气相爆轰波在分叉管中传播现象的数值研究

数值研究气相爆轰波在分叉管中的传播现象.用二阶附加半隐龙格-库塔法和5阶WENO格式求解二维欧拉方程,用基元反应描述爆轰化学反应过程,得到了密度、压力、温度、典型组元质量分数场及数值胞格结构和爆轰波平均速度.结果表明:气相爆轰波在分叉管中传播,分叉口左尖点的稀疏波导致诱导激波后压力、温度急剧下降,诱导激波和化学反应区分离,爆轰波衰减为爆燃波(即爆轰熄灭).分离后的诱导激波在垂直支管右壁面反射,并导致二次起爆.畸变的诱导激波在水平和垂直支管中均发生马赫反射.分叉口上游均匀胞格区和分叉口附近大胞格区的边界不是直线,其起点通常位于分叉口左尖点上游或恰在左尖点.水平支管中马赫反射三波点迹线始于右尖点下游.分叉口左尖点附近的流场中出现了复杂的旋涡结构、未反应区及激波与旋涡作用.旋涡加速了未反应区的化学反应速率.反射激波与旋涡作用并使旋涡破碎.反射激波与未反应区作用,加速其反应消耗,并形成一个内嵌的射流.数值计算得到的波系演变和胞格结构与实验定性一致.     

Flame acceleration and transition to detonation: Effects of a composition gradient in a mixture of methane and air

The effects of a composition gradient on flame acceleration and transition to detonation in a mixture of methane and air were studied by numerically solving the unsteady, fully compressible, reactive Navier–Stokes equations. The specific problem addressed here is for ignition in a two-dimensional, obstructed channel where there is a spatial gradient of equivalence ratios perpendicular to the propagation direction of the reaction wave. The solution method uses a calibrated, optimized chemical-diffusive model that reproduces correct flame and detonation properties for methane–air mixtures over a range of equivalence ratios. Comparisons were made to a stoichiometric, homogeneous mixture in order to focus on the worst-case scenario for safety concerns. The results showed that the flame speed is smaller and the average total heat release are lower, but the maximum flame surface area is larger in the inhomogeneous mixture. This is because there is more unburned material between obstacles but less energy released from this increased flame surface area in the fuel-lean region, leading to the reduction of the total heat release. The transition to detonation is delayed in the inhomogeneous mixture, because the hot spot forms in the fuel-lean region and the strength of the Mach stem that hits the obstacle is weaker. The detonation front tends to decouple into a shock and a flame earlier in the inhomogeneous mixture, due to the incomplete mixing throughout the entire domain during the detonation propagation process.     

Detonation simulations in supersonic flow under circumstances of injection and mixing

The unsteady, reactive Navier-Stokes equations with a detailed chemical mechanism of 11 species and 27 steps were employed to simulate the mixing, flame acceleration and deflagration-to-detonation transition (DDT) triggered by transverse jet obstacles. Results show that multiple transverse jet obstacles ejecting into the chamber can be used to activate DDT. But the occurrence of DDT is tremendously difficult in a non-uniform supersonic mixture so that it required several groups of transverse jets with increasing stagnation pressure. The jets introduce flow turbulence and produce oblique and bow shock waves even in an inhomogeneous supersonic mixture. The DDT is enhanced by multiple explosion points that are generated by the intense shock wave focusing of the leading flame front. It is found that the partial detonation front decouples into shock and flame, which is mainly caused by the fuel deficiency, nevertheless the decoupled shock wave is strong enough to reignite the mixture to detonation conditions. The resulting transverse wave leads to further mixing and burning of the downstream non-equilibrium chemical reaction, resulting in a high combustion temperature and intense flow instabilities. Additionally, the longitudinal and transverse gradients of the non-uniform supersonic mixture induce highly dynamic behaviors with sudden propagation speed increase and detonation front instabilities.     

凝聚炸药爆轰波在高声速材料界面上的折射现象分析

凝聚炸药爆轰在边界高声速材料约束下传播时,爆轰波会在约束材料界面上产生复杂的折射现象.本文针对凝聚炸药爆轰波在高声速材料界面上的折射现象展开理论和数值模拟分析.首先通过建立在爆轰ZND模型上的改进爆轰波极曲线理论给出爆轰波折射类型,然后发展一种求解爆轰反应流动方程的基于特征理论的二阶单元中心型Lagrange计算方法来数值模拟典型的爆轰波折射过程.从改进爆轰波极曲线理论和二阶Lagrange方法数值模拟给出的结果看出,凝聚炸药爆轰波在高声速材料界面上的折射类型有四种:反射冲击波的正规折射、带束缚前驱波的非正规折射、带双Mach反射的非正规折射、带λ波结构的非正规折射.     

