Visualization of the non-steady state oblique detonation wave phenomena around hypersonic spherical projectile

We studied experimentally the shock waves and combustion waves generated by a hypersonic spherical projectile in an explosive mixture. An acetylene/oxygen mixture diluted with argon (2C₂H₂ + 5O₂ + 7Ar) was used with various initial pressures (detonation cell sizes) to observe optically with a shadowgraph imaging system a shock-induced combustion (SIC), a stable oblique detonation wave (ODW), and a wave called a Straw Hat type consisting of a strong SIC and ODW. The criticality of stabilizing an ODW around a projectile is expressed by the ratio of the projectile diameter, d, to the cell size, λ, as d/λ = 3.63–4.84. Although the Straw Hat type wave in the vicinity of criticality is an unstable phenomena, it has been mainly observed by a single frame picture to date, so that it is difficult to discuss the time history of its wave structure. In this study, it was remarkable to directly carry out continuous optical observations using a high speed video camera which can continuously film 100 pictures with a 1 μs frame speed so as to allow an investigation of the sustaining mechanism of the unstable wave structure. Our results allowed the identification of an increase in unsteadiness in the relative distance between the projectile fronts and the transition points to an ODW as the time increased. They also showed local explosions in the SIC region near transition point transformed the ODW front upstream.     

Unstructured Adaptive Grid Flow Simulations of Inert and Reactive Gas Mixtures

Unstructured adaptive grid flow simulation is applied to the calculation of high-speed compressible flows of inert and reactive gas mixtures. In the present case, the flowfield is simulated using the 2-D Euler equations, which are discretized in a cell-centered finite volume procedure on unstructured triangular meshes. Interface fluxes are calculated by a Liou flux vector splitting scheme which has been adapted to an unstructured grid context by the authors. Physicochemical properties are functions of the local mixture composition, temperature, and pressure, which are computed using the CHEMKIN-II subroutines. Computational results are presented for the case of premixed hydrogen–air supersonic flow over a 2-D wedge. In such a configuration, combustion may be triggered behind the oblique shock wave and transition to an oblique detonation wave is eventually obtained. It is shown that the solution adaptive procedure implemented is able to correctly define the important wave fronts. A parametric analysis of the influence of the adaptation parameters on the computed solution is performed.     

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.     

Unstable combustion induced by oblique shock waves at the non-attaching condition of the oblique detonation wave

The instability of oblique shock wave (OSW) induced combustion is examined for a wedge with a flow turning angle greater than the maximum attach angle of the oblique detonation wave (ODW), where archival results rarely exist for this case in previous literatures. Numerical simulations were carried out for wedges of different length scales to account for the ratio of the chemical and fluid dynamic time scales. The results reveal three different regimes of combustion. (1) No ignition or decoupled combustion was observed if a fluid dynamic time is shorter than a chemical time behind an OSW. (2) Oscillatory combustion was observed behind an OSW if a fluid dynamic time is longer than a chemical time behind an OSW and the fluid dynamic time is shorter than the chemical time behind a normal shock wave (NSW) at the same Mach number. (3) Detached bow shock-induced combustion (or detached overdriven detonation wave) was observed if a fluid dynamic time is longer than a chemical time behind a NSW. Since no ignition or decoupled combustion occurs as a very slow reaction and the detached wave occurs as an infinitely fast reaction, the finite rate chemistry is considered to be the key for the oscillating combustion induced by an OSW over a wedge of a finite length with a flow turning angle greater than the maximum attach angle for an ODW. Since this case has not been previously reported, grid independency was tested intensively to account for the interaction between the shock and reaction waves and to determine the critical time scale where the oscillating combustion can be observed.     

