RAD2'ppt Heat Transfer by Both Radiation

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RAD2.pptHeat Transfer by Both Radiation
Convection
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Components of Course What Stage are We Up To?

• Types of exchangers, revision of OHTCs, fouling
  factors.
• Heat exchanger selection.
• Thermal performance analysis (NTUs) for co-
  counter-current exchangers.
• Multi-pass exchangers (ST).
• Condensation boiling.
• Radiation.

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Applications

• Temperature measurement of a hot fluid when walls
  are at a different temperature to gas
• Heat losses from hot surfaces into air or other
  fluids at high temperatures (gt200oC) when
  radiation also becomes significant

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Application

• If hot gas is at a temperature of 500 K and the
  walls of a pipe are at 300 K, then what will be
  the temperature measured by a thermocouple or
  other temperature measurement device which is in
  the pipe?

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Need to Know...

• Emissivity of thermocouple 0.1
• Convective heat-transfer coefficient 50 W
  m-2 K-1

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What Will Happen?

• Heat will flow from the gas to the thermocouple
  by convection, but
• Heat will flow from the thermocouple to the walls
  by radiation

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What Do We Want to Happen?

• Thermocouple should read gas temperature,
  uninfluenced by walls
• Only perfectly true if
• walls at gas temperature
• infinite (very high) convective heat-transfer
  coefficient
• zero (very low) emissivity

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Heat Transfer by Radiation
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Calculations

• Eb1 emissive power of thermocouple 5.67
  x 10-8 T4
• Eb2 emissive power of walls 5.67 x 10-8 3004
  459 W m-2

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Nett Radiative Heat Flux From Thermocouple to
Walls
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Heat Transfer by Convection
Nett convective heat transfer flowrate between
two temperatures T1 and T2 h
A (T1 - T2) Nett convective heat transfer flux
h (T1 - T2)
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Nett Convective Heat Flux From Gas to Thermocouple

• 50 (500 - T)

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At Steady State

• Nett convective heat flux from gas to
  thermocouple
• Nett radiative heat flux from thermocouple to
  walls

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Solution

• Solve (Excel) to get T 494 K (cf. gas
  temperature 500 K)
• Little error here because of relatively low
  temperatures (around 200oC)

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Conclusions

• Can combine any / all mechanisms of heat transfer
• Applications in measurements of high temperatures