Numerical simulations of flame propagation and DDT in obstructed channels filled with hydrogen–air mixture

We study flame acceleration and deflagration-to-detonation transition (DDT) in channels with obstacles using 2D and 3D reactive Navier–Stokes numerical simulations. The energy release rate for the stoichiometric H₂–air mixture is modeled by a one-step Arrhenius kinetics. Computations show that at initial stages, the flame and flow acceleration is caused by thermal expansion of hot combustion products. At later stages, shock–flame interactions, Rayleigh–Taylor, Richtmyer–Meshkov, and Kelvin–Helmholtz instabilities, and flame–vortex interactions in obstacle wakes become responsible for the increase of the flame surface area, the energy-release rate, and, eventually, the shock strength. Computations performed for different channel widths d with the distance between obstacles d and the constant blockage ratio 0.5 reproduce the main regimes observed in experiments: choking flames, quasi-detonations, and detonations. For quasi-detonations, both the initial DDT and succeeding detonation reignitions occur when the Mach stem, created by the reflection of the leading shock from the bottom wall, collides with an obstacle. As the size of the system increases, the time to DDT and the distance to DDT increase linearly with d². We also observe an intermediate regime of fast flame propagation in which local detonations periodically appear behind the leading shock, but do not reach it.     

Detonation development in hydrogen/air mixtures inside a closed chamber: role of a cold wall

Detonation development from a hot spot has been extensively studied, where ignition occurs earlier than that in the surrounding mixtures. It has also been reported that a cool spot can induce detonation for large hydrocarbon fuels with Negative Temperature Coefficient (NTC) behavior, since ignition could happen earlier at lower temperatures. In this work we find that even for hydrogen/air mixtures without NTC behaviors, a cold wall can still initiate and promote detonation. End-wall reflection of the pressure wave and wall heat loss introduce an exothermic center outside the boundary layer, and then autoignitive reaction fronts on both sides may evolve into detonation waves. The right branch can be further strengthened by appropriate temperature gradient near the cold wall, and exhibits different dynamics at various initial conditions. The small excitation time and the large diffusivity of hydrogen provide the possibility for detonation development within the limited space between the autoignition kernel and the cold wall. Moreover, detonation may also develop near the flame front, which may or may not co-exist with detonation waves from the cold wall. Correspondingly, wall heat flux evolution exhibits different responses to detailed dynamic structures. Finally, we propose a regime diagram describing different combustion modes including normal flame, autoignition, and detonation from the wall and/or the reaction front. The boundary of normal flame regime qualitatively agrees with the prediction by the Livengood-Wu Integral method, while the detonation development from both the end wall and the reaction front observes Zel'dovich mechanism. Compared to hydrocarbons, hydrogen is resistant to knock onset but it is more prone to superknock development. The latter mode becomes more destructive in the presence of wall heat loss. This study isolates and identifies the role of wall heat loss on a potential mechanism for superknock development in hydrogen-fueled spark-ignition engines.     

Three-dimensional reactive shock bifurcations

We model interactions of a premixed flame with incident and reflected shocks in a rectangular shock tube using three-dimensional (3D) reactive Navier–Stokes numerical simulations. Shock-flame interactions occur in the presence of boundary layers that cause the reflected shock to bifurcate and form a reactive shock bifurcation (RSB), which contains a flame in the recirculation zone behind the oblique shock. The recirculation zone acts as a flame holder thus attaching the flame to the shock in the vicinity of the wall, and providing a mechanism for a detonationless supersonic flame spread. The accelerated burning induced by an RSB, and Mach stems that may result from RSB–RSB interactions, promote hot-spot formation, and eventually accelerate deflagration-to-detonation transition. Schlieren-type images generated from the simulation results show that the 3D structure of an RSB may not always be easily recognized in experiments if the RSB is attached to the surface of the observation window. The main 3D effect observed in the simulations is caused by the presence of the second no-slip wall in a 3D rectangular channel. Two RSBs that form at adjacent walls interact with each other and produce an oblique Mach stem between two oblique shocks. The oblique Mach stems then interacts with a central Mach stem that forms near symmetry plane, and this interaction creates a hot-spot that leads to a detonation initiation.     

