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.     

Numerical investigation on the mechanism of the effects of plate vibration on mixing and combustion of transverse hydrogen injection

This study numerically investigates the effects of plate vibration and deformation on the combustion performance, the shock wave structure, the mixing characteristics and the flame structure for transverse hydrogen injection. The finite-rate method is used to simulate combustion. Results show that plate vibration causes the combustion performance to oscillate both temporally and spatially, while plate deformation can only change the static characteristics of the flow. Plate vibration and deformation increase the intensity and number of shock wave reflections in different ways. In addition, both plate vibration and deformation increase the momentum flux ratio and the jet penetration depth, which enhances mixing. Finally, plate vibration widens the flame and moves it upward to a greater extent than plate deformation.     

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.     

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.     

Visualization of spontaneous ignition under controlled burst pressure

A high-pressure hydrogen jet released into the air has the possibility of igniting in a tube without any ignition source. The mechanism of this phenomenon, called spontaneous ignition, is considered to be that hydrogen diffuses into the hot air caused by the shock wave from diaphragm rupture and the hydrogen-oxidizer mixed region is formed enough to start chemical reaction. Recently, flow visualization studies on the spontaneous ignition process have been conducted to understand its detailed mechanism, but such ignition has not yet been well clarified. In this study, the spontaneous ignition phenomenon was observed in a rectangular tube. The results confirm the presence of a flame at the wall of the tube when the shock wave pressure reaches 1.2–1.5 MPa in more than 9 MPa burst pressure and that ignition occurs near the wall, followed by multiple ignitions as the shock wave propagates, with the ignitions eventually combining to form a flame.     

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.     

Plasma assisted flame ignition of supersonic flows over a flat wall

A nanosecond pulsed plasma discharge located between two fuel jets is used to ignite and hold jet flames in supersonic crossflows, without the use of additional devices (e.g., cavities or backsteps) for flame holding. The fuel injection nozzles and discharge electrodes are mounted flush with the surface of the flat wall adjacent to the freestream flow. The nonequilibrium plasma is produced by repetitive pulses of 15 kV peak voltage, 10 ns pulse width and 50 kHz repetition rate. Sonic or subsonic fuel jets (hydrogen and ethylene) are injected into a pure oxygen freestream of Mach numbers Ma = 1.7-2.4. The shockwave/flow structures induced by the fuel jets and the OH radical distribution resulting from combustion are characterized by Schlieren photography and planar laser induced fluorescence imaging, respectively. A configuration combining an upstream subsonic oblique jet and a downstream sonic transverse jet is shown to provide adequate flow conditions for jet flame ignition assisted by the plasma discharge. The experimental results are interpreted using a simple model by which the pulsed plasma serves as a source of reactive radicals added to a flammable gas mixture.     

Combustion behavior and stability map of hydrogen-enriched oxy-methane premixed flames in a model gas turbine combustor

The article describes an experimental study and comparison of the combustion behavior and determines the stability map of turbulent premixed H₂-enriched oxy-methane flames in a model gas turbine combustor. Static stability limits, in terms of flashback and blow-out limits, are recorded over a range of hydrogen fraction (HF) at a fixed oxygen fraction (OF) of 30% and a particular inlet bulk velocity, and the results are compared with the non-enriched case (HF = 0%). The static stability limits are also recorded for different inlet bulk velocity (4.4, 5.2, and 6 m/s) and the results are compared to explore the effect of flow dynamics on operability limits of H₂-enriched flames. The stability maps are presented as a function of equivalence ratio (0.3–1.0) and HF (0%–75%) plotted on the contours of adiabatic flame temperature (AFT), power density (PD), inlet Reynolds number (Re) and reacting mixture mass flow rate ($\dot{m}$) to understand the physics behind flashback and blow-out phenomena. The results indicated that both the flashback and blow-out limits tend to move towards the leaner side with increasing HF due to the improved chemical kinetics. The stability limits are observed to follow the Reynolds number indicating its key role in controlling flame static stability limits. The results showed that H₂ enrichment is effective in the zone from HF = 20% up to HF = 50%, and O₂ enrichment is also effective in a similar zone from OF = 20% up to 50%, with wider stability boundaries for H₂ enrichment. Axial and radial temperature profiles are presented to explore the effect of HF on the progress of chemical reactions within the combustor and to serve as the basis for validation of numerical models. Flame shapes are recorded using a high-speed camera and compared for different inlet velocities to explore the effects of H₂-enrichment and equivalence ratio on flame stability. The equivalence ratio at which a transition of flame stabilization from the inner shear layer (ISL) to the outer recirculation zone (ORZ) occurs is determined for different inlet bulk velocities. The value of the transition equivalence ratio is found to decrease while increasing the inlet bulk velocity. Flame shapes near flashback limit, as well as near blow-out limit, are compared to explore the mechanisms of flame extinctions. Flame shapes are compared at fixed adiabatic flame temperature, fixed inlet velocity and fixed flow swirl to isolate their effects and investigate the effect of kinetic rates on flame stability. The results showed that the adiabatic flame temperature does not govern the flame static stability limits.     

