Radiation Exchange

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Radiation Exchange

• EGR 4345
• Heat Transfer

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Blackbody Radiation Exchange
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Blackbody Radiation Exchange
Rate at which radiation leaves surface i and is
intercepted by surface j
For a black surface, Ji Ebi
Net radiative exchange
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Blackbody Radiation Exchange
Therefore
Or
Net radiation transfer from any surface in an
enclosure of black surfaces
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Net Radiation Exchange at a Surface in an
Enclosure
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Net Radiation Exchange at a Surface in an
Enclosure

• Assumptions
• Isothermal surfaces, Ti is known
• Uniform radiosity
• Uniform irradiation
• Opaque
• Diffuse
• Gray surface
• Medium in enclosure is nonparticipating

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Net Radiation Exchange at a Surface in an
Enclosure
Determine the net rate at which radiation leaves
surface i
Where
In terms of emissive power and absorbed radiation
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Net Radiation Exchange at a Surface in an
Enclosure
For an opaque, diffuse, gray surface
Solve for Gi and substitute into qi
Or
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Net Radiation Exchange Between Surfaces in an
Enclosure
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Net Radiation Exchange Between Surfaces in an
Enclosure
Total rate at which radiation reaches surface i
from all surfaces, including i
Reciprocity
Substituting in for Gi
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Net Radiation Exchange Between Surfaces in an
Enclosure
Summation rule
Therefore
Or
Space or Geometric Resistance
Use matrix operations to solve
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Net Radiation Exchange in a Two-Surface Enclosure
Two diffuse, gray surfaces that form an enclosure
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Net Radiation Exchange in a Two-Surface Enclosure
Special equations
General Equation
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Radiation Shields
General Equation
Low emissivity increase resistance
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Reradiating Surface
Zero net radiation, qi 0
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Reradiating Surface