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.     

Numerical investigation of wavelet features in rotating detonations with a two-step induction-reaction model

The wavelet features of rotating detonation waves (RDWs) are numerically investigated using Euler equations and a two-step induction-reaction model. The effects of the inflow stagnation temperature T_st and the heat release rate k_R on the number, height and intensity of the RDWs are discussed in this study. An increase in the stagnation temperature results in more detonation waves in a combustion chamber, which indicates the number of RDWs is sensitivity to the thermodynamic state of the reactants. As the heat release rate decreases, the number of detonation wave decreases and an unstable wavelet pattern is observed. This is represented as the oscillation in height and intensity of the detonation. In addition, some numerical cases are performed to determine the effects of ignition patterns on the number of RDWs. The features of the flow fields are analyzed using varied inflow stagnation temperature and initiation patterns, identifying the co-existence of different wavelet configurations.     

Numerical research of rotating detonation initiation processes with different injection patterns

Three-dimensional numerical simulations are performed to study the initiation process of the rotating detonation combustor initiated by a pre-detonator. Navier-Stokes equations are solved with a 7-species and 8-steps H₂/Air chemical reaction mechanism. Three different injection patterns are considered, including the inner-slit injection, outer-slit injection and mid-slit injection. A single stable rotating detonation wave is established finally for all three injection patterns. Firstly, the initiator dynamics of the pre-detonator is discussed. The initial detonation wave spreads into the combustor from the pre-detonator and two oppositely propagating detonation waves are formed in the combustor in all cases, causing wave collisions. Similar fresh gas injection disturbance is seen, suggesting the little effect of different injection patterns at the beginning. After the recovery of the injection, ‘L-form’ fresh mixture layers are formed in the inner-slit/outer-slit injection patterns, leading to a rapid establishment of stable propagation mode. While in the mid-injection pattern, transition of fresh mixture layer structure is seen from ‘I-form’ to ‘L-form’. Severe lateral expansion exists during this period, resulting in a long-term wave initiation.     

Numerical investigation of the effect of equivalence ratio on the propagation characteristics and performance of rotating detonation engine

The equivalence ratio is one of the significant factors affecting the propagation characteristics and performance of the rotating detonation engine (RDE). Using the compressible reacting flow solver based on the OpenFOAM open-source platform, the effect of different equivalence ratios of premixed H₂/Air gases on the propagation characteristics and performance of RDE under different total inlet pressures (P₀) is investigated. The reactants are injected through the discrete inlet to mimic the spatial inhomogeneity of the reactants in the actual RDE combustor. The results indicate a Y-shaped flow field structure is formed behind the rotating detonation wave (RDW) using the discrete inlet. There is only one RDW in the flow field with the change of the equivalence ratio when the P₀ is 0.5 MPa, and the primary factors affecting the RDW propagation velocity differ under fuel-lean and fuel-rich conditions. The RDW propagation mode switches from single-wave to co-directional double-wave and double-wave collision with the change of the equivalence ratio when the P₀ is 1.2 MPa. The velocity deficit of RDW in the double-wave mode is larger than that in the single-wave mode. Additionally, in the single-wave mode, the specific impulse decreases as the equivalence ratio increases, but the specific thrust increases as the equivalence ratio increases. When the propagation mode changes, the specific impulse and specific thrust show different trends with the change of the equivalence ratio. It demonstrates that multiple co-directional RDWs have the effect of stabilizing the thrust.     

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 study of a hydrogen-air rotating detonation combustor

The rotating detonation engine can generate continuous thrust via one or more detonation waves. In this study, rotating detonation experiments were performed on a combined structure which included a rotating detonation combustor (RDC) and a centrifugal compressor. Air, which functioned as an oxidiser, was obtained from the environment by the compressor, and hydrogen, which was used as fuel, was provided by the supply system. The propagation velocity of the rotating detonation wave (RDW) reached 81% of the Chapman–Jouguet value in experiments. With the increase of the air-injection area, the detonation-wave pressure increased, but the stability decreased. An air-injection area of 495 mm² was selected for long-duration experiments, and the frequency of the RDW ranged from 3 to 3.5 kHz. Through the self-adjustment of the combined structure, the air pressure ultimately reached a stable state after a certain period of time, and a stable detonation wave was formed in the RDC.     

