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 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.     

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.     

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.     

Hydrogen-fueled detonation ramjet model: Wind tunnel tests at approach air stream Mach number 5.7 and stagnation temperature 1500 K

The mode of continuous spinning detonation (CSD) combustion of hydrogen in the annular combustor of a model of a hydrogen-fueled detonation ramjet under conditions of approach air stream Mach number 5.7 and stagnation temperature 1500 K is registered experimentally in a short-duration (pulsed) wind tunnel at the overall air-to-hydrogen equivalence ratio (ER) ranging from 0.7 to 1.4. The maximum values of thrust and specific impulse of the ramjet model are attained at ER = 1.25 and are estimated as 1550 N and 3300 s, respectively. At 1.4 < ER < 1.6, the mode of longitudinally pulsating detonation (LPD) combustion is registered with somewhat lower values of thrust and specific impulse.     

Effects of injection parameters on propagation patterns of hydrogen-fueled rotating detonation waves

Two-dimensional rotating detonation waves (RDWs) with separate injections of hydrogen and air are simulated using the Navier–Stokes equations together with a detailed chemical mechanism. The effects of injection stagnation temperature and slot width on the detonation propagation patterns are investigated. Results find that extremely high temperatures can lead to a chaotic mode in which detonation waves are generated and extinguished randomly. Increasing the slot width can reduce the number of detonation waves and finally trigger detonation quenching at a low injection stagnation temperature. But increasing the slot width can change the RDW propagation pattern from a chaotic to a stable mode under high injection temperature. Furthermore, the kinetic parameter τ (representing the chemical reactivity of the mixture) and the kinematic parameter α (representing the mixing efficiency of hydrogen and oxygen) are introduced to distinguish the RDW propagation patterns.     

Experimental investigation on two-phase pulse detonation engine

This paper presents some results of experimental investigation on a two-phase pulse detonation engine (PDE) model. Proof-of-principle experiments of this model with liquid C₈H₁₆/air mixture were successfully conducted. Efforts were focused on initiation and propagation of detonation waves by means of one-step detonation initiation method, low-energy ignition system (total stored energy of 50 mJ), and effective Schelkin spiral. Three PDE models with different sizes were tested: 30 mm-I.D. by 2 m-length; 56 mm-I.D. by 2 m-length and 50 mm by 1 m, which were operated over a repetition frequency range from 1 Hz to 36 Hz. One-way valves were used to adaptively control intermittent supplies of air and fuel flows. The results of detonation velocity, over-pressure and impulse measurements were presented. The measured pressure ratio of detonation wave was close to that of C-J detonation. The effects of equivalence ratio, PDE diameter, length, and detonation frequency on its performance were experimentally investigated. The obtained results have demonstrated that the averaged thrust of PDE is approximately proportional to the volume of detonation chamber and detonation frequency. For liquid C₈H₁₆/air mixture, the PDE operation with as short a length as 1000 mm and detonation frequency up to 36 Hz was successfully realized, which made an important step to practical PDE.     

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.     

NUMERICAL STUDY OF TURBULENT FLOW AND HEAT TRANSFER IN A CONVEX CHANNEL OF A CALORIMETRIC ROCKET CHAMBER

A numerical study on flow and heat transfer in double-wave cross-corrugated passages with different structure parameters was conducted. The three-dimensional governing equations for mass, momentum, and heat transfer were solved using a control volume finite difference method and a validated low-Reynolds number k-? model. The effects of Reynolds number and structure parameters, including pitch ratio (P₁/P₂) and height ratio (H₁/H₂), were studied. It was found that with a decrease in height ratio, the mainstream flow changed from a pattern dominated by L-shaped flow to one dominated by Z-shaped flow, whereas pitch ratio had almost no influence on the flow pattern. The average Nusselt number Nu_av first increased and then decreased gradually with either an increase in the pitch ratio or a decrease in the height ratio. Pressure drop showed the same trend as heat transfer performance. The best performance evaluation criterion number (g) of double-wave passage was nearly 20% higher than that of the corresponding single-wave passage, whereas the worst was nearly 40% lower. On the whole, the double-wave plate with H₁/H₂ = 5 showed better overall performance. The double-wave plate with P₁/P₂ = 1 had better overall performance for Re < 5,000, whereas that with P₁/P₂ = 3 was better for Re > 7,500.     

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.     

Low frequency instability in a H2/air plane-radial rotating detonation engine

To investigate low frequency instability in a H₂/air plane-radial rotating detonation engine, simultaneous visualization, pressure, and ion measurements are performed. Changing reactants mass flow rate and equivalence ratio, two low frequency instabilities, amplitude low frequency instability (ALFI) and amplitude-frequency low frequency instability (AFLFI) are discovered, characterized by periodic sinusoidal oscillations of detonation-wave parameters. The operating conditions of low frequency instabilities are also summarized. The ALFI with periodic fluctuations of detonation-wave pressure peak mostly occurs in single-wave modes and symmetric dual-wave modes, operating near the critical conditions of modes switching. Almost all asymmetric dual-wave modes and some triple-wave modes exhibit some degree of AFLFI characterized by periodic waxing and waning of detonation-wave pressure peak and velocity. Each low-frequency cycle contains several to dozens of rotating detonation laps. Obvious high-frequency and low-frequency oscillations are observed in air plenum, with exactly the same frequency as that in the combustor but lower amplitudes and some phase difference. The interaction between plenum and combustor affects detonation stability.     

