Flammability limits and near-limit chemistry controlled flame spread over thermally thin paper under sub-atmospheric pressure

Understanding material flammability properties and flame spread rates are important to mitigate fire hazards in high altitude areas, aircraft and spacecraft. Limiting oxygen concentration (LOC) has been widely studied in these conditions, but flame spread near extinction limits has been rarely reported. In this work, the experimental study of horizontal near-limit flame spread over paper is investigated in a sub-atmospheric chamber. We first quantified LOCs at a pressure range from 4 to 45 kPa. Then, the near-limit flame spread was studied by increasing 1mol% oxygen concentration at each pressure. Flame image, flame spread rate and preheating length are measured in this study. Results show that LOC is independent of paper sample thickness but dependent on sample width due to lateral mass and heat losses. Near-limit flames show a two-zone structure, including inner blue flame and outer red flame. We find that the near-limit flame spread rate is dominated by preheating length, and it does not obey the previously proposed the power law relationship between flame spread rate and oxygen concentration and pressure. This work studies flammability and near-limit flame spread at various pressure and oxygen conditions, contributing to understanding the fire risk in the environments different from standard atmospheric conditions.