Heat Transfer : An Action due to Thermal Inequilibrium

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Heat Transfer An Action due to Thermal
Inequilibrium

• P M V Subbarao
• Professor
• Mechanical Engineering Department

A Natural Happening ..
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A Wake-up Call to Earth
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A Natural Engineering Process for the Existence,
Growth and Performance.

• A True Design Reason behind Existence of Natural
  Systems..
• A Strong Design Modification for the Performance
  of Artificial Systems.

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A True Design Reason behind Geometry of Natural
Systems..
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An Evolution to Perfection of Heat Transfer is
Sustainable
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A Reason behind Survival of Electonics
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What is Heat Transfer?

• Thermal energy is related to the temperature of
  matter.
• For a given material and mass, the higher the
  temperature, the greater its thermal energy.
• Heat transfer is a study of the exchange of
  thermal energy through a body or between bodies
  which occurs when there is a temperature
  difference.
• When system and its surroundings are at different
  temperatures, thermal energy transfers from the
  one with higher temperature to the one with lower
  temperature.
• Thermal Energy always travels from hot to cold.
• This spontaneous act is called Heat Action or
  Heat Transfer.

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Heat Transfer Between System Surroundings

• Heat transfer is typically given the symbol Q,
  and is expressed in joules (J) in SI units.
• The rate of heat transfer is measured in watts
  (W), equal to joules per second, and is denoted
  by Q.
• The heat flux, or the rate of heat transfer per
  unit area, is measured in watts per area (W/m2),
  and uses q" for the symbol.

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Heat Transfer in Manufacturing

• The need/role of heat transfer is a two fold
  issue.
• Heat Transfer by design.
• Casting
• Hot working
• Sintering
• Heat Transfer due to unavoidable conditions.
• Machining process.

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What Causes Heat Transfer in Machining?

• First, almost all (90-100) of the work consumed
  in a machining operation finally convert into the
  thermal energy.
• There are several sources of thermal energy
  in cutting with a sharp tool
• Viscous dissipation transformed into heat if the
  cut material are viscoplastic.
• Work done by friction converted to heat.
• Ambient heat source sometimes need be considered
  if thermal deformation is concerned.
• In non-traditional machining, other types of heat
  sources exist

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Modes of Heat Mass Transfer

• Conduction or Diffusion
• Radiation
• Convection

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Conduction Heat Transfer

• Conduction is a significant mode of transfer when
  system and surroundings consist of solids or
  static fluids.
• Two mechanisms explain how heat is transferred by
  conduction lattice vibration and particle
  collision.
• Conduction through solids occurs by a combination
  of the two mechanisms heat is conducted through
  stationery fluids primarily by molecular
  collisions.

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Fourier law of heat conduction
A Constitutive Relation

• The rate of heat transfer through the wall
  increases when
• The temperatures difference between the left and
  right surfaces increase,
• The wall surface area increases,
• The wall thickness reduces,
• The wall is changed from brick to aluminum.
• If we measure temperatures of the wall from left
  to right and plot the temperature variation with
  the wall thickness, we get

This is called as Fourier Law of Conduction
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Circumstances Leading to Heat Conduction
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Circumstances Leading to Heat Conduction in
Machinging
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Thermal Image of Laptop Casing
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Graphite Covering
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Thermal Image of Laptop Casing with Graphite cover
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Statement of Fouriers Law
The (mod of) heat flux, q, (the flow of heat
per unit area and per unit time), at a point in
a medium is directly proportional to the
temperature gradient at the point.
Temperature gradient across the slab of thickness
Dx
T
The heat flux across the slab
x
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Local Heat flux in a slab
Global heat transfer rate
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Mathematical Description

• Temperature is a scalar quantity.
• Heat flux is defined with direction and Magnitude
  A Vector.
• Mathematically it is possible to have

Using the principles of vector calculus