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THERMODYNAMICS II INTRODUCTION TO NAVAL ENGINEERING
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FIRST AND SECOND LAWS OF THERMODYNAMICS INTRODUCTION TO NAVAL ENGINEERING
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FIRST LAW OF THERMODYNAMICS Energy can be neither created nor destroyed but only transformed
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THE GENERAL ENERGY EQUATION Energy In = Energy Out or U 2 - U 1 = Q - W where U 1 : internal energy of the system at the beginning U 2 : internal energy of the system at the end Q : net heat flow into the system W : net work done by the system
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ELEMENTS OF A THERMODYNAMIC CYCLE (1) Working Substance - medium by which energy is carried through the cycle. (2) Heat Source - supplies thermal energy to the working substance. (3) Heat Receiver - absorbs heat from the working substance.
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ELEMENTS OF A THERMODYNAMIC CYCLE (4) Pump - circulates the working substance; acts as a valve between low and high pressure (5) Engine - device which converts the thermal energy of the working substance into useful mechanical energy
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THERMODYNAMIC CYCLES l CLOSED –Working fluid never leaves the cycle, except through accidental leakage (ex: steam cycle) l OPEN –Working fluid is taken in, used, then discarded (ex: internal combustion engine)
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ENGINES l HEATED: –heat is added to the working substance in the engine itself (ex: internal combustion engine) l UNHEATED: –the working substance receives heat in some device that is separate from the engine (ex: steam turbines)
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CLOSEDUNHEATED
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OPENHEATED
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FLOW PROCESSES l NON-FLOW – One in which the working fluid does not flow into or out of its container in the course of the process (ex: air compressors, internal combustion engines) l STEADY FLOW – One in which a working substance flows steadily and uniformly (ex: boilers, turbines, condensers)
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USING THE G.E.E. (Non-Flow) Q 12 = (U 2 - U 1 ) + wk 12 /J where Q 12 total heat transferred (Btu) U 2, U 1 total internal energy at points 1 and 2 (Btu) wk 12 work done between states 1 and 2 (ft-lbs) Jconstant of 778 ft-lbs/Btu
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G.E.E. (Non-Flow) EXAMPLE 5 LBM of a fluid is compressed in the cylinder using 350 Btus of work. If internal energy initially was 100 Btu/lbm and 150 Btu/lbm at the end of the compression, how much heat was added/lost? Q 12 = (U 2 - U 1 ) + wk 12 /J
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STEADY FLOW SYSTEMS AND THE GENERAL ENERGY EQUATION
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ENTHALPY (H or h) l COMBINATION OF: –INTERNAL ENERGY (U) –FLOW WORK Mechanical energy necessary to maintain the steady flow of the fluid Flow work = pV/J (Btu) l H = pV/J + U
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STATE CHANGES l Isobaric –the pressure of and on the working fluid is constant l Isenthalpic –the enthalpy of the working fluid does not change (h 1 = h 2 ) l Isothermal –temperature is constant l Adiabatic –occurs in such a way that there is no transfer of heat to or from the system during the process
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THE SECOND LAW OF THERMODYNAMICS (1) All energy received as heat by a heat engine cycle cannot be converted into work (This means that no cycle can have a thermal efficiency of 100%)
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THE SECOND LAW OF THERMODYNAMICS (2) The transformation of heat to work is dependent on a temperature difference and on the flow of heat from a high temperature reservoir to a low temperature reservoir. (In other word heat must flow from hot to cold)
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THE SECOND LAW OF THERMODYNAMICS (3) It is impossible to construct an engine that, operating in a cycle, will produce no effect other than the transfer of heat from a low temperature reservoir to a high temperature reservoir
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THE SECOND LAW OF THERMODYNAMICS “FRICTION HAPPENS”
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ENTROPY l A THEORETICAL MEASURE OF ENERGY THAT CANNOT BE TRANSFORMED INTO MECHANICAL WORK IN A THERMODYNAMIC SYSTEM. l The total amount of entropy in a system always goes up. l No thermodynamic process can occur without some losses. l First Law: “You can’t win” l Second Law: “You can’t even break even”
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ENTROPY (cont) l A MEASURE OF DISORDER –always growing in our universe l “THE END OF THE UNIVERSE IS UPON US” l CRUCIAL PART OF REAL THERMODYNAMIC EQUATIONS
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CARNOT CYCLE TEMP ENTROPY
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CARNOT EFFICIENCY
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