Assessment of melting and dripping effect on ignition of vertically discrete polypropylene and polyethylene slabs

This contribution experimentally and analytically addresses the dripping and ignition mechanism of vertically discrete polypropylene (PP) and polyethylene (PE) slabs. 3-, 5- and 8-mm-thick samples with 3–15 cm spacings were employed to explore the influence of sample thickness and spacing on dripping behaviors and ignition time of lower slab. An approximate analytical model incorporating the imposed power law heat flux, which is converted from the energy loaded by the dripping droplets, is developed to estimate the ignition time. The results show that PE slabs melt, drip and ignite earlier compared with PP slabs, indicating severer hazard in fire scenarios. For both polymers, thinner slabs result in higher dripping frequency, smaller droplet diameter and consequently shorter ignition time. Nevertheless, vertical spacing has little effect on the former two parameters. When the spacing is lower than 8 cm, irregular variation of ignition time is found due to the combined exertion of thermal radiation from upper slab flame and the droplet heating. However, for spacing larger than 8 cm the upper flame radiation can be ignored and the ignition time increases monotonously. The reliability of the developed model is validated by comparison with experimental measurements.