微重力环境对层流射流扩散火焰形貌特征的影响

研究微重力环境下层流射流扩散火焰的形貌特征,可为航天领域中火灾探测的研究提供理论支持。本文首先选取可控的层流射流扩散火焰,对火焰炭黑特性及其对火焰温度和热辐射的影响进行了理论分析。其次,在国家微重力实验室落塔设备中进行实验,微重力水平可达10-2~10-3 g,实验结果表明,微重力环境下随着伴流空气速度增大火焰亮度增大,火焰辐射分数降低,火焰温度升高,同时火焰内高温区位于火焰两翼处。     

人工扰流条件下喷射火焰的倾角计算

综合考虑喷口直径及射流夹角等因素的影响,利用Thornton模型和相交射流理论推导得出人工扰流条件下的喷射火焰倾角计算公式。通过专门设计的实验装置对人工扰流条件下的丁烷射流燃烧形成的火焰倾角进行实例分析,考察火焰倾角特性及相关影响因素,探讨空气扰流速度变化对火焰倾角的影响,并对公式进行初步验证。结果表明:理论计算结果与实验结果规律吻合;火焰倾角与人工扰流速度以及两射流夹角成正比;同一人工扰流速度下,火焰倾角随着燃气射流速度的增加而减小。     

空气伴流条件下池火脉动特性研究

通过实验与MATLAB程序对空气伴流条件下的池火脉动规律特性进行研究。结果表明:在空气伴流速度不超过0.15 m/s时,火焰脉动频率小,脉动振幅大,燃烧时间短;在空气伴流速度增加至0.19 m/s后,火焰投影面积和平均高度增加,火焰集中性大幅增强,脉动振幅大幅减小,火焰脉动频率增加,燃烧时间增加30%以上。通过实验发现脉动特性改变的主要原因是,在低空气伴流速度下油盘上方存在周期性脱落的旋涡;在高空气伴流条件下由于开尔文-亥姆霍兹效应减弱,漩涡消失,流线平滑波动小。     

多孔陶瓷结构抑制甲烷爆炸实验研究

利用自主搭建的实验平台,对距离点火源 30、50、70 cm 和厚度为 2、4、6 cm 的多孔陶瓷抑制甲烷爆炸效果展开实验研究。结果表明,多孔陶瓷可延长层流火焰的传播时间,减缓火焰由层流向湍流的转变。爆炸火焰接触多孔陶瓷时发生淬熄,导致爆炸反应终止。多孔陶瓷与点火源之间距离的增加或多孔陶瓷厚度增加时,火焰锋面速度下降,但材料厚度对火焰锋面速度的影响效果相比于放置位置更加明显。超压上升速率随着多孔陶瓷与点火源距离的增加而降低,其与点火源距离较近即放置于 30 cm 处时,对超压的衰减效果最佳。增加多孔陶瓷的厚度时,甲烷爆炸超压峰值降低。     

家用燃气灶具火焰长度的试验研究

搭建了家用燃气灶具火焰长度试验台,在不同的工况下测量了灶具的火焰长度。火焰长度随着燃气压力增加和燃气流量增加而增加。燃烧中一次空气量要与燃气量相匹配,一般情况下,一次空气量增加,火焰长度减小。但一次空气量过大,会使火焰长度增加,影响燃气燃烧和锅的吸热。一次空气量过小,会导致扩散燃烧过程延长,火焰长度增加,不利于燃气燃尽。在一次空气调节挡板全关时,火焰由预混燃烧火焰转变为扩散燃烧火焰,火焰长度陡然增加很多。     

丙烷-空气预混火焰在三角形狭缝内传播特性

用计算流体力学方法对以丙烷为燃料的预混火焰进行数值模拟。研究预混火焰在等边三角形狭缝中传播时,狭缝尺寸、壁面初温、火焰初速对火焰淬熄长度的影响,并拟合火焰淬熄长度的计算公式。结果表明：狭缝尺寸、壁面初温以及火焰初速3个因素均与淬熄长度呈正相关。随着火焰初速的变快,淬熄长度增加放缓。在较高的火焰初速下,狭缝尺寸是影响淬熄长度的主要因素。拟合了较准确的淬熄长度计算公式。     

密闭管道内爆炸下限甲烷-空气混合及爆炸规律

探索爆炸下限的甲烷浓度分布对密闭管道内混合气体爆炸传播规律的影响。采用实验研究和数值模拟,分析管道内爆炸下限的甲烷-空气不均匀分布对混合气体爆炸超压、火焰传播距离、火焰传播速度的影响,并分析点火位置对混合气体可燃性的影响、管道长度对火焰传播的影响。相比于均匀混合气体,非均匀混合气体爆炸超压峰值更大,火焰传播速度更快。小尺度管道的长度增大,非均匀混合气体最大瞬态火焰传播速度随之增大。     

