Modes of Heat and Mass Transfer

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Modes of Heat and Mass Transfer

• P M V Subbarao
• Associate Professor
• Mechanical Engineering Department
• IIT Delhi

Just as Intelligent as an Human Being .. ..
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Modes of Heat Mass Transfer

• Conduction or Diffusion
• Convection
• Radiation

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Circumstances Leading to Heat Conduction
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The Chip Carrier

• The chip is housed in a chip carrier or substrate
  made of ceramic, plastic, or glass in order to
  protect its delicate circuitry from detrimental
  effects of the environment.
• The basic part
• The chip
• Bond
• Lead frame
• Bond Wires
• Lid
• Pins
• The design of the chip carrier is the first level
  of thermal control of electronic devices.

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Heat Generation Conduction through A Basic Chip

• The circuitry of an electronic component through
  which electrons flow and thus heat is generated
  is referred as the junction
• Junctions are the sites of heat generation and
  thus the hottest spots in a component.
• Modern chips can with stand a Maximum Junctions
  temperature of
• 150 oC.
• Lower Junction temperatures are desirable for
  extended life and lower Maintenance costs.
• The basic thermal issue is Removal Heat
  generated in 1 micron thick chip made of silicon
  wafer.

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Summary of Fouriers Law of Conduction
Global heat transfer rate due to conduction
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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
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Further Physical Description

• Will k be same in all directions?
• Why k cannot be different each direction?
• Why k cannot be a vector?

Will mathematics approve this ?
What is the most general acceptable behavior of
k, approved by both physics and mathematics?
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Most General form of Fourier Law of Conduction
We are at cross roads !!!!!
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Physical-mathematical Feasible Model

• Taking both physics and mathematics into
  consideration, the most feasible model for
  Fouriers Law of conduction is

Thermal conductivity of a general material is a
tensor.
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Surprising Inventions !!!
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Fire Resistant Wood

• Among the assessment properties of wood composite
  of structural members in building construction,
  fire performance is important and getting more
  attention nowadays.
• A new composite called molded carbon phenolic
  spheres (CPS), a mixture of sugi wood charcoal
  powders and phenol formaldehyde resin molded with
  a hot press is developed by a research group in
  Japan.
• The heat due to a fire accident should be thrown
  out fast outside the building.

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Spread of Fire in A Room
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Micro-structure of CPS
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Thermal Performance
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Another Physical Phenomena analogous to Heat
Conduction

• Diffusive Mass Transfer

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Physics of Mass Transfer

• During a mass transfer process individual
  chemical species travels from high concentration
  region to low concentration region.
• In this regard mass transfer is analogous to
  heat transfer.
• Conduction (Diffusion) Mass Transfer is governed
  by Ficks Law of diffusion.
• Fick's laws of diffusion were derived by Adolf
  Fick in the year 1855.

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Fick's law of diffusion
where C is the concentration, Jx is the rate of
mass diffusion, and D is the diffusion
coefficient.
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In two or more dimensions
Equations based on Fick's law have been commonly
used to model transport processes in foods,
neurons, biopolymers, pharmaceuticals, porous
soils, population dynamics, semiconductor doping
process, etc. A large amount of experimental
research in polymer science and food science has
shown that a more general approach is required to
describe transport of components in materials
undergoing glass transition.
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Heat Convection

• Convection uses the motion of fluids to transfer
  heat.
• In a typical convective heat transfer, a hot
  surface heats the surrounding fluid, which is
  then carried away by fluid movement such as wind.
  
• The warm fluid is replaced by cooler fluid, which
  can draw more heat away from the surface.
• Since the heated fluid is constantly replaced by
  cooler fluid, the rate of heat transfer is
  enhanced.

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Natural Convection

• Natural convection (or free convection) refers to
  a case where the fluid movement is created by the
  warm fluid itself.
• The density of fluid decrease as it is heated
  thus, hot fluids are lighter than cool fluids.
• Warm fluid surrounding a hot object rises, and is
  replaced by cooler fluid.
• The result is a circulation of air above the warm
  surface

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Forced Convection

• Forced convection uses external means of
  producing fluid movement.
• Forced convection is what makes a windy, winter
  day feel much colder than a calm day with same
  temperature.
• The heat loss from your body is increased due to
  the constant replenishment of cold air by the
  wind.
• Natural wind and fans are the two most common
  sources of forced convection.