Characterization of flame spread over PMMA using holographic interferometry sample orientation effects

Abstract

Flame spread over a thermally thick slab of PMMA at several angles of sample orientation from θ = −90° (vertically downward flame spread) to θ = +90° (vertically upward flame spread) in air was investigated by measuring temperature distributions within the PMMA sample in the vicinity of the leading edge of the flame front using holographic interferometry. Samples with widths of 0.32, 0.47, 1.0, and 2.5 cm, a thickness of 2.5 cm, and a length of 30 cm were used. The measured net heat flux from the gas phase to the sample surface at the vaporization front of the sample is about 7 W/cm 2 for downward flame spread (θ < 0δ), 6.5 W/cm 2 at θ = + 10°, and 2.8 W/cm 2 at θ = + 90°. However, the total net heat transfer rate increases with an increase in the angle of sample orientation, because the characteristic heating length, defined as the distance from the adiabatic point on the sample surface to the vaporization point, increases with an increase in the orientation angle of the sample. The total net heat transfer rate into the sample from the gas phase is about 56% of the total net heat transfer input to the sample at θ = − 90°, 78% at θ = 0°, 87% at θ = + 10°, and 99% at θ = + 90°. Therefore, heat transfer from the gas phase into the unburnt fuel ahead of the vaporization point is the dominant heat tranfer path for all angles of orientation. This was clearly demonstrated by the net heat flow vector patterns within the sample. The streamwise conductive heat transfer rate through the sample decreases with an increase in flame spread rate (increase in the sample orientation angle) due to insufficient time being available for the slow thermal wave to travel through the sample.