Downward flame spread, calculation

The emission coefficient was taken to be a constant value close to unity. The configuration factor, F, was calculated in a conventional way, treating the center of each strip as a point. Once the downward flame spread started the radiation from the wall flames and the pyrolysing lining material behind the flames was added to the smoke layer radiation. The heat flux to the walls was then calculated from the expression... 

Figure 6 d) shows the experimental and calculated downward flame spread for material no. 3, full scale test, scenario B. 

Figure 6. Comparison of experimental and calculated data (wall surface temperatures and downward flame spread) in two different experimental setups. Continued on next page.

The test data are used to calculate a flame spread index (h) expressed as <2 x Fg. The Is values represent downward fire propagation and heat release rate characteristics of polymers and decrease with increase in the resistance to fire propagation and heat release rate. Table 11.20 lists h values of some selected polymers as illustrations, where data are taken from Refs. [43] and [68]. The F values vary from a high of 2220 to 0, suggesting large variations in the flame spread behavior of polymers. 

In this small-scale test method, 460-mm (18-in.) x 150-mm (6-in.) wide and up to 25-mm (1-in.) thick vertical sample is used. The sample is exposed to a temperature of 670 + 4 °C at the top from a 300-mm (18-in.) x 300-mm (12-in.) inclined radiant heater with top of the heater closest to and the bottom farthest away from the sample surface. The sample is ignited at the top and flame spreads in the downward direction. In the test, measurements are made for the arrival time of flame at each of the 75-mm (3-in.) marks on the sample holder and the maximum temperature rise of the stack thermocouples. The test is completed when the flame reaches the full length of the sample or after an exposure time of 15-min, whichever occurs earlier, provided the maximum temperature of the stack thermocouples is reached. Flame spread index (7s) is calculated from the measured data, defined as the product of flame spread factor, F, and the heat evolution factor, Q. 

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