Flame radiation feedback to fuel surface in medium ethanol and heptane pool fires with cross air flow

The work in this paper is to clarify and quantify experimentally the effect of cross air flow on flame radiation feedback to fuel surface in medium pool fires. Square pools with dimensions ranged in 10-25 cm are burnt under cross air flow speed ranged in 0-2.5 m/s, using ethanol and heptane as representative fuel producing flame of definitely different sootiness and luminosity as to compare their response to the cross air flow in radiation feedback. The incident radiation heat flux to the fuel surface is measured by a step-sum method using water-cooled radiation gauges on discrete regions of the surface, along with the fuel mass burning history recorded simultaneously by an electronic balance with resolution of 0.1 g. Results show that the cross air flow, by deflecting the flame, reduces the radiation feedback remarkably to finally approach a nearly stable lower value with increase in flow speed. Such reduction relative to that with no wind is quantified, as found to be more effective for heptane pool fire (50-70%) than that for ethanol pool fire (25-35%). However, at the same time, the burning rate is enhanced by the cross air flow. The two fractions, χa (fraction of radiation heat absorbed by fuel surface to that needed for evaporation) and χs (fraction of incident feedback radiation to total heat released by combustion), are then further quantified. They both decrease considerably with increase in flow speed. The χa and χs under no wind are shown to be quite different for heptane (range in 20-60% (χa) and 0.5-1.5% (χs) increasing with pool size) and ethanol (nearly constant 25% (χa) and 2% (χs) independent of pool size). However, when subjected to a relative strong wind as Froude number (Fr) beyond 1, χa is found to be similar for these two different fuels (in 5-15%), but χs found in 0.4-1% for ethanol and in 0.1-0.5% for heptane. The behaviors of pool scale effect on both flame radiation feedback fraction (χa), and accordingly mass burning flux, with a cross air flow are shown to be quite different from those without cross air flow. The flame radiation feedback fraction (χa) increases with pool size and is much higher for heptane than that of ethanol at no cross air flow; while it is similar for these two fuels under a cross air flow and being much lower than that with no cross air flow. Accordingly, the mass burning flux increases with pool size at no cross air flow; while being similar at a relative higher value regardless of pool size for a given Fr with a cross air flow than that with no cross air flow (pool scale effect showing to be well converged by Fr when subjected to a relative strong cross air flow as Fr beyond the critical value of about 1). These quantifications and findings reveal that under a cross air flow, the relative importance of different heat feedback mechanism of a pool fire has been altered prominently due to the deflection of the flame. Flame radiation feedback declines remarkably while those from wall conduction and flame convection should be enhanced to be predominant, which is essential in re-considering the heat balance to characterize and especially to scale the burning behavior of a pool fire in cross air flows.