Numerical investigation on H2/Air non-premixed rotating detonation engine under different equivalence ratios

In this paper, three-dimensional numerical simulations are performed to investigate the formation and propagation characteristics of rotating detonation wave in a non-premixed engine. By changing the mass flow rate of H₂ and fixing air mass flow rate, the effects of equivalence ratio involving fuel lean and rich operating conditions are mainly discussed. Numerical results show that equivalence ratio plays a very critical role in the formation process and propagation mode, which further affects the propulsion performance of rotating detonation engine significantly. For current numerical geometry and operating conditions, the lean limit of equivalence ratio for formatting a stable RDW is about 0.4, dual-wave mode (at equivalence ratio of 0.6, 0.8, 1.0 and 1.4) and single-wave mode (at equivalence ratio of 1.2) are obtained, respectively. When equivalence ratio is 1.0, rotating detonation engine can exhibit excellent operating performance with the shortest formation time, best propagation stability, middling class thrust and specific impulse. Besides, the pressure contour analysis indicates that the effects of equivalence ratio and mass flow rate of H₂ on the collision strength and times during the re-initiation process are the main mechanisms for determining the formation possibility and propagation mode of rotating detonation wave. Besides, the intensity of accumulated pressure wave and distributions of equivalence ratio are two important factors for the generation of new detonation wave front. Furthermore, it is also detected from the comparisons of the propulsion performance that the effects of equivalence ratio on thrust and specific impulse under fuel lean conditions are more significant than those under fuel-rich conditions.     

Numerical study on the interaction characterization of rotating detonation wave and turbine rotor blades

Rotating detonation as a kind of pressure gain combustion is expected to greatly improve efficiency when applied to gas turbine engines. In this paper, the operation of rotating detonation combustor and turbine rotor blade was studied. Firstly, the analysis of the interaction between detonation wave and turbine blade shows that the compression of gas by detonation wave and reflected wave will lead to a sharp increase in the temperature at the wall of blade. When the detonation wave propagates, the oscillation amplitudes of pressure and temperature at the turbine inlet are 70% and 75% respectively, and the detonation oblique shock will change the flow trajectory of the air flow, resulting in the flow direction deviating from the incident angle. Then the comparison between detonation and deflagration shows that the total pressure of detonation is higher and will have greater work potential. The torque generated by the blades under detonation has the characteristics of high-frequency oscillation, which may be detrimental to the operation of the engine.     

Experimental and numerical research on rotating detonation combustor under non-premixed conditions

In order to investigate the formation process and propagation characteristics of detonation wave, developing process of detonation wave from initiation to stable detonation formation under non-premixed conditions has been studied by experiments and numerical simulation. The results show that when mass flow rates of air and hydrogen are 158.957 g/s and 2.728 g/s respectively, stable detonation can be formed in the combustor. Due to the lower inlet pressure, there is an unstable stage in combustor before the stable detonation is formed. Reducing the air pressure will increase the lowest detonation limit of combustor and lead to flame-out and re-initiation in the combustor. The propagation direction of detonation wave may change after re-initiation. Non-premixed intake structure lead to the inconsistency of rotating detonation combustion fluid in the radial direction. Moreover, peak pressure appears near the outer wall, while peak temperature appears near the inner wall.     

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.     

Study of thrust vector control for the rotating detonation model engine

The detonation wave in a rotating detonation engine is highly adaptable to the incoming flow, making the wave easier to control. In this study, a numerical simulation method is used to analyze the working process and flow field structure of a rotating detonation model engine with dual cavity injection of an H₂/air mixture by controlling the injection pressure ratio of the dual cavity and the number of detonation wave heads. It is found that the rotating detonation engine offers the possibility to control the thrust vector with two different modes. The first is a one-cycle alternate control mode with a small injection pressure ratio. Here two deflections occur in different directions occur across one detonation wave propagation cycle, but the overall deflection direction is in the low-pressure region. The second is a one-way control mode, with a large injection pressure ratio, and the deflection direction towards the low-pressure region. For the multi wave-mode, it belongs to one-way control mode because of constant deflection direction in the low-pressure area. From the perspective of thrust distribution along the circumference, the one-way control strategy satisfies the ability of a rotating detonation thrust vector control.     

