Experimental study on the characteristics of horizontal flame spread over XPS surface on plateau.
A series of comparative laboratory-scale experiments were carried out in the Lhasa plateau and the Hefei plain respectively to investigate the characteristics of flame spread over the extruded polystyrene (XPS) foam, a typical thermal insulation material. Flame shape and the temperature profile in solid phase were monitored, and the effects of altitude on the heat transfer process were analyzed. The results show that the temperature rise with time undergoes three stages: the preheating stage, the melting stage and the pyrolysis stage. The durations of the melting and pyrolysis stages on plateau are longer than that in plain, which sequentially results in a lower flame spread rate on plateau. Comparing of the temperature change rate curve on plateau with that in plain, it is found that the peak characteristics of the curves in the pyrolysis stage changed from single peak to multi-peaks, which suggests that the altitude difference might change the pyrolysis mechanisms of XPS material. Moreover, the sample scale effects on flame spread are also explored. Two different regimes are found in flame spread behavior with sample scale at the both altitudes. The spread rate drops with sample scale in convection regime and rises in radiation regime.
Sample width and thickness effects on upward flame spread over PMMA surface.
Upward flame spread has the same propagating direction with air flow and buoyancy, and features as the most hazardous fire case in all flame spread configurations. It has been a long time for fire researchers to find a simple and effective method to evaluate upward flame spread behaviors, especially for different materials and sample sizes. The aim of this work was motivated by the research of sample width and thickness effects on upward flame spread behavior, including flame spread rate during acceleration propagation for different sample thickness and width, theoretical global mass loss prediction based on Emmons's hypothesis, and dimensionless flame height scaling with dimensionless heat release rate for steady stage burning. Four kinds of sample thicknesses were selected, including 1.7, 3.5, 5, and 7mm. For each kind of thickness, six sample widths ranging from 40 to 90mm were prepared. To eliminate the side flame spreading effects, one set of contrast experiments with sample sides sealed was also performed, by which way flame could only spread along sample front surface and flame propagation was inhibited along both sides. Based on Emmons's hypothesis, a method for calculation of global mass loss rate was developed. Theoretical global mass loss rate over pyrolysis surface of upward flame spread configurations was calculated and could fit the experimental data well. Finally, a dimensionless heat-release rate for wall flames of different sample sizes was used to scale the dimensionless flame height with a power-law exponent 0.58. The results of this study have implications concerning designs for high-rise building fire safety problems and can help to get better understandings of upward flame spread mechanism from aspects of heat and mass transfer.
The effects of angular orientation on flame spread over thin materials
Abstract Data were taken to show the flame spread characteristics of thin materials burning on an insulating substrate. Metalized polyethylene terephthalate (0.20 mm) and paper (0.17 mm) were burned on the surface of glass fiber insulation. Flame spread was measured in the upward or downward facing orientation for the material and in the directions of gravity assistance (up) or gravity opposition (down). Measurements were taken at various angles ranging from a vertical to a horizontal orientation. A theoretical analysis was developed to predict the flame spread as a function of material properties, sample orientation, and flame spread direction. The one-dimensional theory was in reasonable agreement with the paper data. Vertical upward spread was found to yield the highest velocity. Critical angles (measured from the vertical) show transition to increasing flame spread for downward bottom spread at −60°, and for upward top surface spread at 60°.