非线性拉伸对甲烷-空气预混火焰燃烧特性的影响

使用定容燃烧弹与高速纹影照相系统研究了不同当量比下甲烷-空气预混气体的层流火焰燃烧特性。实验数据同时应用传统线性模型和非线性模型分析了不同当量比对球形扩展火焰的传播速率和马克斯坦长度的影响。结果显示:随着当量比的增加,层流燃烧速率先增大后减小,直到当量比为1.1时,火焰速率达到最大值。马克斯坦长度始终为正值,且随着当量比的增大而增大。在所有当量比条件下,线性和非线性方法计算的火焰速率大致相同,差值小于0.01 m/s;线性方法得到的马克斯坦长度均大于非线性模型计算的结果,并随着当量比的增大,两种方法得到的马克斯坦长度的差值更加显著。     

甲烷-空气预混气体燃烧特性研究

采用纹影系统、压力传感器和高速相机对甲烷-空气预混气体在定容燃烧弹中的燃烧特性进行研究,分析了当量比对拉伸火焰传播速度、未拉伸火焰传播速度和层流燃烧速度的影响及定容弹中压力的变化规律。结果表明,当量比对预混气体燃烧过程有重要影响,且存在临界当量比1.1,在临界当量比下预混气体燃烧最剧烈,层流燃烧速度达到最大值(0.368 m/s),燃烧压力也达到峰值(0.703 MPa)。当预混气体当量比小于临界值时,拉伸火焰传播速度、未拉伸火焰传播速度、层流燃烧速度和燃烧压力随当量比增加而增加;而当预混气体当量比大于临界值时,速度和压力随当量比增加而减小。     

掺氢天然气层流火焰传播速度试验研究

为准确测量掺氢天然气层流预混火焰传播速度,并研究掺氢比对掺氢天然气层流预混火焰传播特性的影响,通过本生灯法对比试验研究自然光及纹影拍摄条件下掺氢天然气层流预混火焰的传播速度,根据化学动力学机制模拟计算并讨论不同掺氢比条件下的预混燃料层流火焰传播特性。研究表明:利用纹影系统拍摄获得的火焰传播速度更接近燃烧学定义的层流预混火焰传播速度;随着掺氢比的增加,掺氢天然气层流预混火焰传播速度及绝热火焰温度均不断增加,且层流预混火焰传播速度峰值所对应的当量比显著向富燃料侧移动;燃料中氢气组分的不断增加使得H自由基的摩尔分数以及OH自由基的生成速率均显著增加。     

甲烷体积分数对甲烷-煤粉复合火焰传播特性的影响

为探究甲烷体积分数对煤粉爆炸过程的影响,并掌握甲烷-煤粉爆炸火焰传播特征,通过粒度分析仪和同步热分析仪研究2种煤粉样品的粒径大小和热解过程。利用1 500 mm×80 mm×80 mm的半开口竖直燃烧管道,探究不同甲烷体积分数下,中位粒径分别为65和25μm烟煤粉的火焰传播特性,分析甲烷体积分数对甲烷-煤粉复合火焰结构、温度和速度的影响。结果表明:25μm煤粉比65μm煤粉的火焰更加明亮,甲烷体积分数的增加对65μm煤粉火焰有更强的促进作用;当甲烷体积分数越接近当量比时,火焰锋面越规则,火焰速度也越快;随着甲烷体积分数的增加,火焰温度和火焰传播速度均呈现先增大后减小的趋势;甲烷体积分数为9%时,火焰温度达到最大值;甲烷体积分数为8%和10%时,65和25μm煤粉最大火焰速度为分别为26.53和39.28 m/s。     

Hele-Shaw通道内瓦斯爆炸火焰特性试验研究

为探究甲烷/空气预混气体当量比对Hele-Shaw通道内火焰爆燃特性的影响，自行设计搭建尺寸(长×宽×厚)为950 mm×200 mm×6 mm的透明有机玻璃瓦斯爆炸管道试验平台。通过改变试验平台厚度研究通道间隙对甲烷/空气预混气体火焰结构与传播特性的影响。结果表明，不同的通道厚度和当量比对火焰锋面结构和火焰传播动态特性有显著影响。当通道厚度为6 mm时，最大火焰传播速度发生在化学当量比下，为12.84 m/s,且在该工况下火焰最先到通道末端，时间为518.57 ms。当量比为0.8的贫燃状态时，在火焰不稳定性的作用下，火焰传播后期出现二次振荡现象及手指形的火焰锋面。随着通道厚度的减小，火焰到达通道末端的时间逐渐变长，对火焰整体传播速度有明显的抑制作用。     