Detonation propagation characteristic of H2–O2–N2 mixture in tube and effect of various initial conditions on it

As for the premixed H₂–O₂–N₂ gas ignited and induced by flame in tube, this paper represents systemic researches on its detonative formation process and flow field changes under different initial conditions (pressure, temperature, component concentration). The conservational Euler equation set with chemical reaction is taken as the basic gas phase equation model and the reduced elementary chemical reaction and shock wave problem are considered available so as to establish a theoretical model of premixed H₂–O₂–N₂ combustible gas detonation process. A unity coupling TVD format with second-order accuracy is adopted to solve the gas phase equation and deduce the two-dimension Riemann invariant, and the TVD format for solution of the polycomponent convection equation with elementary chemical reaction is proposed. Meanwhile, a time splitting format is adopted to perfectly treat with the rigid problem resulted from the higher time difference value between gas phase flow characteristic time and chemical reaction characteristic time. It is shown by the calculation results that the detonation waves form certain angle with relation to the tube wall surface at the initial stage of ignition, so as to incur reflections and form reflection waves; during the propagation of the detonation waves, the reflection wave structures are propagated backwards the back of waves constantly, so the whole flow field is characterized of obvious two-dimension. Besides, the excessive pressure detonation occurs at first before formation of the stable detonation propagation process, then a stable detonation propagation process forms finally. Mixed gas detonation characteristics resulted from different calculated-initially parameters are different. The higher the initial temperature and pressure of flame is, the shorter the induction time for detonation formed due to combustion acceleration of the mixed gas is, but which nearly brings no great influence on the later propagation process of the detonation waves. The initial mixed gas component can influence the detonation characteristic of the mixed gas observably, when the quantity relative ratio is close to 1 and the mixed gas with larger reaction activity, its detonation propagation speed is rapider and the pressure after detonation waves is higher. The simulation result keeps accordant with the calculated result of the typical C–J detonation theory model.     

Experimental study of a hydrogen-air rotating detonation engine with variable air-inlet slot

Rotating detonation engines have attracted considerable attentions in recent years. In this study, the experiments of initiating rotating detonation waves were performed on a H₂/air rotating detonation wave with the variable air-inlet slot. The results showed that the stability of detonation-wave pressure and velocity both initially increased and then decreased with the increase of slot width, and it could improve the stability of detonation-wave velocity via increasing the equivalence ratio. The intensity of reflected wave was strong for the tests of d = 0.5 mm, which leaded to the advance ignition of fresh mixture and a velocity deficit reaching up to 20%. The strong interaction between air plenum and combustor and bad mixing effect may be the reasons of forming unstable detonation wave for the tests of large-scale slots. The air-inlet slot of d = 1 mm, which got a best experiment results relative to other tests, had a wide equivalence-ratio scope to produce stable detonation wave.     

Three-dimensional simulation of a rotating detonation engine in ammonia/hydrogen mixtures and oxygen-enriched air

Rotating detonation using ammonia as fuel may be a potential carbon free combustion technology for gas turbine. The detonation wave structure and flow field of a rotating detonation annular combustor are investigated by three-dimensional simulation with detailed chemistry of ammonia/hydrogen-air. The detonation properties, propagation mode, combustor performance and emission characteristics are studied by varying the equivalence ratios and hydrogen concentrations. Both the increases of the combustor pressure and the hydrogen concentration promote the chemical reaction rate of the ammonia burn and the detonation wave velocity gradually increases with increasing hydrogen proportion based on one-dimensional simulation. A stable single-rotating waves resulting in ammonia/hydrogen combustor are observed for a wide range of equivalent ratios only when the hydrogen concentration is at least 0.2. The steady run of the single rotating detonation had an optimal cycle efficiency when the hydrogen concentration is increased to a critical value of 0.3. NOx emissions are more dependent on equivalent ratios than hydrogen concentration in equivalence ratios ranging from 0.70 to 1.40.     

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.     

