flames "on edge": what makes them dangerous
Name: Luca Carmignani
Grad Year: 2019
Downward flame spread over solid fuels constitutes a fundamental problem of fire safety, because of the simplicity it offers in terms of steady spread rate, flow encountered by the flame, and because of a rich literature of experimental, theoretical, and computational studies. Downward spread along two-dimensional flat samples has been well studied, and predictive formulae have been developed and refined, but most practical materials are three-dimensional. There are only a handful of work exploring the three-dimensional effects, which can be very significant because of edges or lateral surfaces. A systematic experimental study explores the edge propagation and its effect on the overall downward spread over samples of Polymethyl Methacrylate (PMMA), using many types of cross-sections, from triangular through octagonal, and circular. A MATLAB-based tool is used to produce instantaneous spread rate for central and edge flames; the latter enhances the spread rate as much as five times higher than samples with no edges. This amplification depends primarily on the internal angle at the edge (the smaller the angle, the faster is the flame) and fuel thickness, but not on other factors such as aspect ratio, or cross-sectional area. The edge propagation is mapped for flames over cylindrical fuels (the limiting shape with infinite edges) with an effective radius obtained from the geometry of the edges and the diffusion length scale of the solid phase. A formula for flame spread over cylindrical fuels from literature is expanded to include "thin fuels" and the wide range of three-dimensional spread rates with the effective radius being the independent variable. Based on these results, different types of cross-sectional areas are sorted in the order of their inherent fire safety characteristic.
Industry Application Area(s)
Aerospace, Defense, Security