高原环境下木材表面火蔓延特性的实验研究

在拉萨高原地区和合肥平原地区开展了一系列木材表面火蔓延对比实验研究,通过对火蔓延过程中一些重要特性参数(固相热解温度、气相火焰温度、燃烧区长度和燃烧速率等)的测量,分析了高原地区的火灾危害特性.结果表明,高原地区的固相热解温度与平原差别不大,但是气相火焰温度要略高一些.火蔓延过程中高原地区的燃烧区长度比平原的小,而且两...     

西藏高原环境下油池火的燃烧速率和振荡频率

通过拉萨和合肥两地开展相同尺寸的甲醇油池火燃烧实验,研究了高原油池火燃烧特性参数与平原的不同以及具体的影响机制.研究发现,在燃料类型和燃烧尺寸相同的情况下,燃料在拉萨的质量燃烧速率比合肥慢;基于火焰图像分析,用MATLAB编程对两地火焰图像进行处理,确定了在相同尺寸的油盘下,由于受低氧低压条件的影响,拉萨火焰面积比合肥...     

西藏低压环境对油池火火焰形状的影响

通过拉萨和合肥两地开展相同尺寸的甲醇和汽油油池火燃烧实验,研究了两地油池火燃烧特性参数的不同,分析低氧低压条件对于燃烧特性参数的影响机理.研究发现,在燃料类型和尺寸相同的情况下,燃料在拉萨的失重速率比合肥慢,产生的CO浓度更高;采用火焰图像分析对两地火焰图像进行处理,发现相同小尺寸的油盘燃料燃烧,拉萨火焰面积比合肥要大...     

低压低氧环境下油盆火的燃烧特性和烟气特性

为了研究高原地区低压低氧环境下的燃烧特性和烟气特性,在拉萨和合肥两地区分别进行了正庚烷油盆火实验.实验测量了油盆上方温度、热辐射通量、燃料失重速率、烟气热辐射发射率、烟气组分浓度和火焰高度等燃烧和烟气参数.结果表明:随着海拔高度的升高,油盆火单位面积燃烧速率降低,火焰及热烟气温度降低,对周围环境的热辐射通量和单位面积辐...     

航空煤油火蔓延驰豫脉动特性的实验研究

针对液相控制条件下航空煤油火焰蔓延的驰豫脉动特性,借助CCD摄像和幅频分析技术,对航空煤油火蔓延速度、驰豫脉动模式、脉动频率和脉动幅值等特性进行了实验研究.对航空煤油火焰驰豫脉动蔓延产生机理进行了分析,分析结果表明,在液相控制条件下,前展-回缩-前展-回缩是航空煤油火焰驰豫脉动的主要模式,火焰蔓延平均速度与(To-Tc...     

典型壁面装饰材料的热释放速率模型

根据顺流火蔓延理论推导了适用于ISO 9705墙角火实验的壁面材料火蔓延的热释放速率数学模型,考虑到材料厚度对热释放速率的影响,在模型推导过程中引入了一个与材料厚度相关的参数α,同时在小尺寸实验得到的材料火灾特性数据基础上,利用发展出来的热释放速率模型,对4种不同厚度胶合板的全尺寸热释放速率进行预测,并将预测结果与全尺寸实验结果进行比较.结果表明,本文建立的数学模型能够较好地预测较厚材料的热释放速率变化趋势.     

贮木场木材加工剩余物的燃烧特性

将木材加工剩余物布置成燃烧床,进行木材加工剩余物室外燃烧实验,通过设置不同的木材加工剩余物的含水率、不同的外界风速,分析二者对燃烧床燃烧时周围的温度场、火蔓延速度、火线强度、火焰高度的影响.实验结果表明,在本实验工况下,当木材加工剩余物的含水率增大或风速减小时,温度场、火蔓延速度、火线强度、火焰高度均降低,反之,这4个指标均增大;当木材加工剩余物的含水率为6%,风速为4,m/s时,燃烧床周围温度场最高,平均火蔓延速度最快,火线强度最高.     

不同初始温度对航空煤油表面火蔓延的影响

采用微细热电偶、红外热像仪、CCD摄像及纹影技术对不同初始温度航空煤油表面火蔓现象及机理进行实验研究.结果表明:当航空煤油(开杯闪点为66℃)的初始温度T0≤82.5℃时,其火焰蔓延主要受液相控制,在此阶段表面流预热作用是火焰脉动蔓延的主要控制机制,火焰蔓延平均速度随初始温度升高呈指数增长关系;当航空煤油初始温度T0＞...     

小尺寸火焰传热阻碍规律的实验研究

应用小尺寸火蔓延实验装置(FPA),在不同的外加辐射强度和燃烧环境氧体积分数条件下,对两种常见聚合物PMMA和POM的燃烧特性和传热阻碍的测量方法进行了实验研究.研究发现,两种燃料的无量纲燃烧质量损失速率与无量纲外加辐射热流之间存在确定的通用线性关系,即"燃烧质量损失速率基线";火焰传热阻碍系数分别高达0.4和0.3,它随燃烧环境氧体积分数增加而增大,但不随外加辐射强度变化.研究为发展火蔓延理论模型和燃料可燃性测试方法提供了重要的参考依据.     

