Effects of plate vibration on the mixing and combustion of transverse hydrogen injection for scramjet

Transverse injection is an effective mixing enhancement technique for the combustor of scramjets. Vibration of the plate structure in combustor will easily be induced due to aerodynamic load and harsh aerothermodynamic load simultaneously. Effects of the plate vibration on the mixing and the combustion of the transverse hydrogen injection have been investigated numerically in this study. Finite rate chemistry model is used as combustion model. The supersonic jet experimental model of the Stanford University is modified slightly and used as the analysis model. Effects of the frequency and the amplitude of the plate vibration on combustion performance and flow field structure have been investigated in detail. The results show that the plate vibration increases the mixing efficiency, the combustion efficiency and the total pressure loss coefficient. Besides, it can change the flame structure and the shock wave structure, as well as increase the shock wave intensity at downstream of the injection. The vibration frequency has relatively little effect on the combustion efficiency and the total pressure loss coefficient. When the vibration frequency is large, it presents some high frequency pulsations for the total pressure loss coefficient. However, the vibration amplitude has large effect on combustion efficiency and the total pressure loss coefficient. When the vibration amplitude is small, the combustion efficiency presents regular periodic change with time. When the vibration amplitude is large, it diverges with time, and the flow tends to be unstable. The large vibration amplitude changes the stability of the original flow. Consequently, the combustion with large amplitude fluctuation can critically damage the combustion stability.     

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.     

Effect of initial pressure on flame–shock interaction of hydrogen–air premixed flames

By utilizing a newly designed constant volume combustion bomb (CVCB), turbulent flame combustion phenomena are investigated using hydrogen–air mixture under the initial pressures of 1 bar, 2 bar and 3 bar, including flame acceleration, turbulent flame propagation and flame–shock interaction with pressure oscillations. The results show that the process of flame acceleration through perforated plate can be characterized by three stages: laminar flame, jet flame and turbulent flame. Fast turbulent flame can generate a visible shock wave ahead of the flame front, which is reflected from the end wall of combustion chamber. Subsequently, the velocity of reflected shock wave declines gradually since it is affected by the compression wave formed by flame acceleration. In return, the propagation velocity of turbulent flame front is also influenced. The intense interaction between flame front and reflected shock can be captured by high-speed schlieren photography clearly under different initial pressures. The results show that the propagation velocity of turbulent flame rises with the increase of initial pressure, while the forward shock velocities show no apparent difference. On the other hand, the reflected shock wave decays faster under higher initial pressure conditions due to the faster flame propagation. Moreover, the influence of initial pressure on pressure oscillations is also analyzed comprehensively according to the experimental results.     

Investigation on combustion mode and heat release in a model scramjet engine affected by shocks

A model scramjet engine in which the 1.0 Ma hydrogen jet mixes and reacts with the 2.0 Ma surrounding airstream is investigated using large eddy simulation. The flame structure is analyzed with a focus on the relationship between premixed/diffusion combustion mode and heat release in the supersonic reacting flow. The flame filter is used to evaluate the contributions to heat release rate by different combustion modes qualitatively and quantitatively. Results show that the heat is released from a combination of premixed combustion mode and diffusion combustion mode even when the fuel and airstream are injected into the combustor separately. Local mode-transition occurs as the supersonic jet flame propagates and interacts with shocks. The diffusion combustion mode dominates during the ignition stage and the premixed combustion becomes dominant during the intensive combustion region. When the shock wave impinges on the flame, the combustion area decreases a little due to the compression effects of the shock. However, the heat release rate is significantly improved in the interaction region since the shock could increase the air entrainment rate by directing the airflow toward the fuel jet and enhance the mixing rate by inducing vorticity due to baroclinic effects, which is good for flame stabilization in the supersonic flow. For the present case, 33.3% of the heat is released by diffusion combustion and 66.7% of the heat is released by premixed combustion. Thus the premixed combustion mode is dominant in terms of its contributions to heat release in the model scramjet engine.     

The mechanism of flame propagation affected by flow/shock wave in a confined combustion chamber equipped with a perforated plate

The whole evolution of flame propagation in a confined combustion chamber was firstly experimentally observed in a newly designed experimental apparatus equipped with a perforated plate. The effect of the flame-flow/acoustic/shock wave interaction on the flame propagation was studied. The experiment was conducted with a stoichiometric hydrogen-air mixture. According to the flame morphology and the flame tip velocity, the whole evolution of flame propagation in the experimental apparatus was classified into the following three stages: laminar flame, jet flame and turbulent flame. In the present work, different flame propagation modes were obtained in different conditions. Depending on the initial pressure, three different flame propagation modes were observed. At an initial pressure of 1 bar, the flame propagation after perforated plate was mainly controlled by the interactions of the flame and combustion-generated flow ahead of the flame front. As initial pressures went up to 3 bar and 5 bar, shock waves were clearly observed ahead of the flame, which played a significant role on the flame propagation. The flame decelerated sharply and even propagated backwards, induced by the flame-shock wave interactions. Depending on the intensity of the shock wave, the backward-propagation velocity was higher at 5 bar with a stronger shock wave. In addition, the pressure oscillation at different initial pressures was discussed.     

