Laser-induced spark ignition of H<Subscript>2</Subscript>/O<Subscript>2</Subscript>/Ar mixtures

Laser-induced spark ignition of hydrogen-oxygen-argon mixtures was experimentally investigated using a Q-swiched Nd:YAG laser to break down the gas at 532 nm. The laser-based high-speed schlieren system was employed to record flame front evolution for the gas mixtures with different initial pressure or laser output energy or argon dilution. The results show that the breakdown of the gas leads to the generation of ellipsoidal plasma. The rarefaction waves create the toroidal rings at the leading and trailing edges of the plasma, which provides a reasonable explanation for inward wrinkle of the plasma and the resultant flame. The toroidal rings at leading edge decays more rapidly and a gas lobe is generated that moves towards the laser. The hot gas in the plasma induces the generation of the spark kernel. Affected by the very weak shock wave or compression waves reflected off the wall, the initial laminar flame decelerates. The arc flame front interactions with the wall, reversed shock wave or compression waves, rarefaction waves, etc. induce the transition from laminar flame to turbulent one. These induce the transition from laminar flame to turbulent flame. For stoichiometric hydrogen-oxygen mixtures diluted by 76.92% argon at an initial pressure of 53.33 kPa, the minimum output energy of the laser is 15 mJ for successful laser-induced spark ignition. With increasing initial pressure or the output energy of the laser, or decreasing argon dilution, the speed of the flame front increases.     

瓦斯爆炸火焰的分形特性

通过高速纹影图像从细观角度详细研究了不同传播条件下瓦斯爆炸火焰前锋的细微结构．实验及理论分析表明，瓦斯爆炸火焰有很好的分形特征．瓦斯爆炸火焰分形维数直接反应了火焰前锋皱折对火焰传播的影响．计算出的分形维数是衡量湍流火焰传播速度以及爆炸强度的有效参数．     

Study of the mechanisms of the flame propagation and stabilization in porous media

The CH4/air premixed gas combustion processes in porous media were numerically studied using the two-temperature reacting fluid model with dispersions and detailed chemical reaction mechanism GRI 3.0. The mechanisms of the propagation and stabilization of submerge flames and surface flames in porous media were illuminated distinctly by considering the magnitude of the terms in the two energy equations, analyzing the sensibility of flame propagation speed to flame location, heat exchange coefficient between gas and solid, thermal conductivity and radiative extinction coefficient of porous media. It was concluded that the propagation mechanism of a submerged flame is similar to that of a free flame with an additional preheat zone and that the surface-flame propagation mechanism in porous media is similar to that of a free flame with heat loss in reaction zone. Supported by the National Natural Science Foundation of China (Grant No. 50376060)     

Ignition,flame propagation and extinction in the supersonic mixing layer flow

The supersonic mixing layer flow, consisting of a relatively cold, slow diluted hydrogen stream and a hot, faster air stream, is numerically simulated with detailed transport properties and chemical reaction mechanisms. The evolution of the combustion process in the supersonic reacting mixing layer is observed and unsteady phenomena of ignition, flame propagation and extinction are successfully captured. The ignition usually takes place at the air stream side of braid regions between two vortexes due to much higher temperature of premixed gases. After ignition, the flame propagates towards two vortexes respectively located on the upstream and downstream of the ignition position. The apparent flame speed is 1569.97 m/s, which is much higher than the laminar flame speed, resulting from the effects of expansion, turbulence, vortex stretching and consecutive ignition. After the flame arrives at the former vortex, the flame propagates along the outer region of the vortex in two branches. Then the upper flame branch close to fuel streamside distinguishes gradually due to too fuel-riched premixed mixtures in the front of the flame and the strong cooling effect of the adjacent cool fuel flow, while the lower flame branch continues to propagate in the vortex.     

