Smoldering ignition using a concentrated solar irradiation spot

Ignition of materials by a point source of heating plays an important role in initiating many structure and wildland fires, such as spotting by hot particles, lightning, laser, and concentrated irradiation. Herein, we study the smoldering ignition of tissue paper by a concentrated sunlight spot with heat fluxes up to 780 kW/m², which is focused by a transparent glass sphere. The diameter of the sunlight spot on the paper sample ranges from 1.5 to 20.0 mm by varying the paper position within the focal length, where a smaller spot has a larger intensity of sunlight irradiation. The measured minimum spot irradiation for smoldering ignition is not a constant and is much higher than 11 kW/m² measured in a traditional cone-calorimeter test. As the diameter of the irradiation spot decreases from 20 to 1.5 mm, the minimum irradiation for smoldering ignition increases from 17.5 to 205 kW/m², and the ignition energy increases from 0.084 to 2.0 MJ/m². A simplified heat transfer analysis reveals that the lateral conductive cooling within the fuel becomes dominant for a smaller spot ignition area. This work ultimately quantifies the potential fire risk from concentrated sunlight spots and helps elucidate the underlying mechanisms leading to smoldering ignition.