横向声波扰动下甲烷射流扩散火焰的频率响应

为了探索火焰在横向声波扰动下的振荡现象,基于高速火焰/纹影图像研究了开放环境中横向声波激励作用下甲烷射流扩散火焰的响应。外加声波激励的频率分别为5、10、15、20Hz,声波激励幅值均为3V,采用无声波激励组作为对照组。对时序彩色火焰图像处理得到火焰整体频谱图和火焰振荡主频云图,对时序火焰流场纹影图像处理得到火焰流场振荡主频云图。从火焰整体频谱图中发现:所有实验条件下火焰振荡主频均为10.7Hz,但从火焰振荡主频云图和流场振荡主频云图中却可以发现火焰振荡主频中存在非线性响应(20.5Hz和21.4Hz),横向声波激励和大尺度涡结构的相互作用影响这种局部非线性响应的出现。在流场振荡主频云图中可以发现喷嘴出口存在对应于外部声波激励的振荡频率,这是声波驱使射流周期性横向运动的结果。     

富氧燃烧火焰特性试验研究

利用自行设计的鼓风式燃烧器、火焰测量装置及富氧供气装置组成燃烧试验台,研究燃烧器以液化石油气为燃料,空气中氧体积分数在21％～30％时,富氧燃烧火焰的温度场、长度和锥角等火焰特性的变化情况。随着空气中氧体积分数的增加,火焰长度逐渐变短,火焰出口锥角逐渐增大,火焰温度梯度逐渐变大,燃烧强度逐渐增强。应用燃烧学理论证明了试验结果的正确性。     

苯-空气低压预混层流火焰燃烧特性研究

采用Chemkin软件中的预混层流火焰传播速度模型,对苯化学燃烧中的火焰传播速度、火焰温度、燃烧质量流量、预混火焰结构、苯的生成速率、敏感性分析与燃烧反应路径进行了模拟分析。结果表明:采用的CRECK机理模型可适应于研究混合气预混火焰燃烧;当量比等于1.1时,层流火焰传播速度、火焰温度和层流质量燃烧流量均达到最大值;OH、O和H是激发消耗苯反应的主要自由基;苯发生提氢反应对苯消耗的贡献率为98.5%,其中,由OH提氢反应的比例最高,占43.8%。     

Suppression of fuel and air stream diluted methane–air partially premixed flames in normal and microgravity

The effects of fuel and air stream dilution (ASD) with carbon dioxide on the suppression of normal and microgravity laminar methane–air partially premixed coflow jet flames were experimentally and numerically investigated. Experiments were conducted both in our normal-gravity laboratory and at the NASA Glenn Research Center 2.2 s drop tower. Measurements included flame topology and liftoff heights of diluted flames, critical diluent mole fractions for flame blowout, and the radiant heat loss from flames. The flames were also simulated using an axisymmetric unsteady numerical code that utilizes detailed chemistry and transport models. In addition, counterflow flame simulation results were used to examine similitude between the counterflow and coflow flame suppression, and further characterize the effectiveness of fuel stream versus ASD on flame extinction. A smaller relative fuel stream dilution (FSD) extinguishes partially premixed flames (PPFs) with increasing premixing as compared to dilution of the air stream. Conversely, smaller ASD is required to extinguish PPFs as they become less premixed and approach nonpremixed (NP) behavior. Fuel stream diluted PPFs and air stream diluted NP flames extinguish primarily through a reactant dilution effect while fuel stream diluted NP flames and air stream diluted PPF are extinguished primarily by a thermal cooling effect. Normal gravity flames lift off and blow out with a smaller diluent mole fraction than microgravity flames. The difference between the fuel and ASD effectiveness increases as the gravitational acceleration is reduced. Radiation heat losses are observed to increase with increasing diluent mole fraction and decreasing gravity.     

Laminar smoke points of coflowing flames in microgravity

Laminar smoke points were measured in nonbuoyant laminar jet diffusion flames in coflowing air. Microgravity was obtained on board the International Space Station. A total of 55 smoke points were found for ethylene, propane, propylene, and propylene/nitrogen mixtures. Burner diameters were 0.41, 0.76, and 1.6 mm, and coflow velocities varied from 5.4 to 65 cm/s. These flames allow extensive control over residence time via variations in dilution, burner diameter, and coflow velocity. The measured smoke-point lengths scaled with d^−0.91u_air^0.41, where d is burner diameter and u_air is coflow velocity. The measurements yielded estimates of sooting propensities of the present fuels in microgravity diffusion flames. Analytical models of residence times in gas jet flames are presented, and although residence time helps explain many of the observed trends it does not correlate the measured smoke points.     

