Effect of Low Ambient Pressure on Fire Plume and Ceiling Jet Driven by Liquid Pool Fires

低氣壓環境對液體池火誘導火羽流及頂棚射流的影響機理研究

Student thesis: Doctoral Thesis

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Award date28 Aug 2017

Abstract

    Fires under low pressures have received increasing attention in past few years due to the significant effect of pressure on the physical and chemical reactions in the process of combustion. China possesses four main plateaus, which cover one-third part of its mainland, especially the Qinghai-Tibet Plateau with an average elevation exceeding 4500 meters and considerable precious heritage buildings. Low pressure condition at high altitude leads to different fire behaviors, which need more pragmatic theoretical supports to develop corresponding fire prevention and suppression methods. Furthermore, the burgeoning aviation industry around the whole world poses new challenges to the potential fire hazards in air transportation, where cargoes usually are exposed to a low pressure environment. Meanwhile, the airports constructed at high altitude (above 2438 m elevation) increasingly emerge in recent years, and China has over dozen of them and the fire safety of aviation fuels must be carefully concerned. Understanding the difference caused by the ambient pressure might be helpful to offer guidance for fire protection at high-altitude and aviation environments.

    Two frequently-used methods to simulate the low pressure environment for examining the pressure effect on the fire behaviors are field tests at high altitude and chamber tests by means of a controllable altitude chamber. To investigate whether the experimental conditions at high altitude can be faithfully replicated in a low pressure chamber and the limitations or restrictions on the use of the chamber in experimental study of fire behavior at high altitude, both the two methodologies were employed. The n-heptane pool fires with different sizes were performed, and the differences in the burning intensity, flame envelope and axial temperature distribution were analyzed. It was found that only for 6~12 cm pool dimensions, the burning intensity in quasi-steady stage for chamber test can well simulate that for corresponding filed test. The difference in flame envelope appear for all the configurations, exhibiting a larger slenderness for chamber tests. The axial temperature distribution in quasi-steady stage can be well correlated with the classical theory of fire plume involving the pressure effect, while the exception of the 14 cm pool fires shows apparent distinction for field and chamber tests in plume region.

    Based on the conclusions above, n-heptane pool fires with diameters of 6~10 cm were tested in an altitude chamber under different static chamber pressures ranging from 101~40 kPa. The mass burning intensity is determined by the convective and radiative heat feedback to the fuel, and analysis shows that the convective part is the major contributor to fuel evaporation. A new theoretical model was established to interpret the pressure dependence of burning intensity as the pool diameter equals to 6 cm. Video recordings show that flame height increases with the reduction of pressure, as supported by the dimensionless analysis. To further validate the feasibility of pressure modeling and radiation modeling under low pressure, three typical liquid fuels with different sooting levels, i.e. ethanol, n-heptane and jet-A, were employed to perform a sequence of pool fires in a high-altitude city, Lhasa (3650 m, 64.3 kPa). Mass loss, axial temperature profile and radiative heat flux were recorded in each test. From the assessment of experimental data, it can be concluded that the dimensionless burning intensity mμ/D can be correlated against the Grashof number to different powers for all the three fuels, and the exponent increases with the sooting level of fuels. A correlated relationship can be applied to analyze the axial temperature rises, partitioning flame region, intermittent region and plume region with the modified demarcations, i.e. 0.42 and 1.06. In addition, the averaged flame temperature grows higher with declining sooting level of fuels, while the radiative heat fluxes exhibit the opposite results. Moreover, the measured radiative heat fluxes for different fuels are proportional to LmTf5, and the soot volume fraction apparently increases with the sooting level of the fuels under low pressure condition.

    To examine the effect of varying ceiling height on the combustion behaviors of pool fires whose flame impinged the ceiling, a series of n-heptane pool fires was performed beneath a ceiling in the range of Hef/D=0.43~2.5 using a scaled-down calorimeter. Typical parameters including the burning intensity, combustion yields and combustion efficiency were quantitatively analyzed. It was found that with the decreasing ceiling height, the burning intensity initially increased to a peak at Hef/D=1.38, and then decreased, exhibiting a parabolic variation tendency. The maximum concentration increments of CO and COshowed the parallel results to the burning intensity, while the total amount of CO yield monotonously increased with the declining ceiling height, implying increasing toxicity. Besides, the carbon conversion ratio produced the similar result as the combustion efficiency, and both of them showed the parabolic variation law. Besides, a sequence of pool fires with different dimensions and fuel types was performed under a horizontal unconfined ceiling to measure the maximum excess temperature in Lhasa. The results show that the maximum smoke temperatures beneath the ceiling at high altitude are significant higher than the predicted values by Alpert’s model. Considering the effects of ambient pressure and entrainment coefficient, a new theoretical model for predicting the maximum excess temperature was proposed based on the ideal plume. The current results together with the data in the literature which conform with Alpert’s model successfully converge by employing the proposed correlation.

    Research areas

  • pool fire, high altitude, altitude chamber, enclosure effect, mass burning intensity, ceiling jet, maximum excess temperature