Flame spread downward

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... 

In scenario B, however, both the horizontal concurrent flame spread and the downward flame spread, in and below the hot gas layer, are directly linked to the rate of heat release. 

Downward flame spread for scenario B. Once the horizontal, concurrent flame spread along the wall ceiling intersection has reached an opposite corner in the compartment the downward flame spread in the upper layer starts. In reality, this could possibly start happening during the concurrent flame spread time interval. In the current version of the model, no account is taken of the relatively low oxygen concentration in the upper layer. The flame spread is quite slow at first since the wall material has a relatively low sur-... 

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.

Physically <5f is the flame extension over the new material to ignite. This is illustrated for a match flame, spreading upward or downward in Figure 8.5 or as suggested in Figure 8.1. We shall see that the visible flame is a measure for <5f because at the flame tip in a turbulent flame the temperature can drop from 800 °C in the continuous luminous zone to... 

ASTM D 3675 Surface Flammability of Flexible Cellular Materials Using a Radiant Heat Energy Source This method may be used on cellular elastomeric materials such as flexible polyurethane foam and neoprene foam. It employs a radiant panel heat source consisting of a 300 by 460-nun (12 by 18 in.) panel in front of which an inclined 150 by 460-m (6 by 18 in.) specimen of the material is placed. The orientation of the specimen is such that ignition is forced near its upper edge, and the flame front progresses downward. Factors derived from the rate of progress of the flame front and heat liberated by the material under test are combined to provide a flame spread index. The method was developed to test cellular elastomeric materials which could not be tested by ASTM E 162. 

The flame spread rate depends on the orientation of the specimen which can be horizontal, vertical or at some angle to the horizontal direction. A vertically mounted specimen can burn upward or downward, so that two categories of results can be obtained. When the edges of the specimen are exposed, the combustion is more rapid than that of a specimen with inhibited edges (i.e. the edges are covered by a noncombustible material). 

According to the theoretical considerations of Funt and Magill, the flame spread rate of a vertical specimen with inhibited edges in downward burning is... 

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. 

Three test apparatuses and methods have been developed to determine the fire properties associated with flame propagation (1) the ASTM D 2863 oxygen index test method for downward flame propagation for small samples [14] (2) the ASTM E 1321-90 lateral ignition and flame spread (LIFT) test method for horizontal and lateral flame propagation... 

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. 

Fernandez-Pello, A.C., 1977. Downward Flame Spread Under the Influence of Externally Applied Thermal Radiation. Combustion Science and Technology, 17(1-2) pp. 1-9. 

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