Study on main flow and fuel injector configurations for Scramjet applications

A parametric study of combustor inlet configuration for supersonic combustion ramjet (Scramjet) engine has been conducted by solving two-dimensional full Navier–Stokes equations. The main stream is air of Mach 5 entering through the configured inlet of the combustor and gaseous hydrogen is injected from the configured jet on the side wall. The parameters included are air stream angle and injection angle. On the effect of air stream angle, strong interaction between main and injecting flows can be observed for smaller angle causing sharp increase in mixing efficiency on the top of injector. Also high momentum of air stream towards the side wall causes no recirculation at the upstream of injector and the system becomes unable for flame holding. For the variation of injection angle, results show that in upstream of injector the mixing is dominated by recirculation and in downstream the mixing is dominated by mass concentration of hydrogen. Upstream recirculation is dominant for injecting angle 60° and 90°. Incorporating the various effects, perpendicular injection shows the maximum mixing efficiency and its large upstream recirculation region has a good flame holding capability.     

The Effect of Injection Angle on Mixing and Flame Holding in Supersonic Combustor

IntroductionMixing of fuel with oxidizer and their combustionare encountered in many engineering applicationsincluding hypersonic propulsion system in space vehicles.Particularly, the fuel injection scheme in hypersonicvehicles incorporating Scramet (Supersonic CombustionRamjet) engines, requires special attention for efficientmixing and stable combustion. Though a considerablenumber of researches has been cwhed out on ndxing andcombustion of fuel with oxidizer in Scramet program,still it fa…     

Mixing and flame holding with air inlet configuration in scramjet combustor

A parametric study of combustor inlet configuration for Supersonic Combustion Ramjet (SCRAMJET) Engine has been conducted by solving Two-Dimensional full Navier-Stokes equations. The main stream is air of Mach 5 entering through the configured inlet of the combustor and gaseous hydrogen is injected from the side wall at sonic condition. The parameters that include are the air inlet width and stream angle with a backward facing step under the inlet port. On the effect of inlet width variation, two competing phenomena have been observed: (i) in upstream of injector the strength of recirculation is higher for wider inlet and consequently the mixing increases and (ii) in downstream, the diffusion of hydrogen decreases with the increase of inlet width and eventually mixing decreases. As a result, in far downstream the mixing efficiency increases up to certain inlet width and then decreases for further increment of inlet width. During the variation of air stream angle, strong interaction between main and injecting flows can be observed for small angle causing sharp increase in mixing efficiency on the top of injector. Due to high momentum of air stream towards the side wall, no recirculation occurs at the upstream of injector for small angle and the system becomes unable for flame holding. Among the cases considered, the configuration of moderate air stream angle shows higher mixing efficiency and good flame holding capability.     

Effects of incident shock wave on mixing and flame holding of hydrogen in supersonic air flow

The effects of incident shock wave on mixing and flame holding of hydrogen in supersonic airflow have been studied numerically. The considered flow field was including of a sonic transverse hydrogen jet injected in a supersonic air stream. Under-expanded hydrogen jet was injected from a slot injector. Flow structure and fuel/air mixing mechanism were investigated numerically. Three-dimensional Navier–Stokes equations were solved along with SST k-ω turbulence model using OpenFOAM CFD toolbox. Impact of intersection point of incident shock and fuel jet on the flame stability was studied. According to the results, without oblique shock, mixing occurs at a low rate. When the intersection of incident shock and the lower surface is at upstream of the injection slot; no significant change occurs in the structure of the flow field at downstream. However when the intersection moves toward downstream of injection slot; dimensions of the recirculation zone and hydrogen-air mixing rate increase simultaneously. Consequently, an enhanced mixing zone occurs downstream of the injection slot which leads to flame-holding.     

Combustion efficiency improvement for scramjet combustor with strut based flame stabilizer using passive techniques

In a Supersonic combustion ramjet (Scramjet) engine, combustion occurs at supersonic velocity as incoming air remains supersonic. Scramjet engines have complex flow phenomena taking place inside the combustor. In a scramjet combustor, mixing and combustion should take place within few milliseconds. In the present study, two additional trailing struts with no fuel injection are placed a short distance downstream of the fuel injection strut. Effect on combustion performance using these strut-based flame stabilizer configurations are assessed. Reynolds averaged Navier-Stokes equations are solved with turbulence model and species transport equations. Validated results of the single strut are compared with the different strut configurations. It is found that the placement of trailing struts have a vital role. High-pressure zones, shock reflections, recirculation region, and expansion fan in between combustion region and the trailing struts enhance the combustion efficiency. Strut configuration introducing high-pressure regions outperformed other strut configurations due to the formation of a strong recirculation region in the core combustion region. Combustion efficiency is found to have a maximum improvement of 67.14% compared with single strut-based flame stabilizer. Total pressure loss also increased due to the introduction of additional struts.     

