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.     

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.     

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%.     

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.     

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.     

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.     

Performance evaluation and outlet load improvement of a rotating detonation combustor with different outlet nozzles

Outlet nozzles for a rotating detonation combustor were designed to meet a downstream turbine and reduce the high pressure and heat load caused by the oblique shock wave at the outlet. The effects of the rotating detonation combustor with two types of outlet nozzles were studied, and the performance and outlet parameters of the combustor were measured at an elevated chamber pressure and preheating temperature based on gas turbine conditions. The results showed that the outlet nozzles could cause changes in the wave collisions and folding of the weak flame front in the detonation formation process, but the basic propagation process was similar to that without a nozzle. The pressure ratio changed from 1.427 in the original model to 1.392 and 1.304 with the two types of outlet nozzles. Meanwhile, the outlet load was greatly improved. The peak values of the static temperature at the outlet dropped by 22.423% and 27.572% with the two types of outlet nozzles compared to the original model. In addition, the peak static pressures dropped by 75.737% and 83.722%, respectively. In addition, the outlet nozzles significantly reduced the unevenness of the outlet static temperature and static pressure distributions. This created a better outlet operating environment, thereby improving the performance of the rotating detonation combustor.     

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.     

The effect of premixed stratification on the wave dynamics of a rotating detonation combustor

Typical injection schemes of rotating detonation combustors inject fuel locally into the combustion channel, creating stratified fuel-rich and fuel-lean mixing regions. In this study, premixed hydrogen and air rotating detonations are explored in a rotating detonation combustor through premixing part of the fuel into the oxidizer flow. The objective is to investigate the effect of premixing on the operation of the combustor. Three premixing schemes are examined where the detonation wave speeds are analyzed. The results show that in premixing, the fuel-lean regions became more favorable for continuous detonation propagation when premixed with the bypass fuel, resulting in higher detonation wave speeds. This phenomenon is shown to be independent of the global fuel-air equivalence ratio and the amount of fuel premixed into the oxidizer. As such, combustor performance and the operational regime could be improved with lean hydrogen premixing amounts in the main flow oxidizer.     

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.     

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.     

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 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.     

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.     

Investigation of counter-rotating shock wave phenomenon and instability mechanisms of rotating detonation engine with hollow combustor and Laval nozzle

The counter-rotating shock wave phenomenon and instability mechanisms in the hollow rotating detonation combustor with a Laval nozzle are investigated. The in-house solver BYRFoam based on the OpenFOAM platform and the detailed chemical reaction kinetic mechanism are used. The coupling of the detonation wave and the shock wave is revealed, with the continuous presence of the counter-rotating shock wave in the combustor that propagates in the opposite direction to the detonation wave and oscillates in intensity. The intensities of both the detonation wave and the counter-rotating shock wave are found to be constantly oscillating, and this instability is referred to as the collision-oscillation instability. It is caused by the complex interaction between the detonation wave, the fresh gas and the counter-rotating shock wave. The velocity difference between the detonation wave and the counter-rotating shock wave is found to lead to the migration of the collision point, which in turn leads to the peak periodic undulation phenomenon of the pressure curve, that is, the wave packet. This instability is called the counter-rotating instability, which co-exists with the collision-oscillation instability for a long time. The results of this study are compared with the experimental data, and the reasons for the oscillation of the experimental pressure signal are explained.     

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 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 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.     

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.