Fire Simulation for Engineers/FDS/Radiation

Radiation transport, RADI

For most FDS simulations, thermal radiation transport is computed by default and you need not set any parameters to make this happen. However, there are situations where it is important to be aware of issues related to the radiative transport solver.

The most important issue involves the fraction of energy released from the fire as thermal radiation, commonly referred to as the radiative fraction. It is a function of both the flame temperature (T⁴ dependence) and chemical composition, neither of which are reliably calculated in a large scale fire calculation. In fact, because of the size of the mesh cells, the flame sheet is not well-resolved.

To compensate the underestimation of the fire radiation, the RADIATIVE_FRACTION is not calculated and is set to 35% by default: every mesh cell cut by the flame radiates that fraction of the chemical energy being released into it. Some of that energy may be reabsorbed elsewhere, yielding a net radiative loss that is less than RADIATIVE_FRACTION, depending mainly on the size of the fire and the soot loading.

For example:

 &RADI RADIATIVE_FRACTION=0.45 /

sets the fraction of energy released from the fire as thermal radiation to 45%. The following table summarizes some RADI parameters:

Parameter	Type	Description	Unit	Default
NUMBER_RADIATION_ANGLES	Integer	Number of solid angles		104
RADIATIVE_FRACTION	Real	Radiative Loss Fraction		0.35