv and du Tds pdv Tv then du T dT T p dv (2)
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ME300Lecture19 - Lecture 19: Evaluating Property Changes...

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