Numerical study on supersonic combustion with cavity-based fuel injection

The present study describes the numerical investigations concerning the combustion enhancement when a cavity is used for the hydrogen fuel injection through a transverse slot nozzle into a supersonic hot air stream. The cavity is of interest because recirculation flow in cavity would provide a stable flame holding while enhancing the rate of mixing or combustion. Several inclined cavities with various aft wall angle, offset ratio and length are evaluated for reactive flow characteristics. The cavity effect is discussed from a viewpoint of total pressure loss and combustion efficiency. The combustor with cavity is found to enhance mixing and combustion while increasing the pressure loss, compared with the case without cavity. But it is noted that there exists an appropriate length of cavity regarding the combustion efficiency and total pressure loss.     

超音速燃烧的喷嘴模型研究

在综合现有的强化混合技术基础上，提出了一种新的喷嘴模型－低角度分级喷嘴模型，并设计了缩尺寸燃烧室。通过燃烧室入口马赫数为２的氢燃烧超音速燃烧实验，获得了超音速燃烧室中壁压沿轴向的变化规律，证明了低角度分级喷嘴模型燃烧室比目前公认的后掠斜坡喷嘴模型燃烧室有更高的燃烧效率。     

Parametric effect on the flow and mixing properties of transverse gaseous injection flow fields with streamwise slot: A numerical study

Mixing process between the injectant and air in supersonic crossflow depends on the injector configuration and the jet-to-jet spacing heavily. In the current study, the three-dimensional Reynolds-averaged Navier–Stokes (RANS) equations coupled with the two equation SST k-ω turbulence model were employed to simulate the mixing process induced by an array of three spanwise-aligned small-scale rectangular portholes, and the influences of the jet-to-jet spacing, the jet-to-crossflow pressure ratio and the aspect ratio of the injector on the flow field properties were evaluated. Two quantitative objectives were considered in this article, namely the fuel penetration depth and the mixing efficiency. The obtained results show that the flow field induced by the array of three spanwise-aligned small-scale rectangular portholes is a multiobjective design optimization problem, and the large aspect ratio is beneficial for the mixing enhancement in supersonic crossflow. However, it is not beneficial for the flame holding. The interaction between the adjacent injectors has a great impact on the fuel penetration depth in the far-field, especially for the larger jet-to-crossflow pressure ratio, and this is due to its wider fuel plume.     

Dynamics of upstream flame propagation in a hydrogen-enriched premixed flame

An unconfined strongly swirled flow is investigated to study the effect of hydrogen addition on upstream flame propagation in a methane-air premixed flame using Large Eddy Simulation (LES) with a Thickened Flame (TF) model. A laboratory-scale swirled premixed combustor operated under atmospheric conditions for which experimental data for validation is available has been chosen for the numerical study. In the LES-TF approach, the flame front is resolved on the computational grid through artificial thickening and the individual species transport equations are directly solved with the reaction rates specified using Arrhenius chemistry. Good agreement is found when comparing predictions with the published experimental data including the predicted RMS fluctuations. Also, the results show that the initiation of upstream flame propagation is associated with balanced maintained between hydrodynamics and reaction. This process is associated with the upstream propagation of the center recirculation bubble, which pushes the flame front in the upstream mixing tube. Once the upstream movement of the flame front is initiated, the hydrogen-enriched mixture exhibits more unstable behavior; while in contrast, the CH₄ flame shows stable behavior.     

