【作者】 陈健；

【导师】 陆守香；

【作者基本信息】 中国科学技术大学 ， 安全科学与工程， 2020， 博士

【摘要】 舱室火灾是火灾科学研究的经典问题,然而部分理论并没有考虑舱室压力的影响。随着社会的发展,频繁发生的压力舱室内火灾逐渐引起人们的重视,诸如打压期间的核电安全壳、飞机巡航时的货舱等。由于此类舱室功能特殊且灭火难度大,往往一旦受到火灾影响即导致严重的后果。因此,研究舱室压力对火灾燃烧的影响,指导此类压力舱室的火灾防治,是急需解决的科学问题。在火灾初期增长阶段,火源功率较小,舱室内环境参数(压力、温度和气体组分)相对稳定;随着火源功率的增大,火灾进入充分发展阶段,燃烧过程会受到舱室内环境参数的耦合影响。针对这两个火灾发展阶段的特点,本文开展了环境压力对小尺度油池火燃烧特性的影响研究、封闭舱室初始压力对油池火燃烧特性的影响研究。具体工作包括:(1)环境压力对小尺度油池火燃烧特性的影响研究为了研究环境压力对小尺度油池火燃烧特性的影响,搭建了压力舱实验平台,其中气氛控制系统可以实现小尺度油池在压力、气氛相对恒定的环境下稳定燃烧,得到了直径1.5 cm和2.0 cm乙醇油池火在latm-5atm环境压力下的燃烧速率、火焰图像和辐射分数。实验结果发现随着环境压力的增大,小尺度油池的燃烧速率呈现先逐渐减小后逐渐增大的趋势,转折点对应的环境压力约为2.5 atm。这不同的变化趋势是由于当环境压力小于2.5 atm时,压力对传导热反馈的影响占主导,燃烧速率随压力的增大略有减小;随着压力的进一步升高,压力对对流和辐射项热反馈的影响逐渐占主导,导致燃烧速率随着压力的增大而增大。利用无量纲分析,提出了影响火焰形态特征参数(火焰长度、火焰宽度和火焰体积)的无量纲量,包括无量纲火焰振荡长度,无量纲油池直径和燃料在出口处的雷诺数,并根据本文的实验结果,得到了无量纲火焰振荡长度与火焰形态特征参数之间的关系式,建立了油池层流火焰形态特征参数预测模型。并进一步基于火焰形态特征参数预测模型和前人建立的碳烟辐射模型,分析了环境压力对火焰中碳烟生成时间、滞留时间的影响,提出了无量纲火焰辐射分数,并根据实验结果拟合得到了无量纲火焰辐射分数与无量纲振荡长度的关系式,建立了耦合环境压力、燃料质量流量、燃料性质的辐射分数模型。(2)封闭舱室初始压力对油池火燃烧特性的影响研究为了研究封闭舱室初始压力对油池火燃烧特性的影响,在封闭舱室(封闭舱室与外界有能量交换,无物质交换)内开展直径从4 cm到10 cm的乙醇油池火燃烧实验,得到了不同舱室初始压力(0.6 atm-2.5 atm)下油池火的燃烧速率、火焰特征和自熄灭火行为。实验结果发现舱室内油池火的平均燃烧速率(从点燃到自熄灭)与舱室平均压力的幂次方成正比,其中指数随着油池直径的增加而增加。舱室的初始压力和油池直径对自熄灭时舱室内氧气浓度的影响较小,分布在12%-14%之间,且绘制了氧气浓度值的概率分布图,发现其符合高斯分布,平均值约为12.9%,标准差约为0.37%。基于浮力主导的层流边界层控制方程推导了控制池火火焰结构的格拉晓夫数,其中将自然对流火灾的固有长度尺度视为特征长度,发现随着格拉晓夫数的增大,池火火焰从层流向湍流转捩,并根据火焰形态的特征划分了四种典型的池火火焰结构:Laminar flame,Tip flickering flame,Sinuousmeanderingflame和Turbulentflame。基于火焰图像的概率分布云图量化了池火火焰的振荡强度,根据实验结果发现振荡强度与格拉晓夫数的幂次方成正比。(3)压力舱室内池火燃烧参数预测模型为了解释封闭舱室初始压力对油池火燃烧特性的影响,针对不同封闭舱室初始压力下油池火燃烧实验结果,结合传热学和燃烧学,建立了舱室内池火传热模型、自熄灭时间预测模型和池火火焰振荡频率预测模型。提出了舱室热环境(舱室热烟气和热壁面)对油池热反馈的计算方法,发现舱室压力的增大会导致舱室内烟气发射率增大、透过率降低,进而使得舱室内烟气对油池的热反馈增大、舱室壁面对油池的热反馈减小。并利用热反馈项修正了舱室内平均压力与油池燃烧速率的关系,建立了舱室内池火传热模型。根据舱室内氧气浓度的变化,建立了考虑舱室初始压力的池火自熄灭时间预测模型,并根据实验结果对模型进行了验证。利用碳化硅纤维搭建了大尺度油池火测温平台,得到了甲醇油池火火焰温度在振荡中的演化规律并探索了池火振荡的机理,将池火的振荡模式划分为尖部振荡和非尖部振荡,针对两种不同的振荡模式,揭示了环境压力、油池直径等对池火火焰振荡频率的影响规律,得到了反映压力对振荡频率影响规律的斯特劳哈尔数与格拉晓夫数关系式,并根据实验结果和前人的数据对比验证了模型。更多还原

