Ignition limit of EPS foam by a hot particle under cross wind

The ignition of the building insulation materials by a hot particle is a typical spot fire phenomenon, but the scientific understanding is still limited. In this work, a hot steel spherical particle (6-16 mm and 800-1200 °C) was dropped onto the low-density expandable polystyrene (EPS) foam with an external airflow velocity of 0-4 m/s to obtain the ignition limit at the flash point and fire point. Airflow provides an alternative shortcut transition of unstable flash flame to a strong fire point and fuel burnout, because airflow increases the oxygen supply and flame heating rather than cooling the particle. As the airflow velocity increases, both flash and fire points first become easier to reach because airflow facilitates the mixing of pyrolysates and oxygen in the Smothering Regime. When the airflow velocity increases to the Thermal Regime, the delay time remains stable. Further increasing the airflow velocity to the Chemical Regime, the ignition delay time slightly increases until the airflow blows off the flash flame by cooling the particle or blowing away the flammable mixture from the hot surface. Such a competitive effect of airflow on hot particle ignition is also qualitatively verified by theoretical analysis. Flame retardants inside EPS foam do not change the flash ignition but inhibit the transition to fire point and burnout, even under the assistance of airflow. This work enhances the comprehension of the complex interactions between flash points and fire points in the spotting or hot-particle ignition of the building facades.