The effects of flame generated turbulence for turbulent-induced deflagration to detonation transition

The turbulent deflagration to detonation transition (DDT) process occurs when a subsonic flame interacts with intense turbulence resulting in spontaneous acceleration and the onset of DDT. The mechanisms that govern the spontaneous ignition are deduced intricately in numerical simulations. This work experimentally explores the conditions that are known precursors to detonation initiation. More specifically, the experiment presented investigates the role of flame-generated compression as a cycle that continuously amplifies until a hotspot forms on the flame front and ignites. The study quantifies the compression comparatively against other flame regimes through ultra-high speed pressure measurements while qualitatively detailing flame generated compression through density gradients via schlieren imaging. Additionally, flow field measurements are quantified throughout the flow using simultaneous particle image velocimetry (PIV) and OH* chemiluminescence. The turbulence fluctuations and flame speeds are extracted from these measurements to identify the reactant conditions where flame-generated compression begins. Collectively, these simultaneous high-speed measurements provide detailed insight into the flame and flow field characteristics where the runaway process occurs. This work ultimately documents direct flow field measurements to extract the contribution of flame-generated turbulence on the turbulent deflagration to detonation transition process.     

On the mechanism of the deflagration-to-detonation transition in a hydrogen-oxygen mixture

The flame acceleration and the physical mechanism underlying the deflagration-to-detonation transition (DDT) have been studied experimentally, theoretically, and using a two-dimensional gasdynamic model for a hydrogen-oxygen gas mixture by taking into account the chain chemical reaction kinetics for eight components. A flame accelerating in a tube is shown to generate shock waves that are formed directly at the flame front just before DDT occurred, producing a layer of compressed gas adjacent to the flame front. A mixture with a density higher than that of the initial gas enters the flame front, is heated, and enters into reaction. As a result, a high-amplitude pressure peak is formed at the flame front. An increase in pressure and density at the leading edge of the flame front accelerates the chemical reaction, causing amplification of the compression wave and an exponentially rapid growth of the pressure peak, which “drags” the flame behind. A high-amplitude compression wave produces a strong shock immediately ahead of the reaction zone, generating a detonation wave. The theory and numerical simulations of the flame acceleration and the new physical mechanism of DDT are in complete agreement with the experimentally observed flame acceleration, shock formation, and DDT in a hydrogen-oxygen gas mixture.     

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.     

Controlled detonation initiation in hypersonic flow

Experimental evidence of controlled detonation initiation and propagation in a hypersonic flow of premixed hydrogen-air is presented. This controlled detonation initiation is created in a hypersonic facility capable of producing a Mach 5 flow of hydrogen-air. Flow diagnostics such as high-speed schlieren and OH* chemiluminescence results show that a flame deflagration-to-detonation transition occurs as a combined result of turbulent flame acceleration and shock-focusing. The experimental results define three new distinct regimes in a Mach 5 premixed flow: deflagration-to-detonation transition (DDT), unsteady compressible turbulent flames, and shock-induced combustion. A two-dimensional implicit-LES (ILES) simulation, which solves the compressible, reactive Navier-Stokes equations on an adapting grid is conducted to provide additional insight into the local physical mechanism of detonation transition and propagation.     

Reaction propagation modes in millimeter-scale tubes for ethylene/oxygen mixtures

Effects of tube diameter and equivalence ratio on reaction front propagations of ethylene/oxygen mixtures in capillary tubes were experimentally analyzed using high speed cinematography. The inner diameters of the tubes investigated were 0.5, 1, 2 and 3 mm. The flame was ignited at the center of the 1.5 m long smooth tube under ambient pressure and temperature before propagated towards the exits in the opposite directions. A total of five reaction propagation scenarios, including deflagration-to-detonation transition followed by steady detonation wave transmission (DDT/C–J detonation), oscillating flame, steady deflagration, galloping detonation and quenching flame, were identified. DDT/C–J detonation mode was observed for all tubes for equivalence ratios in the vicinity of stoichiometry. The velocity for the steady detonation wave propagation was approximately Chapman–Jouguet velocity for 1, 2, and 3 mm I.D. tubes; however, a velocity deficit of 5% was found for the case in 0.5 mm I.D. tube. For leaner mixtures, an oscillating flame mode was found for tubes with diameters of 1 to 3 mm, and the reaction front travelled in a steady deflagrative flame mode with velocities around 2–3 m/s when the mixture equivalence ratio becomes even leaner. Galloping detonation wave propagation was the dominant mode for the fuel lean regime in the 0.5 mm I.D. tube. For rich mixtures beyond the detonation limits, a fast flame followed by flame quenching was observed.     

