Propagation and extinction of downward burning fires

Abstract Measurements are reported on downward burning of PMMA fuel rods in an apparatus similar to that used in the Limiting Oxygen Index flammability test. As the ambient oxygen concentration is reduced the flame propagation rate decreases. The propagation rate V does not go to zero as the oxygen concentration Y ox is reduced, rather extinction occurs at a finite value of V . Although the propagation rate decreases as Y ox is reduced, the surface regression rate is not reduced. In agreement with this finding, the calculated heat flux per unit area to the pyrolysis region does not decrease as Y ox decreases. Thus, extinction is not caused by reduction of heat transfer from the flame to the burning region. To investigate the cause of extinction a previously developed analysis for the effect of surface heat flux on flame propagation is applied to the present problem. It is found that flame propagation is sensitive to the amount of heat transferred to the fuel surface ahead of the flame, and that extinction occurs (at a finite propagation rate) for sufficiently low values of forward heat flux. Comparisons of predicted and measured values of V are made using heat flux distributions measured at values of Y ox for atmospheric and extinction conditions and reasonable agreement is found. A transient measurement near the extinction limit confirms that the Limiting Oxygen Index is a measure of the critical condition for downward flame propagation rather than a more general limit for burning.