The Velocity and Structure of the Flame Front at Spread of Fire Across the Pine Needle Bed Depending on the Wind Velocity

Abstract

The paper addresses a comprehensive experimental laboratory-scale study of the characteristics and regularities of fire spread across a bed of pine needles of Siberian boreal forests (SBF) and the impact of wind velocity on these regularities. We used such precision physical and physicochemical methods as in situ mass spectrometry, PIV, microthermocouple technique, etc. We measured fire spread rates, spatial gas temperature distribution near the surface and inside the pine needle bed, the pine needle temperature distribution on the bed surface, and gas flow velocity fields before and behind the flame front. Data were obtained on concentration profile of ethanol as the main pine needle pyrolysis product, on the concentration profiles of О₂ and СО₂, on the angle of slope of the flame sheet, and on the impact of the wind velocity on these characteristics. It was established that as the wind velocity changes in the range of 0.15–0.2 m/s, the regularities and characteristics of fire spread drastically change. PIV measurements have demonstrated high turbulence near the flame front. It was established that at the wind velocity of 0.2 m/s, СО₂ concentration grows, while О₂ drops inside the pine needle bed before the flame front more significantly than at the wind velocity of 0.1 m/s. This is attributed to the increase of turbulent mass transfer before the flame front. At that, the pyrolysis rate of the pine needles slows down, and concentration of the pyrolysis products inside the bed in the flame front decreases. The data obtained throw light on the physicochemical processes taking place during fire spread across the bed of pine needles.