柴油机湍流扩散燃烧火焰面分形特性的研究

利用分形几何学的基本原理对柴油机湍流扩散燃烧火焰的表现特性进行了研究,对火焰前锋轮廓线进行了测量和分析,所得结果表明：柴油机中的湍流扩散燃烧火焰面也具有分形特性,从而实现了对扩散燃烧火焰面的定量描述     

Gas dynamics and heat transfer inside a solid propellant crack during ignition transient

To study the gas dynamic and heat transfer phenomena inside a single isolated longitudinal solid propellant surface crack, two 3-D geometric models with different crack shapes were constructed. Concerning the influence of propagation of jet from the igniter on the flame spreading phenomena in the crack, flow region around the opening of the crack was also included in the above geometric models. A theoretical framework was then adopted to model the conjugate heat transfer in the combustion channel and the crack cavity. Numerical simulation results indicate that the ignition shock wave can spread into the crack cavity. Extremely high overpressure and pressurization rate were observed along the crack front. It is possible that the crack may propagate before the flame front reaches it. An ignited region located at the crack front near to the channel surface in downstream direction was generated long before the flame front reached the crack opening in both models.     

超细水雾抑制瓦斯燃烧火焰的实验研究

通过搭建模拟回采工作面小尺寸实验平台,利用改造后的家用燃气灶作为燃烧器,研究了可燃预混气体火焰在超细水雾作用下火焰的变化过程,并测得了超细水雾作用于火焰过程中火焰的温度.通过改变通风速度和超细水雾的雾化速率,揭示了可燃预混气体在超细水雾氛围中火焰与温度的变化规律.通过建立稳定火焰面的力和速度平衡,找出了火焰形状拉伸扭曲的原因是破坏了火焰外焰和热气层交界面上力和速度的平衡;在其他条件一定的情况下,向模拟工作面上施加超细水雾后,火焰周围流场扰动明显增大,与在单独通风条件下相比,火焰温度跳动变化幅度和频率增大;在保证超细水雾质量浓度不变条件下,通风速度越大,则混合气体扰动越大,温度的跳动就越大,火焰的燃烧就越不稳定.实验结果表明超细水雾能很好地抑制瓦斯火焰的燃烧.     

受限空间瓦斯爆炸火焰与毒气传播研究

为揭示瓦斯爆炸过程中火焰、毒气及压力三者间相互关系,采用一端封闭的爆炸试验装置,通过改变瓦斯聚集长度和点火强度,研究了瓦斯爆轰及爆燃状态下火焰、毒气及压力传播变化规律.结果表明,管道内瓦斯爆燃状态下火焰的传播速度远小于爆轰状态下的传播速度,变化趋势呈线性;瓦斯爆炸火焰传播速度的大小直接影响爆轰的形成以及爆炸强度和爆炸传播距离;爆燃状态下火焰和毒气传播的距离基本相当,均为原始瓦斯聚集总长度的2倍左右;爆轰状态下火焰和毒气传播的距离基本相当,均大于原始瓦斯聚集长度,但传播距离不确定.     

Effect of variation in gas distribution on explosion propagation characteristics in coal mines

In order to investigate the effect of variation in the distribution of gas on explosion propagation characteristics in coal mines, experiments were carried out in two different channels with variation in gas concentration and geometry. Flame and pressure transducers were used to track the explosion front velocity. The flame speed (Sf) showed a slight downward trend while the methane concentration varied from 10% to 3% in the experimental channel. The peak overpressure (Pmax) dropped dramatically when com-pared with normal conditions. As well, the values of Pmax and Sf decreased when the methane concentration dropped from 8% to 6%. The flame speed in the channel, connected to a cylinder with a length varying from 0.5 to 2 m, was greater than that in the normal channel. The peak overpressure was also higher than that under normal conditions because of a higher flame speed and stronger pressure piling up. The values of Pmax and Sf increased with an increase in cylinder length. The research results indicate that damage caused by explosions can be reduced by decreasing the gas concentration, which should be immediately detected in roadways with large cross-sections because of the possibility of greater destruction caused by more serious explosions.     

