Numerical study of acoustic coupling in spinning detonation propagating in a circular tube

Spinning detonations propagating in a circular tube were numerically investigated with a two-step reaction model by Korobeinikov et al. The time evolutions of the simulation results were utilized to reveal the propagation behavior of single-headed spinning detonation. Three distinct propagation modes, steady, unstable, and pulsating modes, are observed in a circular tube. The track angles on a wall were numerically reproduced with various initial pressures and diameters, and the simulated track angles of steady and unstable modes showed good agreement with those of the previous reports. In the case of steady mode, transverse detonation always couples with an acoustic wave at the contact surface of burned and unburned gas and maintains stable rotation without changing the detonation front structure. The detonation velocity maintains almost a CJ value. We analyze the effect of acoustic coupling in the radial direction using the acoustic theory and the extent of Mach leg. Acoustic theory states that in the radial direction transverse wave and Mach leg can rotate in the circumferential direction when Mach number of unburned gas behind the incident shock wave in the transverse detonation attached coordinate is larger than 1.841. Unstable mode shows periodical change in the shock front structure and repeats decoupling and coupling with transverse detonation and acoustic wave. Spinning detonation maintains its propagation with periodic generation of sub-transverse detonation (new reaction front at transverse wave). Corresponding to its cycle, whisker is periodically generated, and complex Mach interaction periodically appears at shock front. Its velocity history shows the fluctuation whose behavior agrees well with that of rapid fluctuation mode by Lee et al. In the case of pulsating mode, as acoustic coupling between transverse detonation and acoustic wave is not satisfied, shock structure of spinning detonation is disturbed, which causes failure of spinning detonation.     

Numerical investigation of two-wave collision and wave structure evolution of rotating detonation engine with hollow combustor

A three-dimensional numerical simulation of rotating detonation engine (RDE) with hollow combustor is performed to analyze wave structure evolution systematically. Wave structure evolution is classified into four categories, namely two-wave collision (counter-rotating waves), abscission of detonation tail, and shock wave to detonation transition. Two-wave collision consists of symmetric detonation collision, asymmetric detonation collision, and detonation/shock collision. Two symmetric detonation waves turn into shock waves after collision. Collision of asymmetric detonation waves creates single detonation wave. The detonation/shock collision decreases the detonation wave intensity. Abscission of detonation tail and shock to detonation transition can both create single detonation wave or two opposite-direction detonation waves, depending on the wave hitting angle and the amount of fresh gas. All phenomena mentioned above affect the number of detonation waves in the combustion chamber.     

双点激光点火直接起爆的数值模拟

针对双点激光点火直接起爆过程中爆轰波的形成、发展和传播问题,采用高精度数值模拟方法求解带化学反应的二维欧拉方程组,研究了不同环境压力情况对流场结构与波系变化的影响.结果表明,环境压力会影响激波强度与爆轰波的传播速度,是决定双点激光点火形成的火核在碰撞过程中能否实现爆轰并维持爆轰波传播的重要因素,利用双激光点相互作用形成...     

Effect of obstacles on the detonation diffraction and subsequent re-initiation

The DCRFoam solver (density-based compressible solver) built on the OpenFOAM platform is used to simulate the reflection and diffraction processes that occur when detonation waves collide with various objects. Static stoichiometric hydrogen–oxygen mixtures diluted with 70% Ar are used to form stable detonation waves with large cells, with initial conditions of 6.67 kPa pressure and 298 K temperature. The diameters of the cylindrical obstacle range from 6 mm to 22 mm, with x = 230 mm, x = 244 mm, and x = 257 mm being the chosen position. Cylindrical, square, triangular, and inverted triangular obstacles are used, and the quenched detonation re-initiation processes behind them are investigated. In the detonation diffraction process, four triple points exist at the same time due to the effect of cylindrical obstacles of smaller diameters. The re-initiation distance of the detonation wave increases with the increase of cylindrical obstacle diameter. Both the Mach reflection angle and the decoupled angle decrease as the diameter increases. When the location of the cylindrical obstacles is changed, the detonation wave dashes into the obstacles with its different front structures, it is easier to realize the detonation re-initiation when the weak incident shock at the front of a detonation wave strikes the obstacles, and the re-initiation distance decreases by 17.1% when compared with the longest re-initiation distance. The detonation re-initiation distance is shortest under the action of cylindrical obstacles, however the quenched detonation cannot be re-initiated when the inverted triangle and square obstacles are used. The suppression effects of inverted triangle and square obstacles on detonation waves are more evident.     

