Heat transfer mechanisms in horizontal flame spread over wood and extruded polystyrene surfaces

The heat transfer in horizontal flame spreading over the surface of construction materials was experimentally investigated at two different altitude conditions (the Lhasa plateau 3658 m, and the Hefei Plain, 50 m). The experiments were carried out with the charring material Whitewood and the thermoplastic material the extruded polystyrene (XPS) over a wide range of sample widths. The temperature profile in solid phase was measured. Different flame spread behaviors were found from the temperature profiles of the XPS and Whitewood. The XPS undergoes the melting stage in the preheated stage while the Whitewood undergoes the char burning stage after the pyrolysis stage. The heat transfer analysis suggests that the gas phase heat transfer is dominant for whitewood and XPS, even though the XPS is thermally thick. The flame spread rate and the surface heat flux with various sample widths were measured. They both dropped firstly and then rose with the increasing sample width. Furthermore, the spread rate was correlated with a pseudo-property Ф, which is the product of the surface heat flux and the preheated length. A good linear relationship was found between the spread rate and Ф, which agrees well with Quintiere's model. Moreover, the sample width effects on flame spread rate were well explained by the effects on heat transfer. The horizontal flame spread behaviors with sample width were dominated by two different regimes: the spread rate drops for small sample width in convection regime but rises for large sample width in radiation regime.