Burning rates of plastics The small B assumption and radiation effects

Burning Rates of Plastics The Small B Assumption and Radiation Effects 

Recall that for these considerations the most convenient expression for B is 

Equation (6.179) shows why good straight-line correlations are obtained when Gf is plotted as a function of mo0o for burning rate experiments in which the dynamics of the air are constant or well controlled (i.e 6 is known or constant). One should realize that 

The consequence of this small B assumption may not be immediately apparent. One may obtain a physical interpretation by again writing the mass burning rate expression for the two assumptions made (i.e., B 1 and B = [im , H]/Lw) 

If flame radiation occurs in the mass burning process—or any other radiation is imposed, as is frequently the case in plastic flammability tests—one can obtain a convenient expression for the mass burning rate provided one assumes that only the gasifying surface, and none of the gases between the radiation source and the surface, absorbs radiation. In this case Fineman [32] showed that the stagnant film expression for the burning rate can be approximated by 

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