Simulations of detonation wave propagation in rectangular ducts using a three-dimensional WENO scheme

This paper reports high resolution simulations using a fifth-order weighted essentially non-oscillatory (WENO) scheme with a third-order TVD Runge-Kutta time stepping method to examine the features of detonation front and physics in square ducts. The simulations suggest that two and three-dimensional detonation wave front formations are greatly enhanced by the presence of transverse waves. The motion of transverse waves generates triple points (zones of high pressure and large velocity coupled together), which cause the detonation front to become locally overdriven and thus form “hot spots.” The transversal motion of these hot spots maintains the detonation to continuously occur along the whole front in two and three dimensions. The present simulations indicate that the influence of the transverse waves on detonation is more profound in three dimensions and the pattern of quasi-steady detonation fronts also depends on the duct size. For a “narrow” duct (4L×4L where L is the half-reaction length), the detonation front displays a distinctive “spinning” motion about the axial direction with a well-defined period. For a wider duct (20L×20L), the detonation front exhibits a “rectangular mode” periodically, with the front displaying “convex” and “concave” shapes one following the other and the transverse waves on the four walls being partly out-of-phase with each other.     

Induction for multiple rotating detonation waves in the hydrogen–oxygen mixture with tangential flow

Rotating detonation engines are studied more and more widely because of high thermodynamic efficiency and high specific impulse. Generally one detonation wave exists in the engines but sometimes multiple detonation waves appear, as is complicated and difficult to explain. Increasing the number of rotating detonation waves uniforms the flow field and weakens the combustion instabilities. A controllable way to induce multiple detonation waves is introduced here. Rotating detonation engine runs with a single detonation wave or multiple detonation waves were both conducted. Pressure sensors were used to record the pressure traces of rotating detonation waves and gas flow controllers controlled the flow rates of reactants. Tangential flow of reactants from the predetonator produces shock waves moving upstream, inducing multiple rotating detonation waves when there is axial flow of reactants from the head of the combustor. The maximum number of detonation waves is subject to the flow rates.     

Numerical application of additive Runge-Kutta methods on detonation interaction with pipe bends

A detailed reaction model comprised of 9 species and 48 reactions is employed in simulating two-dimensional cellular detonations propagating through smooth pipe bends in a stoichiometric H₂/O₂ mixture diluted by Argon. Additive Runge-Kutta (ARK) methods are applied to solve the stiff reactive Euler equations, in which the stiff and non-stiff terms are solved implicitly and explicitly. The numerical results indicate that, as the regular cellular detonation wave propagating through the bend section, the diffraction near the inner wall causes an increase in detonation cell size while the detonation reflection occurring on the bottom wall leads to a decrease in cell size. In addition, an expansion wave is generated continuously. The expansion wave causes the failure as well as the partial failure of the detonations near the inner and outer walls, respectively. On the contrary, the transverse re-initiation waves evolve into a detonation in the decoupling zone just downstream of the bend outlet owing to continuous compression imposed by other transverse waves propagating right behind. Meanwhile, there exists a transition length after the detonation propagating out of the bend and entering the sloped tube section.     

多循环喷雾爆震的特性研究

在无阀式脉冲爆震发动机模型机上进行了多循环喷雾两相爆震的实验研究．点火后爆震管内压力上升需要一定的延迟时间,但是迅速增压过程是在火焰传播到一定区域后开始的,在该区域形成向两个方向传播的压缩波,向未燃区传播的压缩波不断加强,形成爆震波,向已燃区传播的压缩波不断衰减;爆震峰值压力沿流向不断增加,压力上升速度加快,峰值随机差异放大;通过对压力历程的分析,用两种方法估算了两相爆震波诱导区的长度．实验中发现,两相爆震的点火延迟时间远大于爆燃向爆震转变的时间,两者之和相对于高频爆震循环非常可观,是限制两相脉冲爆震发动机频率提高的关键因素,并分析了多循环工作时的吸气和排气过程．     