Knocking combustion in spark-ignition engines

Knocking combustion research is crucially important because it determines engine durability, fuel consumption, and power density, as well as noise and emission performance. Current spark ignition (SI) engines suffer from both conventional knock and super-knock. Conventional knock limits raising the compression ratio to improve thermal efficiency due to end-gas auto-ignition, while super-knock limits the desired boost to improve the power density of modern gasoline engines due to detonation. Conventional combustion has been widely studied for many years. Although the basic characteristics are clear, the correlation between the knock index and fuel chemistry, pressure oscillations and heat transfer, and auto-ignition front propagation, are still in early stages of understanding. Super-knock combustion in highly boosted spark ignition engines with random pre-ignition events has been intensively studied in the past decade in both academia and industry. These works have mainly focused on the relationship between pre-ignition and super-knock, source analyses of pre-ignition, and the effects of oil/fuel properties on super-knock. The mechanism of super-knock has been recently revealed in rapid compression machines (RCM) under engine-like conditions. It was found that detonation can occur in modern internal combustion engines under high energy density conditions. Thermodynamic conditions and shock waves influence the combustion wave and detonation initiation modes. Three combustion wave modes in the end gas have been visualized as deflagration, sequential auto-ignition and detonation. The most frequently observed detonation initiation mode is shock wave reflection-induced detonation (SWRID). Compared to the effect of shock compression and negative temperature coefficient (NTC) combustion on ignition delay, shock wave reflection is the main cause of near-wall auto-ignition/detonation. Finally, suppression methods for conventional knock and super-knock in SI engines are reviewed, including use of exhaust gas recirculation (EGR), the injection strategy, and the integration of a high tumble - high EGR-Atkinson/Miller cycle. This paper provides deep insights into the processes occurring during knocking combustion in spark ignition engines. Furthermore, knock control strategies and combustion wave modes are summarized, and future research directions, such as turbulence-shock-reaction interaction theory, detonation suppression and utilization, and super-knock solutions, are also discussed.     

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 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.     

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.     

Effect of orifice shapes on the detonation transmission in 2H2–O2 mixture

In this study, the effect of orifice geometries on the detonation propagation is considered systematically in stoichiometric 2H₂–O₂ mixture. Three various orifice shapes with the same blockage ratio (BR = 0.889) are used firstly, i.e., round, square and triangular. Eight PCB pressure transducers are employed to obtain the average velocity through two adjacent signals while the smoked foil technique is used to record the detonation cellular pattern. The experimental results indicate that three different propagation modes can be observed: (1) when the initial pressure (P₀) is smaller than the critical value (P_c), the steady detonation wave cannot be produced before the orifice plate, afterwards, the mechanism of deflagration to detonation transition (DDT) is seen; (2) near the critical pressure, a steady detonation wave is formed prior to the obstacle, but the failure of detonation is seen after its propagation through the orifice plate due to the diffraction effect and the mass and momentum loss from the wall, and then the phenomenon of detonation re-initiation is observed due to the reflection from the wall; (3) at the initial pressure larger than the critical value, the steady detonation wave can propagate through the orifice plate without decay. Moreover, although the effect of orifice shapes on the critical pressure can be nearly ignored, the re-ignition position is different among three various orifice geometries. For the cases of round and square orifices, the ignition position is produced near the center of the wall. However, the detonation wave is re-ignited from the corner in the case of triangular orifice. Finally, the critical condition of detonation propagation can be quantified as D_H/λ > 1. But the critical values of D_H/λ are not uniform among three different orifice geometries. For the cases of round, square and triangular orifices, the critical values of D_H/λ are 8.94, 5.88 and 3.84, respectively.     

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.     

Transition to detonation in a hydrogen-air mixtures due to shock wave focusing in the 90 - deg corner

This work presents experimental observation of the ignition modes due to shock wave focusing in a 90 - deg corner in a mixture of 20%–55% H₂ in air with the main purpose of recognizing critical conditions for transition to detonation. The results showed three ignition modes, first ‘weak’ ignition followed by deflagration with ignition delay time higher than ∼1 μs, second ‘strong’ with instantaneous transition to detonation, and third with deflagrative ignition and delayed transition to detonation. The transition was observed only when specific shock wave velocity was reached. The transition velocity for stoichiometric mixture was approx. 715 m/s corresponding to M = 1.75 and 71% of speed of sound in products. For leaner or richer mixtures, the transition velocity increased approaching the speed of sound in products at approx. 18% and 58% H₂.     

Numerical study on transition of hydrogen/air flame triggered by auto-ignition under effect of pressure wave in an enclosed space

End gas auto-ignition and transition of flame front are considered as the main causes of severe pressure oscillation in spark-ignition engines, which is one of the major features of knock and super-knock. The knowledge of characteristics of auto-ignition, flame front development, propagation of pressure wave and relations between them, still needs to be maintained. In this study, flame front transition induced by pressure wave and auto-ignition are investigated using one-dimensional simulation with detailed chemistry in an enclosed space Calculation cases with different initial thermodynamic conditions are investigated. Mass fraction of OH is employed as indicator of auto-ignition progress under variable conditions caused by pressure wave. Different propagation modes of flame front, including subsonic deflagration, detonation and supersonic deflagration, are developed under the effects of both pressure wave and auto-ignition. Results show that mass fraction of OH could successfully reflect auto-ignition progress, thus indicating occurrence and sequence of auto-ignition at different locations. Transitions from deflagration to detonation and detonation to supersonic deflagration are found to be triggered by sequential auto-ignition with different gradient of auto-ignition progress ahead of flame front induced by pressure wave.     