Mixing performance of transverse hydrogen/air multi-jet through coaxial injector arrays in supersonic crossflow

Increasing the fuel mixing performance substantially improves the overall performance of the scramjet engine for a long-distance flight. In this paper, the influence of coaxial injector arrays of hydrogen/air multi-jet on the mixing performance of the fuel in supersonic crossflow is fully investigated. Our main goal is to examine the impacts of air and fuel coaxial injector on fuel distribution and penetration downstream of injectors in different operating conditions. In this study, fuel and air are simultaneously injected through coaxial multi-jet at sonic condition while of free-stream Mach number is 4. Computational Fluid Dynamic is applied for simulation of the transverse coaxial jet at supersonic crossflow. The effect of jet diameter with the same mass flow rate of air and hydrogen on fuel mixing is also investigated. The mixing efficiency of different jet spaces and pressures is also examined to obtain an optimum jet arrangement in the combustor chamber. Our study shows that the injection of the coaxial air/hydrogen jet noticeably improves mixing downstream by augmentation of fuel interaction with an air jet. Our results also show that fuel jet space of 7 Dj offers maximum fuel mixing by the formation of multi vortices with uniform strength.     

Large eddy simulation of hydrogen combustion in supersonic flows using an Eulerian stochastic fields method

An Eulerian Monte-Carlo approach, the so-called Eulerian stochastic fields (ESF) method is implemented and evaluated for simulation of non-premixed hydrogen/air combustion in supersonic flows. The ESF method is integrated into a compressible flow large eddy simulation (LES) solver, and validated on a supersonic combustor with a strut as flame-holder. Comparison with experimental data and with results from a well-stirred reactor (WSR) model demonstrates the advantage of the LES-ESF method for simulation of local-extinction and re-ignition phenomena. The hydrogen/air flame structure and the stabilization of the combustion process in the supersonic combustor are analysed based on the present LES-ESF method. Oscillation of the recirculation zones is found to be the dominant mechanism for the local-extinction/re-ignition and the flame stabilization under the present condition.     

Hydrogen/air supersonic combustion for future hypersonic vehicles

In this work, supersonic hydrogen combustion in the Hyshot II scramjet engine is investigated. In particular, fundamental physics of mixing, combustion and vorticity generation as well as the interaction between shock waves, boundary layer and heat release are analyzed by means of 3D Large Eddy Simulations (LES) using detailed chemistry. Results show very complex structures due to the interaction between the four sonic H₂ crossflow injections and the airstream flowing at M = 2.79. A bow shock forms ahead of each H₂ injector: the interaction between bow shocks and boundary layers leads to separation zones where H₂ recirculates. In these recirculation zones, OH radicals are produced, indicating that a flame already starts upstream of the injectors and downstream of the flow separation. The formation of barrel shocks due to the H₂ expansion and recompressions is also predicted. Comparison of pressure distribution along the wall centreline at 1.3 ms shows agreement with experimental results, mostly in the first part of the combustor, where the grid is very fine. The combustion is very fast and efficient: only 12.35% of hydrogen is found unburned at the combustor exit. This confirms that burning hydrogen is efficient and feasible also in supersonic flows and therefore it is a good candidate for hypersonic airbreathing applications.     

Numerical investigation on implications of four strut injectors on combustion characteristics of a Doubly-Dual cavity-based scramjet combustor

The present study numerically investigates the implications of four strut injector on combustion characteristics of a hydrogen fueled doubly-dual cavity based scramjet combustor. Large Eddy Simulation (LES) is adopted to reveal the combustion characteristics of the doubly-dual cavity based scramjet combustor. Firstly, the fluid flow characteristics of a three strut injector are compared with four strut injector to highlight the influence of 4-strut injector. Thereafter, a separate study is carried out to investigate the influence of Mach number on the performance of the combustor. Our study reveals that the four strut injector arrangement greatly affects the shock wave interaction like, shock-shock and the shock-shear layer which leads to enhance the formation of vortices and recirculation region as compared to the three strut injector arrangement. In addition, introduction of 4-strut injectors found to result in better mixing and complete combustion as evidenced by the higher static temperature and the lesser amount of oxygen mass fraction at the outlet of the combustor.Also, the flame blow out condition is seen at the trailingedge of a strut and found to be minimum when 4 strut arrangement is use dowing to maximum value of OH mass fraction. Further, the higher air-fuel mixing rate is observed at Mach 2.5 of the free stream of their inlet which aids to stabilize the flame leading to complete combustion.     