Entrainment in fire plumes

A new technique for measurement of mass flow rates in buoyant fire plumes is described. The characteristics of 10–200 kW methane diffusion flames stabilized on porous-bed burners of 0.10–0.50 m dia. are described. A transition in the dependence of flame height on heat input and burner size was observed when the flame height was about four times the burner diameter. The mass flow rates in the buoyant plumes produced by the fires were measured for a range of elevations starting just below the time-averaged top of the flame and extending to six times this flame height. The mass flow rates in this region of the plume were correlated by the use of a simple plume model. Atmospheric and forced disturbances in the air being entrained increased the entrainment rate of the plume.     

Experimental study of suppressing effect of fine water droplets on propane/air premixed flames stabilized in the stagnation flowfield

Effect of fine water droplets on the laminar burning velocity of propane/air premixed flame was investigated by using a single jet-plate configuration. For the case without water droplets, the measured laminar burning velocities are in reasonably good agreement with previously reported data and the numerical simulation. The dependence of the burning velocity on the stretch rate for the case without water droplets is positive for all mixtures tested, resulting in the negative Markstein length, which coincides with previous experimental and theoretical studies. Water droplets lower significantly the laminar burning velocity and change its dependence on the stretch rate to negative. This leads to apparent positive Markstein lengths. The positive Markstein length was discussed on the basis of the droplets behavior in the stagnation flow field adopted in the present study. Even if the droplet mass loading was kept constant, the water droplets do not follow the diverting flow field when the stretch rate is high and the droplets accumulation occurs in the stagnation region where the burning velocities were measured. This fact results in the lower burning velocity as compared to that measured for uniformly dispersed water droplets.     

Acoustic extinction of laminar line-flames

A systematic study was conducted to elucidate the effects of acoustic perturbations on laminar diffusion line-flames burning in air, and to determine the conditions required to cause acoustically-driven extinction. Line-flames were produced from the fuels n-pentane, n-hexane, n-heptane, and n-octane using fuel-laden wicks. The wicks were housed inside a burner whose geometry produced line-flames that approximated a two-dimensional flame sheet. The acoustics utilized ranged in frequency from 30 to 50 Hz, and acoustic pressures from 5 to 50 Pa. Prior to acoustic testing, the unperturbed mass loss rates and flame heights were measured. These quantities were found to scale linearly, which is consistent with the Burke-Schumann theory. The mass loss rates associated with hexane-fueled flames experiencing acoustic perturbations were then studied. It was found that the strongest influence on the mass loss rate was the speed of oscillatory air movement experienced by the flame. It was also found that the average mass loss rate increased linearly with the increasing air movement speed. Finally, acoustic perturbations were imposed on the flames from all fuels to determine acoustic extinction criterion. To ascertain if the observed phenomenon was unique to the alkanes tested, flames fueled by JP-8 (a kerosene-based fuel) were also examined. Using the data collected, a model was developed which characterized the acoustic conditions required to cause flame extinction. The model was based on the ratio of a modified Nusselt number to the Spalding B number of the fuel. It was found that at the minimum speaker power required to cause extinction, this ratio was a constant (independent of the chemical nature of the fuel).     

Heat feedback to the fuel surface of a pool fire in an enclosure

As a part of an effort to determine the energy balance at the pool fire surface in compartments, a series of fire experiments were conducted to study heat flux of the flame in a vitiated environment formed with air and combustion products gases. This paper presents experimental results of the burning behaviour of a heptane pool fire in a reduced scale compartment equipped with a mechanical ventilation network. Measurements of heat fluxes, fuel mass loss rate, oxygen concentration and temperature are performed for heptane fires of 0.26 and 0.3 m diameter pans at different ventilation flow rates. An original method to separate effects of the radiant heat flux of the flame and of the external heat feedback to the fuel surface is developed. This was achieved by using an additional heat flux measurement located under the pool fire. A correlation was also developed to determine the temperature rise on the plume centerline in the compartment as a function of the heat release rate. The results indicate a decrease in the fuel mass loss rate, flame temperature and heat fluxes to the fuel surface as the oxygen concentration measured near the fuel decreases by varying the air refresh rate of the compartment. The flame radiation fraction shows a similar behaviour, whereas the convective fraction of the flame heat flux increases when oxygen concentration decreases. Based on these experimental findings, it was discussed that any classification of the burning regime of a pool fire should consider both the effects of pan diameter and the burning response to vitiated air.     

Metrological Tool for the Characterization of Flame Fronts Based on the Coupling of a Heat Flux Approach with Image Processing Data

The aim of this paper is to present some improvements in the metrology of forest fire flames by coupling image processing and radiative heat flux measurement. A new metrological tool using a visual video camera and a specific multiple thermal sensors is proposed. By means of an appropriate segmentation algorithm and the Direct Linear Transformation, the image processing methodology gives the forward or the backward fire front positions as input data to a radiative heat flux approach. Using a simplified flame model, this technique provides fire front positions versus time, and average values for the flame length, flame depth, flame tilt angle, apparent flame temperature and flame emissivity. The rates of spread, obtained by a linear regression of the determined fire front positions, as well as the thermal and radiative properties of the flame, are compared favorably to those given in the literature.