Absorption Coefficient Glass

[José Felipe Pérez Segovia]

Hello, 

I am modelling a fire enclosure where the walls are entirely made of glass.

I am bit loss/confused regarding the absorption coefficient to prescribe.

I have calculated a value of  2.23 1/m.

This comes from

Absorption coefficient=(2.303*A)/t

where t is the width of the glass assumed as 0.1m and A is the Absorbance.

Absorbance = 2 – log(%T) 

T is the Transmittance, assumed as 80% for glass.

I know that the Absorption coefficient should take into account the weighlenghtss. This calculation is only to give me an order of magnitude . Could you let me know if I am missign some fundamental aspect? 

Thank you!

[Kevin McGrattan]

I suggest the following: create a very simple test case where you have a flat hot plate and a target obstruction at some short distance away. There are some cases in the Examples folder that are documented in the FDS Verification Guide. Check that FDS is computing the correct heat flux based on the plate temperature, shape and distance. Now put your glass between the source and target. Run the case again. Adjust the ABSORPTION_COEFFICIEINT until you get the desired reduction in the heat flux to the target. 

Even if you find a value in the literature or deduce a value yourself, you will want to do this simple exercise. The reason is that FDS assumes a gray gas (i.e. wavelength independent) radiative flux, but glass absorbs certain wavelengths more than others. So you are going to calculate some effective value based on the thickness of the glass.