初始温度对柴油表面火蔓延的影响

通过自行搭建的液体火蔓延实验系统,运用高速纹影摄像技术和数码摄像机实时记录火蔓延的整个过程,分析了表面流和闪火焰出现的成因;结合微细热电偶测温技术,对火蔓延过程中的油面温升规律、速度变化规律进行了探讨;研究发现当燃油初始温度小于35,℃时,随温度升高,表面流作用被削弱,平均火蔓延速度降低;当燃油初始温度大于35,℃时,火蔓延速度随温度升高而逐渐增大.     

Sub-Atmospheric Pressure Coupled with Width Effect on Downward Flame Spread over Energy Conservation Material Polyurethane Foam

Rigid polyurethane(PUR)foam,a sustainable thermosetting building facade porous polymer material,has been widely applied in the construction industry for energy conservation.Additional knowledge of the fire safety performance of PUR foam at different altitudes and sample widths is required.Comparative lab-scale experiments were conducted in the Lhasa plateau(66.5 kPa)and the Hefei plain(99.8 kPa)in China.Flame propagation characteristics(average flame spread rate and flame height)were measured at different widths and atmospheric pressures of the test locations.Experimental results show that the dependence of dimensionless flame heights on sample width shows negative power law relationships with index of−w/5.4 to−w/5.8.Both flame height and flame spread rate were lower under low ambient pressure conditions as H fP0.26~0.33 and VfP0.057~0.568.Flame spread rate decreased with increasing sample width in the convection regime before a critical width of 4 cm–8 cm,after which the flame spread rate increased in the radiation regime.Results of this study contribute to the science of combustion,fire safety and energy conservation,and provide a basis for fire safety protocols for historical heritage buildings in the Lhasa plateau.     

Prediction of the critical condition for flame acceleration over wood surface with different sample orientations

To understand the inclination effects on flame spread over wood surface, a set of flame spread experiments were carried out for different sample orientation angles from ?50° to 20° in the Hefei Plain (at the altitude of 50 m) and in the Tibetan plateau (at the altitude of 3658 m). At both altitudes, a transition zone was found at 10–20° orientation for flame spread rate, the preheated length and flame angle from the horizontal. The transition zone was an external manifestation of the change of flame spread from steady state to acceleration. A new relationship of Π_c = 1 was established to predict the occurrence of acceleration based on theoretical analysis. Experimental data at the two altitudes suggested that the critical value of Π_c is about 1.1–1.2, which has a good agreement with the theoretical value.     

Postulations of flame spread mechanisms

An experimental study was conducted to explore the flame spread mechanism over thin solid fuel sheets. Flame spread rates over paper at various inclined angles were measured, and Schlieren photography was used to qualitatively assess heat transfer to the unburnt material in front of the pyrolysis zone. Two different types of flame spread were observed. One is te downward flame spread observed in the range of ?90 to ?30 deg from the horizontal. In this region, flame spread rate was almost constant with time, although it increased slightly with increasing angle. The other type of spread observed was the accelerative upward flame spread at angles of zero to 90 deg. Flame spread at angles from ?30 deg to zero seemed unsatable and increased by repetitive acceleration and deceleration In the range of inclined angles from ?90 to ?30 deg, heat transfer from the flame zone to the unburnt material seemed to take place mainly through the gas phase in the region 0.2 ～ 0.4 cm in front of the pyrolysis zone. In this case, the direction of the gas stream could be considered to oppose that of the flame spread. In the case of upward flame spread, the unburnt material in front of the pyrolysis zone seemed to be heated by convection of the bottom side, where the direction of the gas stream was obviously parallel with that of flame spread.     

Microgravity experiments on flame spread along fuel-droplet arrays at high temperatures

Microgravity experiments on droplet-array combustion were conducted under high-ambient-temperature conditions. n-Decane droplet arrays suspended on SiC fibers were inserted into a high-temperature combustion chamber and were ignited at one end to initiate the flame spread in high-temperature air. Flame-spread modes, burning behavior after the flame spread, and flame-spread rate were examined at different ambient temperatures. Experimental results showed that the appearance of flame-spread modes and the flame-spread rate were affected by the ambient temperature. The flame-spread rate increased with the ambient temperature. These facts are discussed based on the temperature effects on the droplet heating and the development of a flammable-mixture layer around the next droplet. A simple model was introduced to analyze these effects. The effects of the ambient temperature on the appearance of group combustion of the array after the flame spread and the scale effect in the flame spread are also discussed.     