一种基于二级轻气炮平台的超高速弹丸发射装置设计

 基于强爆轰效应,提出一种新的超高速弹丸发射装置。在二级轻气炮平台上,设计内置炸药柱的三级加速腔,将二级轻气炮发射的克质量弹丸由6~7 km/s,再次加速至9~10 km/s的超高速。通过数值模拟优化设计药柱与飞片的形状,以及弹丸的树脂封装,从而抑制强爆轰产生的中心射流对弹丸的破坏,并且在短脉冲强冲击过程中,避免在直径达厘米级的铝或钛合金弹丸内的层裂破坏。数值模拟结果表明,经过优化设计的三级加速腔可以将气炮出膛速度约6.0 km/s的1.03 g铝合金弹丸加速至9.6 km/s,弹丸最终形状仍保持类球形,速度增益约1.6。     

Initiation of a detonation wave by resonant laser radiation in a hydrogen-oxygen mixture flowing about a wedge

The formation of an oblique detonation wave in a supersonic hydrogen-oxygen flow about a planar wedge is considered. It is shown that the excitation of the electronic state b ¹Σ _g ⁺ in oxygen molecules by resonant laser radiation with a wavelength of 762 nm makes it possible to initiate detonation combustion at a distance of ≈1 m from the tip of the wedge at low temperatures (500–600 K). Notably, it suffices to irradiate the gas in the narrow (0.5–1.0 cm across) paraxial region of the flow near the tip of the wedge. It is found that the laser-induced excitation of molecular oxygen is several times more efficient than ordinary heating of the mixture to initiate a detonation wave.     

Formation of transverse waves in oblique detonations

The structure of oblique detonation waves stabilized on a hypersonic wedge in mixtures characterized by a large activation energy is investigated via steady method of characteristics (MoC) calculations and unsteady computational flowfield simulations. The steady MoC solutions show that, after the transition from shock-induced combustion to an overdriven oblique detonation, the shock and reaction complex exhibit a spatial oscillation. The degree of overdrive required to suppress this oscillation was found to be nearly equal to the overdrive required to force a one-dimensional piston-driven detonation to be stable, demonstrating the equivalence of two-dimensional steady oblique detonations and one-dimensional unsteady detonations. Full unsteady computational simulations of the flowfield using an adaptive refinement scheme showed that these spatial oscillations are transient in nature, evolving in time into transverse waves on the leading shock front. The formation of left-running transverse waves (facing upstream) precedes the formation of right-running transverse waves (facing downstream). Both sets of waves are convected downstream away from the wedge in the supersonic flow behind the leading oblique front, such that the mechanism of instability must continuously generate new transverse waves from an initially uniform flow. Together, these waves define a cellular structure that is qualitatively similar to a normal propagating detonation.     

Initiation and sustaining mechanisms of stabilized Oblique Detonation Waves around projectiles

Direct initiations and stabilizations of three-dimensional conical detonation waves were attained by launching spheres with 1.06–1.31 times the C–J velocities into detonable mixtures. We conducted high time-resolution Schlieren visualizations of the whole processes over unsteady initiations to stable propagations of the stabilized Oblique Detonation Waves (ODWs) using a high-speed camera. The detonable mixtures were stoichiometric oxygen mixtures with acetylene, ethylene or hydrogen. They were diluted with argon in a 50% volumetric fraction, and a 75% diluted mixture was also tested for the acetylene/oxygen. The direct initiation of detonation by the projectile and the DDT process like the re-initiation appeared in the initiation process of stabilized ODW. This process eventually led to the stabilized ODW supported by the projectile velocity and the ringed shape detonation wave originating in the re-initiation. We modeled the spatial evolution of stabilized ODW after the re-initiation based on its C–J velocity and angle. The model qualitatively reproduced the measured development rate of stabilized ODW. We also discussed about the detonation stability for the curvature effect arising from the three-dimensional nature of stabilized ODW around the projectile. The curvature effect attenuated the detonation wave below its C–J velocity at the vicinity of projectile. The propagation limits of curvature effect will be responsible for the criticality to attain the stabilized ODWs. By accessing the detailed distributions of propagation velocities and curvature radiuses, the critical curvature radiuses normalized by the cell sizes experimentally revealed to be 8–10 or 15–18 for mixtures diluted with each 50% argon or 75% argon/krypton.     

