Difference between ABSORPTION_COEFFICIENT and EMISSIVITY

[CK Lam]

Dear All,

I am trying to model a glass wall with a flame nearby, and I would
like to know the temperature change of the glass under the effect of
radiant heat flux, and the amount of flame radiation penetrating
through the glass.

Under &MATL, I have input SPECIFIC_HEAT, CONDUCTIVITY, DENSITY of the
glass. As I search in the FDS discussion forum, I also need to input
Absorption Coefficient and Emissivity.

In reference with FDS Users' Guide,

ABSORPTION_COEFFICIENT (1/m) would affect the radiation penetrate and
absorb into the solid.

EMISSIVITY is the percentage of incident heat flux to heat up the
solid (say 0.9 as default in FDS).

(1) However, I am not sure whether my understanding on the physical
meanings of Absorption Coefficient and Emissivity, and their
relationship are correct:

Considering a glass is 0.01m thick and has an absorption coefficient
of 20m-1, and it is exposed to 10kW/m2 radiant heat flux, then there
would be 10kW/m2 x exp(- 0.01 x 20) = 8.187kW/m2 radiant heat flux
penetrating through the glass and 1.812kW/m2 radiant heat flux will be
absorbed by the glass.

If the emissivity of the glass is 0.9, then 90% of the total absorbed
radiant heat flux 1.812kW/m2 (i.e. 1.631kW/m2) will heat up the glass,
and 0.181kW/m2 heat flux will be radiating out from the glass.

I am not sure whether my interpretation is correct. Please correct me
if I am wrong.

(2) is there any MATL property I need to set in the FDS model?

Really thanks for your help.

Best Regards,
Lam

[dr_jfloyd]

Emissivity defines how much of the incident radiation is either
absorbed or reflected. The absorption coefficient for the material
defines how deep into the surface the absorption takes place.

[Jens]

You have to be carefull with emissivity for glass - the emissivity is
dependent of the wavelengt of the radiation - i.e. a lot of glass
stops infrared light but not light in the visible spectre.

Try reading more about emissivity here http://en.wikipedia.org/wiki/Emissivity

I think you problem sounds more like you need to make some
meassurements with some devices like: "incident heat flux" and
"radiative heat flux gas", then you will be able to do some post
processing of these results. That is how I would do it - and then make
a matlab(octave) script to do the post processing.

Kind regards Jens