添加H2对生物质气层流燃烧特性的影响

为研究H₂对生物质气层流燃烧特性的影响，在内径和长度均为200 mm,点火位置为正中央的圆柱形定容燃烧弹内进行当量比为0.6～1.4、H₂体积分数为7.15%～33.15%的掺氢生物质气/空气燃烧试验。利用高速摄像仪采集火焰传播图片，并通过计算分析软件Chemkin-Pro进行化学反应路径分析。研究结果表明：添加H₂对生物质气/空气的层流燃烧速度起促进作用，在贫燃侧增长效应平稳而在富燃侧增长效应显著，使火焰的不稳定性增强；火焰结构中主要自由基浓度的增大是引起层流燃烧速度增大的主要化学动力学原因；H₂主要通过参与HCO基团氧化加快反应速率，富燃侧层流燃烧速度显著增大的原因是H₂作用的路径多为有O₂参与的反应。     

甲烷对合成气层流预混火焰传播特性的影响

为研究甲烷对合成气层流预混火焰化学动力学特征及合成气层流预混火焰传播特性的影响,选用GRI3.0-机理,模拟研究300 K条件下含0～70%甲烷的合成气层流预混火焰传播速度、火焰温度、反应敏感性及重要自由基浓度等。结果表明:合成气层流预混火焰传播速度、绝热火焰温度随甲烷比例的增加非线性下降,火焰传播速度峰值对应的当量比随甲烷增加显著向贫燃侧发展;富燃条件下,随着甲烷少量加入(≤30%),火焰中自由基H浓度显著下降,火焰传播速度受抑制作用显著;随着甲烷的继续增加,不足的自由基OH抑制自由基CH_3的生成,影响自由基H的消耗,削弱CH_4对层流预混火焰传播速度的抑制作用。     

置障长管内丙烷-空气爆炸过程的流场特性研究

本文构建了12 m×0.125 m的大长径比密闭管道的二维模型,运用计算流体动力学软件Fluent,基于Realizable k-ε湍流模型和预混燃烧模型,对有障碍物条件下丙烷-空气爆炸过程中湍流对火焰的加速机理进行数值模拟研究,重点分析不同阻塞率对流场微观特性的影响规律。结果表明,阻塞率对管道内流场特性的影响十分明显,在一定范围内,阻塞率越大,火焰锋面前后的速度梯度越高,引起的湍流涡旋规模越大,导致火焰阵面的变形程度越严重,使得火焰锋面传播速度以及气体的扩散速度也越快。     

基于Matlab图像处理的瓦斯爆炸火焰传播速度研究

基于自制小尺寸试验平台进行不同体积分数瓦斯爆炸试验,运用高速摄像机拍摄爆炸图像,直观分析火焰传播过程。利用Matlab软件对拍摄图像进行数字化处理,求得不同瓦斯体积分数下的爆炸火焰传播速度。结果表明:基于图像处理的方法能够简单准确地计算火焰传播速度,火焰亮度阈值的取值对火焰传播速度计算结果有重要影响;火焰传播速度随时间逐渐增大,当出现"郁金香"火焰后略有降低,随后继续增大;瓦斯体积分数对平均火焰传播速度有较大影响,混合当量比为1时火焰传播速度最大,在混合当量比小于1时,火焰传播速度受混合当量比影响较大,而当混合当量比大于1时,火焰传播速度受混合当量比影响较小。     

聚磷酸铵对糖粉火焰传播特性的影响研究

为明确聚磷酸铵(APP)对糖粉粉尘火焰燃烧特性的影响,采用竖直开口方管道燃烧试验平台,研究不同质量分数APP对糖粉粉尘爆燃火焰特征、火焰速度、火焰温度等参数的影响,并采用热分析仪对糖粉及其APP混合物的热解行为进行了分析。结果表明:随着APP质量分数的增加,糖粉粉尘火焰的亮度逐渐降低;添加质量分数为6%的APP后,糖粉火焰在管道中的最大传播速度由17. 3 m/s降低到2. 8 m/s,火焰最高温度降低了46. 46%,糖粉热解残余量由0. 71%提高到16. 06%; APP对糖粉火焰抑制作用包括物理抑制和化学抑制,APP热分解反应可吸收燃烧反应中的热量,分解产物可捕捉提燃烧反应中的自由基,从而达到抑制糖粉火焰传播的目的。     

