lecture 4 - Thermal and Fluids Engineering I Lecture 4...

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Thermal and Fluids Engineering I Lecture 4 Page 1 Lecture 4 – Specific Heat Specific Heat of Ideal Gases Specific heat is the energy required to raise the temperature of a unit mass of a substance by one degree For gases, we distinguish between constant pressure processes and constant volume processes: Constant pressure: Constant volume: More heat is needed to raise a unit mass by one degree in a constant pressure process than in a constant volume process. In a constant pressure process, the added heat is converted to both internal energy and work to raise the weight. In a constant volume process, all added heat is converted to internal energy.
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Thermal and Fluids Engineering I Lecture 4 Page 2 First consider the constant volume heating of a gas in a rigid tank. From the first law UQ W ∆=− Since no work is done ∆= If the gas is ideal, internal energy is only a function of temperature (an empirical fact). We define the specific heat at constant volume c v so that v Um c T constant specific heat, ideal gas This equation applies to all processes of ideal gases, not just constant volume ones. In differential form v dU mc dT = In general, c v is a function of temperature. () v dU mc T dT = and v Ud c T d T = ∫∫ The specific internal energy u , is defined as U u m = Specific internal energy is a thermodynamic property, like specific volume, temperature or pressure. There is an EES function for specific internal energy which is u = INTENERGY(H2, T=T1)
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Thermal and Fluids Engineering I Lecture 4 Page 3 Now consider heat addition in a constant pressure process. The first law is UQ W ∆=− If the process is quasi-equilibrium, WP d V = Because pressure is constant 21 () d VP VV P V == = P V 2 1 QU P V U U P V V =∆ + = + Since 12 P PP we may arbitrarily rewrite this equation in the form ( ) 22 2 11 1 P V UP V =+ + We define a new property, enthalpy, as H V so that, QH H =− constant pressure, closed system
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Thermal and Fluids Engineering I Lecture 4 Page 4 As it happens, the enthalpy of an ideal gas is only a function of temperature . We prove this by substituting the ideal gas law into the definition of enthalpy to get mRT HU M =+ Since U
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This note was uploaded on 04/08/2008 for the course ENGR 2250 taught by Professor Borca-tasciuc during the Spring '08 term at Rensselaer Polytechnic Institute.

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lecture 4 - Thermal and Fluids Engineering I Lecture 4...

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