A flame technique to isolate the detonation/product interface relevant to rotating detonation engines

A novel experimental technique is proposed to study the detonation propagation in a layer of non-reacted gas weakly confined by combustion products. This problem is relevant to rotating detonation engines, where transverse detonations are confined by products of a previous rotation cycle, and other applications such as industrial safety. The experimental technique utilizes a flame ignited along the top wall in a long channel. The preferential growth of the flame along the long direction of the channel creates a finger flame and permits to create a narrow layer of unburned gas. A detonation ignited outside of this layer then propagates through the layer. This permits to conduct accurate observations of the detonation interaction with the inert gas and determine the boundary condition of the interaction. The present paper provides a proof-of-concept demonstration of the technique in a 3.4 m by 0.2 m channel, in which long finger flames were observed in ethylene-oxygen mixtures. The flame is visualized by high-speed direct luminosity over its entire travel, coupled with pressure measurements. A direct simulation of the flame growth served to supplement the experiments and evaluate the role of the induced flow by the flame growth, which gives rise to a non-uniform velocity distribution along the channel length. Detonation experiments were also performed at various layer heights in order to establish the details of the interaction. The structure was visualized using high speed Schlieren video. It was found that an inert shock always runs ahead of the detonation wave, which gives rise to a unique double shock reflection interaction.     

收缩楔腔内激波汇聚诱导点火实验

文章提出了一种采用圆柱形汇聚激波实现可燃气体点火特性研究的新方法.通过采用激波动力学理论合理地设计壁面型线, 将激波管中产生的平面运动激波近乎连续地转变为扇形区内圆柱形汇聚激波.以氢氧预混气体为考察对象, 开展了相关激波管实验, 实现了可控圆柱面激波汇聚诱导点火.实验发现两种点火现象:强点火和弱点火.在强点火过程中, 点火由入射激波直接诱导产生; 而在弱点火过程中, 点火则是在波后气流经历热压缩过程后发生.      

High resolution numerical simulation of the structure of 2-D gaseous detonations

Two dimensional numerical simulation of the structure of gaseous detonation is investigated by utilizing the single step Arrhenius kinetic reaction mechanism in both high and low activation energy mixtures, characterized by their irregular and regular detonation structure, respectively. All the computations are performed on a small Beowulf cluster with six nodes. The dependency of the structure on the grid resolution is performed and it is found that, resolution of more than 300 cells per hrl is required to demonstrate the role of hydrodynamic instabilities, (KH and RM instabilities) in detonation propagation in irregular structures, while due to the absence of fine-scale structures, resolution of 50 cells per hrl, gives the physical structure of detonation with regular structures. Results show that the transverse waves in irregular structure are significantly stronger than the transverse wave in regular structure detonation, which can enhance the burning rate of the unburned pockets behind the shock front. Results for resolution of 600 cells per hrl illustrate that, in addition to the primary mode, the interaction of large vortices with the shock front provides secondary modes in the structure which leads to the irregularity of the structure in high activation energy mixture. In contrast with the results obtained for regular structure, which no unburned gas pockets and vortices observed behind the front, the results for irregular structure reveal that most portions of the gases, escape from shock compression and create large unburned gas pockets behind the both weak section of the Mach stem and the incident wave, which will burn eventually by the turbulent mixing due to the vortices associated with hydrodynamic instabilities. Therefore, the ignition mechanism in irregular structure is due to the both shock compression and by turbulent mixing associated with hydrodynamic instabilities, while the shock compression yields the ignition mechanism in regular structure detonation.     

高焓横向射流尾迹区掺混燃烧的数值特性

尾迹区作为横向射流流场的重要结构受到广泛关注,其掺混和燃烧特性对近壁面区域的流场特性有重要影响.文章在对仿真充分验证的基础上,采用Reynolds平均模拟方法对Ma=8飞行条件下高焓横向射流尾迹区中的掺混和燃烧特性进行了数值研究.探究了冷热流场尾迹区中的氢气掺混特性,冷态流场尾迹区中的激波结构对氢气分布产生一定影响,热态流场尾迹区中存在多种氢气掺混路径.V形回流区中的高浓度氢气对燃烧产生了一定的阻碍作用.定量测量了尾迹区中的火焰结构,尾迹区火焰的顶点位置随高度增加向下游线性移动,受射流主流影响,尾迹区火焰的展向宽度在距离壁面一定高度后开始增大.对冷热流场中的主要参数进行了对比,燃烧消耗了氢气使温度升高,但是尾迹区中的流动速度没有明显增加,燃烧放出的热量没有完全转化为流体的动能.      