Implication of diamond shaped dual strut on combustion characteristics in a cavity-based scramjet combustor

The present study deals with the implication of the novel diamond-shaped dual strut with a backward-facing step on the combustion characteristics of a cavity-based scramjet combustor. The dual strut with diamond shape is considered for the investigation since it triggers the flow separation leading to the disturbances in the flow especially at the top and bottom wall thus serving the flame holding purpose. Firstly, the combustion characteristics of a combustor with a single strut are compared and evaluated with a dual strut based combustor to portray the effect of a dual strut. A separate study is carried out to investigate the influence of spacing in a dual strut on the performance of the scramjet combustor. Our study reveals that the dual strut with a cavity greatly affects the formation of vortices, separation region, and recirculation region which is evident from an increase in the mixing and combustion efficiency. It can be observed that the formation of vorticity and the recirculation region is found to be larger due to the strong and multiple reflections of a shock in the case of the diamond-shaped dual strut with a backward-facing step injection as compared to the single diamond-shaped strut. Further, it is observed that the spacing(D) in the dual strut also affects the mixing and combustion performance. It is found that the value of mixing and combustion efficiency decreases with an increase in spacing independent of Mach number due to the presence of larger separation regions. It can also be observed that the length of the recirculation region occupies entire the cavity making flame stable when the spacing is the least. This is desirable as far as engine performance is concerned.     

Numerical simulation on the spontaneous ignition of high-pressure hydrogen release through a tube at different burst pressures

Spontaneous ignition induced by high-pressure hydrogen release is one of the huge potential risks in the promotion of hydrogen energy. However, the understanding of the microscopic dynamic characteristics of spontaneous ignition, such as ignition initiation and flame development, remains unresolved. In this paper, the spontaneous ignition caused by high-pressure hydrogen release through a tube is investigated by two-dimensional numerical simulation at burst pressure ranging from 2.67 to 15 MPa. Especially, the thermal and species characteristics in hydrogen shock-induced ignition under different strengths of shock wave are discussed carefully. The results show that the stronger shock wave caused by higher burst pressure leads to larger heating area and higher heating temperature inside the tube, increasing the possibility of spontaneous ignition. The shortening effect of initial ignition time and initial ignition distance will decrease with the increase of the burst pressure. Ignition will be initiated when the temperature is raised to about 1350–1400 K under the heating effect of shock waves. It is also found that the ignition occurs under the lean-fuel condition firstly on the upper and lower walls of the tube. The flame branch after spontaneous ignition is observed in the mixing layer. Two ignition kernels show different characteristics during the process of combustion and flow. The evolution of HRR and mass fraction of key species (OH, H, HO₂) are also compared to identify the flame front. The mass fraction of H has the better trend with HRR. It is suggested that H radical is a more reasonable choice as the indicator of the flame front.     

Stabilization performances and mechanisms of a diffusion-like vortex-tube combustor for oxygen-enriched combustion

The stabilization performance and mechanisms in a diffusion-like vortex-tube combustor is investigated for oxygen-enriched combustion. The stability limit, flame configuration, and pressure fluctuation are investigated under various conditions. Results show that a diffusion-like flame structure is established in the combustor and the nonpremixed peculiarity becomes more prominent with the increase of oxygen mole fraction. The steady combustion can be achieved in the range of global equivalence ratio 0.01 to 1.0 with a low-pressure fluctuation amplitude always less than 1300 Pa, indicating a good combustion stability of this combustor. Additionally, the stabilization mechanism is discussed from the time matching and velocity matching. Based on the axial fuel entry method, the Damköhler number (Da) is always less than 1.0 as a whole, which is the principal reason for the tubular flame shape and the steady combustion procedure in this vortex-tube combustor. The intensified combustion under oxygen-enriched combustion can increase the flame speed, and subsequently reduce the mixing quality and make the yellow flame more visible. Besides, the temperature distribution and the flow field structure can explain the corrugation and deformation of the flame front under oxygen-enriched conditions.     

