Thermodynamics III: 2nd Law

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Thermodynamics III 2nd Law Cycles

• It just dont get no better than this

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Objectives

• Understand types of state changes
• Comprehend thermodynamic cycles
• Comprehend the 2nd Law of Thermodynamics to
  include entropy, reversibility, the Carnot
  cycle
• Determine levels of output and efficiency in
  theoretical situations

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State Changes

• In addition to using flow/no-flow classifications
  for thermo processes, it is helpful to look at
  what happens to a medium also
• Isobaric pressure remains constant throughout
  process (some pistons)
• q12 h2 - h1

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State Changes

• Isometric volume remains constant during entire
  process
• q12 u2 - u1
• Adiabatic no transfer of heat to or from medium
  during process -gt usually in a rapid process
• w u2 - u1

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Thermodynamic Cycles

• Defn a recurring series of thermodynamic
  processes through which an effect is produced by
  transformation or redistribution of energy
• One classification
• Open working fluid taken in, used, discarded
• Closed working medium never leaves cycle, except
  through leakage medium undergoes state changes
  returns to original state

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Five Basic Elements of all Cycles

• Working substance transports energy within
  system
• Heat source supplies heat to the working medium
• Engine device that converts the thermal energy
  of the medium into work
• Heated heat added in engine itself
• Unheated heat received in some device separate
  from engine

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Five Basic Elements of all Cycles

• Heat sink/receiver absorbs heat from the working
  medium
• Pump moves the working medium from the
  low-pressure side to the high-pressure side of
  the cycle
• Examples
• Closed, unheated engine steam cycle
• Open, heated engine gasoline engine

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Basic Thermodynamic Cycle
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Second Law of Thermodynamics

• Reversibility
• the characteristic of a process which would allow
  a process to occur in the precise reverse order,
  so that the system would be returned from its
  final condition to its initial condition, AND
• all energy that was transformed or redistributed
  during the process would be returned from its
  final to original form

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Second Law of Thermodynamics

• Defn 1 (Clausius statement) no process is
  possible where the sole result is the removal of
  heat from a low-temp reservoir and the absorption
  of an equal amount of heat by a high temp
  reservoir
• Defn 2 (Kelvin-Planck) no process is possible
  in which heat is removed from a single reservoir
  w/ equiv amount of work produced

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Second Law of Thermodynamics

• Overall NO thermodynamic cycle can have a
  thermal efficiency of 100 (i.e., cannot convert
  all heat into work)
• Quick review
• 1st Law Conservation/transformation of energy
• 2nd Law Limits the direction of processes
  extent of heat-to-work conversions

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Entropy

• Defn theoretical measure of thermal energy that
  cannot be transformed into mech. Work in a
  thermodynamic system
• It is an index of the unavailability of energy or
  the reversibility of a process
• In all real processes, entropy never decreases -gt
  entropy of universe is always rising

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Carnot Cycle

• Second Law states that no thermo system can be
  100 efficient, and no real thermal process is
  completely reversible
• A French engineer, Carnot, set out to determine
  what the max efficiency of a cycle would be if
  that cycle were ideal and completely reversible

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Carnot Cycle

• All heat is supplied at a single high temp and
  all heat is rejected at a single low temp
• Carnot used a simple cycle

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Carnot Cycle
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Carnot Cycle

• Carnot Principle the max thermal efficiency
  depends only on the difference between the source
  and sink temps
• Does not depend on property of fluid, type of
  engine, friction, or fuel
• Example

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Questions?