FIRST LAW OF THERMODYNAMICS

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INTRODUCTION TO NAVAL ENGINEERING
THERMODYNAMICS II
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INTRODUCTION TO NAVAL ENGINEERING
FIRST AND SECOND LAWS OF THERMODYNAMICS
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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
• U2 - U1 Q - W
• where
• U1 internal energy of the system at the
  beginning
• U2 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
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ELEMENTS OF A THERMODYNAMIC CYCLE
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THERMODYNAMIC CYCLES

• CLOSED
• Working fluid never leaves the cycle, except
  through accidental leakage (ex steam cycle)
• OPEN
• Working fluid is taken in, used, then discarded
  (ex internal combustion engine)

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ENGINES

• HEATED
• heat is added to the working substance in the
  engine itself (ex internal combustion engine)
• UNHEATED
• the working substance receives heat in some
  device that is separate from the engine (ex
  steam turbines)

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CLOSED UNHEATED
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OPEN HEATED
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FLOW PROCESSES

• 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)
• 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)

• Q12 (U2 - U1) wk12/J
• where
• Q12 total heat transferred (Btu)
• U2, U1 total internal energy at points 1 and
  2 (Btu)
• wk12 work done between states 1 and 2
  (ft-lbs)
• J constant 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?
• Q12 (U2 - U1) wk12/J

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STEADY FLOW SYSTEMS AND THE GENERAL ENERGY
EQUATION
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ENTHALPY (H or h)

• COMBINATION OF
• INTERNAL ENERGY (U)
• FLOW WORK
• Mechanical energy necessary to maintain the
  steady flow of the fluid
• Flow work pV/J (Btu)
• H pV/J U

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STATE CHANGES

• Isobaric
• the pressure of and on the working fluid is
  constant
• Isenthalpic
• the enthalpy of the working fluid does not change
  (h1 h2)
• Isothermal
• temperature is constant
• 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

• A THEORETICAL MEASURE OF ENERGY THAT CANNOT BE
  TRANSFORMED INTO MECHANICAL WORK IN A
  THERMODYNAMIC SYSTEM.
• The total amount of entropy in a system always
  goes up.
• No thermodynamic process can occur without some
  losses.
• First Law You cant win
• Second Law You cant even break even

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ENTROPY (cont)

• A MEASURE OF DISORDER
• always growing in our universe
• THE END OF THE UNIVERSE IS UPON US
• CRUCIAL PART OF REAL THERMODYNAMIC EQUATIONS

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CARNOT CYCLE
TEMP
ENTROPY
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CARNOT EFFICIENCY
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Reading

• Chapter 18 Read 10-15
• Skim 16-20
• Chapter 19 Read 1-5
• Skim 6-13,15-18
• HOME WORK 1 HANDOUT