End-gas autoignition and knocking combustion of ammonia/hydrogen/air mixtures in a confined reactor

End-gas autoignition and detonation development in ammonia/hydrogen/air mixtures in a confined reactor is studied through detailed numerical simulations, to understand the knocking characteristics under IC engine relevant conditions. One-dimensional planar confined chamber filled with ammonia/hydrogen/air mixtures is considered. Various initial end-gas temperature and hydrogen concentration in the binary fuels are considered. Homogeneous ignition of stochiometric ammonia/hydrogen/air mixtures is firstly calculated. It is found that H₂ addition significantly promotes autoignition, even if the amount of addition is small. For ammonia/air mixtures and ammonia/hydrogen/air mixtures with low hydrogen mole ratios, it is found from chemical explosive mode analysis results that NH₂ and H₂NO are most important nitrogen-containing species, and R49 (NH₂+NO<=>N₂+H₂O) is a crucial reaction during thermal runaway process. When the hydrogen mole ratio is high, the nitrogen-containing species and reactions on chemical explosive mode becomes less important. Moreover, a series of one-dimensional simulations are carried out. Three end-gas autoignition and combustion modes are observed, which includes forcibly ignited flame propagation, autoignition (no detonation), and developing detonation. These modes are identified within wide ranges of hydrogen contents and initial end-gas temperatures. Furthermore, chemical kinetics at the reaction front and autoignition initiation locations are also studied with chemical explosive mode analysis. Finally, different thermochemical conditions on knocking intensity and timing are investigated. It is found that a higher initial temperature or a higher H₂ content does not always lead to a higher knocking intensity, and the knocking timing decreases with the reactivity of end-gas.     

Investigation of rotating detonation wave fueled by “ethylene-acetylene-hydrogen” mixture

In this study, the stable operating range and basic characteristics including the pressure and speed of a rotating detonation are researched. The fuel is an “ethylene–acetylene–hydrogen” mixture, examined at three mixing ratios of 2:1:4, 2.2:1:4, and 1.8:1:4 (ethylene:acetylene:hydrogen). The pressure of the rotating detonation wave (RDW) increases when the equivalence ratio (ER) is near the stoichiometric ratio, but it is little affected by the flow rate. The detonation wave speed maintains at 1200–1400 m/s, approximately 70% of the Chapman-Jouguet (C-J) speed, which is hardly impacted by the ER and flow rate. The speed of the RDW in the long-duration tests is higher than in the short-duration tests, and the time taken for the formation of a stable RDW is longer. The stable operating range is broadened and speed is increased with the increase in the acetylene and hydrogen in the mixture. The instabilities in the RDW are found to be correlated with the planar acoustic waves, whereas the mechanisms of the decoupling and re-ignition of the RDW are explained from the perspective of thermoacoustic coupling.     

Numerical simulation of radial-stratified rotating detonation flow field structures with different injection patterns

Rotating detonation engine has been widely studied in recent years because of its high theoretical efficiency and heat release rate. In many numerical simulations, the combustible mixture is injected and fully filled at the head of the combustor. In this paper, annular injection slits are proposed and three representative injection patterns are simulated by changing the injection directions. Stable single-wave modes are formed in all three patterns and two kinds of combustible mixture layer structures are found, namely “L-shape” and “T-shape” structures. Following the combustible mixture layer, the detonation wave is not fully filled in the radial direction, thus radial and circumferential shock waves are induced from the detonation wave, forming more complex wave structures. After the radial shock wave, velocity vortex and significant deflagration are found and propagate with the shock wave, thus maintaining a higher pressure and temperature there.     

Stable detonation wave propagation in rectangular-cross-section curved channels

The detonation propagation phenomena in curved channels were experimentally studied in order to determine the stable propagation condition. A stoichiometric ethylene–oxygen mixture gas and five types of rectangular-cross-section curved channels with different inner radii of curvature were employed. The detonation waves propagating through the curved channels were visualized using a high-speed video camera. Multi-frame short-time open-shutter photography (MSOP) was developed in the present study to simultaneously observe the front shock shape of the detonation wave and the trajectories of triple points on the detonation wave. The detonation wave became more stable under the conditions of a higher filling pressure of the mixture gas and/or a larger inner radius of curvature of the curved channel. The critical condition under which the propagation mode of the detonation wave transitioned from unstable to stable was having an inner radius of curvature of the curved channel (r_i) equivalent to 21–32 times the normal detonation cell width (λ). In the stable propagation mode, the normal detonation velocity (D_n) increased with the distance from the inner wall of the curved channel and approached the velocity of the planar detonation propagating through the straight section of the curved channel (D_str). The smallest D_n was observed on the inner wall and decreased with decreasing r_i/λ. The distribution of D_n on the detonation wave in the stable mode was approximately formulated. The approximated D_n given by the formula agreed well with the experimental results. The front shock shape of the detonation wave could be reconstructed accurately using the formula. The local curvature of the detonation wave (κ) nondimensionalized by λ decreased with increasing distance from the inner wall. The largest λκ was observed on the inner wall and increased with increasing r_i/λ. D_n/D_str decreased with increasing λκ. This nondimensionalized D_n–κ relation was nearly independent of r_i/λ.     