Deflagration to detonation transition in aluminum dust-air mixture under weak ignition condition

Deflagration to detonation transition (DDT) in flake aluminum dust-air mixture was studied in a 199 mm inner diameter and 29.6 m long horizontal tube. 40 sets of dust dispersion system were used to disperse flake aluminum into the experimental tube. An electric spark of 40 J was used to ignite the aluminum-air mixture. Self-sustained detonation was observed and the characteristics of deflagration to detonation transition process were studied. Contributed to the transverse wave and cellular structure of detonation wave front in aluminum-air mixture, the propagation velocity of detonation wave ranges from 1480 m/s to 1820 m/s and the maximum overpressure oscillates between 40 and 102 bar. Single head spinning detonation wave in aluminum dust-air mixture was observed and the cell size was evaluated.     

Experimental investigation on propagation characteristics of liquid-fuel/preheated-air rotating detonation wave

The rotating detonation combustor can be applied to the turbine engine to develop into a new power device, and the liquid-fuel/air rotating denotation has important research significance for engine applications. In this research, the propagation characteristics of liquid-fuel/air rotating detonation wave were experimentally investigated. A hydrocarbon mixture—liquid gasoline was employed for the fuel, the oxidizer was high-temperature air preheated by a hydrogen-oxygen heater, and the rotating detonation wave was initiated via a hydrogen-oxygen pre-detonator. The effects of the equivalence ratio, ignition pressure, and air total temperature on the propagation characteristics of the liquid-fuel rotating detonation wave were analyzed. The liquid-fuel/air continuous rotating detonation wave can be successfully obtained with a single-wave mode, and the velocity and peak pressure of the rotating detonation waves increase as the equivalence ratio increases. As the detonation-wave pressures at the outlet of the pre-detonator increase, the establishment time of the rotating detonation wave gradually decreases, and the average establishment time is 4.01 ms. Stable rotating detonation waves are obtained with the air total temperature of 600–800 K, but the intensity of the detonation wave has a large deficit due to some instabilities.     

Cellular pattern evolution in gaseous detonation diffraction in a 90°-branched channel

This paper presents recent results of an experimental investigation on gaseous detonation diffraction in a 90°-branched channel. The entire process of diffraction is demonstrated by cellular patterns and the analysis is mainly based on their evolution. Detonation pressure history and velocity are measured and the corresponding cellular patterns are recorded on soot foils around the branched segment. Results show that detonation propagation is notably disturbed by the branched wall geometry and that a complex wave configuration appears in both channels. Cellular patterns show that an expansion fan appears at the T-junction area with a Mach reflection taking place in the horizontal channel, while regular reflection takes place in the vertical channel. Subsequently, it appears that there is a transition from a regular reflection to a Mach reflection in the vertical channel. Details of the cellular pattern indicate that from the early stage to the end of diffraction, the detonation wave sequentially experiences attenuation, front decoupling, and degradation into deflagration, reinitiation, and recuperation. According to cellular pattern evolution and velocity measurement, a recuperated detonation with nearly the same velocity as the undisturbed incoming wave finally develops downstream in both channels, at a distance of about four times the channel height (160 mm). The mechanism of diffraction is explored based on the ZND (Zel'dovich-von Neumann-Döring) model, and the soot foils in both channels show a pattern consistent with air shock-wave diffraction in a 90°-branched channel.     

Characteristic of rotating detonation wave in the H2/Air hollow chamber with Laval nozzle

To pinpoint the relationship between high frequency tangential instability(HFTI) and continuous rotating detonation (CRD), series of H₂/Air rotating detonations are experimentally achieved in the hollow chamber with Laval nozzle. The contraction ratio of the nozzle has a significant effect on the detonation. The detonation waves number increases with the increasing of equivalence ratio (ER) or nozzle contraction ratio. Based on its character, a new type of detonation is defined as two dominant peak one wave mode (TDPO). The velocities of detonation waves propagating in this new mode are larger than the Chapman-Jouguet (CJ) theoretic value. On the assumption that the reflection wave is rotated with the detonation wave, this mode is well illustrated. The forming process of two waves is also given. The results show that the appearance of combustion mode is relative to the reflection wave generated at the contraction section of the nozzle. The inner mechanism of the refection wave is illustrated. These works make a foundation to investigate the relationship between rotating detonation and tangential instability.     

