【作者】 李海航；

【导师】 汪箭；

【作者基本信息】 中国科学技术大学 ， 安全科学与工程， 2014， 博士

【摘要】 随着人类在高原、高空环境活动的增多,低压下火灾为人类带来了新的问题和挑战。发展高原和高空环境下火灾的防治技术显得越来越重要,因此研究低压下火灾燃烧特性具有重要的现实意义。根据前人的研究发现,对于火灾燃烧条件来说,低气压环境区别于正常大气环境的最大不同就是空气密度和氧气分压的减小。不同压力下空气密度和氧气分压的改变,分别直接导致了浮力作用和碳黑产生量的不同。浮力和火焰抖动、火焰形态等息息相关,一般用弗劳德数表征浮力对火焰作用的大小。而碳黑与燃烧效率、火焰形态、产烟倾向等有关。因此自然地确定浮力和碳黑作为研究低压对火灾燃烧作用的途径,并选取在不同压力下有着相等质量损失速率的气体燃料作为研究对象。低气压环境的实现主要有高海拔现场自然环境和低压舱人工模拟环境两种方式。现场自然环境实验虽然花费大开展难,但是它适合开展大尺度的火灾燃烧实验,特别适合用来发现不同气压下火灾宏观现象的不同,因此在拉萨和合肥两地分别搭建了小型锥形量热仪进行实验,主要用气体分析仪测量排烟管道内的燃烧产物、热电偶树测量火焰羽流温度、辐射计测量火焰对周围的辐射热流、光学烟密度计测量烟气的消光系数,并使用简化的化学量热法计算热释放速率。在揭示不同海拔高度火灾宏观现象的基础上,在低压舱内开展多级低压条件下适于研究燃烧机理的多种实验。实验中,主要采用不加装滤光片的相机测量发光火焰形状、加装CH滤光片的相机尝试获取化学当量比火焰形状,采用高速相机和快速傅里叶变换先后拍摄火焰抖动过程和计算火焰抖动频率。在拉萨和合肥两个海拔高度上,通过锥形量热仪测量甲烷、乙炔和丙烷的热释放速率和辐射热流等参数,进一步揭示和明确了低气压对燃烧效率和辐射分数等主要参数的影响。通过总结不同含碳量燃料的类似结果,可以表明：低压下火焰的热释放速率和燃烧效率更高；低压下火焰总羽流温度更高；低压下火焰辐射热流和辐射分数更小；低压下火焰产生烟气的透射率更大,说明低压下火焰的产烟率更小在低压舱内通过实验研究了多个等级低压下甲烷、乙烯和丙烷扩散火焰抖动行为,主要是研究火焰抖动频率和压力之间的关系以及出口速度对抖动频率的影响。主要发现总结如下：层流扩散火焰抖动行为可以分为顶部抖动、间歇抖动、和连续抖动3个区域,连续抖动发生在较大燃料流量或较大压力下；抖动频率对燃料流量和类型不敏感,随流量的增加只略微增加,相同实验压力下3种燃料火焰的抖动频率几乎相等；抖动频率随压力升高而增大,实验压力范围内从8Hz增加到12Hz,测量的抖动频率随压力增加的0.27次幂而增大；另外,在一些条件下观察到了丙烷火焰存在多重频率现象。在低压舱内测量了0.03到O.1MPa压力范围内,不发烟甲烷、乙烯和丙烷稳定火焰的发光形状,经过对高度和宽度使用无量纲数缩放分析,得到了如下结论：Reynolds数线性缩放模型对烃类燃料基本适用,在不同的Re数下浮力和碳黑的共同作用使得线性关系的斜率发生了轻微变化；在最大的Fr数或最低压力下,浮力对火焰高度的影响不重要,相反,在最小Fr数区域,浮力对火焰高度作用占主导地位；在对火焰宽度进行Froude数缩放时,发现归一化火焰宽度和Froude数存在着很好的线性关系。在低压舱测量了多级压力下,不发烟甲烷、乙烯和丙烷稳定火焰的发光形状,特别为甲烷火焰进行了0.2个大气压的实验,发光火焰高度和宽度随压力的变化规律总结如下：第一种情形,对于较大燃料流量和较高压力范围条件,火焰高度呈现“先增后减”趋势；第二种情形,在较小流量和较低压力范围内,可总结出“先减后增”趋势。第三种情形,在合适的燃料流量和压力范围下,火焰高度会呈现“先减后增再减”趋势。在低压舱内进行了乙炔层流射流扩散火焰烟点实验,实验压力范围为0.03～0.1MPa。利用建立的反应射流中心线速度变化模型对实验结果进行深入分析,发现烟点火焰高度、燃料质量流量、存留时间和压力之间存在如下关系：低压下乙炔烟点火焰高度随压力升高而减小,这个变化趋势与高压下乙烯和甲烷火焰的变化规律相同；烟点燃料质量流量随压力的幂指数变化,幂指数为负值,与先前多数实验结果一致；在一个很大的压力范围(0.03MPa到1.6MPa)内,烟点火焰存留时间随压力升高而减小。更多还原

