Shock-induced Combustion Ramjet Research Articles

Effects of non-thermal termolecular reactions on wedge-induced oblique detonation waves

The shock-induced combustion ramjet (Shcramjet) based on oblique detonation waves (ODWs) is among the promising choices for hypersonic propulsion systems. An understanding of the ignition, propagation, and stability of ODWs is critical to harnessing their propulsive potential. In such high speed reacting flows, there is a high probability of occurrence of non-thermal reactions due to the presence of non-trivial amounts of highly reactive radicals including H, O and OH apart from O2 as demonstrated recently [M. P. Burke, S. J. Klippenstein, Nat. Chem. 9 (2017) 1078–1082, Y. Tao, A. W. Jasper, Y. Georgievskii, S. J. Klippenstein, R. Sivaramakrishnan, Proc. Combust. Inst. 38 (2021) 515–522]. The present work focuses on examining the initiation, propagation and structure of oblique detonation waves in stoichiometric H2-air mixtures through numerical simulations with and without non-thermal reactivity on a two-dimensional adaptive grid. Non-thermal reactions were included in the macroscopic kinetic model as chemically termolecular reactions facilitated by the H + OH radical-radical recombination and the H + O2 radical-molecule association reactions. Since, the non-thermal reactions result in a corresponding decrease in the reaction fluxes of the incipient recombination/association reactions, an additional simulation was performed by applying corrections to the respective incipient recombination/association rate constants using the methodology demonstrated by Tao et al. [Y. Tao, A. W. Jasper, Y. Georgievskii, S. J. Klippenstein, R. Sivaramakrishnan, Proc. Combust. Inst. 38 (2021) 515–522]. Results show that, under ODWE relevant conditions, non-thermal reactivity fundamentally alters the induction length, intensity as well as the structure of the ODW. Specifically, it is found that non-thermal reactivity leads to a noticeable reduction in initiation length and a simultaneous increase in instantaneous peak heat release rate and the degree of unsteadiness of the ODW. Statistical analysis of key thermo-chemical variables is performed to elucidate the important species as well as reactions responsible for the observed variations.

Parametric study on mixing augmentation mechanism induced by cantilevered ramp injectors in a shock-induced combustion ramjet engine

The mixing process is very important for the shock-induced combustion ramjet engine, and the fuel and air should be mixed completely before entering into the combustion chamber. In the current study, the studies on standard and swept cantilevered ramp injectors are provided to reveal the mixing enhancement mechanism and the formation of the complex shock wave system. At the same time, some parameters are also taken into consideration to evaluate the flow field properties, including the mixing efficiency, the intensity of streamwise vorticity and total pressure recovery efficiency. The obtained results predicted by the three-dimensional Reynolds-average Navier-Stokes (RANS) equations coupled with the two equation shear stress transport (SST) k-ω turbulence model show that the mixing enhancement mechanism of the cantilevered ramp injector is the shear action in the near flow field and the large-scale vortex structure caused by the cantilevered configuration in the far flow field. The cantilevered ramp injector is beneficial for the mixing enhancement, and it can reduce the total pressure loss as well.

Aerodynamic principles of shock-induced combustion ramjet engines

The shock-induced combustion ramjet engine is the most favorable air breathing propulsive system and a suitable option for high-speed flight. In this paper, theoretical analysis and numerical simulations were conducted to study the thrust performance of shock-induced combustion ramjet engines. Firstly, the propulsive performance of supersonic combustion ramjet engines was theoretically analyzed by using the Chapman-Jouguet detonation theory. Then, the aerodynamic principles of shock-induced combustion ramjet engines were put forth on the basis of the theoretical analysis results. Finally, a full-scale shock-induced combustion ramjet engine was designed according to the aerodynamic principles. Two-dimensional numerical simulations were conducted to simulate its combustion flow field and propulsive performance. The numerical results demonstrate the correctness and application of the theoretical aerodynamic principles.