Autoignition and detonation development from a hot spot inside a closed chamber: Effects of end wall reflection

The advancement of highly boosted internal combustion engines (ICEs) with high thermal efficiency is mainly constrained by knock and super-knock, respectively, due to the end gas autoignition and detonation development. The pressure wave propagation and reflection in a small confined space may strongly interact with local end gas autoignition, leading to combustion characteristics different from those in a large chamber or open space. The present study investigates the transient autoignition process in an iso-octane/air mixture inside a closed chamber under engine-relevant conditions. The emphasis is given to the assessment of effects of the pressure wave-wall reflection and the mechanism of extremely strong pressure oscillation typical for super-knock. It is found that the hot spot induced autoignition in a closed chamber can be greatly affected by shock/pressure wave reflection from the end wall. Different autoignition modes respectively from the hot spot and the end wall reflection are identified. A non-dimensional parameter quantifying the interplay between different length and time scales is introduced, which helps to identify different autoignition regimes including detonation development near the end wall. It is shown that detonation development from the hot spot may cause super-knock with devastating pressure oscillation. However, the detonation development from the end wall can hardly produce pressure oscillation strong enough for the super-knock. The obtained results provide a fundamental insight into the knocking mechanism in engines under highly boosted conditions.     

氢-空气绕流无限长斜劈斜爆震波数值模拟

采用二维多组分有化学反应的Euler方程,考虑单步化学反应,采用有限反应速率模型,对斜爆震波进行计算,通过将数值模拟结果与实验结果对比,验证了数值模拟方法的有效性。在此基础上,对氢空气绕流无限长斜劈的斜爆震过程进行了数值模拟,分析了斜劈角度对斜爆震波特性的影响。结果表明:当斜劈角度增大时,爆震波中的化学反应速率随之增大,相应的斜爆震波的角度也增加,波后的温度更高,同时波后法向马赫数减小。     

Numerical study of self-sustained oblique detonation in a non-uniform mixture

Self-sustained oblique detonation behind a spherical projectile formed in a non-uniform H₂/O₂/Ar mixture was numerically investigated. A hypersonic combustible mixture flow around a 4.76 mm diameter body was modeled to be flowing at 2500 m/s and 100 kPa. The concentration gradient was prescribed applying the Gaussian distribution to hydrogen concentration. Axisymmetric Euler equations including a detailed kinetics of 9 species and 27 elementary reactions were solved with an explicit 2nd-order time integration scheme combined with point implicit method for chemical reaction. Oblique detonation was always obtained when the mixture on the centerline was stoichiometric, as it is for a uniform mixture, and a broader range of equivalence ratio could sustain oblique detonation far from the sphere. Local detonation angle was revealed to reasonably match Chapman-Jouguet analytical solutions with a minor difference attributed to curvature, less reactive composition, and the concentration gradient. Also, a strongly fuel-rich region encountered decoupling of the shock-flame, in which an abrupt deflection of the shock front appeared. These decoupling phenomena can be attributed to a slower kinetics of a less reactive mixture. All of interesting findings in this study will also benefit understanding of various form of detonation in non-uniform mixture taking advantage of the analogy between them.     

The effects of mixture preburning on detonation wave propagation

Pressure gain combustion in the form of continuous detonations can provide a significant increase in the efficiency of a variety of propulsion and energy conversion devices. In this regard, rotating detonation engines (RDEs) that utilize an azimuthally-moving detonation wave in annular systems are increasingly seen as a viable approach to realizing pressure gain combustion. However, practical RDEs that employ non-premixed fuel and oxidizer injection need to minimize losses through a number of mechanisms, including turbulence-induced shock-front variations, incomplete fuel-air mixing, and premature deflagration. In this study, a canonical stratified detonation configuration is used to understand the impact of preburning on detonation efficiency. It was found that heat release ahead of the detonation wave leads to weaker shock fronts, delayed combustion of partially-oxidized fuel-air mixture, and non-compact heat release. Furthermore, large variations in wave speeds were observed, which is consistent with wave behavior in full-scale RDEs. Peak pressures in the compression region or near triple points were considerably lower than the theoretically-predicted values for ideal detonations. Analysis of the detonation structure indicates that this deflagration process is parasitic in nature, reducing the detonation efficiency but also leading to heat release far behind the wave that cannot directly strengthen the shock wave. This parasitic combustion leads to commensal combustion (heat release far downstream of the wave), indicating that it is the root cause of combustion efficiency losses.     