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 investigation of a non-premixed hollow rotating detonation engine

Rotating detonation engines (RDEs) are widely studied because of their compact configurations and high thermal cycle efficiency. In this paper, a series of numerical investigations of a non-premixed hollow RDE are performed. The transient explicit density-based solver in ANSYS Fluent is used to perform the simulations. For a hollow RDE without Laval nozzle, there is only one rotating detonation wave in the combustion chamber. Compared to the traditional annular RDE, the mixing quality is deteriorated, and the thrust of the engine decreases and becomes more unstable. When the hollow RDE is attached with a Laval nozzle, there are two rotating detonation waves in the combustion chamber. The pressure within the combustion chamber increases while the axial velocity decreases. The mixing quality is improved. The height of detonation waves decreases with larger contraction ratio of the nozzle. A Laval nozzle is beneficial for a hollow RDE to achieve steadier operation and higher thrust output. When the contraction ratio is 4, the propulsive performance of the engine is the highest. The maximum thrust achieved is 840 N.     

Pulsating one-dimensional detonation in ammonia-hydrogen–air mixtures

In this study, one-dimensional detonations in ammonia/hydrogen-air mixtures are numerically investigated by solving the fully compressible Navier-Stokes equations with detailed chemistry. Pulsating instabilities with single-mode are observed during the detonation wave propagation, accompanied by periodic coupling and decoupling of the lead shock wave and the reactive front. The ratio between driver pressure and initial pressure determines the overdrive degree and thus the oscillatory mode of detonation for a premixture with certain composition. The effects of hydrogen dilution and mixture equivalence ratio on pulsating detonations are also examined under a constant driver pressure. The growing hydrogen fraction in fuel blends significantly increases the oscillation frequency. In addition, the pulsating detonation frequency rises with increasing equivalence ratio under fuel-lean conditions, peaks under stoichiometric conditions, and falls under fuel-rich conditions as the equivalence ratio increases further. Evolutions of reactants, main intermediate radicals, and products are analysed in both fuel-lean and fuel-rich conditions. A chemical explosive mode analysis further confirms the highly-autoignitive nature of the mixture in the induction zone between reaction front and shock front where thermal diffusion plays a negligible role.     

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 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.     

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 the mixing performance of H2 and air in curved micro-channel combustors

Curved micro-channels are frequently used in micro-Swiss roll combustors and other applications. The secondary flows (i.e., Dean vortices) play an important role in both the mixing performance of fuel and oxidant and the flame propagation characteristics in curved micro-channel combustors. In the present study, the helicity method was adopted and a mixing performance evaluation criterion (MPEC) was proposed to investigate the impacts of inlet velocity, nominal equivalence ratio (φ) and curvature radius on the Dean vortices and mixing performance of H₂ and air in curved micro-channels. First, it is found that with the increase of inlet velocity, the intensity of Dean vortices increases and their shape grow asymmetrical. When the inlet velocity is high enough, another pair of smaller vortices appear near the outer wall. Meanwhile, the mixing performance of H₂ and air becomes worse due to the reduced residence time of gas flow. Second, as the nominal equivalence ratio is decreased, the Dean vortices are intensified and the vortices shape become asymmetrical. Moreover, the mixing process is improved owing to the enhanced secondary flows. Thirdly, the intensity of Dean vortices increases significantly as the curvature radius is decreased. However, the mixing performance becomes worse due to the shortened length of the curved micro-channel. Finally, an empirical correlation between MPEC and the Reynolds number (Re), Dean number (De) and φ was obtained based on the numerical results, which may provide a guidance for the design and operation of curved micro-channel combustors.     

The influence of heat transfer and friction on the impulse of a detonation tube

In the present study, we experimentally and numerically investigated the influence of heat transfer and friction on the performance of a single-shot detonation tube open at one end. Two kinds of specific impulse measurement were carried out with various tube lengths and levels of surface roughness, one by using a ballistic pendulum arrangement and the other by integrating the pressure history measured at the thrust wall. These measurements revealed the degree to which potential impulse can be exploited by the detonation tube after the impulse losses due to various wall loss mechanisms such as heat transfer and friction. The detonation tube obtained 89%, 70%, and 64% of the theoretical ideal impulse for electropolished tubes at a ratio of tube length to diameter (L/D) of 49, 103, and 151, respectively. The impulse losses due to shear stress on the side wall of the detonation tube were found to have a dominant influence on the performance of the detonation tubes of L/D = 103 and 151, but the loss was remarkably small for L/D = 49 relative to that of the longer tubes. In addition to the experiments, a simplified one-dimensional gas-dynamic model was developed by considering heat transfer and friction as wall loss mechanisms and validated by the experimental results. This simplified model was found to predict the experimental results very well, especially in the range of L/D 103–151.     

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.