【Abstract】 Compartment fire is one of the most classical issues in the fire research history,and only a few studies have focused on the chamber pressure.With the development of economy,there are more and more pressure chamber.Fires can also occur in pressure chamber,where the internal pressure is not equal to a standard atmospheric pressure,such as aircraft cargo compartments and nuclear containment shells during a pressurized period.The understanding of the influence of chamber pressure on fire behavior can provide theoretical guidance for fire safety design and promote fire prevention technology.In the initial stage of the fire in the pressure chamber,the heat release rate is small,while during the full development stage,the enviromental parameters(pressure,temperature,and gas atmosphere)would influence the development of fire.In this paper,the influence of chamber pressure(environmental pressure,initial pressure)on the burning characteristics of pool fire was studied through experimental investigations and theoretical analyzes.Main fundings include:(1)The effect of enviromental pressure on burning characteristics of small scale pool firesFirstly,the influence of enviromental pressure on the burning characteristics of small scale pool fires was investigated.To achieve the stable burning of pool fire,the experimental platform of pressure chamber was designed.In the chamber,ethanol pool fires in 2.0-cm-and 1.5-cm-diameter small scale vessels were conducted under a wide pressure range of 1-5 atm,where burning rate,flame shape and radiation fraction were measured.The effect of pressure on the heat transfer from the flame to the fuel pool controls the change of burning rate with pressure.As pressure increases,burning rate decreased slightly because the effect of pressure on conductive heat transfer was dominant.Then,burning rate increased significantly with pressure,because the effect of pressure on convective heat transfer and radiant heat transfer gradually became important.The prediction model for the flame geometric parameters of laminar diffusioin flame was proposed based on the dimensionless analysis,where the influencing factors of the flame shape parameters were summarized.To verify the proposed model,the dimensionless oscillation length was correlated with dimensionless flame geometric parameters.Base on the flow model above,a soot model similarity was developed,where the soot concentration was expressed in terms of the laminar smoke point height and the flow conditions,and a global relationship for non-dimensional radiation fraction was introduced and verified.(2)The effect of inital pressure on burning characteristics of pool fires in the closed chamberSecondly,the effect of initial pressure on the burning behavior of ethanol pool fire was studied.A series of ethanol pool fires with diameters from 4 cm to 10 cm was carried out in a pressure chamber with interior dimensions of 0.6 m(height)× 0.4 m(diameter),and the different initial pressures range from 0.6 atm to 2.5 atm.The fuel mass,flame video,and oxygen concentration were measured in the experiment,furthermore the change in flame appearance,evolution of mass burning flux,and self-extinction time were analyzed and discussed.The mass burning flux increased significantly with initial pressure and the mean mass burning flux can be correlated with the average pressure using exponential function,where the value of exponent increased with dimater.The results showed that the pressure has little influence on the remaining oxygen concentration at extinction.The oxygen concentration follows Gaussian distribution.The mean value is about 12.9%,and the variance value is about 0.37%.Based on the momentum conservation equation for the gas movement driven by buoyant force,the non-dimensional parameter was derived to illustrate the change of flame structure.The flame structure of pool fire was firstly divided:laminar flame,tip flickering flame,sinuous meandering flame and turbulent flame.The oscillation intensity,which was redefined using the probability contour of the flame,could be related with the proposed non-dimensional parameter.(3)The development of fire models for fire pool fire in the pressure chamberFire models for fire pool fire in the pressure chamber,including the heat transfer model,the prediction correlation of self-extinction time and the model for the oscillation of pool fire were poposed.Considering the efect of pressure,a theoretical formulation was proposed to estimate the radiation flux received by the fuel surface from the chamber wall and hot gases,with which the relationship between the modifed mass burning fux of pool fire and pressure was obtained.By considering the effect of initial pressure,an empirical form for predicting the self-extinction time was developed,which showed a good agreement with the experimental value in our study and previous test.Measurements of the time-varying temperature field in a l m pool fire burning in a quiescent environment using a thin filament pyrometry(TFP)method were presented.The change of temperature field in one cycle was obtained,and the analysis provided quantitative insight into the complex and dynamic structure of the oscillation.The influence of ambient pressure and gravity on oscillation frequency depended on the flameoscillation mechanisms,the tip flickering flame or the non-tip flickering flame.Based on the one-dimensional movement of vortex along the axis,a theoretical model was proposed to explain the effect of ambient pressure and gravity on frequency,and then the obtained relationship between Strouhal and Grashof was verified by the measured value.更多还原