Smouldering and its transition to flaming combustion of polyurethane foam: An experimental study

Author：Wang, Z. Z., Liu, N. A., Yuan, H., Chen, H. X., Xie, X. D., Zhang, L. H., Rein, G.

Journal：Fuel

DOI:  10.1016/j.fuel.2021.122249

Keywords: Smouldering, Transition to flaming, Polyurethane foam, Inclination angle, Buoyancy, buoyancy, Energy & Fuels, Engineering

Abstract: 

Polyurethane foam smouldering continues to attract the attention of the fire community and has been intensively investigated. However, scarce studies have compared the differences of smouldering spread in the vertical and horizontal configurations. In this work, smouldering experiments of polyurethane foam under different inclination angles, sample lengths and densities were conducted. The smouldering temperature and spread rate were measured. It was observed that in horizontal smouldering, only with a proper sample length, sufficient oxygen supply can achieve a self-sustained smouldering spread and yield enough char for the subsequent second char oxidation, and eventually trigger a transition to flaming. Results also show that the transition to flaming is more likely to occur with the increase of the inclination angle. The effective average spread rate increases with the inclination angle and sample length. A higher-density foam resulted in a longer smouldering-to-flaming transition time and a lower spread rate, possibly attributable to a decreased oxygen supply to the reaction zone due to decreased foam porosity and permeability. However, the higher-density foam generates more residual char for further oxidation and accumulates more heat and flammable pyrolysate gases to trigger the transition to flaming. The effect of buoyancy was theoretically interpreted based on the energy balance. An expression for the smouldering spread rate was proposed by considering the buoyancy-induced flow in the energy balance equation. A good agreement between the predictions and the experimental results confirms the critical effect of inclination angle on the spread rate of smouldering combustion.