Spatio-temporal structure of vertically spreading flame over non-planar PMMA surfaces

Abstract

Burning of PMMA slabs with non-planar surfaces is presented in this paper. Non-planar surfaces have been produced using steps fabricated in transparent thermally thick PMMA slabs. The step size is kept constant and three step locations have been used in this study. Either the plate portion or the step portion of the slab is ignited, but keeping the ignition location at the bottom. Concurrent flame spread over stepped slabs is analysed in detail. Time-dependent %mass loss, surface regression, and OH*, CH* chemiluminescence, and planar laser induced incandescence of soot and planar laser induced fluorescence of polyaromatic hydrocarbons (PAH) are determined. The results indicate that the main influence of the step at any location has been to increase the mass loss rate and therefore, the burning rate. The flow fields are quite different between the step ignition and the plate ignition cases. As a result, secondary flame zones are observed in the wake zone downstream of the step for step ignition cases and the flames influenced by stagnation point flow field are observed for plate ignition cases. The influences of step size for step ignition and location of step for plate ignition cases are also studied. The OH*/CH* maps show their overlap region as that of high heat release rates. The PAH precursor to soot is spatially mapped relative to the latter. Although the plate-injection cases are sooty, LII indicates otherwise because of poor mixing in a forward-facing step leading to substantial sooting above the sample outside the field of view. On the other hand, the effect of step recirculation zone in promoting mixing and suppressing soot is evident from the results.