Docstoc

Radiation Exchange

Document Sample
Radiation Exchange Powered By Docstoc
					Radiation Exchange



               EGR 4345
               Heat Transfer
Blackbody Radiation Exchange
 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
Blackbody Radiation Exchange
 Therefore

  Or


  Net radiation transfer from any surface in an
        enclosure of black surfaces
Net Radiation Exchange at a
Surface in an Enclosure
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
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
Net Radiation Exchange
at a Surface in an Enclosure

For an opaque, diffuse, gray surface



Solve for Gi and substitute into qi




 Or                                    Driving Potential

                                        Surface Radiative
                                        Resistance
Net Radiation Exchange
Between Surfaces in an Enclosure
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
Net Radiation Exchange
Between Surfaces in an Enclosure

Summation rule



 Therefore


Or
                                 Space or
                                 Geometric
Use matrix operations to solve   Resistance
Net Radiation Exchange
in a Two-Surface Enclosure

Two diffuse, gray surfaces
that form an enclosure
Net Radiation Exchange
in a Two-Surface Enclosure

      Special equations




General Equation
Radiation Shields

                    General Equation




                         Low emissivity
                         increase resistance
Reradiating Surface
Zero net radiation, qi = 0
Reradiating Surface

				
DOCUMENT INFO
Shared By:
Categories:
Tags:
Stats:
views:3
posted:3/7/2012
language:
pages:16