Effects of a turbine guide vane on hydrogen-air rotating detonation wave propagation characteristics

The rotating detonation engine is a new machine that can generate thrust via continuous rotating detonation waves (RDWs). In this study, experiments were performed on a structure combining a rotating detonation combustor (RDC) and a turbine guide vane to investigate the propagation characteristic of hydrogen-air RDW. The results showed that the velocity of detonation wave initially increased and then decreased with the increase of equivalence ratio, and it got a velocity of 84% Chapman-Jouguet value. The velocity of detonation wave generally rose by 4.31% comparing with the no guide vane tests, while the scope of steady-operation state became narrow. The oscillation pressure was reduced by 64% after passing through the guide vane, and the magnitude of pressure was only 0.4 bar at the guide vane exit. Meanwhile, part of the shock wave was reflected back to combustor resulting in some small pressure disturbances, and the propagation mode of reflected wave was related to the propagation direction of RDW.     

Experimental and numerical studies on detonation failure and re-initiation behind a half-cylinder

Mach reflection causes the re-initiation of decoupled detonation owing to changes in the boundary. A complementary series of experiments and numerical simulations, illustrating detonation failure and subsequent reinitiation processes, have been presented. Immediately across the half-cylinder, the decoupled detonation owing to the diffraction effect wave is reflected on the bottom wall to form a regular reflection, and then changes into the Mach reflection, which further determines the detonation reinitiation. Two different reinitiation modes after detonation wave diffraction were observed for the stable mixtures: the direct Mach reflection re-initiation mode and Mach reflection combined with the transverse detonation. However, for unstable detonations, a different reinitiation mode was obtained, whereby the development of intrinsic instabilities resonates with the reflection on the bottom wall, rendering the Mach reflection randomly occurring or even absent. The critical limit of detonation failure is characterized by the radius of the half-cylinder and the cell size. In addition, the transition length from regular to Mach reflection was measured to reveal the length-scale effect on the process.     

Supersonic driven detonation dynamics for rotating detonation engines

In this paper we present the first exploration of detonation wave propagation dynamics in premixed supersonic flows using a novel rotating detonation engine (RDE) configuration. An RDE with a coupled linear extension, referred to as ρDE, is used to divide detonations traveling radially in the RDE into linearly propagating waves. A tangential propagating wave is directed down a modular tangential linearized extension to the engine for ease of optical diagnostics and hardware configuration investigations. A premixed Mach 2 supersonic linear extension is coupled to the ρDE to investigate the effects of varying crossflow configurations for detonation propagation, particularly the interaction between detonations and supersonic reactive mixtures. Detonation waves are generated at the steady operating frequency of the RDE and visualized using high speed schlieren and broadband OH* chemiluminescence imaging. The stagnation pressure was varied from over- to ideally-expanded supersonic regimes. Experimental analysis of detonation interaction with the supersonic regimes show that the detonation propagates freely in the ideally-expanded regime. Deflagration-to-detonation transition (DDT) occurs in the over-expanded regime. Based on the data collected, the DDT process favors supersonic flow with higher source pressures.     

Numerical study on the characteristics of rotating detonation wave with multicomponent mixtures

In order to investigate the effects of gas mixture components on the combustion characteristics of rotating detonation wave, two-dimensional simulation is presented to simulate the propagation process of rotating detonation wave with different methane conversions. The results indicate that there are five propagation modes of rotating detonation wave with different components: single-wave mode, single wave with counter-rotating components mode, double-waves mode, triple-waves mode and quadruple-waves mode. The detonation wave propagates along the forward direction in all five modes. With the increase of methane conversion, multi-wave mode appears in the combustion chamber. The fuel component has a great influence on the heat release ratio of detonation combustion. The velocity of detonation wave decreases with the increase of methane conversion. With the increase of methane conversion, the chemical reaction rate gradually increases, which leads to the intensification of chemical reaction on the deflagration surface. The reaction on the deflagration surface develops to the unburned fuel zone, which eventually leads to the formation of compression waves and shock waves in the fuel refill zone. When the shock wave sweeps through the fresh premixed gas, the reactant is compressed to form a detonation point and then ignite the fuel. A new detonation wave is finally formed. The total pressure ratio decreases with the increasing methane conversion, and the uniformity of the total pressure of outlet decreases with increasing methane conversion.     

