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.
Characteristics of flame spread over the surface of charring solid combustibles at high altitude
To explore the characteristics of flame spread over the surface of charring solid combustibles at high altitude, the whitewood with uniform texture was chosen to conduct a series of experiments in Lhasa and Hefei, with altitude of 3658 m and 50 m respectively. Several parameters, including the flame height, flame spread rate, flame temperature, surface temperature, were measured on samples with different width and inclinations. A quantitative analysis of flame spread characteristics over sample surface at high altitude was performed. Results showed that, in the environment of lower pressure and oxygen concentration at high altitude, the flame height and flame spread rate over sample surface decreased, but the flame temperature increased slightly. However, with increasing of sample width, the relative difference between the flame spread rates at different altitudes decreased.
Propagation and extinction mechanisms of opposed-flow flame spread over PMMA for different sample orientations
Abstract To understand the propagation and the extinction mechanisms of flame spreading along a combustible solid, opposed-flow flame spread along a thick slab of PMMA was experimentally investigated for four different sample orientation angles from 0° (horizontal) to −180° (ceiling flame spread) in several airflow rates. The detailed temperature distribution in the condensed phase during flame spread was measured using holographic interferometry (HI) and infrared thermography (IR). The particle-track laser sheet (PTLS) technique was employed to measure the local entrainment velocity to the flame front at the leading edge. This study found that a flame spread rate with a constant speed is proportional to the net total heat transfer rate. The heat transfer rate from the gas phase, Q y , is about 60% of the total heat transfer rate, Q T , under the no-imposed-flow horizontal flame spread condition ( u ¯ = 0 m / s ). However, the heat transfer rate through the condensed phase, Q x , increases with increasing airflow above 80% of Q T near the extinction limit. The radiative heat loss from the surface, Q R , increases with increasing opposed-flow rate and reaches a maximum of 13% of Q T at u ¯ = 0.65 m / s , while the heat transfer rate in ceiling flame spread is different than in horizontal or downward flame spread under nonopposed and slow-opposed flow conditions. The Q y in nonopposed flow ceiling flame spread is double that in horizontal flame spread, and is 85% of Q T . Despite adequate heat feedback to the condensed phase near the flame leading edge, the flame spread rate decreases rapidly as it approaches the extinction limit. In order to interpret the absence of flame spread or the extinction mechanism, we introduce the Damkohler number including the local air entrainment velocity and the burning rate at finite flame thickness. PTLS showed that the local entrainment velocity at the flame leading edge increased the opposed-flow rate. This leads to a decrease in the Damkohler number. When the Damkohler number reaches a critical value (at most