Large eddy simulation of reacting flow in a hydrogen jet into supersonic cross-flow combustor with an inlet compression ramp

Large eddy simulation (LES) has been performed to investigate transverse hydrogen jet mixing and combustion process in a scramjet combustor model with a compression ramp at inlet to generate shock train. Partially Stirred Reactor (PaSR) sub-grid combustion model with a skeleton of 19 reactions and 9 species hydrogen/air reaction mechanism was used. The numerical solver is implemented in an Open Source Field Operation and Manipulation (OpenFOAM) and validated against experimental data in terms of mean wall pressure. Effects of a shock train induced by the inlet compression ramp on the flame stabilization process are then studied. It can be observed that the interaction of the oblique shock and the jet mixing layer enhance the combustion and stabilize the flame. Symmetrical recirculation zone, which contributes to the flame anchoring of the supersonic transverse jet combustion, is observed in the near wall region of 10 < x/D < 20. The hydrogen fuel is transported from the center of jet plume to the near wall region on both sides of the central plane (z/D = 0) and thus intense combustion near the wall is observed due to the enhanced mixing and shock compression heating. Besides, the jet penetration in the reacting field is different from that in non-reacting case with the influence of the interaction between the reflected oblique shock and the jet shear layer on the windward side.     

Release of chemical energy by combustion in a supersonic mixing layer of hydrogen and air

The process involved in chemical energy release by combustion in a supersonic, constant pressure, hydrogen-air laminar mixing layer was studied computationally, with a chemical kinetics model involving nineteen reactions and eight species. To try to find out the physical reason for the different trends of the pressure curves observed in an experimental supersonic combustor at two different initial air stream temperatures. Two initial air stream temperatures corresponding to the two experimental cases are chosen such that the higher temperature yielded a shorter ignition distance, and the lower temperature yielded a longer ignition distance. For both cases the stream wise rate of energy release rises rapidly to a peak after ignition then falls to a post-ignition value which decreases very slowly with distance. A single premixed flame occurs at ignition for both cases, but then develops into a triple flame structure in the high temperature case, and a flame with only two branches in the low temperature case. The flames move from the airside to hydrogen side consuming the oxygen as they go, until the post-ignition phase is reached. There the dominant energy release arises from the formation of a diffusion flame. In the high temperature case a narrow lean premixed flame accompanies this diffusion flame on the airside. The flame structure, but not the energy release, is effected by the initial temperature distribution across the mixing layer, which is found to be influenced by the velocity difference between the faster air stream and the slower hydrogen stream. Increasing the concentration of oxygen atoms in the oncoming air stream was found to cause substantial reduction in the ignition distance, but did not significantly effect the flame structure, or the rate of heat release in the post-ignition phase. Finally, the different trends of pressure curves observed in the experiment can be reconstructed when pressure variation was considered in this model. Thus we can conclude that the difference in the trends of the pressure curves is caused by the difference in the initial air stream temperature.     

Effect of jet schemes of the double-nozzle strut injector on mixing efficiency of air and hydrogen for a scramjet combustor

This work presents the numerical analysis of the DLR scramjet combustor for different jet schemes of the double-nozzle injector, namely the various injection directions, injection angles, and nozzle spacings. After comparing various jet schemes, it is found that the optimal jet scheme for the double-nozzle strut is to set the angle of 60° for the inward injection direction and the nozzle spacing of 3 mm. Furthermore, the mixing efficiency of the optimal jet scheme is investigated at different Mach numbers. The current research focuses on the mixing mechanism of air and hydrogen by analyzing the flow structures in the strut's wake region. It is observed that the double-nozzle configuration increases the number of vortexes behind the strut and creates a recirculation zone between the two jet streams. The mixing efficiency of the scramjet combustor improves significantly with an increase in the injection angle, but the spacing and direction of the double-nozzle have little effect on the mixing efficiency. It is found that the additional total pressure loss generated by the double-nozzle configuration can be negligible. In addition, the results show that the mixing efficiency of the optimal jet scheme for the double-nozzle is improved more significantly at low Mach numbers (e.g., Ma = 2 and 3).     