Impacts of jet angle and jet-to-crossflow pressure ratio on the mixing augmentation mechanism in a shcramjet engine

Abstract The mixing process between the fuel and the incoming flow should be taken into consideration for the shock-induced combustion ramjet (shcramjet) engine, and this is pivotal for its successful engineering implementation. In the current study, the impacts of the gaseous fuel jet angle and the jet-to-crossflow pressure ratio are investigated numerically in order to make a foundational study and reveal the mixing augmentation mechanisms of different conditions. Flow fields windward and leeward are studied, and some parameters are provided to evaluate the flow field properties quantitatively, namely the mixing efficiency, the total pressure recovery coefficient, the fuel penetration depth and the mixing length. The obtained results predicted by the three-dimensional Reynolds-Averaged Navier-Stoke (RANS) equations coupled with the two equation shear stress transport (SST) k-ω turbulence model show that both the jet angle and the jet-to-crossflow pressure ratio have a great influence on the flow field, and the hydrogen distribution and streamwise vorticity downstream of the injectant orifice are affected seriously. The small jet angle and low jet-to-crossflow pressure ratio are more beneficial for the air-fuel mixing but with some shortcomings, and there needs an optimum result in the future to capture better achievement.

Parametric study on mixing augmentation mechanism induced by air injection in a shock-induced combustion ramjet engine

The fuel-air mixing process plays an important role in the shock-induced combustion ramjet (shcramjet) engine. In the current study, the investigation of front fuel jet and rear air jet is provided in order to study the mixing effect induced by the air jet and reveal the mixing augmentation mechanism and formation of several recirculation zones. Some parameters are also provided to evaluate the flow field properties quantitatively, namely the mixing efficiency, the total pressure recovery coefficient, the fuel penetration depth and the mixing length. The obtained results predicted by the three-dimensional Reynolds-average Navier-Stokes (RANS) equations coupled with the two equation shear stress transport (SST) k-ω turbulence model show that the additional air jet is really beneficial for accelerating the mixing process and improving the mixing efficiency. The shear vortex and air injected are the key sources for the mixing enhancement mechanism of the dual injection strategy. The formations of recirculation zones midstream and downstream are provided as well. The midstream recirculation zone “R2” is related to both front fuel jet and rear air jet, and “R3” is related to the air jet only. The downstream recirculation zone1 is composed by flows from the fuel jet and the air jet, and the downstream recirculation zone2 is formed by hydrogen flows.

Reynolds-average Navier-Stokes study of steady and pulsed gaseous jets with different periods for the shock-induced combustion ramjet engine

The mixing process is very important for the shock-induced combustion ramjet engine. In the current study, the steady jet, as well as pulsed jets with different periods, is investigated in order to achieve adequate fuel/air mixing in the supersonic flow. Flow field properties are studied numerically based on grid independency analysis and code validation. The influence of the hydrogen distribution, as well as the flow field parameters such as mixing efficiency, total pressure recovery coefficient, and fuel penetration depth, is deeply analyzed for different jet-to-crossflow pressure ratios, namely, 10.29 and 25.15. The obtained results predicted by the three-dimensional Reynolds-averaged Navier-Stokes equations coupled with the two equation shear stress transport k-ω turbulence model show that the grid scale makes only a slight difference to wall pressure profiles. The pulsed jets with different periods are beneficial for the mixing process, especially when the jet-to-crossflow pressure ratio is high, and it has special advantages on reducing the total pressure loss and improving the fuel penetration depth. Among the pulsed jets considered in the current study, the T1 pulsed jet with higher frequency has the best performance, and its mixing augmentation mechanism is predicted. Its mixing enhancement mechanism is focusing on merging a mass of air around into the fuel core by the intermittent injection.