Numerical simulation of the interaction between shock train and combustion in three-dimensional M12-02 scramjet model

The characteristics of combustion flow fields and performance for hypersonic M12-02 scramjet were numerically simulated and analyzed. The compressible two-equation k-w SST turbulence model was employed for the turbulence model and the 9-species, 27-reaction-step hydrogen-air reaction mechanism was used as the reaction kinetics model. The numerical method was verified and a good agreement was obtained between the results of the numerical simulations and the experimental data. The results showed that shock waves from the upper and lower walls respectively crossed with each other near the central axis, forming a ‘diamond’ shape in the high-temperature combustion region. Compared to the conventional scramjet engine, most of the fuel reaction was in pure supersonic combustion mode for this hypersonic scramjet engine. Changes in the distribution of fuel on the upper and lower walls could have an appreciable impact on the combustion flow field. Average fuel distribution between upper and lower walls is benefit for combustion enhancement while the heat transfer in the corner of the side wall is severe and should be avoided during operation. The flame investigation showed that it cannot automatically predict the flame surface temperature in advance only based on the equivalence ratio Φ according to diffusion combustion theory. Compared to Φ = 1.0 condition, the flame surface temperature for Φ = 0.8 condition is higher as the complicated interaction between shock waves and combustion, which makes the local air temperature and mixing extent in flame surface is more appropriate. However, in terms of the overall engine performance, the Φ = 1.0 condition has the better combustion efficiency along the whole flow path.     

Experimental study on induced accelerated combustion of premixed hydrogen-air in a confined environment

The study on induced accelerated combustion of premixed hydrogen-air in a confined environment is of great significance for the efficient utilization of hydrogen energy in internal combustion engines. The accelerated flame induced by the orifice plate is more stable and easy to control, which is beneficial to achieve controlled and rapid turbulent combustion. In this work, the accelerated combustion process induced by the orifice plate, and the influence of the orifice structure and initial conditions on the flame propagation and combustion characteristics were investigated by constant volume combustion bomb and schlieren method. The results show that the combustion process induced by the orifice plate consists of three stages: the initial stage of propagation, the accelerated stage of the orifice plate, and the end combustion stage. The reduction in aperture induces greater turbulence intensity and increases the perturbation of the orifice plate to the flame, resulting in a substantial increase in flame propagation speed through the orifice plate. As the initial pressure and the equivalence ratio increase, the velocity of turbulent flame induced by the orifice plate and the change rate of the velocity before and after the orifice plate increase. As the initial temperature increases, the turbulent flame propagation velocity does not change much, and the velocity change rate before and after the orifice plate decreases. The effect of the initial conditions on flame acceleration induced by the orifice plate is essentially the influence of flame propagation speed and instability. The greater the flame propagation speed and the stronger the flame instability, the stronger the induced turbulence and the greater the influence of the turbulent flow disturbance, and the greater the velocity of the turbulent flame induced by the orifice plate. There exists an optimum aperture for the shortest combustion duration at any initial conditions, but the optimal diameter is not sensitive to changes in initial conditions. The effect of orifice-induced combustion acceleration is remarkable, and the combustion durations induced by each orifice plate are shortened by more than 50%.     

Numerical simulation of the influence of confined multi-point ignited H2–O2 mixture on the propagation of shock waves towards a deformable plate

The study of shock wave propagation in a detonation chamber is of great importance as a part of the plate forming process. Investigations related to the effects of premixed gas detonation on the deflection of a plate require in-depth examination. An Eulerian-Lagrangian numerical simulation is conducted using the space-time conservation element and solution element method of LS-DYNA software to study the effect of confined multi-point ignited gaseous mixture on the dynamic response of thin plates clamped at the end of a combustion chamber. The FSI couples a Lagrangian finite element solver with a Eulerian fluid solver in a 2D space with detailed chemistry of H₂–O₂ mixture. The solution contains the detonation wave propagation through the combustion chamber and its interaction with the plate. The influence of variation in the multi-point ignition locations and combustion chamber dimensions on the pressure history and plate deflection is studied. To verify the model, a comparison with the experimental study is carried out using an adjustable model representative of the real experiment. The verified model is used to link the evolution of plate shape with the arrival time and intensity of shock waves within the chamber. It is found that a longer distance between the ignition point and the plate intensifies the ultimate deflection of the plate. In addition, a fairly large combustion area employed in a direction rather than transverse to the plate surface is unable to influence the ultimate deformation of the plate.     

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.     

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.     

脉动供燃料燃烧技术及火焰频率特征

介绍了一种天然气的新型脉动供燃料燃烧技术,该技术能有效降低NOx,提高传热效率,节约能源,且改造简易,运行经济;阐述了其技术原理,综述了其发展状况,指出对我国燃烧技术改进的重要意义.利用直接摄像技术,对一个采用该燃烧方式的射流火焰在不同流量脉动频率下的火焰类型和结构特征进行了研究.结果发现,随脉动频率变化出现了丰富的火焰类型.燃气脉动与系统共振特性相耦合影响火焰脉动特性,在系统共振和谐振频率附近,火焰根部出现崩溃混合,且火焰湍动升起,火焰长度变短.在某些频率下火焰直长,具有清晰的贫富燃交替结构.     

