Ignition of thin fuel by thermoplastic drips: An experimental study for the dripping ignition theory

Ignition by dripping flame is widely observed in wire fires and facade fires, but little research has quantified its fire hazard. This work studies the ignition of thin papers (0.07-0.32 mm) by burning polyethylene drips with four sizes (2.6-6.2 mg) and dripping frequencies (0.8-1.8 Hz). The probability of dripping ignition as a function of key dripping parameters is quantified to determine the ignition limits. As the paper thickness increases, more drips and longer time are required for ignition, similar to the classical pilot ignition of thin fuels. The attached flame acts as the piloted source; heating effects from hot drips and dripping flame are comparable; and ignition occurs to the paper rather than landed drips. Moreover, the dripping-ignition capability is controlled by the dripping mass rate, which is the product of the drip mass and the dripping frequency. For the dripping mass rate of about 4.5 mg/s, the equivalent heat flux is 15 +/- 3 kW/m(2). The dripping-ignition time is inversely proportional to the mass of drip and the square of the dripping frequency, different from the piloted ignition under irradiation. This work provides important information to quantify the fire hazard of dripping and explores the ignition mechanism in dripping fire.