Smoldering combustion and blow-off in incense sticks (Effect of wind directions)

The smoldering spread and extinction behavior are significantly impacted by both wind velocity and angle. However, this combined effect has not been extensively quantified. This study investigates the influence of the wind angle, ranging from θ₀ = 0° (concurrent smoldering spread) to θ₀ = 180° (opposed smoldering spread), which is defined as the angle between the directions of the forced airflow and the reaction propagation, on blow-off limit and smoldering spread rate of 2.5 mm incense sticks using a rotating apparatus. The experimental results indicate an increase in smoldering rates and blow-off wind velocity as θ₀ decreases. The maximum average smoldering rate is about 2.5 cm/min at θ₀ = 0°, while it is less than 1 cm/min at θ₀ = 180°. The blow-off wind velocity is 15 m/s at θ₀ = 0° and 5 m/s at θ₀ = 180°. The blow-off limit was discussed based on Damköhler number (Da). A simplified heat transfer analysis, coupled with computational fluid dynamics simulation, was used to calculate the smoldering rate. The simple model captures the experimental trend of the increasing smoldering rate at low wind velocity. A higher smoldering temperature results in a higher smoldering rate. Additionally, the higher smoldering rate observed at θ₀ = 0° was reproduced, emphasizing the significant role of heat transfer. To maintain a steady smoldering rate, all the heat produced by the reactions must be removed, which occurs most efficiently at θ₀ = 0°, leading to the highest smoldering rate.