Flow structure and flame stability in a micro can combustor with a baffle plate

Flow structure and flame stability to be formed inside a micro can combustor, with a baffle plate having a central fuel nozzle and multi air holes located annularly were investigated experimentally. The structures of the isothermal flow and the reacting flow behind the baffle plate are measured by using a particle image velocimetry (PIV). The result shows that generation of the flow recirculation region enhances the mixing most effectively and is useful to make the combustion chamber compact. However, for the reacting flow condition, the flow structure behind the baffle plate will be changed drastically. The flame stabilization mechanisms have to be discussed in terms of local conditions of fuel and air mixing, flame propagation speed, and so on. These local structures seem to play an important role for the lifted flame location and stability of this type of burner.     

初始温度对氢-空气预混诱导湍流燃烧特性的影响

通过在火焰传播路径上布置孔板实现诱导湍流燃烧,利用纹影技术和压力采集系统研究了初始温度对孔板诱导氢-空气预混湍流燃烧特性的影响。试验结果表明:穿越孔板前火焰传播速度略有下降,穿越孔板后火焰被诱导为湍流燃烧,火焰发展进程加快;随着初始温度的升高,最高燃烧压力和最大压升率减少,两者出现的时刻提前,添加孔板后的燃烧持续期变化率降低,但穿越孔板后的火焰传播速度的差异不显著。     

掺氢比对低排放燃烧室性能影响研究

为了探究传统天然气燃气轮机对氢气燃料的适应性,基于现役某型工业低排放燃气轮机结构和性能,用数值模拟方法分析了燃料中氢气比例对低排放燃烧室性能的影响,确定了燃烧室燃用甲烷和氢气燃料的换用性能。研究表明：在1.0额定工况,掺氢比小于等于30%时,燃烧室不发生回火,喷嘴内部和火焰筒肩部回流区的温度以及燃烧室的总压损失随掺氢比的升高而升高,NOx排放体积分数小幅升高,CO排放体积分数减少;当掺氢比大于30%时,燃烧室发生回火,喷嘴和火焰筒肩部回流区温度、总压损失、NOx排放体积分数大幅升高,CO排放基本为零。在其他工况下,负荷变化对燃烧室边界条件影响较为复杂,对喷嘴回火边界影响无单调性变化规律。     

Experimental investigation of deflagration to detonation transition in hydrocarbon-air gaseous mixtures

The paper presents the results of investigation of deflagration to detonation transition in gas mixtures with exothermic chemical reaction using the experimental method of nonintrusive diagnostics of the process. Schlieren photochronography in the optical sections in different places of the tube is performed using the laser as a source of light. Experimental results of visualization of the transition process in hydrocarbon-air gas mixtures show several different flow patterns: (1) The detonation wave originates in the flame zone. (2) The detonation wave originates between the flame zone and primary shock wave. (3) The secondary combustion zone originates between primary shock and the flame and causes the detonation. (4) Spontaneous flame occurs that leads to the combustion to detonation transition. The influence of the flame zone on the originating strong detonation wave is noticed.     

THERMALLY INDUCED VIBRATION IN A THIN PLATE UNDER THE WAVE HEAT CONDUCTION MODEL

Thermally induced vibration in a thin plate under a thermal excitation is investigated. The excitation is in the form of a suddenly applied laser pulse (thermal shock). The resulting transient variations of temperature are predicted using the wave heat conduction model (hyperbolic model), which accounts for the phase lag between the heat flux and the temperature gradient. The resulting heat conduction equation is solved semianalytically using the Laplace transformation and the Riemann sum approximation to calculate the temperature distribution within the plate. The equation of motion of the plate is solved numerically using the finite difference technique to calculate the transient variations in deflections.     

Effect of hydrogen-air mixture diluted with argon/nitrogen/carbon dioxide on combustion processes in confined space

The effects of the dilution with inert gas on different combustion processes in confined space are investigated by utilizing a newly designed constant volume combustion bomb (CVCB) equipped with a perforated plate. Hydrogen-air mixture diluted with argon, nitrogen and carbon oxide of different proportions is employed in the present work. Combustion phenomena were all captured by high-speed Schlieren photography including flame propagation, compression wave formation as well as pressure oscillation. The results show that the dilution of inert gas slows down flame propagation in the combustion chamber. The velocity deficit increases in the order of Ar/N₂/CO₂, which indicates that CO₂ is a better inhibitor of flame propagation than Ar and N₂. The evaluated jet flow accelerates continuously driven by the forward spreading laminar flame and the velocities at different inert gas conditions decrease in the sequence of Ar/N₂/CO₂. No shock wave occurs during the combustion process when inert gases are introduced into the chamber. The amplitude of pressure oscillations decreases with diluted mixture due to the absence of flame-shock interactions. Besides, the peak pressure shows difference among different inert gases.