Flame acceleration process and detonation transition in a channel with roughness elements on a wall

We experimentally investigated the effect of small roughness elements, which could be regarded as the wall roughness, on flame acceleration and deflagration-to-detonation transition (DDT). Our previous experiments (Maeda et al., 2019) using the sandpaper-like irregular roughness indicated that the flame acceleration and the associated DDT were greatly enhanced by the roughness. In this study, CH* chemiluminescence imaging as well as schlieren imaging was conducted in parallel with pressure measurements using an ethylene-oxygen combustion in the channel (486 mm long, 10 mm square cross-section) with the regular roughness (square pyramid elements with a base length and a height of 1 mm) in order to directly link the interference between the flow-field affected by the roughness and the propagating flame surface resulting the enhancement of chemical reactions, whereas the schlieren imaging alone could not allow to discuss the chemical reaction field in the previous study. After the leading shock wave was formed by the initial finger flame acceleration process, multiple interactions were observed on the flame front with the flow-field and pressure disturbances of the unreacted gas near the roughness elements. The results provided clear evidence that the roughness emphasized the effect of boundary layer, and the region where the disturbance layer and the flame were interacting coincided with the strong chemical reaction in the chemiluminescence image, indicating increase of the flame surface area caused by the turbulence on the flame front, which was also validated by the rough estimation of the burning velocity. The detonation onset was observed at the flame surface near the wall with the roughness elements. The possible factors of the final detonation transition were deduced to be the hot spot formation based on the multiple interactions of pressure waves with the roughness elements and entrainment of the unreacted gas of the highly turbulent flame front.     

氢氧爆轰波在激波管中的成长机制研究

 针对气相爆轰波成长机制研究,采用压力传感器和高速摄影技术,测试了氢氧混合气体在点火后的火焰波、前驱冲击波以及爆轰波的成长变化过程,计算了冲击波过程参数和气体状态参数,分析了火焰加速机制。实验结果表明,APX-RS型高速摄影系统可用于拍摄气相爆轰波的成长历程;氢氧爆轰波的产生是由于湍流火焰和冲击波的相互正反馈作用,导致反应区内多处发生局部爆炸,爆炸波与冲击波相互耦合,最终成长为定常爆轰波。     

Experimental characterization of detonation initiation modes in a confined chamber flush-mounted with small-diameter ignition tubes

This work reports the experimental characterization of detonation initiation modes in a confined chamber in respect to the different types of reacting waves generated in various small-diameter ignition tubes. Depending on the length of the tube and mixtures composition, four types of reacting waves can be generated and utilized to initiate detonation in the main chamber, namely the over-driven detonation ignition wave, CJ detonation ignition wave, high-speed deflagration ignition wave and deflagration ignition wave. Based on the mechanisms of detonation initiation in the main chamber, four initiation modes can be observed: the direct initiation, the local explosion initiation, and the fast and slow deflagration-to-detonation transition (DDT) initiation. By comparing the detonation initiation positions and flame-tip velocities, the first two modes show appreciably shorter initiation distances compared to the DDT modes. The over-driven detonation ignition wave is shown to yield a high probability of direct initiation, while contrary to expectation, the high-speed deflagration ignition wave exhibits superior initiation performance compared to the CJ detonation ignition wave. It is illustrated that the energy decay through diffraction and the effect of precursor shock wave reflection on the wall of the rectangular chamber are viable factors responsible for this observation. The deflagration ignition wave is also shown to be able to rapidly initiate the detonation near the inlet of the chamber, albeit with a lower success rate.     

Experimental characterization of RDE combustor flowfield using linear channel

An experimental study was conducted to characterize fundamental behavior of detonation waves propagating across an array of reactant jets inside a narrow channel, which simulated an unwrapped rotating detonation engine (RDE) configuration. Several key flow features in an ethylene-oxygen combustor were explored by sending detonation waves across reactant jets entering into cold bounding gas as well as hot combustion products. In this setup, ethylene and oxygen were injected separately into each recessed injector tube, while a total of 15 injectors were used to establish a partially premixed reactant jet array. The results revealed various details of transient flowfield, including a complex detonation wave front leading a curved oblique shock wave, the unsteady production of transverse waves at the edge of the reactant jets, and the onset of suppressed reactant jets re-entering the combustor following a detonation wave passage. The visualization images showed a complex, multidimensional, and highly irregular detonation wave front. It appeared non-uniform mixing of reactant jets lead to dynamic transverse wave structure. The refreshed reactant jets evolving in the wake of the detonation wave were severely distorted, indicating the effect of dynamic flowfield and rapid pressure change. The results suggest that the mixing between the fuel and oxidizer, as well as the mixing between the fresh reactants and the background products, should affect the stability of the RDE combustor processes.     