The study of geometric effects on the explosion front propagation in a horizontal channel with a layer of spherical beads

Experiments were performed in a horizontal channel partially filled with a layer of 12.7 mm ceramic-oxide beads filled with a nitrogen-diluted stoichiometric methane–oxygen mixture, i.e., CH₄ + 2(O₂ + 2/3N₂). Ionization probes and pressure transducers were used to track the explosion front velocity in the 1.22 m long, 76 mm wide and 152 mm high horizontal channel. Schlieren photography and smoked foil techniques are used to gain insight into the explosion front structure. The explosion propagation phenomenon was characterized by the combustion in the bead layer and the unobstructed gap above. It was determined that for a fixed gap height the bead layer thickness had very little effect on the explosion propagation phenomenon. However, for a fixed bead layer height the explosion propagation was strongly influenced by the gap height. The combustion products vented from the bead layer behind the flame propagating in the gap affects the structure of the shock-flame front in the gap and the maximum flame velocity achieved. The coupling between the vented products and the flame velocity in the gap was strongly influenced by the gap height. The gap height also affects the structure of the detonation wave propagating in the gap following DDT that always occurred in the gap. The DDT run-up distance was found to increase with increasing gap height and inversely with initial pressure.     

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。     

Statistical analysis of detonation wave structure

Hydrocarbon fueled detonations are imaged in a narrow channel with simultaneous schlieren and broadband chemiluminescence at 5 MHz. Mixtures of stoichiometric methane and oxygen are diluted with various levels of nitrogen and argon to alter the detonation stability. Ethane is added in controlled amounts to methane, oxygen, nitrogen mixtures to simulate the effects of high-order hydrocarbons present in natural gas. Sixteen unique mixtures are characterized by performing statistical analysis on data extracted from the images. The leading shock front of the schlieren images is detected and the normal velocity is calculated at all points along the front. Probability distribution functions of the lead shock speed are generated for all cases and the moments of distribution are computed. A strong correlation is found between mixture instability parameters and the variance and skewness of the probability distribution; mixtures with greater instability have larger skewness and variance. This suggests a quantitative alternative to soot foil analysis for experimentally characterizing the extent of detonation instability. The schlieren and chemiluminescence images are used to define an effective chemical length scale as the distance between the shock front and maximum intensity location along the chemiluminescence front. Joint probability distribution functions of shock speed and chemical length scale enable statistical characterization of coupling between the leading shock and following reaction zone. For more stable, argon dilute mixtures, it is found that the joint distributions follow the trend of the quasi-steady reaction zone. For unstable, nitrogen diluted mixtures, the distribution only follows the quasi-steady solution during high-speed portions of the front. The addition of ethane is shown to have a stabilizing effect on the detonation, consistent with computed instability parameters.     

Formation and characteristics of composite reaction – Shock clusters in narrow channels

High-speed schlieren visualizations show that a composite reaction-shock cluster structure is formed in the last flame acceleration stage prior to detonation transition for ethylene/oxygen mixture in a narrow channel. The composite structure is bounded by a normal shock at the leading edge of the structure, and series of parallel oblique shocks interweave with reaction front on the other end in the cluster. Propagating velocity of the reaction front at the inception of the cluster is ～ 45–50% of Chapman-Jouguet detonation velocity of the mixture. Reaction front accelerates rapidly after the formation of the reaction-shock cluster, and run into detonation in tens of microseconds except for very lean mixtures. The angle between the parallel oblique shocks in the cluster and the side wall, defined as ω-angle, is found to be constant for a specific mixture as the reaction wave propagates. Dependence of ω-angle on mixture equivalence ratio and channel size are investigated in the study. Analysis shows that DDT distance is linearly proportional to ω-angle, and an empirical correlation is derived.     