Triboelectric charge variability in firearm particulates and projectiles

In this paper, we investigate the variability of dynamic triboelectric charging of both particulate emissions (smoke, unburned propellant and other debris) and projectiles launched from a 9 mm Beretta pistol. We measured charge transients both at the weapon muzzle and at a location 10 m downrange using cylindrical charge sensors to characterize the charge associated with the muzzle blast and that of the projectile. We derive an analytical expression for the time-domain voltage response of cylindrical charge sensors, and use it to quantify the charge present on the projectiles. Our results indicate that the magnitude and polarity of charge present on a given projectile depends strongly on the ammunition type, even if the primer and propellant chemistries are identical. Projectiles, on average, carry a charge opposite in polarity to that of the particulates present in the muzzle blast. We hypothesize that the relative motion of the gas-solids flow in the annulus between the barrel and the projectile leads to charge transfer between the particulates and the projectile, resulting in the case of oppositely charged projectiles and particulates.     

The dynamics of a non-premixed rotating detonation engine from time-resolved temperature measurements

The structure and dynamics of a hydrogen-air rotating detonation engine (RDE) are described based on 100-kHz laser absorption spectroscopy measurements of water temperature at four simultaneous locations within the detonation channel. The analysis focuses on the evolution of the flowfield over a 200 ms period for three separate air mass flow rate cases. Two-dimensional unwrapped visualizations of the temperatures show a flowfield structure containing regions with the detonation front, combustion products, oblique shock, and refilling reactants, qualitatively agreeing with previous simulations and experiments. A major conclusion is that water from the combustion products is measured throughout all space and time in the RDE, including near the injector, implying the presence of performance loss processes such as burning upstream of the detonation wave or the back recirculation of combustion products with fresh fuel–air. By analyzing the elevated temperatures of the reactants during the refill process, one estimation for the mass fraction of combustion products in the reactants is as high as 20–30% on average. This product mass fraction is found to be inversely proportional to the bulk air mass flow rate and decreases as time progresses. This indicates these non-ideal processes are more significant closer to RDE ignition for poorer performing operating conditions. For the largest air mass flow case, water temperatures near the nominally cold plenum conditions likely corroborate the presence of a recirculation region on the RDE inner body. Analysis of inter- and intra-cycle temperature dynamics further support non-ideal processes occurring behind the detonation wave and during the refill process. As a whole, the data indicates that the RDE performance is better as time progresses away from ignition or for higher air mass flow rates. These data are also important for comparison with numerical models.     

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.     

The re-initiation of cellular detonations downstream of an inert layer

The current work aims to examine how the nature of cellular instabilities controls the re-initiation capability and dynamics of a gaseous detonation transmitting across a layer of inert (or non-detonable) gases. This canonical problem is tackled via computational analysis based on the two-dimensional, reactive Euler equations. Two different chemical kinetic models were used, a simplified two-step induction-reaction model and a detailed model for hydrogen-air. For the two-step model, cases with relatively high and low activation energies, representing highly and weakly unstable cellular detonations, respectively, are considered. For the weakly unstable case, two distinct types of re-initiation mechanisms were observed. (1) For thin inert layers, at the exit of the layer the detonation wave front has not fully decayed and thus the transverse waves are still relatively strong. Detonation re-initiation in the reactive gas downstream of the inert layer occurs at the gas compressed by the collision of the transverse waves, and thus is referred to as a cellular-instability-controlled re-initiation. (2) If an inert layer is sufficiently thick, the detonation wave front has fully decayed to a planar shock when it exits the inert layer, and re-initiation still occurs downstream as a result of planar shock compression only, which is thus referred to as a planar-shock-induced re-initiation. Between these two regimes there is a transition region where the wave front is not yet fully planar, and thus perturbations by the transverse waves still play a role in the re-initiation. For the highly unstable case, re-initiation only occurs via the cellular-instability-controlled mechanisms below a critical thickness of the inert layer. Additional simulations considering detailed chemical kinetics demonstrate that the critical re-initiation behaviors of an unstable stoichiometric mixture of hydrogen-air at 1 atm and 295 K are consistent with the finding from the two-step kinetic model for a highly unstable reactive mixture.     