Hydrogen mixing augmentation mechanism induced by the vortex generator and oblique shock wave in a scramjet engine

The mixing process between the fuel and the incoming air is extremely important for the engineering implementation of the scramjet engine. In the current study, the vortex generator coupled with the oblique shock wave is utilized to promote the hydrogen mixing process in a supersonic crossflow. The configurations of the vortex generator are put into investigation, namely typical ramp, split ramp and ramp vane. Some parameters are provided to evaluate the flow field properties quantitatively. The obtained results predicted by the three-dimensional Reynolds-average Navier-Stokes (RANS) equations show that the method of shock wave/jet shear layer interaction coupled with the vortex generator can effectively improve the mixing efficiency. Different vortex generator structures all have great effect, especially for Case SR (split ramp), with the mixing efficiency raised by 36.27%. The streamwise vorticity plays an important role in the mixing process.     

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 mixing performance and diffusion combustion characteristics of H2 and air in planar micro-combustor

In the present work, the effects of inlet velocity and channel height (H₀ = 0.6 mm, 1.0 mm and 1.4 mm) on the mixing performance, flame stability limit and combustion efficiency of H₂ and air in a 2D planar micro-combustor with a separating plate were studied numerically. The results demonstrate that improved mixing can be achieved with a decrease in inlet velocity and channel height. Moreover, the flame blow-off limit is the largest for a micro-combustor with H₀ = 0.6 mm; the flame becomes inclined at a high velocity and the direction varies with the inlet velocity. Furthermore, a micro-combustor with a medium height (H₀ = 1.0 mm) can achieve the largest blowout limit among the three cases. Finally, for identical inlet velocities, the combustion efficiency increases with decreasing combustor height. In summary, these findings can provide a guideline for the optimal design of such micro-combustors.     

Mixing augmentation induced by the combination of the oblique shock wave and secondary recirculation jet in a supersonic crossflow

The mixing process of fuel-air in the supersonic crossflow is a pivotal technology for the scramjet engine. In this paper, numerical simulation of the transverse sonic hydrogen jet into a supersonic Mach 3 crossflow with the mixing augmentation strategy induced by the combination of the oblique shock wave and secondary recirculation jet has been carried out. Detailed flow field structures, hydrogen mass fraction distributions, vortex structures, heat flux and some parameters have been explored in order to investigate its mixing enhancement mechanism. Results of the three-dimensional Reynolds-average Navier-Stokes (RANS) equations coupled with the two-equation shear stress transport (SST) κ-ω turbulence model show that the combined strategy of the oblique shock wave and secondary recirculation jet device can effectively improve the mixing speed and mixing efficiency with little total pressure loss. Also, the secondary recirculation jet device can reduce the peak of the heat flux effectively. In this study, the case with the single bleed hole owns the best effect with improving the mixing efficiency by 82.75% locally and reducing the maximum heat flux by 15.24% respectively。     

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.     

