Wind-aided turbulent flame spread and burning over large-scale horizontal PMMA surfaces

Abstract

Measurements of concurrent fire spread and burning over horizontal polymethylmethacrylate (PMMA) surfaces exposed to air flows ranging from 1 to 2.1 m/s in a 2.4 m × 5.4 m wind tunnel are presented and analyzed. The fire propagation occurs in two successive modes. In the first mode, when the flame is confined within a boundary layer, the pyrolysis mass loss flux is essentially constant. Transition to the second mode, occurring earlier at lower wind velocities, is accompanied by a rapid increase in the pyrolysis flux because the flame stands up into a plume. For the first boundary layer mode, the flame is an order of magnitude thicker than the theoretical turbulent boundary layer over a flat plate due to upward buoyancy and blowing effects. The measured flame spread rate in the first mode increases during propagation and is independent of the wind velocity U ∞. Over the whole range of data, the flame length x f is approximately 1.25 times the pyrolysis length. A correlation of x f in terms of the energy release rate Q′ ̇ shows in the second mode. As the fire propagates, radiation increasingly becomes the dominant mode of heat transfer from the flame to the fuel surface. Predictions of the spread velocity using Quintiere's [ J. Res. Natl. Bur. Stand. 93:61–70 (1988)] model follow the trend in the measurements reasonably well, though the actual predictions are sensitive to the uncertain material property values.