Cell-like structure of unstable oblique detonation wave from high-resolution numerical simulation

A comprehensive numerical study was carried out to investigate the unsteady cell-like structures of oblique detonation waves (ODWs) for a fixed Mach 7 inlet flow over a wedge of 30° turning angle. The effects of grid resolution and activation energy were examined systematically at a dimensionless heat addition of 10. The ODW front remains stable for a low activation energy regardless of grid resolution, but becomes unstable for a high activation energy featuring a cell-like wave front structure. Similar to the situation with an ordinary normal detonation wave (NDW), a continuous increase in the activation energy eventually causes the wave-front oscillation to transit from a regular to an irregular pattern. The wave structure of an unstable ODW, however, differs considerably from that of a NDW. Under the present flow condition, triple points and transverse waves propagate downstream, and the numerical smoke-foil record exhibits traces of triple points that rarely intersect with each other. Several instability-driving mechanisms were conjectured from the highly refined results. Since the reaction front behind a shock wave can be easily destabilized by disturbance inherent in the flowfield, the ODW front becomes unstable and displays cell-like structures due to the local pressure oscillations and/or the reflected shock waves originating from the triple points. The combined effects of various instability sources give rise to a highly unstable and complex flow structure behind an unstable ODW front.     

Oblique detonation waves stabilized in rectangular-cross-section bent tubes

Oblique detonation waves, which are generated by a fundamental detonation phenomenon occurring in bent tubes, may be applied to fuel combustion in high-efficiency engines such as a pulse detonation engine (PDE) and a rotating detonation engine (RDE). The present study has experimentally demonstrated that steady-state oblique detonation waves propagated stably through rectangular-cross-section bent tubes by visualizing these waves using a high-speed camera and the shadowgraph method. The oblique detonation waves were stabilized under the conditions of high initial pressure and a large curvature radius of the inside wall of the rectangular-cross-section bent tube. The geometrical shapes of the stabilized oblique detonation waves were calculated, and the results of the calculation were in good agreement with those of our experiment. Moreover, it was experimentally shown that the critical condition under which steady-state oblique detonation waves can stably propagate through the rectangular-cross-section bent tubes was the curvature radius of the inside wall of the rectangular-cross-section bent tube equivalent to 14–40 times the cell width.     

涡轮导向器对旋转爆轰波传播特性影响的实验研究

为了研究涡轮导向器对旋转爆轰波传播特性的影响,以氢气为燃料,空气为氧化剂,在不同当量比下开展了实验研究.基于高频压力传感器及静态压力传感器的信号,详细分析了带涡轮导向器的旋转爆轰燃烧室的工作模式以及涡轮导向器对非均匀不稳定爆轰产物的影响.实验结果表明:在当量比较低时,爆轰燃烧室以快速爆燃模式工作;逐渐增大当量比,爆轰燃烧室开始以不稳定旋转爆轰模式工作;继续增大当量比,爆轰燃烧室以稳定旋转爆轰模式工作,且旋转爆轰波的传播速度和稳定性均随当量比的增大逐渐提高.爆轰波下游的斜激波与涡轮导向器相互作用,涡轮导向器对压力振荡的幅值具有明显的抑制作用,但对压力振荡频率的影响较小.随着当量比的增大,涡轮导向器上下游的静压均同时增大,经过涡轮导向器的作用,涡轮下游静压明显降低.     

Numerical simulation of modes of combustion and detonation of hydrogen-air mixtures in porous medium in the framework of the mechanics of two-phase reaction mediums

Numerical simulation is carried out for combustion and detonation waves propagating through a motionless gas mixture in a porous inert charge. Computations are performed in a one-dimensional approximation by means of an EFAE computer program that was developed in the framework of the mechanics of multiphase reaction mediums. The chemical conversion of gas is modeled by a one-stage reaction of the Arrhenius type with constants selected based on existing experimental data on the ignition lags behind the reflected shock waves. Computations are performed for hydrogen-air mixtures with 35 and 15% hydrogen and compared with literature experimental data in which the initial pressure and the diameter of charged particles are varied. All three combustion modes (slow, fast, and supersonic) observed in the experiment and combustion failure under conditions lower than threshold are followed by numerical simulation. In addition, the computations qualitatively reproduced experimental data on the change of the combustion mode in the case of transfer from stoichiometric to a lean mixture and data on the combustion wave velocity and limiting conditions of combustion mode transition and failure of flame as a function of the initial pressure and the charged particle size. It is shown that supersonic waves propagating with a velocity of lower than 1100 m/s do not have a Chapman-Jouguet surface in the end of the reaction zone and it is evident that they can be related to detonation, as in the cited literature.