| Fire hazard performs a serious threat to personal and property safety, while the decreasing pressure affects the physical and chemical reaction in the process of combustion, and changes the fuel combustion characteristics which are significantly different from those in the ordinary ambient pressure. Special high-altitude (low air pressure) environments universally exist in plenty of scenarios, e.g. the plateau region and the aviation fight environment. The plateau region covers about one-third percent of Chinese land area, especially for the Qinghai-Tibet Plateau with the mean altitude over4000meters, where more than1000relic constructions have been built. Recently, the aviation industry is developing rapidly, while the aviation flight environment is of the typical low air pressure environment characteristics, with the cruising altitude of passenger Jets basically above9000m. Thus, the in-depth understanding of fire burning behavior under low air pressure will be of great scientifically significance and engineering application value for fire prevention at high altitude.For the assessment on large scale fire hazard at high altitude, pool fire tests on Jet-A fuel and N-Heptane were conducted in Hefei (a sea-level city,50m/100.8kPa) and Lhasa (Tibet city,3,650m/64.3kPa). The experimental results indicates that the mass burning rate is proportional to the2/3power of the ambient pressure, in accordance with the theoretical prediction from pressure modeling, where the convective term was emphasized and scaled. Whereas, the burning rate variation of N-Heptane thin layer fires shows weekly dependent on pressure, which may be explained by the boiling burning. The addition of ceiling increases the mass burning rate and the centerline temperature, although the effect on centerline temperature is observed to be weaker in Lhasa. The heat release rate of N-Heptane was also measured, and the acquired results shows that the combustion efficiency Xα and the convective heat fraction Xc rises a little with the decreasing pressure, while the radiative fraction Xr shows independent (or weakly dependent) on pressure.To further examine the pressure effects on fuel burning rate and fire plume characteristics, experimental measurements and theoretical analysis on circular N-Heptane fires with serial sizes (D:4cm-14cm) were also conducted in Hefei and Lhasa. From the results, the mean burning rate at quasi-steady stage and boiling stage consistently implied that the exponent a (m"-Pα) varies for different heat transfer domination stages, i.e. α≤0for conductive stage and α=2/3for convective stage. Analysis shows that the flame height, the axial flame and plume temperatures are all well correlated with the dimensionless heat release rate Q*～Q/(PD5/2), with the correlation coefficients derived from the current low-pressure measurements. It indicates that the flame height and the plume temperature increase with the pressure rise as a power function of pressure for the same pool size.More investigation on pressure effects on pool fire behaviors were conducted in an "altitude chamber"(i.e. controllable pressure conditions), which was located in the State Key Laboratory of Fire Science at Hefei. Experiments on N-Heptane pool fires were performed with the small round burners under the different static chamber pressures, ranging from40kPa,60kPa,80kPa to100kPa. Mass burning rate and flame video were recorded during the whole burning process of each test. The mass burning rate is affected by the flame heat feedback to the fuel, and analysis shows that the convective part is the major contributor to the fuel evaporation for these experimental pools. Flame videos show that flame height increases with the reduction of pressure, as validated by the dimensionless analysis. The flame puffing amplitude increases under low pressure, resulting a portion of the flame being quenched. A special phenomenon was observed from the flame videos that flame rotation emerged at the ending burning stage, which was attributed to the Coriolis force effect. |
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