review - Review of Thermodynamics 1st and 2nd Law...

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1 Review of Thermodynamics 1 st and 2 nd Law Applications Conservation of Energy: The 1 st Law of Thermodynamics KE PE U Q W Δ+ Δ = Change in amount of energy contained within the system during some time interval = Net amount of energy transferred in across the system boundary by heat transfer during the time interval - Net amount of energy transferred out across the system boundary by work during the time interval
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2 Alternative Forms of the Energy Balance • Differential Form: dE Q W δ = • Time Rate Form: dE QW dt = First law for Cycles • First law for an arbitrary system: Δ E=Q-W • For a cycle, Δ E cycle =0 since it begins and ends at the same state. Thus cycle cycle =
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3 Power Cycle • A Power system receives heat Q in from a hot reservoir and rejects heat Q out to a cold reservoir. • Net cycle heat Q cycle =Q in -Q out • Thus W cycle =Q in -Q out • Thermal efficiency: in out in out in in cycle Q Q Q Q Q Q W = = = η 1 Refrigeration Cycle • A Refrigeration cycle extracts heat from a cold reservoir and rejects heat Q out to a hot reservoir. • Net cycle heat Q cycle =Q in -Q out • Thus W cycle +Q cycle =0 • Coefficient of Performance (COP): 1 > = = β in out in cycle in Q Q Q W Q
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4 Heat Pump Cycle • Heat pump is similar to refrigeration except that Q out is the output of interest . • Coefficient of Performance (COP): 1 > = = γ in out out cycle out Q Q Q W Q Summary of Cycle Analysis Power Cycles Refrigeration & Heat Pump Cycles cycle cycle cycle EQ W Δ= cycle cycle QW = cycle in W Q η = in cycle Q W β = out cycle Q W γ =
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5 Chapter 3: Evaluating Properties Property Relations in Engineering Thermodynamics Lava flowing into the Pacific Ocean in Hawai . Photo courtesy of Mike Benson. The State Principle Two independent, intensive, thermodynamic properties are required to fix the state of a simple compressible system. For example: P and v T and u x and h Intensive thermodynamic properties: Less used: g - Gibbs free energy a - Helmholz free energy v – specific volume T – absolute temperature P –absolute pressure s –specific entropy x – quality (steam only) u – specific internal energy h – specific enthalpy
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6 P-v-T Relation Note the location of the following: - Single phase regions - Two phase regions - Saturation states - Triple line - Critical Point T-v diagram Diagram courtesy of Jerry M. Seitzman, 2001.
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7 Linear Interpolation: Between values in the tables LL H LH L TT v v v v −− = Subscripts: L – Value in table at lower end H – Value in table at upper end None – value of interest Quality For use in Tables A-2 and A-3 • For Saturated Mixture (Liquid-Vapor) Region Quality; x; an intensive property x gives fraction that is vapor (gas) (1-x) gives Moisture Content g f g m m m x + 0 x 1; x = 0 Saturated Liquid (subscript ‘f’) x = 1 Saturated Vapor (subscript ‘g’) ‘fg’ ‘g’-’f’
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8 Quality Relations LET b = ANY INTENSIVE PROPERTY – (b = v, u, h, s, etc.) (1 ) f f gf f g ff g fg g f g f bb x b x b b bx b x b == =+ =− =⋅ +
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This note was uploaded on 05/12/2009 for the course ME 3322 taught by Professor Neitzel during the Spring '07 term at Georgia Tech.

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review - Review of Thermodynamics 1st and 2nd Law...

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