Experimental study of the effect of nitrogen addition on gas explosion

Flame propagation and combustion characteristics of methane/air mixed gas in gas explosion were studied in a constant volume combustion bomb. Stretched flame propagation velocity, unstretched laminar flame propagation velocity, unstretched laminar combustion velocity and Markstein length were obtained at various ratios of nitrogen to gas mixture. Combustion stability at various ratios of nitrogen to gas mixture was analyzed by analyzing the pictures of flame propagation. Furthermore, the effect of initial pressure on the flame propagation and combustion characteristics of methane/air mixed gas in gas explosion was analyzed. The results show that the unstretched laminar flame propagation velocity, the unstretched laminar combustion velocity, Markstein length, flame stability, and the maximum combustion pressure decrease distinctly with the increase of nitrogen fraction in the gas mixture. At the same ratios of nitrogen to gas mixture, Markstein length, unstretched laminar flame propagation velocity and unstretched laminar combustion velocity decrease and the maximum combustion pressure increase with the increase of initial pressure of the gas mixture. When nitrogen fraction in the gas mixture is over 20%, the flame will be unstable and is easy to exterminate.     

Modeling the initial flame acceleration in an obstructed channel using large eddy simulation

The propagation and acceleration of a flame surface past obstructions in a closed square channel was investigated using large eddy simulation. The dynamic Smagorinsky–Lilly subgrid model and the Boger flame surface density combustion model were used. The geometry is essentially two-dimensional with fence-type obstacles distributed on the top and bottom surfaces, equally spaced along the channel length at the channel height. Flame propagation, however, is three dimensional as ignition occurs at a point at the center of the channel cross-section. The effect of obstacle blockage ratio on the development of the flame structure was investigated by varying the obstacle height. Three-dimensional cases were simulated from the initiation of a combustion kernel through spark ignition to the acceleration of the flame front at speeds up to 80 m/s. The transition from laminar flame propagation to turbulent flame propagation within the “thin reaction zone” regime was observed in the simulations. By analyzing the development of the three dimensional flame surface and unburned gas flow field, the formation of several flame structures observed experimentally are explained. Global quantities such as the total flame area and centerline flame velocity were ascertained and compared to the experimental data. High amplitude oscillations in the centerline flame velocity were found to occur from a combination of the unburned gas flow field and fluctuations in the volumetric burning rate.     

Effects of particle characteristics on flame propagation behavior during organic dust explosions in a half-closed chamber

To reveal the effects of particle characteristics on the mechanisms of flame propagation during organic dust explosions clearly, three long chain monobasic alcohols which are solids at room temperature and have similar physical–chemical properties were chosen to carry out experiments in a half-closed small chamber. A high-speed video camera was used to record the flame propagation process and to obtain the direct light emission photographs. Flame temperature was detected by a fine thermocouple. Based on the experimental results above, analysis was conducted on flame propagation characteristics and temperature profiles of organic particle cloud. As a result, it was found that the particle materials, especially volatility, strongly affected the flame propagation behavior. Particle concentration also affects the combustion zone propagation process significantly. With increasing the particle concentration, the maximum temperature of the combustion zone increases at the lower concentration, reaches a maximum value, and then decreases at the higher concentration. The propagation velocity of the combustion zone has a linear relationship with the maximum temperature, which implies conductive heat transfer is dominant in the flame propagation process of the three different volatile dusts.     

Effect of flame propagation regime on pressure evolution of nano and micron PMMA dust explosions

Experiments using an open space dust explosion apparatus and a standard 20 L explosion apparatus on nano and micron polymethyl methacrylate dust explosions were conducted to reveal the differences in flame and pressure evolutions. Then the effect of combustion and flame propagation regimes on the explosion overpressure characteristics was discussed. The results showed that the flame propagation behavior, flame temperature distribution and ion current distribution all demonstrated the different flame structures for nano and micron dust explosions. The combustion and flame propagation of 100 nm and 30 μm PMMA dust clouds were mainly controlled by the heat transfer efficiency between the particles and external heat sources. Compared with the cluster diffusion dominant combustion of 30 μm dust flame, the premixed-gas dominant combustion of 100 nm dust flame determined a quicker pyrolysis and combustion reaction rate, a faster flame propagation velocity, a stronger combustion reaction intensity, a quicker heat release rate and a higher amount of released reaction heat, which resulted in an earlier pressure rise, a larger maximum overpressure and a higher explosion hazard class. The complex combustion and propagation regime of agglomerated particles strongly influenced the nano flame propagation and explosion pressure evolution characteristics, and limited the maximum overpressure.     

