Thermodynamics II: 1st Law of Thermodynamics

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Thermodynamics II 1st Law of Thermodynamics
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Objectives

• Comprehend the principles of operation of various
  heat exchangers
• Understand boundary layers
• Comprehend the First Law of Thermo
• Comprehend the basic principles of open/closed
  thermo systems
• Comprehend thermo processes

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Heat Exchangers

• Defn device used to transfer thermal energy
  from one substance to another
• Direction of Flow
• -gt Parallel not used by Navy
• -gt Counter more efficient used by Navy
• -gt Cross used extensively
• Number of passes (single or multiple)

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Heat Exchangers

• Type of Contact
• Direct mixing of substances pour hot into cold
• Indirect/surface no direct contact some thin
  barrier used
• Phases of Working Substance
• liquid-liquid PLO cooler
• liquid-vapor condenser
• vapor-vapor radiator in home steam-heat

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Heat Exchangers

• Boundary layer/film w/in pipes or channels of
  fluid flow, the fluid adjacent to the wall is
  stagnant
• -gt local temp increases
• -gt DT metal decreases
• -gt amount of heat transfer decreases
• -gt reduced efficiency possible damage
• Try to minimize film by adjusting flow or
  increasing turbulence

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Heat Exchangers

• Should be made of materials that readily conduct
  heat have minimal corrosion
• Maximize surface area for heat transfer
• Minimize scale, soot, dirt, fouling -gt reduces
  heat transfer, efficiency, causes damage

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First Law of Thermodynamics
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First Law of Thermodynamics
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First Law of Thermodynamics

• Principle of Conservation of Energy
• energy can neither be created nor destroyed, only
  transformed (generic)
• energy may be transformed from one form to
  another, but the total energy of any body or
  system of bodies is a quantity that can be
  neither increased nor diminished (thermo)

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First Law of Thermodynamics

• General Energy Equation
• Energy In Energy Out, OR
• U2 - U1 Q - W (or u2 - u1 q - w)
• Where
• U1 internal energy of system _at_ start
• U2 internal energy of system _at_ end
• Q net thermal energy flowing into system during
  process
• W net work done by the system

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Thermodynamic System

• Defn a bounded region that contains matter
  (which may be in gas, liquid, or solid phase)
• Requires a working substance to receive, store,
  transport, or deliver energy
• May be open (mass can flow in/out) or closed (no
  flow of mass out of boundaries)

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Thermodynamic Processes

• Defn any physical occurrence during which an
  effect is produced by the transformation or
  redistribution of energy
• Describes what happens within a system
• Two classifications non-flow steady flow

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Non-Flow Process

• Process in which the working fluid does not flow
  into or out of its container in the course of the
  process (closed system)
• Energy In Energy Out
• Q - W U2 - U1
• Example Piston being compressed

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Steady Flow Process

• Process in which the working substance flows
  steadily and uniformly through some device (i.e.,
  a turbine) (open system)
• Assumptions (at any cross section)
• Properties of fluid remain constant
• Average velocity of fluid remains constant
• System is always filled so volin volout
• Net rate of heat xfer work performed is constant

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Processes - Flow Work

• Defn mechanical energy necessary to maintain
  the flow of fluid in a system
• Although some energy has been expended to create
  this form of energy, it still represents a stored
  (kinetic) energy which can be used
• Flow work pressure x volume (PV)

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Processes - Enthalpy

• Enthalpy the total energy of the fluid due to
  both internal energy flow energies
• Represents the heat content or total heat
• Enthalpy (H)
• H U PV (in ft-lb, BTU, or Joules)
• h u Pv (divide by lbm)

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Questions?