The visibility range when there is smoke (visibility) is related to the light extinction coefficient as follows:
{width=20%}
where:
-
C - dimensionless constant characterizing an object viewed through smoke (C = 8 for light-emitting objects and C = 3 for light-reflecting objects). By default, FDS takes C = 3. The methodology for calculating the estimated fire risk values [1] recommends using the value C = 2.38. In the FDS input file, the value of C is set by the VISIBILITY_FACTOR parameter in the MISC parameter group;
-
К - light extinction coefficient (in [1] the term “optical smoke density” is used).
With values of K close to zero (which occurs at the beginning of fire development, when there is still no smoke), the visibility range S, in accordance with formula (1), tends to infinity. FDS cannot operate with infinitely large values, so the concept of “maximum visibility” is introduced (MAXIMUM_VISIBILITY parameter in the MISC parameter group). Accordingly, if the value obtained from formula (1) exceeds the maximum visibility, then the visibility is considered to be equal to the maximum visibility. By default, FDS sets maximum visibility at 30 meters.
In fact, formula (1) can be extended:
{width=40%}
To measure visibility at a specific point, you need to create a parameter group of the following form in the input file:
&DEVC ID='5' FYI='Dver 1_1' QUANTITY='VISIBILITY' XYZ=5.025,4,1.7/
The QUANTITY parameter defines the physical quantity to be measured. For visibility, the parameter value must be ‘VISIBILITY’.
To measure the light extinction coefficient, the QUANTITY parameter must be ‘EXTINCTION COEFFICIENT’:
&DEVC ID='5' FYI='Dver 1_1' QUANTITY=’EXTINCTION COEFFICIENT’ XYZ=5.025,4,1.7/
The picture below shows typical graphs of visibility change at various MAXIMUM_VISIBILITY parameter values (15, 30, and 45 meters).
It can be seen that the graphs completely coincide when the visibility is less (или below&&&) than MAXIMUM_VISIBILITY.
In accordance with the methodology for determining the fire risk values [1], escape route visibility blocking occurs when the visibility becomes less than 20 meters. This means that for the correct determination of the visibility blocking time, the MAXIMUM_VISIBILITY parameter value in the FDS input file must not be less than 20 meters. If the MAXIMUM_VISIBILITY is below 20 meters, it is considered that from the very beginning of the fire development, the escape routes are blocked.
The Fenix+ 3 application allows you to set an individual critical visibility value for each registering device. If a critical visibility of more than 30 meters is set for any registering device, this registering device is considered blocked from the very beginning of the fire development, provided that the MAXIMUM_VISIBILITY parameter has the default value adopted in FDS (30 meters). Therefore, when generating the FDS input file, Fenix+ 3 sets the MAXIMUM_VISIBILITY parameter value to 10 meters higher than the maximum critical visibility of the registering devices, but not less than 30 meters.
The picture below shows graphs of visibility change at various VISIBILITY_FACTOR (C) parameter values (2.38, 3, 3.78).
All these graphs are obtained from one by stretching vertically (along the Visibility axis) in proportion to the VISIBILITY_FACTOR parameter value. This means that changing the VISIBILITY_FACTOR parameter by a certain factor does not lead to a corresponding change in the time at which the visibility is the same.
For example, in the figure above, the VISIBILITY_FACTOR parameters are sequentially increased by 1.26 times (26%). However, the time when visibility drops to 20 meters changes by a much smaller factor. While at VISIBILITY_FACTOR=2.38 it is 16.2 s, at VISIBILITY_FACTOR=3 it is 16.6 s, and at 3.78 it is 17.0 s, an increase of less than 1.025 times (2.5%).
Currently, you cannot set the MAXIMUM_VISIBILITY and VISIBILITY_FACTOR parameters in Fenix+ 3 interface.