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Lect17FirstLawOpenSysDiv2&5SP2012

Lect17FirstLawOpenSysDiv2&5SP2012 - ME 200...

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ME 200: Thermodynamics I Lecture 17: First Law of Thermodynamics for Open Systems
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First Law of Thermodynamics for Closed Systems First Law of Thermodynamics – Closed Systems : General Form Q W U KE PE    ,: Neglect KE PE Q W U  : Per unit mass q w e : Differential form q w de  : CV CM dE dE Rate form Q W dt dt 
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This Lecture First Law of Thermodynamics » Open Systems
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CV CS Inlet Exit Consider a closed system that contains a fixed quantity of mass m . At time t , the mass in the closed system is distributed between the control volume and an inlet to the control volume:   iC V mm m t     CV CV mt E t i m For the closed system that contains the total mass m :   2 1 2 CV i ii i Et E t V mu g z     Conservation of Energy – Control Volume Analysis for an Open System
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CV CS Inlet Exit    CV CV mt t E tt     e m We have assumed that the velocity and flow properties are uniform at the inlet. Now at some time t + t , the quantity of mass m i in the inlet will have just moved completely into the control volume. At that time, a quantity of mass m e will have moved out of the control volume, as shown in the following diagram: Conservation of Energy – Control Volume Analysis for an Open System
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  CV e mm t t m   2 2 2 e CV e e e V Et t E t t m u gz            3 t Et Q W   The quantity of mass in the closed system is unchanged at time t + t , but the energy for the closed system may have changed: Now for the closed system of fixed mass we can apply the first law to determine the change in energy: Conservation of Energy – Control Volume Analysis for an Open System
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Lect17FirstLawOpenSysDiv2&5SP2012 - ME 200...

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