激光作用铝靶爆轰波流场观测与理论分析

进行了强激光作用铝靶实验,采用纹影照相法,观察爆轰波流场演化过程,分析了爆轰波衰减规律。根据冲击波运动的自相似性,采用点爆炸模型描述了激光作用下爆轰波流场的演化,计算了波阵面速度、压力和温度。结果显示:爆轰波阵面沿迎着激光光源方向较快传播,波阵面形状由最初的半椭球形逐渐向半球形转变,在演化过程中扰动区结构复杂,存在多个密度间断层。在爆轰波开始传播阶段,波阵面的压力和温度较高但下降很快。     

Effects of heat loss and viscosity friction at walls on flame acceleration and deflagration to detonation transition

The coupled effect of wall heat loss and viscosity friction on flame propagation and deflagration to detonation transition(DDT) in micro-scale channel is investigated by high-resolution numerical simulations.The results show that when the heat loss at walls is considered, the oscillating flame presents a reciprocating motion of the flame front.The channel width and Boit number are varied to understand the effect of heat loss on the oscillating flame and DDT.It is found that the oscillating propagation is determined by the competition between wall heat loss and viscous friction.The flame retreat is led by the adverse pressure gradient caused by thermal contraction, while it is inhibited by the viscous effects of wall friction and flame boundary layer.The adverse pressure gradient formed in front of a flame, caused by the heat loss and thermal contraction, is the main reason for the flame retreat.Furthermore, the oscillating flame can develop to a detonation due to the pressure rise by thermal expansion and wall friction.The transition to detonation depends non-monotonically on the channel width.     

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.     

An experimental investigation of the onset of detonation

An experimental configuration is devised in the present investigation whereby the condition at the final phase of the deflagration to detonation transition (DDT) process can be generated reproducibly by reflecting a CJ detonation from a perforated plate. The detonation products are transmitted downstream through the plate, generating a turbulent reaction front that mixes with the unburned mixture and that drives a precursor shock ahead of it at a strength of about M = 3. The gasdynamic condition that is generated downstream of the perforated plate closely corresponds to that just prior to the onset of detonation in the DDT process. The turbulence parameters can be controlled by varying the geometry of the perforated plate; thus, the condition leading to the onset of detonation can be experimentally investigated. A one-dimensional theoretical analysis of the steady wave processes was first performed, and the experimental results show good agreement, indicating that the present experimental condition can be theoretically described. Two different detonation tube geometries (one with a square cross-section of 300 mm by 300 mm and the other with a circular cross-section of 150 mm) are used to demonstrate the independence of the tube diameter at the critical condition for DDT. Perforated plates with different hole diameters (d = 8, 15, and 25 mm) were tested, and the hole spacing to hole diameter ratio was maintained at 0.5. Different hydrogen–air mixtures were tested at normal temperature and pressure. For the plate with 8 mm holes, the onset of detonation is never observed. For the plate with 15 mm holes, successful initiation of a detonation is achieved for 0.8 < < 1.75 in both detonation tubes. For the plate with 25 mm holes, detonation initiation is observed for 0.7 < < 2.1 in the square detonation tube and for 0.8 < < 1.6 in the smaller circular detonation tube.     

Formation of the preheated zone ahead of a propagating flame and the mechanism underlying the deflagration-to-detonation transition

The Letter presents analytical, numerical and experimental studies of the mechanism underlying the deflagration-to-detonation transition (DDT). Insight into how, when, and where DDT occurs is obtained by analyzing analytically and by means of multidimensional numerical simulations dynamics of a flame accelerating in a tube with no-slip walls. It is shown that the deflagration-to-detonation transition exhibits three separate stages of evolution corroborating majority experimental observations. During the first stage flame accelerates and generates shocks far ahead of the flame front. During the second stage the flame slows down, shocks are formed in the immediate proximity of the flame front and the preheated zone ahead of the flame front is created. The third stage is self-restructuring of the steep temperature profile within the flame, formation of a reactivity gradient and the actual formation of the detonation wave itself. The mechanism for the detonation wave formation, given an appropriate formation of the preheated zone, seems to be universal and involves a reactivity gradient formed from the initially steep flame temperature profile in the presence of the preheated zone. The developed theory and numerical simulations are found to be well consistent with extensive experiments of the DDT in hydrogen-oxygen and ethylene-oxygen mixtures in tubes with smooth and rough walls.     