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.     

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.     

Effect of orifice plate on the transmission mechanism of a detonation wave in hydrogen-oxygen mixtures

In this paper, a square orifice plate with 60 mm thick and the blockage ratio (BR) of 0.889 is employed to systematically explore the transmission regime of a steady detonation wave in hydrogen-oxygen mixtures. The influence of hydrogen mole fraction is also considered. The average velocity of combustion wave can be determined by evenly mounting eight high-speed pressure sensors on the tube wall, and the detonation cellular patterns can be also registered by the soot foil technique. The experimental results indicate that for the condition of smooth tube, the hydrogen concentration limits range of detonation successful propagation is 37.5%–73.68%. Two propagation modes can be obtained, i.e., the regimes of fast flame and steady detonation. The hydrogen concentration limits range is narrowed to 42.53%–69.51% in the tube with a square orifice plate. Three propagation regimes are observed: (1) near the low limit, a steady detonation wave can be produced before the obstacle, and the phenomenon of detonation decay is seen across the square orifice plate because of the influence of diffraction resulting in the mechanism of detonation failure. The failed detonation wave is not re-ignited because of the lower hydrogen concentration; (2) as the hydrogen mole fraction is increased to 42.53%, the mechanism of detonation re-ignition can be seen after the detonation decay. Well within the limits, the same detonation re-initiation phenomenon also can be observed; (3) as the hydrogen concentration is further enhanced to 69.7% beyond the upper limit, a stable detonation wave is not produced prior to the orifice plate, and the combustion wave front maintain the mode of fast flame until the end of the channel. Finally, it can be found that the detonation wave can successfully survive from the diffraction only when the effective diameter (d_eff) is at least greater than one cell size (λ).     

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.     

Experimental and numerical study on multi-wave modes of H2/O2 rotating detonation combustor

H₂/O₂ are desirable propellants for rocket-based rotating detonation engines but are rarely reported. This report presents an experimental study on rotating detonations powered by H₂/O₂. A non-premixed three-dimensional numerical simulation was conducted via OpenFOAM-based code. The experimental results revealed more than five co-rotating detonation waves at various flow rates with a propagation speed below 2000 m/s. Furthermore, an adjustment stage was observed prior to the stabilization of the detonation in the N-wave mode. The wavenumber in the adjustment stage varied between N and N+1 when the flow rate was 153 g/s and between N-1 and N+1 at 186 g/s. The simulation results revealed that multiple waves and low filling heights characterized the flow field of the H₂/O₂ rotating detonation. The severe deflagration of the contact surface led to new detonation waves at excessive filling heights. This supports further exploration of the potential application of H₂/O₂ propellants in rotating detonation rocket engines.     

Detonation wave propagation in semi-confined layers of hydrogen–air and hydrogen–oxygen mixtures

This paper presents results of an experimental investigation on detonation wave propagation in semi-confined geometries. Large scale experiments were performed in layers up to 0.6 m filled with uniform and non-uniform hydrogen–air mixtures in a rectangular channel (width 3 m; length 9 m) which is open from below. A semi confined driver section is used to accelerate hydrogen flames from weak ignition to detonation. The detonation propagation was observed in a 7 m long unobstructed part of the channel. Pressure measurements, ionization probes, soot-records and high speed imaging were used to observe the detonation propagation. Critical conditions for detonation propagation in different layer thicknesses are presented for uniform H₂/air-mixtures, as well as experiments with uniform H₂/O₂ mixtures in a down scaled transparent channel. Finally detail investigations on the detonation wave propagation in H₂/air-mixtures with concentration gradients are shown.