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   Radiation from an incremental area
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   Form:
   q = RadDa( T, epsilon, da )
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   ------
   Inputs
   ------
   T              (1,:) Temperature (deg-K)
   epsilon        (1,1) Emissivity
   da             (1,1) incremental area (m^2)

   -------
   Outputs
   -------
   q              (1,:) Heat flux (watts)

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