有障碍物半开口管道内氢气燃烧数值模拟研究

基于有障碍物氢气燃烧实验装置进行数值模拟研究,采用Fluent软件分析了半开口管道内障碍物对氢气/空气燃烧特性的影响。结果表明:障碍物会促进实验管段内氢气火焰加速,随着障碍物阻塞率和数量的增加,火焰加速更快且燃烧压力峰值更大;在相同阻塞率下,障碍物形状对氢气火焰速度和燃烧压力峰值的影响很小;燃烧压力随障碍物间距的增大先增大后减小,障碍物间距为3倍管道内径时产生的燃烧压力峰值最大。     

Flame propagation through aluminum particle cloud in a combustion tube

The propagation of a flame is investigated experimentally and theoretically for a large, horizontal combustion tube containing a mixture of air and aluminum powder with pre-existing turbulence. One end of the tube is closed and the other is connected to a large dump-tank. Twenty dispersion systems are used on the tube to produce a uniform suspension of aluminum dust in the tube with a mean diameter of 6 μm. The characteristics of a flame front from the ignitors at the closed end are measured using photodiodes and the development of pressure is monitored by transducers. Experimental results revealed the entire process of an accelerating flame and the development of shock waves. A set of conservation equations for two-phase turbulent combustion flow is derived, using the two-fluid model, k–ε model, Hinze–Tchen model and EBU-Arrhenius model for turbulent combustion. The SIMPLE scheme usually applied to the homogeneous turbulent combustion is extended to fit this two-phase, reactive behavior. The results of calculations show the positive feedback coupling among combustion, expansion and turbulence during flame propagation. Computed and measured results are generally in good agreement.     

Flame propagation behaviors of nano- and micro-scale PMMA dust explosions

Flame propagation behaviors of nano- and micro-polymethyl methacrylate (PMMA) dust explosions were experimentally studied in the open-space dust explosion apparatus. High-speed photography with normal and microscopic lenses were used to record the particle combustion behaviors and flame microstructures. Simple physical models were developed to explore the flame propagation mechanisms. High-speed photographs showed two distinct flame propagation behaviors of nano- and micro-PMMA dust explosions. For nano-particles, flame was characterized by a regular spherical shape and spatially continuous combustion structure combined with a number of luminous spot flames. The flame propagation mechanism was similar to that of a premixed gas flame coupled with solid surface combustion of the agglomerates. In comparison, for micro-particles, flame was characterized by clusters of flames and the irregular flame front, which was inferred to be composed of the diffusion flame accompanying the local premixed flame. It was indicated that smaller particles maintained the leading part of the propagating flame and governed the combustion process of PMMA dust clouds. Increasing the mass densities from 105 g/m³ to 217 g/m³ for 100 nm PMMA particles, and from 72 g/m³ to 170 g/m³ for 30 μm PMMA particles, the flame luminous intensity, scale and the average propagation velocity were enhanced. Besides, the flame front became more irregular for 30 μm PMMA dust clouds.     

基于燃烧相互作用的PMMA相向火蔓延特性实验研究

为探究耦合燃烧作用对固体可燃物火蔓延的影响,开展基于燃烧相互作用的聚甲基丙烯酸甲酯(polymethyl methacrylate, PMMA)相向火蔓延特性实验研究。通过对不同宽度PMMA板进行相向火蔓延实验,获取火焰图像、温度场、质量损失速率等燃烧特性参数,分析相向火蔓延的过程特点与燃烧机理。研究结果表明：相向火蔓延过程中存在4个典型阶段,即快速发展阶段、相对稳定阶段、相互作用阶段、融合燃尽阶段;PMMA板宽度对相向火蔓延燃烧特性的影响较为显著,体现在热解区长度、相对稳定状态维持时间、质量损失速率等参数变化上。研究结果可为建筑物保温材料的火灾预防抑制提供参考。     

Temperature profile across the combustion zone propagating through an iron particle cloud

Knowledge of the mechanism of combustion zone propagation during dust explosion is of great importance to prevent damage caused by accidental dust explosions. In this study, the temperature profile across the combustion zone propagating through an iron particle cloud is measured experimentally by a thermocouple to elucidate the propagation mechanism. The measured temperature starts to increase slowly at a position about 5 mm ahead of the leading edge of the combustion zone, increases quickly at a position about 3 mm ahead of the leading edge, reaches a maximum value near the end of the combustion zone, and then decreases. As the iron particle concentration increases, the maximum temperature increases at lower concentration, takes a maximum value, and then decreases at higher concentration. The relation between the propagation velocity of the combustion zone and the maximum temperature is also examined. It is found that the propagation velocity has a linear relationship with the maximum temperature. This result suggests that the conductive heat transfer is dominant in the propagation process of the combustion zone through an iron particle cloud.