Mixing augmentation mechanism induced by the dual injection concept in shcramjet engines

As one of the key techniques, achieving adequate fuel/air mixing before combustion is a task that must be addressed seriously for shock-induced combustion ramjet (shcramjet) engines. In this paper, dual injectors which have been combined by a front hydrogen jet and a rear air jet with different aspect ratios are placed on the second ramp of a shcramjet inlet to promote mixing process between fuel and hypersonic crossflow. At the same time, dual injectors with pure hydrogen injection and the single hydrogen injection with the same injection area are studied as well for comparison. Flow field properties are investigated numerically based on grid independency analysis and code validation. Obtained results predicted by the three-dimensional Reynolds-average Navier-Stokes (RANS) equations coupled with the two equation SST κ-ω turbulence model show that the grid scale makes only a slight difference to wall pressure profiles for all cases studied in this article. The dual transverse injection system with a front hydrogen jet and a rear air jet is beneficial for improvement of the mixing efficiency between hydrogen and air. Mixing efficiency in the near field increases with increase of aspect ratio of the air porthole. The air porthole in dual transverse injections would accelerate mixing process when compared with the single injection strategy irrespective of aspect ratio of the air porthole.

Investigation on gaseous jet in forebody/inlet for shock-induced combustion ramjet (shcramjet) engines

As one of the most promising propulsion systems in the future, there are unusual advantages of the shock-induced combustion ramjet (shcramjet) engine. In the current study, the slot is placed on the second ramp of the shcramjet inlet to promote the mixing process between the fuel and the hypersonic crossflow. The influences of the injection angle and the jet-to-crossflow pressure ratio have been investigated numerically based on the grid independency analysis and the code validation. The obtained results predicted by the two-dimensional Reynolds-average Navier-Stokes (RANS) equations coupled with the two equation RNG k-ε turbulence model show that the grid scale makes only a slight difference to the wall pressure profiles for all jet-to-crossflow pressure ratios employed in this article. With the smaller injection angle, higher fuel/air mixing efficiency, less loss of the stagnation pressure and lower fuel penetration depth are shown. Low jet-to-crossflow pressure ratio can increase the mixing efficiency and decrease the loss of the stagnation pressure. High jet-to-crossflow pressure ratio results in higher penetration depth. There is an optimum injection angle for each jet-to-crossflow pressure ratio to achieve the maximization of the fuel/air mixing in hypersonic flows.

悬臂斜坡喷注构型对流场与燃料混合特性影响研究

激波诱燃冲压发动机作为超燃冲压发动机的一种，更加适合高超声速飞行器机体与发动机的一体化设计。为了研究其前体/进气道的燃料/空气混合特性和增强机理，本文采用地面试验方法对不同悬臂斜坡喷注器的流场特性进行了研究。研究表明，悬臂斜坡上形成的斜激波呈很强的三维效应。燃料喷注后会改变悬臂斜坡上斜激波的方向，使其与斜坡的夹角增大。悬臂斜坡的存在使流场中产生了压差，影响了气流的运动轨迹，进一步导致了流向涡的产生。后掠喷注构型和双喷口构型的混合特性更好，三角构型喷注不能形成明显的斜激波压缩波系，混合效果不佳。 Shock-induced combustion ramjet (shcramjet), as one of the scramjet engines, is more suitable for the integration design between forebody and combustion of hypersonic vehicle. In order to investigate its flowfield characteristics and mixing-enhanced mechanism of h2/air mixture for the forebody/inlet, experiment investigations are conducted to study the mixing enhancement and flow-field of the cantilevered ramp injector. The obtained results show that, it is very obvious for the three-dimensional effect of cantilevered ramp injector. The direction of oblique shock will be changed when the fuel is added, at the same time the angle of shock wave will increase. The cantilevered ramp results in the appearance of pressure gradients. The phenomenon will affect the moving tracks of flow, then leads to the generation of flow vortex. The fuel mixing performance of the sweepback and doublejet configurations are better than that of benchmark configuration. The fuel mixing performance for the triangle jet configuration is not good because the shock wave compress is not obvious for the flowfield.