Numerical study of detonation diffraction through 90-degree curved channels to expansion area

The adaptive mesh refinement program AMROC was adopted to conduct numerical study of detonation diffraction through 90-degree curved channels to expansion area. The detonation diffraction processes through straight and curved channels were compared. The effect of curved channel on the re-initiation of diffracted detonation and its mechanism were analyzed in depth. The influence of curvature radius on the deflection of detonation front was discussed. Results indicate that the detonation wave is highly asymmetric when diffracted through the curved channel, which results in the wave front deflection with a certain angle. The value of deflection angle θ decreases with the increase of inner wall curvature radius r₀, while the deflecting direction is the same as the propagating direction of the triple point in the Mach reflection. The utilization of the curved channel can promote the re-initiation of diffracted detonation. The mechanism of this promoting effect is the redistribution of detonation energy produced by the interaction from the curved boundaries, which concentrates most of the energy into the compressed area to initiate local detonation and realizes re-initiation eventually. The promoting effect is weakened as r₀ gets enlarged and the initial pressure p₀ decreases. The critical condition for the re-initiation of diffracted detonation wave through the curved channels was obtained by analyzing the detonation re-initiation results.     

Coupling characteristic analysis and propagation direction control in hydrogen–air rotating detonation combustor with turbine

Two-dimensional numerical simulations are performed to investigate the interactions between a rotating detonation combustor (RDC) and turbine. The flowfield structure and coupling characteristics are analyzed under different equivalent ratios. Furthermore, the formation mechanism of the detonation wave direction is illustrated. Results show that although RDCs increase the average turbine work, they enhance the flowfield oscillation and maintain the quasi-periodic fluctuation in the turbine work. Compared to counter-clockwise propagation, the pressure oscillation attenuation and total pressure loss through the turbine guide vane increase when the detonation wave propagates clockwise. When the detonation wave height is low, RDCs are more prone to quenching due to the impact of reflected waves. After re-ignition, the detonation wave direction is unrelated to the initial ignition direction and is random. Introduction of deflecting wedges into RDCs enables the automatic control of the detonation wave direction but causes extra total pressure loss of at least 3.6%.     

Numerical investigation of pre-detonator in rotating detonation engine

Pre-detonators are commonly used in rotating detonation engine (RDE) experiments. Current experimental studies focus on the performance of pre-detonator while ignoring the influence of pre-detonator on the flow field. In numerical simulations one-dimensional detonation wave is usually used to ignite the fresh gas in RDE. This is a simplification of the pre-detonator used in practical hotfire tests. But the coupling between the pre-detonator and the combustor is ignored. The aim of the present study is to study the influence of pre-detonator on the flow field in the RDE. A model of RDE with a pre-detonator is built, in which three-dimensional numerical simulations fueled with hydrogen/air is performed. The influence of pre-detonator on the combustor in different stages is studied. After initiation, detonation wave from the pre-detonator forms two counter-rotating detonation waves. The tangential installation of pre-detonator fails in directional initiation of detonation wave. The coupling effect is shown as the reflection and expulsion of shock wave. Detonation wave or oblique shock wave in the combustion chamber enters the pre-detonator and turns into shock wave before colliding with the end and re-entering the combustion chamber. Under some circumstances, the reflected shock wave will initiate a detonation wave and affect the wave structure in the combustion chamber. In the stable stage, the reflected shock wave has no effect on the flow field. However, periodic collision of reflected shock wave with detonation wave at the junction causes ablation in long-time experiments. Increasing the axial distance between pre-detonator and injection wall is expected to be a solution for the ablation problem.     

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.     

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.     

负温度梯度对预混气体反应波传播模态的影响

基于一维数值模拟研究了负温度梯度对氢气/空气混合气反应波传播模态的影响,重点论述燃烧模态的转变机理.研究表明:当反应波沿着负温度梯度传播时,可以观测到超声速爆燃向爆震、爆震向亚声速爆燃的转变;其中,超声速爆燃传播过程由顺序自燃控制;当其传播速度低于当地Chapman-Jouguet爆震速度时,会发生超声速爆燃向爆震的转变.此外,在低温区域也观测到了爆震向亚声速爆燃的转变.Zel’dovich(ZND)结构和RankineHugoniot曲线表明,诱导激波后混合气反应活性对维持爆震传播具有重要作用.     

Propagation process of H2/air rotating detonation wave and influence factors in plane-radial structure

The rotating detonation wave (RDW) propagation processes and influence factors are simulated in the plane-radial structure. The effects of inner radii of curvature, domain widths and stagnation pressures on propagation mode are studied. The RDW is initiated, and two kinds of propagation mode are obtained and analyzed. The flow field structure, parameters variation and influence factors on unstable propagation mode are explored in depth, and the geometrical and injection conditions of the unstable propagation are obtained. Results indicate that the decoupling and re-initiation occur repeatedly during the unstable propagation mode of the RDW, and the angular velocities of leading shock wave vary accordingly. When the domain width remains constant, the range of stagnation-pressure under unstable propagation mode increases as the inner radius increases. But the RDW propagates steadily when the inner radius increases to a certain value (Larger than 40 mm in this study). The effect of curvature radius and initial pressure ahead of detonation wave on the unstable propagation mode in this calculation model is similar to that in a curved channel. When r_i +0.464p_a > 80.932 or r_i ≥ 40 mm, the detonation wave can propagate steadily in the annular domain. When the curvature radius remains constant, the stagnation-pressure range of the unstable propagation mode decreases as the domain width increases.