Flame acceleration and the transition to detonation of stoichiometric ethylene/oxygen in microscale tubes

Flame propagation in capillary tubes with smooth circular cross-sections and diameters of 0.5, 1.0, and 2.0 mm are investigated using high-speed photography. Flames were found to propagate and accelerate to detonation speed in stoichiometric ethylene and oxygen mixtures initially at room temperature in all three tube diameters. Ignition occurs at the midpoint along the length of the tube. We observe for the first time transition to detonation in micro-tubes. Detonation was observed with both spark and hot-wire ignition. Tubes with larger diameters take longer to transition to detonation. In fact, transition distance scales with the diameter in our 1.0 and 2.0 mm cases with spark ignition. Flame structures are observed for various stages of the process. Three types of flame propagation modes were observed in the 0.5 mm tube with spark ignition: (a) acceleration to Chapman–Jouguet (CJ) detonation speed followed by constant CJ wave propagation, (b) acceleration to CJ speed, followed by the detonation wave failure, and (c) flame acceleration to a constant speed below the CJ speed of approximately 1600 m/s. The current detonation mechanism observed in capillary tubes is applicable to predetonators for pulsed detonation, micro propulsion devices, safety issues, and addresses fundamental issues raised by recent theoretical and numerical analyses.     

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。     

Chemical-diffusive models for flame acceleration and transition-to-detonation: genetic algorithm and optimisation procedure

This paper presents a general approach for developing an automated, fast and flexible procedure to determine the reaction parameters for a simplified chemical-diffusive model to simulate flame acceleration and deflagration-to-detonation transition (DDT) in a stoichiometric methane–air mixture. The procedure uses a combination of a genetic algorithm and Nelder-Mead optimisation scheme to find the optimal reaction parameters for a reaction rate based on an Arrhenius form for conversion of reactants to products. The model finds six optimal reaction parameters that reproduce six flame and detonation properties. Results show that the reaction parameters closely reproduce their intended flame and detonation properties. The laminar flame profile computed using the reaction parameters in a 1D Navier-Stokes code matches the profile obtained when using a detailed chemical reaction mechanism. The optimal reaction parameters are then used in a 2D simulation of flame acceleration and DDT in an obstacle-laden channel containing stoichiometric methane–air, and the results show that the computation closely follows the transition-to-detonation observed in experiments. This automated procedure for finding parameters for a proposed reaction model makes it possible to simulate the behaviour of flames and detonations in large, complex scenarios, which would otherwise be an incalculable problem.     

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.     

Energy coupling mechanism for pulse detonation ignition of a scramjet cavity

The effect of detonation decoupling on the ignition capability of a pulse detonation igniter in a scramjet cavity is investigated. It was observed that a strongly coupled detonation is required for igniting a fueled cavity and the ignition capability rapidly decays for a weak or slightly decoupled detonation. The pulse detonation igniter pressure and wave speed were measured at subatmospheric pressure to characterize the pressure and fill characteristics dependence on backpressure. Temperatures measurements using 100 kHz H₂O absorption measurements showed an increase in peak temperature for critically filled conditions but a decrease in bulk fluid temperature with decreasing fill time. Simultaneous schlieren and chemiluminescence measurements demonstrate that a fully coupled detonation entrains greater quantities of high-temperature and reacting fluid in the vortex formed on the edges of the detonation plume, enhancing the mixing and spreading of products into the cavity. This shedding of high-temperature intermediate species is the primary mechanism governing successful ignition in the scramjet cavity.     

A numerical modeling of the acceleration of a flame by an additional energy input ahead of its front

The acceleration of a flame after an additional energy input ahead of its front was simulated using numerical methods. The combustion of a hydrogen-air mixture in a semiopen channel was considered. The calculations were performed within the framework of a two-dimensional hydrodynamic model of premixed flames, with consideration given to heat transfer, multicomponent diffusion, and chemical kinetics. It was demonstrated that, when the interaction of the flame front with the near-wall boundary layer is taken into account, even a moderate energy input could substantially promote the development of the Landau-Darrieus instability and, possibly, deflagration to detonation transition.