Investigations on multicycle spray detonations

Experimental investigations were carried out on a 50-I.D. multicycle pulse detonation engine (PDE) model, and liquid fuel (gasoline) was used. The average of pressure peak, as measured by piezoelectricity pressure transducer, increased versus distance to thrust wall before fully-developed detonation came into being. According to the pressure history, the pressure in detonation tube would not rise abruptly until the flame front advanced a certain distance downstream the spark. Just at that moment, two compression waves spreading to opposite direction were formed. One was enforced by combustion and became detonation rapidly. The other was weakened because of obstacles and insufficiency of fuel. Two methods were used to determine the induction length of two-phase detonation wave through the pressure history. Ignition delay time was found to be longer than deflagration-to-detonation transition (DDT) time, and the sum of the two would change little as cycle frequency increased. So they could be the most important factors controlling two-phase PDE frequency. Filling process and blowdown process were also analyzed. Translated from Journal of Combustion Science and Technology, 2006, 12(1): 90–95 [译自: 燃烧科学与技术]     

Numerical simulation on the operating characteristics of rotating detonation ramjet engines at high flight mach number

For high-Mach-number incoming flow circumstances, a rotating detonation ramjet engine configuration is proposed in this research. By installing supporting blocks at the rear of the combustor, this configuration achieves continuous rotating detonation operation. Based on the Comparison of the flow structures obtained from the engine configuration with and without the supporting block before and after detonation ignition respectively, we obtain the intrinsic mechanism of detonation wave's propagation and re-initiation under the action of the supporting block. The supporting block creates a deflagration wave that is almost stationary before detonation ignition. In the detonation-ignited state, the deflagration wave is continually formed and traveling upstream under the influence of the supporting block, which is analogous to the periodical before detonation ignition of a transverse wave structure. The dynamic deflagration wave will cause the incomplete reactants behind the detonation wave to burn as the intensity of the detonation wave decreases. As a result, the incident shock wave is transformed into a Mach stem to achieve the re-initiation of the detonation wave.     

烈性爆震下活塞材料破坏实验

已有研究表明,乙炔/氧气混合气在燃烧室内产生的爆轰波会发生汇聚现象并且容易破坏活塞材料.为了揭示烈性爆震下影响爆轰波破坏力的关键因素,通过改变活塞试件的厚度和混合气的初始状态来进行活塞材料破坏实验.结果表明,对于烈性爆震,活塞顶部中心厚度的选择存在最佳值.当初始压力超过一定的安全值之后,试件的中心挠度会急剧增大.当量比从1.0增加到1.5时,混合气变浓,爆轰波破坏力并不会减弱.当混合气当量比从1降低到0.5时,爆轰波的破坏力明显减弱.     

Numerical investigation of the effect of inlet mass flow rates on H2/air non-premixed rotating detonation wave

In this paper, a three dimensional numerical investigation was carried out to study the formation and propagation characteristics of non-premixed rotating detonation wave using H₂/air as reactive mixtures. At a constant global equivalence ratio, the effects of inlet mass flow rates of H₂ and air on various performance parameters of rotating detonation wave and based on it combustor were analyzed in detail. On this basis, the mode switching process of rotating detonation wave caused by transiently changing the inlet mass flow rates was also discussed. The numerical results showed that inlet mass flow rates of H₂ and air played a very critical role in the formation, propagation and mode switching of rotating detonation wave. With the increase of inlet mass flow rates, rotating detonation wave could be switched from single wave to double waves. The propagation direction of double waves depended on the changing process of inlet mass flow rates. Meanwhile, compared to the single wave, double waves or its based combustor had the obvious advantages in formation time, stability and thrust, but had disadvantage in pressure ratio. In addition, both fill characteristics and mixing quality of fresh reactive mixtures are the underlying important mechanisms to explain the effects of inlet mass flow rates on rotating detonation waves.