A study of flame spread along a droplet array at elevated pressures up to a supercritical pressure

Experimental investigations on flame spread along a droplet array have been conducted at elevated pressures up to supercritical pressures of the fuel droplet under normal gravity and microgravity. The flame spread rate is measured using high‐speed chemiluminescence images of OH radicals and direct visualization is employed to observe the images of the vaporizing fuel around the unburnt droplet. The mode of flame spread is categorized into two: a continuous mode and an intermittent one. There exist a limit droplet spacing and a limit ambient pressure in normal gravity, above which flame spread does not occur. It is seen that flame spread rate is dependent upon the relative position of flame to droplet spacing. In microgravity, the limit droplet spacing of flame spread and the droplet spacing of maximum flame spread rate are larger than those in normal gravity. In microgravity, the flame spread rate with ambient pressure decreases initially, shows a minimum, and then decreases again after taking a maximum. Flame spread time is determined by competing effects between the increased transfer time of the thermal boundary layer due to reduced flame diameter and the decreased ignition delay time in terms of the increase of ambient pressure. In normal gravity, the flame spread rate with ambient pressure decreases monotonically and there exists a limit ambient pressure, except at small droplet spacing, under which flame spread extends to the range of supercritical pressures of fuel. This is because natural convection induces the upward flow of hot gases into a plume above the burning droplets and limits the lateral transfer of thermal boundary layer. Consequently, it is found that flame spread behaviour under microgravity is considerably different from that under normal gravity due to the absence of natural convection. Copyright © 1999 John Wiley & Sons, Ltd.     

Experimental studies on the three-dimensional effects of opposed-flow flame spread over thin solid materials

The three-dimensional effects of flame spread over thin solid materials were experimentally studied using a natural-convection-suppressing horizontal narrow-channel. In a sufficiently wide narrow-channel, the variation of flame spread against the width of the material sample showed different trends for different gas flow speeds and oxygen concentrations. The extent of three-dimensional effects was inversely proportional to the gas flow speed or its square. Near quenching extinction limits, the effects were significant because weak combustion is sensitive to a slight variation of heat loss and oxygen concentration. The effects may be due to different factors such as side heat loss, side oxygen diffusion, or both. Far away from quenching extinction limits, the effects were weak because vigorous combustion is insensitive to a small variation of oxygen concentration and heat loss. In all tests, the effects were limited to the samples of width less than 10 times of the diffusion length. Moreover, a higher oxygen concentration suppressed the effects at a lower gas flow speed. For sufficiently wide samples, in the most range of gas flow speeds, the channel width had almost no effect on flame spread. However, near extinction limits, the flame spread rate decreased with the increasing channel width.     

An embedded upward flame spread model using 2D direct numerical simulations

A fully coupled 2D fluid-solid direct numerical simulation (DNS) approach is used to simulate co-flow flame spread over poly(methyl methacrylate) (PMMA) at different angles of inclination. Comparison of simulations and experimental measurements are conducted over a range of flame spread rates. Results show that the heat flux to the preheating region varies considerably in time — contradicting often employed assumptions used in established flame spread theories. Accounting for the time dependent behavior is essential in accurate predictions of flame spread, however, a universal characterization in terms of easily defined parameters is not found. Alternatively, a reaction progress variable based embedded flame model is developed using mixture fraction, total enthalpy and surface temperature. State maps of the gas-phase properties and surface heat flux are constructed and stored in pre-computed lookup tables. The resulting model provides a computationally efficient and a local formulation to determine the flame heat flux to the surface resulting in excellent agreement to DNS and experiments for predictions of flame spread rate and position of the pyrolysis front.     

热薄材料表面火焰传播的三维效应

利用能够限制自然对流的水平窄通道,对薄材料表面逆风传播火焰的三维效应进行了实验研究,参数包括气流速度、氧气浓度、燃料宽度等.结果表明,在足够宽的通道内,火焰传播随燃料宽度的变化,表现出随氧气浓度和气流速度的不同而变化的三维特性.侧面热损失和氧气扩散对火焰传播的影响,在各种氧气浓度和气流速度下,都限于燃料宽度小于10倍扩散长度.     

TIME-DEPENDENT MODEL OF A BUOYANT DOWNWARD FLAME SPREAD OVER A THERMALLY THIN SOLID FUEL

A time-dependent model was developed and solved numerically to study a purely buoyant downward flame spread over a thermally thin solid fuel in various gravity environments. According to the specified burn-out solid density and no retained ash, the flame propagation behavior over a thin solid fuel surface could be simulated. At ignition, the flame is premixed. After several transition burnouts the flame transitions into a self-sustained steady spread diffusion flame. When the gravity level was varied, the Damkohler number effects were verified. An unstable flame spread was noted near the extinction limit at which the flame spread rate decelerated. Blow-off extinction was predicted after the flame spread a short distance. The ignition delay time increased with increasing gravity level. Compared with the experimental measurements, the predicted blow-off extinction limit was closer than that predicted by the steady combustion model.