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 pi June 1, 2004 09:39

Hi there,

I'd like to know what means the variable IRRADIAT in radiation using Monte Carlo modelling.

 Juan Carlos June 1, 2004 12:22

Hello Pi,

IRRADIAT is the internal name for the Wall Irradiation Flux in the CFX-5.x solver. I guess you are using CFX-5.6, so please discard the variable altogether since the contents may be questionable depending of your setup.

In CFX-5.7, this variable is output using its external name (Wall Irradiation Flux) and its content is correct.

Wall Irradiation Flux is the radiative energy arriving towards the wall from everywhere else. It differs from the Wall Radiative Heat Flux by the Radiative Emission, that is Wall Radiative Heat Flux = Wall Radiative Emission (not available in Post) - Wall Irradiation Flux. From a theoretical point of view, it is the integral over 2*pi of the incoming Radiation Intensity times (normal vector dotted incoming direction). Please see Siegel and Howell, or Modest book for a more detail explanation.

Hope this helps, Juan Carlos

 pi June 2, 2004 09:34

All clear JC. Just curious, why is neccesary to define the several bands in multiband option when you'll do anyway to set the trasmisivity, emmisivity, etc..as CEL functions (otherwise this values would be constant along all the spectrum and multiband would be no use)?

thanks

pi

 Juan Carlos June 2, 2004 13:56

Dear Pi,

The multiband spectral model solves/ray track for radiation within each band. The spectrum must be discretized (spectral mesh) ahead of time.

The CEL expressions you are referring to only describe the functional form of the quantity, but it does not describe the finite width of the band. The expressions for Emissivity,Absorption Coefficient,Scattering Coefficient, Refractive Index and Radiation Sources are evaluated at the midpoint of the band (uniform value within each band). The use of CEL expressions will allow for future development of spectral sub-models within a band.

If your radiation quantities have important spectral variations on different parts of the spectrum, your spectral discretizations must reflect these as well. That is, the minimum number of spectral bands must be able to capture those variations (keeping in mind the midpoint as described above). You could relate it to a trapezoidal/Simpson integration rule. Please see the Radiation Theory/Spectral Models documentation p200(5.6) or p222(5.7)

If all your input radiation quantities are spectrally uniform, then there is no point to use multiband except to burn some floating point operations.

All the best, Juan Carlos

 pi June 7, 2004 08:14

Hi Juan Carlos, my problem is a pretty simple box with a window which receives solar radiation at a given angle, transmites most of it, absorbing a small part (in the solar band) and reemitting it in the thermal band. As you probably know the solar band covers a very small region between 0.2 and 2.5 microns wavelength aproximately. I am considering a two band model so the first one goes from 0 to 2.5 and the second one from there up to 1000 microns, corresponding the later to thermal radiation. In the beginning of the out file, right before to start to compute, a message "first band contribution is negligible" shows. Do I have to worry by that? Am I doing anything wrong? Besides that I think that most of the radiation inside the box will come from the part of the floor "bathed" by the sun, first transmitted through the glass and then absorbed by that area of the floor in the solar wavelenghts and then reradiated as heat. If I´m right then a higher temperature should be visible there...or not?

thanks

pi

 Juan Carlos June 7, 2004 12:04