Heat Transfer Overview

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Heat Transfer Overview
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How Do We Define Heat Transfer?

• Energy in transit due to a temperature difference
• Electrical analogy
• Concerned with time rate of change
• In this well calculate things such as
• What rate of heat transfer will occur in a
  certain situation?
• What temperatures can we expect?
• How long will it take to heat/cool something?
• What are some specific applications?

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Examples of Where You May Use Heat Transfer in
Industry

• Estimating the temperature of components on a
  circuit board to determine if failure rates are
  acceptable and then designing the cooling system
• Choosing a heat sink and fan that will cool a
  transistor appropriately
• Determining the orientation and size of solar
  panels needed to power a micro-satellite
• Designing the cooling system for an ice-hockey
  table
• Choosing the type and size of heat exchanger to
  be used to heat a chemical used in semi-conductor
  manufacturing

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More Examples

• Choosing the type and size of radiator needed for
  a new type of automobile
• Calculating the optimal amount of insulation to
  be placed on a steam pipe to reduce heating costs
• Determining how much power must be applied to an
  antennae on an airplane to keep water from
  freezing on it
• Designing a quick-freezing system for a meat
  distributor to seal in flavor and juices
• Calculating heat losses and gains from a building
  to size their heating and air conditioning systems

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Types of Heat Transfer

• Conduction due to interaction between particles
  usually through a solid
• Convection gas to solid (or vice versa)
• Includes both conduction between particles and
  advection (heat transfer due to mixing)
• Radiation radiated energy (no medium needed)

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Why not just use computer programs? Why take a
class?

• How will you set up the model? What boundary and
  initial conditions?
• Are the simplifications chosen OK?
• 3-D convection at moderate/high velocities is
  impossible to solve without significant
  simplifications
• How will you interpret the results? Are the
  results accurate?

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Conduction

• Energy transfer from more to less energetic
  particles due to particle interactions
  diffusion of energy due to molecular activity
• Examples
• Usually involves solids
• Rate f(
  )
• What is the driving potential?

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Fouriers Law of Heat Conduction
kthermal conductivity (W/mC or Btu/h ft F) --
a measure of how fast heat flows through a
material -- k(T), but we usually use the value
at the average temperature q can have x, y,
and z components its a vector quantity
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Special Case

• If T(x) is linear, Fouriers Law for the 1-D case
  becomes
• When will this happen?
• Example

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Conduction Definitions

• Heat capacity rcp (J/m3C)
• Amount of heat needed to raise a unit volume of
  material one degree
• Thermal diffusivity a k/rcp (m2/s)
• How fast heat diffuses through a material

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Convection

• Energy transfer due to both
• molecular motion (diffusion, like conduction) and
• bulk motion of fluid (motion of gas or liquid)
• Advection
• Convectiondiffusionadvection
• Three kinds
• Forced convection external fluid motion
• Natural (free) convection motion due to
  buoyancy effects
• Latent heat exchange due to phase change
  condensation, boiling (covered in ME 211 but not
  ME 114)

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Newtons Law of Cooling

• hheat transfer coefficient (W/m2C)
• Tssolid surface temperature
• T temperature of fluid far from surface
• hf(
  )

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Boundary Layer
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Example

• A 0.4 cm x 2 cm computer chip must dissipate 5 W
  of heat. Air with a heat transfer coefficient of
  80 W/m2K and a temperature of 20C blows over
  the chip. The chip is in danger of overheating if
  it reaches 90C. Is the chip in danger? Should
  you attach a heat sink?

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Thermal Radiation

• Emitted by all matter above 0 Kelvin
• Due to changes in electron configurations
• Requires no medium
• Emissive power of a blackbody (ideal radiator)
• Tssurface temp in Kelvin
• sStefan-Boltzmann Constant 5.67x10-8 W/m2K4

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Thermal Radiation, Cont.

• eemissivity how efficiently a surface emits
  compared to a blackbody
• aabsorptivity percent of incident flux absorbed
• e,a f(temp, wavelength, surface condition)

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Thermal Radiation, cont.

• Special case ea if surface temperatures of all
  surfaces in an enclosure are close
• Special case surface completely surrounded by
  another isothermal surface, no intervening medium

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Total Heat Transfer

• Only conduction, convection, or radiation can
  occur or else a combination can occur
  simultaneously
• QconvQrad or QcondQrad