Influences of Angles of Attack and Sideslip on the Flow Field of a Cantilevered Ramp Injector in Supersonic Flows

The cantilevered ramp injector has been employed as an effective fuel injection strategy in the shock-induced combustion ramjet engine, and its flow field characteristics have attracted increasing attention. Three-dimensional ReynoldsAveraged Navier-Stokes (RANS) equations coupled with the RNG k- turbulence model have been employed to investigate the flow field of a cantilevered ramp injector in a freestream with a Mach number of 6.0. The effects of the angles of attack and sideslip on the flow field of a cantilevered ramp injector have been studied, and the predicted axial distribution of the maximum injectant mole fraction shows good agreement with the available experimental data in public literature. The obtained results show that the case with an angle of attack 3 � can promote the mixing process between the fuel and air most efficiently in a typical cantilevered ramp injector. However, it cannot prevent the premature ignition of the premixed combustible flow. The mixing process cannot be promoted when the normalized axial distance is larger than 7.55 in cases with different angles of sideslip, and larger angles of sideslip can reduce the effect of premature ignition for the premixed combustible flow.

Numerically Simulated Comparative Performance of a Scramjet and Shcramjet at Mach 11

The aeropropulsive performance characteristics of a scramjet and a shock-induced combustion ramjet (shcramjet) are compared at a flight Mach number of 11 and an altitude of 34.5 km. The vehicles share the same inlet type, fuel injection system, fuel/air equivalence ratio, mixing/combustor duct, methodology of nozzle design, and gridding technique. The numerical simulation of the three-dimensional vehicle flowfields from tip to tail are performed by the window allocatable resolver for propulsion code, in which the multispecies Favre-averaged Navier―Stokes equations are closed by the Wilcox k-ω turbulence model. Combustion is simulated by the H 2 -air chemical kinetics model of Jachimowski. Magnitudes of the thrust, fuel-specific impulse, pressure, and frictional forces acting on the vehicles are determined. Results show that the scramjet outperforms the shcramjet with a fuel-specific impulse of 1450 s as opposed to 1109 s developed by the shcramjet. However, the shcramjet is appreciably smaller and thus lighter than the scramjet with a combustor length which is one-fifth of the scramjet combustor length, requiring much less cooling.

Research status of key techniques for shock-induced combustion ramjet (shcramjet) engine

As one of the most promising propulsion systems in the future, shock-induced combustion ramjet engine can remedy the disadvantages in the integrated design of scramjet engine and airframe. It can shorten the length of the combustor, lighten the structure weight of the engine and keep better performance in a broad range of flight Mach number. The elementary principle of shock-induced combustion ramjet engine is introduced. The key technologies of this kind of propulsion system are described, while their research status is presented in detail. Suggestion on the development of shcramjet engine in China is put forward.

Computational Study of the Propulsive Characteristics of a Shcramjet Engine

A shock-induced combustion ramjet is considered wherein gaseous hydrogen is injected via cantilevered ramp injectors located in a staggered manner on opposite walls of the internal duct of a mixed-compression inlet. The nonhomogeneous combustible mixture thus formed at the exit of the duct is then ignited through a shock generated by a wedge located on the lower wall of the duct. The products of the ensuing combustion process are then expanded in a specifically designed nozzle. The numerical simulation of the three-dimensional shcramjet flowfield at Mach 11 and an altitude of 34.5 km is performed by the Window Allocatable Resolver for Propulsion code, in which the multispecies Favre-averaged Navier-Stokes equations are closed by the k-w turbulence model and the Wilcox dilatational dissipation correction, to account for compressibility effects at high convective Mach number. The H 2 -air chemical reactions are modeled by a nine species, 20 reaction model based on that of Jachimowski. Magnitudes of the thrust, fuel specific impulse, and frictional forces on the entire shcramjet configuration are numerically determined. The moderate value of fuel specific impulse of 683 s is primarily due to the high equivalence ratio adopted.

