Engineering Lesson Guide 2

1
INTRODUCTION TO NAVAL ENGINEERING
THERMODYNAMICS I
2
INTRODUCTION TO NAVAL ENGINEERING
ENERGY AND HEAT TRANSFER
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THERMODYNAMICS

• The science concerned with the inter-relationship
  between thermal energy and mechanical energy

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WHAT ARE THE TYPES OF ENERGY?

• Stored
• Potential (based on position)
• Kinetic (based on velocity)
• Transitional
• Energy that is in the process of being
  transferred from one object or system to another.
  All energy in transition begins and ends as
  stored energy

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HOW CAN KINETIC ENERGY BE STORED ENERGY?

• Definition of energy
• the ability to produce an effect
• The Bullet example
• Has the capability for an effect
• But it needs something to hit in order to
  transfer that energy (the effect)

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MECHANICAL ENERGY

• Potential Energy (PE)
• PE mgh
• Kinetic Energy (KE)
• KE (1/2)mV2
• Mechanical Energy in TRANSITION
• Called Work
• Wk FD

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THERMAL ENERGY

• Stored Thermal Energy Called Internal
• Internal Potential Energy
• Associated with the force of attraction that
  exist between molecules.
• Internal Kinetic Energy
• Associated primarily with the activity of the
  molecules
• Thermal Energy in TRANSITION
• Called Heat
• Requires a temperature difference between two
  systems

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MEASURING THERMAL ENERGY

• In the real world we use CALORIES and JOULES
  but...
• BRITISH THERMAL UNIT
• Quantity of Heat required to raise the
  temperature of 1 lbm of water from 50.9F to 60.9F
• Please dont write that down

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MODES OF HEAT TRANSFER
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(1) CONDUCTION

• Heat flows from hotter to colder region when
  there is physical contact between the regions

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CONDUCTION (cont.)

• The total quantity of heat passed
• is proportional to the cross-sectional area of
  the conductor over a given time
• is proportional to the time of heat flow
• in a given length of time is proportional to the
  thermal gradient (temp difference)
• depends on the thermal conductivity of the
  substances

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GENERAL CONDUCTION EQUATION

• Q kTA (t1-t2)/L
• where
• Q Quantity of heat (Btu or cal)
• k Coefficient of thermal conductivity
  (Btu/((hr)(F)(ft))
• T Time (hr)
• t1 Temp at hot end (F)
• t2 Temp at cold end (F)
• L Distance between the two ends (in)
• A Cross sectional area (sq ft)

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CONDUCTION EXAMPLE
LENGTH
HOT
COLD
AREA
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(2) RADIATION

• Mode of heat transfer that does not involve any
  physical contact between the emitting and
  receiving regions

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(3) CONVECTION

• The mechanical transportation of a mass of fluid
  from one place to another
• Beyond the molecular level
• Movement of fluid within fluid
• Transportation, not transfer
• Fluids thermal energy remains in stored form
  unless it is transferred by radiation or
  conduction

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CONVECTION (cont)

• TWO TYPES OF CONVECTION
• Natural
• Occurs when there are differences in the density
  of different parts of the fluid. The difference
  in density are usually caused by a temperature
  difference.
• Forced
• Occurs when some mechanical device, such as a
  pump or a fan, produces movement

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DEFINITIONS

• Sensible Heat
• When heat added results in the change in
  temperature (kinetic energy)
• Latent Heat
• When the heat added results in a physical change
  of the substance (potential energy)
• Saturation Temperature/Pressure
• Psat/Tsat
• The point at which liquid and vapor may exist in
  equilibrium contact with each other

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DEFINITIONS (cont)

• Saturated Liquid/Vapor
• A liquid/vapor at a specified pressure which is
  at Tsat for the pressure
• Wet vapor
• Subcooled Liquid
• A liquid at that specified pressure which is
  below the Tsat
• Superheated Vapor
• A vapor that has been raised above Tsat for a
  given pressure

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DEFINITIONS (cont)

• Latent Heat of Vaporization
• Amount of heat necessary to change a mass of
  liquid to vapor without changing the temperature
• Latent Heat of Fusion
• Amount of heat that must be added/removed to a
  unit mass to melt/solidify it

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GENERAL CONDUCTION EQUATION REVISITED

• Q kTA (t1-t2)/L

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TYPES OF HEAT EXCHANGERS
Parallel Heat Exchanger
Counterflow Heat Exchanger
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TYPES OF HEAT EXCHANGERS
Crossflow Heat Exchanger
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FACTORS FOR K

• THE BOUNDARY LAYER
• SCALE/CHEMICAL DEPOSITS
• SOOT/DIRT BUILDUP