The effect of hydrogen injection parameters on the quality of hydrogen–air mixture formation for a PFI hydrogen internal combustion engine

To research the quality of the hydrogen–air mixture formation and the combustion characteristics of the hydrogen fueled engine under different hydrogen injection timings, nozzle hole positions and nozzle hole diameter, a three-dimensional simulation model for a PFI hydrogen internal combustion engine with the inlet, outlet, valves and cylinder was established using AVL Fire software. In the maximum torque condition, research focused on the variation law of the total hydrogen mass in the cylinder and inlet and the space distribution characteristics and variation law of velocity field, concentration field and turbulent kinetic energy under different hydrogen injection parameters (injection timings, nozzle hole positions and nozzle hole area) in order to reveal the influence of these parameters on hydrogen–air mixture formation process. Then the formation quality of hydrogen–air mixture was comprehensively evaluated according to the mixture uniformity coefficient, the remnant hydrogen percentage in the inlet and restraining abnormal combustion (such as preignition and backfire). The results showed that the three hydrogen injection parameters have important influence on the forming quality of hydrogen–air mixture and combustion state. The reasonable choice of the nozzle hole position of hydrogen, nozzle hole diameter and the hydrogen injection time can improve the uniformity of the hydrogen–air mixing in the cylinder of the hydrogen internal combustion engine, and the combustion heat release reaction is more reasonable. At the end of the compression stroke, the equivalence ratio uniform coefficient increased at first and then decreased with the beginning of the hydrogen injection. When hydrogen injection starting point was with 410–430°CA, equivalence ratio uniform coefficient was larger, and ignition delay period was shorter so that the combustion performance index was also good. And remnant hydrogen percentage in the inlet was less, high concentration of mixed gas in the vicinity of the inlet valve also gathered less, thus suppressing the preignition and backfire. With the increase of the distance between the nozzle and the inlet valve, the selection of the hydrogen injection period is narrowed, and the optimum hydrogen injection time was also ahead of time. The results also showed that it was favorable for the formation of uniform mixing gas when the nozzle hole diameter was 4 mm.     

Effects of cavity-induced mixing enhancement under oblique shock wave interference: Numerical study

In this research, the effects of oblique shock on the mixing characteristics in a supersonic combustor equipped with a cavity is numerically investigated. To reveal the flow structure of the supersonic flow field under oblique shock wave interference, three-dimensional steady RANS equations and SST k-ω turbulence model are adopted. The current work focuses on comparing the interaction effects between oblique shock wave and bow shock wave, which are formed by fuel jet on fuel mixing under different conditions. The numerical analysis demonstrates that an optimal angle exists for the mixing efficiency of the ramp. The optimal angle diminishes as the jet-to-crossflow pressure ratio increases. The oblique shock wave in a certain range is conducive to enhance the penetration depth of ethylene. The smaller angle of the ramp does not cause large stagnation pressure losses.     

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.     

Hysteresis in flame stabilization in a hydrogen fueled supersonic combustor

Hysteresis in flame stabilization mode transitions in a hydrogen-fueled strut-stabilized supersonic combustion test rig was experimentally observed and studied. Air was vitiated using H₂–O₂ combustion products to stagnation conditions of 8.65 bar and 1350 K and was expanded through a rectangular nozzle to Mach number 2.5. H₂ fuel was injected transversely using a strut positioned at the center of the combustor. The equivalence ratio (ER) was changed in time to study its effects on flame stabilization modes. Shadowgraph and wall pressure measurements were used to study the shock system generated by the strut in the supersonic combustor. High-speed OH1 chemiluminescence and high-speed flame imaging were used to study the heat release zones and flame structure of different combustion modes and transitions between them. Three different combustion modes were observed, namely: divergent section flame (CM1), strut wake stabilized flame (CM2), and jet stabilized flame (CM3). CM1 was observed at a very low ER, where the H₂ was ignited by the normal shock positioned in the divergent section. At this point, the weak shock system at the strut is unable to ignite the fuel. At higher ER, CM2 was observed, as a stronger shock system ignites the richer mixture at the wake of the strut. It was observed that the mixture auto-ignites in the strut wake and doesn't flashback from the divergent section. When the ER is further increased, the stronger injection shock reduces the local velocity and increases the static temperature, enhancing the flame speed of the richer mixture. Thus, the flame flashes back to the fuel jet. Two hysteresis were observed in the supersonic combustor based on ER as a time-varying input. The flame stabilization mode has two solutions based on the history of the change in ER, hence indicating hysteresis. The hysteresis between CM1 and CM2 is because of the retention of the temperature and radicals in the recirculation zone at the wake of the strut. The hysteresis between CM3 and CM2 is because of the retention of the temperature and radicals in the horseshoe vortices around the fuel jets. Understanding hysteresis will help design scramjets with wider operability.     

