Study on the transition mechanism for broadleaf foliage from smoldering to flaming combustion under external radiant heat flux

This paper explores the behaviors of piloted ignition of fresh broadleaf foliage under external radiant heat flux. A total of 12 plant species were adopted, with volatile matter content varying from 15.4 to 41.4%. Measurements were conducted using a cone calorimeter with the radiant heat flux set at 35, 55, 70, and 85 kW m⁻². Under the same radiant heat flux, the samples underwent combustion in two major modes. Some of the samples appeared short time flame and followed by smoldering, whereas the others maintained smoldering throughout the measurements. With an increase in the radiant heat flux or volatile matter content of a sample, the combustion mode of the samples may shift from smoldering to flaming combustion at initial stage of measurement. The critical heat release rate of a sample to form flaming combustion is between 20.9 and 22.5 kW m⁻². Further analysis confirmed that the flaming combustion formed at initial stage of measurement mainly relies on the pyrolysis process of the sample surface layer. The initial volatile matter content of a sample and the rate of temperature rise at the surface layer are major factors governing the mass flux of volatiles generated and subsequently determining whether the sample undergoes flaming combustion.