预混气体爆轰波传播定性仿真建模研究

借助定性仿真,建立预混气体爆轰波的传播定性模型,推理分析不同初始压力下,预混气体爆轰波传播速度,研究预混气体爆轰波爆轰过程中传播速度与初始压力的关系,且采用定量的实验验证定性的建模推理分析结果.结果表明,定性推理分析的结果与实验结果是一致的,即随初始压力的逐渐增大,爆轰波的传播速度逐渐加大.爆轰波传播定性建模推理分析对安全传输可燃性气体和管道阻火器的安全使用具有重大意义.     

Influence of obstacle-produced turbulence on development of premixed flames

An investigation into influence of obstructions on premixed flame propagation has been carried out in a semi-open tube. It is found that there exists flame acceleration and rising overpressure along the path of flame due to obstacles. According to the magnitude of flame speeds, the propagation of flame in the tube can be classified into three regimes: the quenching, the choking and the detonation regimes. In premixed flames near the flammability limits, the flame is observed first to accelerate and then to quench itself after propagating past a certain number of obstacles. In the choking regime, the maximum flame speeds are somewhat below the combustion product sound speeds, and insensitive to the blockage ratio. In the more sensitive mixtures, the transition to detonation (DDT) occurs when the equivalence ratio increases. The transition is not observed for the less sensitive mixtures. The dependence of overpressure on blockage ratio is not monotonous. Furthermore, a numerical study of flame acceleration     

陶瓷窑炉中富氧燃烧火焰特性的试验研究

以单节辊道窑的结构形式设计物理模型进行富氧燃烧试验研究,在不同氧浓度条件下,从火焰形貌、火焰温度分布、火焰传播等方面分析富氧燃烧技术对火焰特性的影响,为窑炉设计及富氧喷枪设计与布置提供依据。研究结果表明:当氧浓度从21%增加至30%时,火焰亮度逐渐增加;火焰长度逐渐缩短并向喷枪口处收缩;最高火焰温度明显提高,且火焰最高温度出现位置向喷枪口靠近;轴向火焰温度分布趋向集中,在设计窑炉时应该注意火焰长度方向的尺寸;火焰前沿速度逐渐降低,引起温度梯度变大,火焰覆盖面积减小,需要设计可用于富氧燃烧的可调节式燃烧器,以保证在不同氧气浓度下,火焰处于稳定燃烧状态。     

瓦斯爆炸过程中爆炸波的结构变化规律

在实验的基础上，研究了管内瓦斯爆炸过程中爆炸波结构变化规律。研究结果表明，管内瓦斯爆炸过程中所产生的爆炸波有两个峰值，第一峰值为爆炸波压力，第二峰值压力为火焰到达后加热气体升压所致。形成激波后，爆炸波两峰值往下游传播越来越靠近，测试结果与爆炸传播理论是相互验证的。     

二级环流对双级贫预混火焰熄火边界的影响

为改善预混火焰的燃烧稳定性,提出双级贫预混燃烧的火焰组织思路,对预混气流进行不同当量比的径向分级,使其能在较低当量比时稳定燃烧.以一定流动条件下直管射流火焰和双级贫预混火焰的中心气流所能达到的最小稳燃当量比作为熄火边界,实验测量并分析火焰不同组织方式和环流条件变化对熄火边界的影响.结果表明:布置在射流火焰外围的环流对火焰根部附近的浓度和速度分布有一定影响;双级贫预混火焰的二级预混环流能够通过减弱外界环境气氛对锋面前预混气的稀释作用而拓宽火焰的熄火边界;环流的预混当量比是影响火焰熄火边界的关键.与均一当量比的直管射流火焰相比,双级贫预混火焰能够在更低的当量比条件下稳定燃烧,对工业上控制NOx排放有着重要的意义.     