Hypervelocity Fuel/Air Mixing in Mixed-Compression Inlets of Shcramjets

This paper investigates the mixing of hydrogen fuel with air in the mixing duct of a mixed-compression shock-induced combustion ramjet (shcramjet) inlet. Mixing augmentation through the use of cantilevered ramp injector arrays on opposite shcramjet inlet walls is studied and the influence of relative array locations is quantified. Studies were undertaken numerically using the WARP code that solves the Favre-averaged Navier-Stokes equations closed by the Wilcox k-w turbulence model. Air-based mixing efficiencies of up to 0.58-0.68 were achieved with thrust potential losses less than that gained from high-speed fuel injection. Shocks created from the fuel injector structures play a major role in the mixing behavior of the fuel jets on the opposing side of the mixing duct. Chemically reacting studies verified for the correct selection of spanwise displacement of the fuel injectors, an air buffer created between the fuel and walls suppresses premature ignition while still allowing for a mixing efficiency of up to 0.46-0.54.

Numerical Simulation of a Real Shcramjet Flowfield

A shock-induced combustion ramjet (shcramjet) geometry is considered wherein the fuel, gaseous hydrogen, is injected in a two-oblique shock external compression inlet via cantilevered ramp injectors and a wall slot. The combustible mixture formed at the exit of the inlet is then ignited through the shock generated by the cowl of the engine. The numerical simulation of the three-dimensional flowfield of a shcramjet flying at M = 11 and at an altitude of 35 km was performed using the WARP code, in which multispecies Favre-averaged Navier-Stokes equations are closed by the k-w turbulence model and the Wilcox dilational dissipation correction, to account for compressibility effects at high turbulence Mach numbers. The hydrogen/air chemical reactions are modeled by Jachimowsky's nine species, 20 reaction model. It has been found that the combustor length resulting from the shock-induced process is of the order of 25-30% of the inlet length. The relatively low value of the fuel specific impulse obtained, 573 s, is mainly due to incomplete mixing achieved in the adopted inlet model. To the authors' knowledge, the paper contains the first ever proof, in the open scientific literature, of the feasibility of this hypersonic propulsion concept in realistic flow situations, by numerical simulation.

Blunt-Body Generated Detonation in Viscous Hypersonic Ducted Flows

Heat release from detonation waves due to blunt bodies embedded into a constant area channel flow was studied numerically, solving laminar Navier-Stokes equations for chemically reacting nonequilibrium flows with a steady-state and time-accurate formulation. A maximum heat release is of interest in the context of propulsion applications, where the energy added to the flow can be utilized to increase thrust. Hence, channel blockage ratios (CBR), defined as the ratio of blunt-body diameter over channel height, in the range from 1/18 to 1/3 were examined for inflow conditions typical for a shock-induced combustion ramjet combustor of a hypersonic vehicle operating in a flight altitude of 38 km and a Mach number in excess of 10. The flow entering the domain at Mach number 5 is a stoichiometrically premixed hydrogen-air gas mixture that was modeled with 13 species and 33 chemical reaction equations.

Suppression of Premature Ignition in the Premixed Inlet Flow of a Shcramjet

This paper addresses the problem of premature ignition in a shock induced combustion ramjet (shcramjet) inlet. Previous studies have developed fuel injectors and inlet configurations that maximize the mixing efficiency in a shcramjet inlet while maintaining inlet losses at a minimum. A chemically reacting study of previously recommended shcramjet inlets finds premature ignition to occur primarily in the boundary layer in the last 15% of the inlet, spreading into the core flow prior to the inlet exit. Both gaseous nitrogen and additional hydrogen are then injected into the inlet flowfield in an attempt to suppress the flame. Various inflow conditions are considered, and injected using various geometries. Premature ignition is suppressed most feasibly by the injection of additional hydrogen through a backward facing step (slot injector) located just before the second inlet shock, such that the global equivalence ratio of the premixed flow exiting the inlet is one. The performance of the original inlet remains unaltered and the frictional force on the inlet wall is reduced by 10% due to the hydrogen slot injection. All turbulent, chemically reacting, three dimensional, mixing flowfields are solved using the WARP code which solves the FANS equations closed by the Wilcox kω turbulence model and the Wilcox dilatational dissipation correction. Chemical kinetics are modeled by a 9 species, 20 reaction model by Jachimowski.