Numerical study on supersonic combustion of hydrogen and its mixture with Ethylene and methane with strut injection

In this paper, supersonic combustion and flow field of hydrogen and its mixture with ethylene and methane from strut injections in a Mach 2 supersonic flow are studied numerically. The fuel mixture of hydrogen, methane and ethylene represents the major products of pyrolysis of hydrocarbon fuels with large molecules such as kerosene as it acts as coolant and flows through cooling channels and absorbs heat. Detached Eddy Simulation with a reduced kinetic mechanism and steady flamelet model are applied to simulate turbulent combustion. The calculated temperature profiles of hydrogen are compared to the experimental results of DLR supersonic combustor for validation of the present numerical method. The supersonic combustion flows associated with shock waves, turbulent vortices and flame structures are studied. With addition of methane and ethylene, the flame zone moves further downstream of the strut and the maximum flow temperature at chamber exit decreases by 200 K. With analysis of total temperature ratios, it is found that combustion efficiency for hydrogen combustion is 0.91 and it decreases to 0.78 for the fuel mixture. The calculation of ignition delay time and flame speed reveals that fuel mixture of hydrogen and hydrocarbons has considerably larger delay time and smaller flame speed, that contributes to the weakened flame zone and lower combustion efficiency.     

Experimental study of propane/air premixed flame dynamics with hydrogen addition in a meso-scale quartz tube

The combustion process of propane/air premixed flame in meso-scale quartz tubes with different hydrogen additions was investigated experimentally to explain the flame-wall interaction mechanism. The ranges of different flame regimes were obtained by changing the flow rates of propane and hydrogen. The effects of hydrogen addition, inlet velocity and equivalence ratio were analyzed. The results show that the hydrogen addition broadens the operation ranges of fast flame regime and slow flame regime significantly. The flame propagation speed is in the same order of the thermal wave speed in solid wall for the slow flames. In fast flame regime, the flame propagation speed has an inverse correlation with the inlet flow velocity irrespective of the equivalence ratio. With the increase of the equivalence ratio, the maximum flame speed in fast flame regime decreases gradually, while the maximum flame speed in slow flame regime increases continually. It indicates that rich fuel condition suppresses the fast flame and promotes the slow flame. In slow flame regime, the output thermal efficiency is dominated by the inlet velocity and equivalence ratio.     

High-fidelity resolution of the characteristic structures of a supersonic hydrogen jet flame with heated co-flow air

In the present paper, direct numerical simulation (DNS) is performed to analyze the characteristic structures of a supersonic jet lifted hydrogen-air flame with Reynolds number of 22, 000, and Mach number of 1.2. The fuel consisting of 85% H₂ and 15% N₂ by volume is injected into hot co-flow air from a round orifice. Overall 975 million grids are used to compute the complex multi-scales phenomena. A Damköhler number and a flame index are defined to analyze combustion modes and the mixedness of the flame. Complicated characteristic elements of the supersonic jet lifted flame are observed, i.e. a stable laminar flame base with auto-ignition as the stabilization mechanism, a violent mixing region in which vigorous turbulent combustion occurs with both fuel-lean and fuel-rich mixtures, and a flame region consisting of outer diffusion combustion and inner weaker premixed combustion in the far field. At the leading edge of the fame base, auto-ignition takes place primarily in the fuel-lean mixture where the mixedness mode is opposed. Downstream of the laminar flame base, the combustion becomes turbulent due to the intensified mixing of fuel and air, which results in the subequilibrium values of temperature and OH concentration. Detonation occurs near the sonic layer, and then sustains the combustion in higher dissipative mixture. The flame near the stochiometric condition keeps non-premixed, and the other non-premixed flame elements could be observed in the very fuel-rich region. Through the reacting field the premixed flame appears near the shear layer. The combustion intensity decreases in the far field where the inner non-premixed flame disappears gradually.