预混火焰胞状不稳定性研究

采用高速纹影系统和定容燃烧弹对预混球形膨胀火焰的胞状不稳定性进行研究,分析火焰胞状不稳定性的表现、原因以及影响因素。研究结果表明：火焰胞状不稳定性主要指不等扩散不稳定和流体力学不稳定;流体力学不稳定的原因为,曲率的关系会使其流线发散或汇聚,因而相应地降低或增加了流场的速度,其主要影响因素为火焰厚度和密度比,表现为火焰前锋面产生裂纹随后出现规则的胞;不等扩散不稳定的原因为前锋面包络的控制体由于热质不等扩散的缘故,主要影响因素为刘易斯数,表现为火焰表面布满不规则的胞。     

瓦斯爆炸过程中火焰传播规律的模拟研究

在模拟实验和数值计算的基础上 ,研究了瓦斯爆炸过程中火焰传播规律及其加速机理 .研究结果表明 ,障碍物对瓦斯爆炸过程中火焰传播规律有重要影响 .障碍物的存在将使瓦斯爆炸过程中火焰的传播速度迅速提高 .瓦斯爆炸时 ,火焰阵面附近温度较高 ,阵面前附近区域温度梯度变化较大 ,阵面后区域的温度变化较小 .障碍物附近温度很快上升到最大值 ,然后由于化学反应结束及管道壁吸热 ,温度开始下降 .在火焰传播通道上设置的障碍物对气相火焰具有加速作用 ,加速作用的机理主要是由于障碍物诱导的湍流区对燃烧过程的正反馈造成的     

瓦斯爆炸运动火焰生成压力波的数值模拟

从三维N-S方程出发，用TVD格式，对瓦斯爆炸过程中火焰产生压力波的过程进行了数值模拟，在此基础上，模拟了氢氧燃烧驱动的破膜过程以及破膜前后压缩波、稀疏波对火焰阵面的影响，同时，也研究了瓦斯爆炸过程中，压力波、火焰与障碍物的相互作用，数值模拟结果与理论分析吻合较好，从而进一步验证了该程序能处理含有化学反应和复杂管道的预混可燃气体爆炸问题。     

可控活化热氛围下喷射燃料自燃的研究方法

阐述了湍流反应流中自燃的研究方法及现状,对可控活化热氛围下的气体燃料及液体燃料的自燃现象分别开展了试验研究,提出了一个研究液体燃料均质混合气自燃的新方式。结果表明:甲烷气体燃料火焰起升高度随协流温度的升高而降低,呈现出两阶段变化特点,低温段发生了明显的自燃现象,其火焰稳定由均质混合气自燃所主导;柴油液雾燃烧试验观察到了快速的多点自燃现象,形成了独特的起升火焰,有三重火焰的特征。试验结果验证了所提出的研究液体燃料自燃方法的可行性。     

Numerical simulation for explosion wave propagation of combustible mixture gas

A two-dimensional multi-material code was indigenously developed to investigate the effects of duct boundary conditions and ignition positions on the propagation law of explosion wave for hydrogen and methane-based combustible mixture gas. In the code,Young’s technique was employed to track the interface between the explosion products and air,and combustible function model was adopted to simulate ignition process. The code was employed to study explosion flow field inside and outside the duct and to obtain peak pressures in different boundary conditions and ignition positions. Numerical results suggest that during the propagation in a duct,for point initiation,the curvature of spherical wave front gradually decreases and evolves into plane wave. Due to the multiple reflections on the duct wall,multi-peak values appear on pressure—time curve,and peak pressure strongly relies on the duct boundary conditions and ignition position. When explosive wave reaches the exit of the duct,explosion products expand outward and forms shock wave in air. Multiple rarefaction waves also occur and propagate upstream along the duct to decrease the pressure in the duct. The results are in agreement with one-dimensional isentropic gas flow theory of the explosion products,and indicate that the ignition model and multi-material interface treatment method are feasible.