隔板深度对激波聚焦起爆影响的数值模拟研究

考虑几何结构参数对激波聚焦触发爆轰波的复杂影响,对H2/Air预混气的环形射流激波聚焦起爆现象开展了数值模拟研究,详细分析了不同隔板深度条件下的激波聚焦过程、流场演化特征以及爆轰波参数变化规律。研究结果表明,凹腔内激波聚焦诱导的局部爆炸以及隔板前缘处射流形成"卷吸涡"是引起爆轰波触发的两个重要机制,而隔板深度是影响环形射流激波聚焦起爆性能的关键因素。随着隔板深度的增加,凹腔内激波聚焦的强度逐步增强,回传的能量损失有所减小,进而导致爆燃转爆轰的距离与时间显著缩短。此外,当隔板深度由1 mm逐渐增加至3 mm时,爆轰波自持传播稳定性呈现出先降低后升高的变化趋势,产生这一现象的主要原因是爆轰波强度与三波点运动的相互作用。     

Enhanced DDT mechanism from shock-flame interactions in thin channels

We show experimentally and numerically that when a weak shock interacts with a finger flame in a narrow channel, an extremely efficient mechanism for deflagration to detonation transition occurs. This is demonstrated in a 19-mm-thick channel in hydrogen-air mixtures at pressures below 0.2 atm and weak shocks of Mach numbers 1.5 to 2. The mechanism relies primarily on the straining of the flame shape into an elongated alligator flame maintained by the anchoring mechanism of Gamezo in a bifurcated lambda shock due to boundary layers. The mechanism can increase the flame surface area by more than two orders of magnitude without any turbulence on the flame time scale. The resulting alligator-shaped flame is shown to saturate near the Chapman–Jouguet condition and further slowly accelerate until its burning velocity reaches the sound speed in the shocked unburned gas. At this state, the lead shock and further adiabatic compression of the gas in the induction zone gives rise to auto-ignition and very rapid transition to detonation through merging of numerous spontaneous flames from ignition spots. The entire acceleration can occur on a time scale comparable to the laminar flame time.     

Detonation initiation from shock and material interface interactions in hydrogen-air mixtures

A numerical study was conducted to explore the mechanisms of detonation initiation in a stoichiometric hydrogen-air mixture resulting from the interaction between a Mach 2.8 shock and a perturbed material interface. The simulations used a high-order compressible numerical method for fluid dynamics with both detailed and simplified chemical-diffusive models. Three material interfaces were considered: no interface, a perturbed planar flame, and a perturbed helium interface. The case with no interface did not evolve into a detonation. The case with the flame produced a series of additional shock-flame and shock-shock interactions. The shock-shock interactions produced a series of contact surfaces and sliplines with increasing temperature. Hot spots eventually formed along these sliplines and a detonation was initiated shortly thereafter through a reactivity gradient mechanism. The overall process of detonation initiation was similar for both detailed and simplified chemical-diffusive models. Only the fine details, such as the precise time and location of the hot spots, were different. This indicates that simplified chemical-diffusive models are adequate to describe the initiation of detonations in the present configuration. The processes that ignited the detonation were also similar in the case where the flame was replaced with a helium interface. Helium has a similar acoustic impedance to the products and produced similar wave refraction patterns. Thus, the primary effect of the flame is to facilitate the shock-shock interactions that produce hot spots and initiate the detonation. The chemical energy released by the flame has a secondary influence.     

气相爆轰波沿胞格的动力学机理研究

 建立了一种以子单元分析为基础,研究气相爆轰波沿胞格运动时的动力学机理的新方法。根据该子单元的性质和斜冲击波关系,首先推导了对撞前后前导冲击波沿胞格对称轴的马赫数之比和入射冲击波入射角及胞格几何性质的关系,求解了胞格结构中的三波点对撞问题。然后,采用爆炸波模拟前导冲击波的自持运动过程,求解气相爆轰波沿胞格的动力学过程,理论分析表明,气相爆轰波在胞格起点首先经历一个增长过程,然后才出现衰减。理论分析结果与实验和数值计算结果的比较表明符合得较好。     

Inward-propagating cylindrical flames under different ignition conditions

Inward-propagating cylindrical flames are studied numerically by high-resolution simulations using a one-step Arrhenius kinetics. Emphasis is placed on the effect of shock waves on the flame propagation by setting initial ignition conditions with and without shock wave. It is found that without initial shock wave, the inward-propagating flame propagates initially at a constant speed, while in the later stage of the propagation, it shows a small-amplitude oscillatory motion. When the shock wave initially introduced is medium, a large-amplitude oscillatory motion is caused by the interaction of shock waves with the inward-propagating flame. Moreover, autoignition occurs at the center and develops outwardly into a cellular flame. However, as the introduced shock wave is strong, autoignition created at the center evolves outwardly a cellular detonation.