Limiting conditions of smoldering-to-flaming transition of cellulose powder

Abstract

Smoldering-to-flaming (StF) transition is an abrupt initiation of homogeneous gas-phase burning (i.e., flaming) preceded by heterogeneous solid-phase burning (i.e., smoldering), which can lead to fast spreading fires both in residential and wildland fires. Despite its hazard, StF transition remains difficult to predict because of its sensitive nature to external conditions, the inherent variability of porous biomass fuels, and a limited understanding of its dominating mechanisms. In this work, we apply a new experimental apparatus to investigate the limiting conditions of StF transition of uniform cellulose powders, at different oxidizer flow velocities (1.5–10.5 cm/s), oxygen concentrations (21, 19, 15, and 10%), and external heat fluxes (0–60 kW/m2). We find that the external heat flux required for StF transition increases as the O2 concentration decreases from 21% to 10% at fixed flow velocities, agreeing with both phenomenological arguments and a simplified energy balance. However, for a fixed O2 concentration, as the flow velocity increases, the required heat flux increases, due to the increased importance of convective heat losses. Furthermore, tests performed at 8% and 5% O2 do not undergo transition even at higher heat fluxes up to a threshold oxygen concentration of 10%. Under low-velocity conditions (airflow <5 cm/s), smoldering may still co-exist with a discrete weak blue flame because oxygen can diffuse into reacting (smoldering) cellulose powder. An oxidizer flow velocity of 5 cm/s appears to be a limiting value for the StF transition, at least for the present experimental conditions. It was also found that increasing the density of cellulose powder required a lower external heat flux for StF transition. This work advances the fundamental understanding of the StF transition, and thus helps guide the prevention of extreme fire events.