Combustion stabilization based on a center flame strut in a liquid kerosene fueled supersonic combustor

A newly designed strut is proposed in this paper for fuel injection and flame holding in a liquid-kerosene-fueled supersonic combustor.The thickness of the strut is 8mm and the front blockage is about 8%.The characteristic of this strut is that extra oxygen can be injected through a set of orifices at the back of the strut,which can change the local flow field structure and ER(Equivalence Ratio).Based on the above mentioned strut,a stable local flame is generated at the back of the strut and the main combustion can be organized around this local fire.Numerical simulation is conducted to compare the local flow field distribution at the back of the strut with/without extra oxygen injection.Experiments are conducted to test the combustion characteristics based on this fuel injection and flame holding strategy.The temperature distribution which can reflect the local flame characteristic has been measured in the experiments conducted under cold incoming supersonic air flow condition.In addition,the overall combustion performance in a full-scale supersonic combustor has been evaluated in the experiments conducted under hot incoming supersonic air flow condition.Results show that this strut strategy is very promising since it can organize stable supersonic combustion at the center of the combustor without any cavity or rearward facing step.Besides that,even with the 8mm thick strut,the combustion can be stable in a wide range of ER from 0.25-1 by using liquid room-temperature kerosene.     

Mechanism of fuel mixing of hydrogen multi-jet in presence of upstream convergent/divergent ramp at supersonic flow

The enactment of the fuel mixing structure is crucial for the advance of supersonic vehicles. All-inclusive efforts have been done to disclose the impacts of different parameters on instrument of the fuel combination with air within the combustion chamber. In the present work, comprehensive computational investigations have been done to explore the importance of oblique ramp upstream on the fuel mixing process of hydrogen multi-jets at supersonic cross airflow. The primary attention of the current study is to compare the role of interaction of air and fuel by the existence of an oblique ramp upstream of four cross jets. Flow analysis is also done to unveil the main difference of convergent and divergence ramps located upstream of each injector. For simulation of the proposed models, Computational Fluid Dynamics (CFD) is employed to resolve RANS equations with the SST turbulence model in high-speed free stream. The main significant factors i.e. mixing efficiency and circulation factor are also compared in our work for comparison of the flow parameters and mixing concepts. According to our investigations, the presence of the upstream oblique jet meaningfully enhances the fuel mixing as flow moves downstream of injectors. The outcomes also showed that productivity of the divergent ramp is higher than that of the convergent one due to high jet diffusion in the depth of the domain by the creation of a strong horseshoe vortex.     

Simulations of combustion oscillation and flame dynamics in a strut-based supersonic combustor

This paper numerically investigated the dynamic characteristics of combustion in a model scramjet. Three-dimensional compressible large eddy simulation was performed on a hydrogen fueled combustor and pressure fluctuations were recorded. The analysis of pressure data showed that the combustion processes are intrinsically unstable under supersonic air inflow conditions. Flame dynamics were convinced by the fluctuations in flame lift-off distance away from the strut base. Combined with the corresponding time interval, instantaneous flame speed was calculated. Results indicated that pressure oscillations at different locations show difference in amplitude, frequency, and the underlying control mechanism. Flame front oscillation analysis showed that the flame–shock interaction in the strut recirculation zone was responsible for the combustion instability. Flame dynamics were compared with low-speed turbulent lifted flames. The transition between flame propagation just after the strut and shock-induced combustion in the subsonic bubble at the intersection of two wall-reflected oblique shocks made for the flame stabilization.     

Numerical investigations on the performance of a multistrut hydrogen injector in a scramjet combustor

This study aims at investigating the effect of a multistrut-based hydrogen injector in a scramjet combustor underreacting case. The numerical analysis is carried out using two-dimensional Reynolds-averaged Navier–Stokes equations with the Shear Stress Transport k − ω turbulence model in contention to comprehend the flow physics during scramjet combustion. The three major parameters, such as the shock wave pattern, wall pressures, and static temperature across the combustor, are validated with the reported experimental results. The results comply with the range, indicating that the adopted simulation method for single strut injection can be extended for other investigations. It is noticed that with multistrut injectors, as hydrogen jet pressure increases in the supersonic flow field, the jet penetration rate in the lateral direction of the flow and the total pressure loss as compared with the baseline injection pressure conditions has increased. The supersonic flow characteristics are determined based on the flow properties, combustion efficiency, mixing efficiency, and total pressure loss. Compared with the single-strut output of a scramjet combustor, multistruts inclusion increased the combustion efficiency by almost 18%, the mixing efficiency attained the maximum with 16% fewer lengths. The total pressure loss in single-strut is 14% lower than that of multistrut.