Hypervelocity Fuel/Air Mixing in a Shcramjet Inlet

The mixing of fuel with air in the inlet of a shock-induced combustion ramjet (shcramjet) is presented. The study is limited to nonreacting hydrogen/air mixing in an external-compression inlet at a flight Mach number of 11 and at a dynamic pressure of 1400 psf (67,032 Pa), with use of an array of cantilevered ramp injectors. Results are obtained using the WARP code solving the Favre-averaged Navier‐Stokes equations closed by the Wilcox kω turbulence model and the Wilcox dilatational dissipation correction, discretized by the Yee‐Roe flux-limited scheme. Because of the fuel being injected at a very high speed, fuel injection in the inlet is found to increase the thrust potential considerably, with a gain exceeding the losses by 40‐120%. Losses due to skin friction are seen to play a significant role in the inlet, because they are estimated to make up as much as 50‐70% of the thrust potential losses. The use of a turbulence model that can predict the wall shear stress accurately is, hence, crucial in assessing the losses accurately in a shcramjet inlet. Substituting the second inlet shock by a Prandtl‐Meyer compression fan is encouraged because it decreases the thrust potential losses and reduces the risk of premature ignition by reducing the static temperature, while decreasing the mixing efficiency by a mere 6%. One approach that is observed to be successful at increasing the mixing efficiency in the inlet is alternating the injection angle along the injector array. The use of two injection angles of 9 and 16 deg is seen to result in a 32% increase in the mixing efficiency at the expense of a 14% increase in the losses when compared to a single injection angle of 10 deg. When alternating injection angles are used, the mixing efficiency reaches as much as 0.47 at the inlet exit.

Numerical Investigation of the Turbulent Mixing Performance of a Cantilevered Ramp Injector

An injector geometry is considered for fuel delivery in a high-enthalpy, high-Mach-number airflow typical of that found in hypersonic propulsion systems such as the scramjet and shock-induced combustion ramjet or shcramjet. It is thought to embody the characteristics of both conventional ramp and low-angle wall injection techniques. The objective is to investigate the turbulent hypervelocity fuel/air mixing characteristics of the considered injector geometry, with particular emphasis on the effect of the convective Mach number and global equivalence ratio on its mixing efficiency. The analysis of the three-dimensional hypervelocity mixing flow is based on the Favre-averaged Navier-Stokes equations closed by the Wilcox kω turbulence model for a multispecies gas. A Roe scheme turned second-order accurate through a symmetric total-variation-diminishing limiter is used for the spatial discretization while approximate factorization is used to iterate in pseudotime

Propulsive Performance of Hypersonic Oblique Detonation Wave and Shock-Induced Combustion Ramjets

A comparative study is presented of the propulsive characteristics of hypersonic detonation wave and shockinduced combustion ramjets. The same ramjet design methodology is used to assess the propulsive performance of both types of ramjets. The lower ‐upper symmetric Gauss ‐Seidel scheme combined with a symmetric shockcapturing total variation diminishing scheme are used to solve the Euler equations describing thetwo-dimensional hydrogen/air combustible e owe eld with nonequilibrium chemical reactions including 13 species (H2, O2, H, O, OH, H2O, HO2, H2O2, N, NO, HNO, N 2, and NO2). Results obtained for e ight Mach numbers 12< ‐ M1 < ‐ 16 and for a e ight dynamic pressure of 67,032 Pa (1400 psf) show that combustor entrance temperatures Tce (or inlet compression ratios ) substantially lower than the near ignition values of hydrogen/air mixture, adopted for the generation of a near Chapman ‐Jouguet oblique detonation wave in the combustor, entail signie cant performance augmentation. For the considered range of e ight Mach numbers and value of e ight dynamic pressure, the thrust of a shock-induced combustion ramjet is maximum for 650 <‐ Tce < ‐ 700 K, and at this point the combustion is entirely shock induced. The thrust generation can be enhanced by more than 10% and the fuel specie c impulses improved by more than one-third over their magnitudes corresponding to maximum thrust detonation wave ramjets.