【Abstract】 Fires at subatmospheric pressures bring mankind new problems and challenges with more activities in plateau and sky. The fire prevention technology at high altitudes is becoming increasingly important, therefore, studying fire burning characteristics under low pressures is of important practical significance. According to previous studies, the condition of the low pressure environment distinguishing normal atmospheric environment is the reduction of air density and partial pressure of oxygen. Buoyancy effect and soot production are largely influenced by air density and partial pressure of oxygen respectively. Buoyancy is closely related with flame flickering and flame morphology and Froude number is generally used to characterize the role of buoyancy, while soot relates to combustion efficiency, flame shape, and smoky tendency. Therefore, buoyancy and soot were chosen as the paths when studying effects of subatmospheric pressure on combustion. Besides, gaseous fuels were selected as research objects since their mass loss rates are equal under different pressures.Natural environment at high altitude and artificial environment in hypobaric chamber are the two main methods to achieve subatmospheric pressures. Natural environment are suitable for conducting large scale fire experiments although it is expensive and difficult. Comparative experiments were implemented in each cone calorimeter in Lhasa and Hefei. Gas analyzer, thermocouple tree, radiometer, and kapnometer were employed to measure combustion products, fire plume temperature, radiation flux, and extinction coefficient respectively. A simplified thermochemistry based on the measured concentrations of O2and CO2was used when calculating heat release rates. In view of distinct macroscopical phenomena at two different altitudes, we continued experiments in a hypobaric chamber for studying combustion mechanism under multilevel low pressures. A combined normal video and the CH filtering technique was employed in the flame image recording to facilitate determining the flame shape and smoke point. The flame images of flickering flames were recorded by a480fps high-speed camera, and Fast Fourier Transform were carried out to measure flickering frequency.Fire behaviors such as heat release rate and radiation flux of three gaseous hydrocarbon fuels were experimentally measured in each cone calorimeter at two different altitudes in Lhasa and Hefei. The effects of subatmospheric pressure on combustion efficiency and radiation fraction were further revealed when summarizing results of methane, acetylene, and propane fires. The heat release rates, fire plume temperatures, and smoke transmittances at the lower pressure are higher while the radiation fluxes are lower. Hence, the combustion efficiencies at the lower pressure are higher and the radiation fractions and smoke production rates are lower.The flickering behavior was studied for methane, ethylene, and propane flames at eight subatmospheric pressure levels. The relationship between flickering frequency and pressure and the influence of exit velocity on frequency were highlighted. Generally, the flickering behaviors were observed falling into three regimes, i.e. tip flickering, intermittent flickering, and continuous flickering. The continuous flickering appears when the fuel flow rate or pressure is increased above a particular value. The flickering frequency is found insensitive to fuel type or flow rate. The observed frequencies generally vary from about8Hz at0.3atm to12Hz at1.0atm. The logarithmic relationship between the measured frequency and pressure is close to the previous theoretical analysis. In addition, an important phenomenon of multiple frequencies was observed in the tests.The luminous shapes were presented to examine the scaling of flame heights and widths of laminar stable flames burning methane, ethylene, and propane in a subatmospheric pressure chamber. The linear Reynolds scaling was generally suitable for hydrocarbon fuels although slopes were changing under the combined action of buoyant and soot. The buoyant was unimportant in determining flame height when under the largest Fr or at0.3atm. Oppositely, at the smallest Fr, buoyancy dominant tendency was reached. In Froude scaling of width, the unchanged slopes between normalized flame width and Fr indicated a good linearity relationship.The luminous shapes were measured for laminar stable non-smoky flames burning methane, ethylene, and propane under ambient pressures of0.02-0.1MPa. Diverse changing trends happen under various fuel flow rates and different pressure ranges by accounting micro flames at subatmospheric pressures and results over smoke points in references. First, under conditions of relatively higher fuel flow rates and pressure ranges, flame heights follow an increasing-decreasing trend. Second, a decreasing-increasing trend can be concluded under relatively lower fuel flow rates and pressure ranges. Third, a decreasing-increasing-decreasing trend will happen under proper fuel flow rate and pressure range.Experimental study on smoke point of acetylene laminar jet diffusion flame was performed in a chamber with subatmospheric pressures of0.03-0.1MPa. The relationships between smoke point flame height, fuel mass flow rate, residence time and pressure are summarized as follows:The smoke point flame height was observed to vary with pressure in the same pattern as ethylene and methane flames at elevated pressures. The scaling of the smoke point fuel mass flow rate with the pressure to a power law with a negative exponent is qualitatively consistent with results found in most earlier works. The residence time at smoke point has been found to increase with decreasing pressure over a wide pressure range.更多还原