Visualization of auto-ignition and pressure wave during knocking in a hydrogen spark-ignition engine

Visualizations of auto-ignition inside the end-gas region due to flame propagation and pressure waves that occur during knocking were carried out in a hydrogen spark-ignition engine using a high-speed camera. Our results demonstrated that auto-ignition in an end-gas region that was compressed by the propagating flame front could be visualized using the high-speed color camera. A large amount of unburned mixture caused by the auto-ignited kernel explosion generated the strong pressure waves. Strong pressure wave oscillations induced by the initial auto-ignition could be visualized using a video camera with a speed of 250 kframes/s. The auto-ignition and pressure waves caused the thermal boundary layer to breakdown near the cylinder wall and piston head, therefore combustion of the lubricant oil grease was visible inside burned gas region. This auto-ignition and pressure waves may result in damage to the cylinder wall and piston head during engine knocking.     

A numerical study on pressure wave-induced end gas auto-ignition near top dead center of a downsized spark ignition engine

Auto-ignition of end gas is known as a main cause of knock in SI engine. In order to study the characters of auto-ignition induced by pressure wave, different levels of hot zones characterized by temperature gradients are created in end gas, which are then ignited by incident pressure wave developed from main flame. Evolutions of pressure and temperature in end gas are monitored to investigate pressure incidence and end gas auto-ignition. Computational Fluid Dynamics (CFD) calculations are carried out in a simplified two-dimensional symmetrical computational domain. Turbulence is modeled by renormalization-group (RNG) k–ε model and the turbulence-chemistry interaction is modeled using Eddy Dissipation Concept (EDC) with a detailed chemical kinetic mechanism for hydrogen oxidation. Ignition delay sensitivity analysis is also employed to investigate chemical kinetics during the incidence of pressure wave. The results show that the incidence of pressure wave has significantly different effects on auto-ignition characteristics, thus resulting in different ignition delays, pressure oscillations and enhancements of reflected pressure wave.     

Dynamics of premixed hydrogen/air flame in a closed combustion vessel

The dynamics of a premixed hydrogen/air flame propagating in a closed vessel is investigated using high-speed schlieren cinematography, pressure measurement and numerical simulation. A dynamically thickened flame approach with a 19-step detailed chemistry is employed in the numerical simulation to model the premixed combustion. The schlieren photographs show that a remarkable distorted tulip flame is initiated after a classical tulip flame has been fully produced. A second distorted tulip flame is generated with a cascade of indentations created in succession before the vanishing of the first one. The flame dynamics observed in the experiments is well reproduced in the numerical simulation. The burnt region near the flame front is entirely dominated by a reverse flow during the formation of the distorted tulip flame. The distorted tulip flame can be formed in the absence of vortex motion. The pressure wave leads to periodic flame deceleration and plays an essential role in the distorted tulip formation. The numerical results corroborate the mechanism that the distorted tulip flame formation is a manifestation of Taylor instability.     

Numerical simulation and validation of flame acceleration and DDT in hydrogen air mixtures

Combustion of hydrogen can take place in different modes such as laminar flames, slow and fast deflagrations and detonations. As these modes have widely varying propagation mechanisms, modeling the transition from one to the other presents a challenging task. This involves implementation of different sub-models and methods for turbulence-chemistry interaction, flame acceleration and shock propagation. In the present work, a unified numerical framework based on OpenFOAM has been evolved to simulate such phenomena with a specific emphasis on the Deflagration to Detonation Transition (DDT) in hydrogen-air mixtures. The approach is primarily based on the transport equation for the reaction progress variable. Different sub-models have been implemented to capture turbulence chemistry interaction and heat release due to autoignition. The choice of sub-models has been decided based on its applicability to lean hydrogen mixtures at high pressures and is relevant in the context of the present study. Simulations have been carried out in a two dimensional rectangular channel based on the GraVent experimental facility. Numerical results obtained from the simulations have been validated with the experimental data. Specific focus has been placed on identifying the flame propagation mechanisms in smooth and obstructed channels with stratified initial distribution. In a smooth channel with stratified distribution, it is observed that the flame surface area increases along the propagation direction, thereby enhancing the energy release rate and is identified to be the key parameter leading to strong flame acceleration. When obstacles are introduced, the increase in burning rate due to turbulence induced by the obstacles is partly negated by the hindrance to the unburned gases feeding the flame. The net effect of these competing factors leads to higher flame acceleration and propagation mechanism is identified to be in the fast deflagration regime. Further analysis shows that several pressure pulses and shock complexes are formed in the obstacle section. The ensuing decoupled shock-flame interaction augments the flame speed until the flame coalesces with a strong shock ahead of it and propagates as a single unit. At this point, a sharp increase in propagation speed is observed thus completing the DDT process. Subsequent propagation takes place at a uniform speed into the unburned mixture.     

Auto-ignition and flame stabilization of hydrogen/natural gas/nitrogen jets in a vitiated cross-flow at elevated pressure

The influence of natural gas (NG) on the auto-ignition behavior of hydrogen (H₂)/nitrogen (N₂) fuel jets injected into a vitiated cross-flow was studied at conditions relevant for practical combustion systems (p = 15 bar, T_cross-flow = 1173 K). In addition, the flame stabilization process following auto-ignition was investigated by means of high-speed luminosity and shadowgraph imaging. The experiments were carried out in an optically accessible jet in cross-flow (JICF) test section. In a H₂/NG/N₂ fuel mixture, the fraction of H₂ was stepwise increased while keeping the N₂ fraction approximately constant. Two different jet penetration depths, represented by two N₂ fraction levels, were investigated. The results reveal that auto-ignition kernels occurred even for the lowest tested H₂ fuel fraction (XH_2/NG=XH₂/(XH₂+XNG)=80%)$\left(X_{{\text{H}}_2/\text{NG}}=X_{{\text{H}}_2}/\left(X_{{\text{H}}_2}+X_{\text{NG}}\right)=80\%\right)$, but did not initiate a stable flame in the duct. Increasing XH_2/NG$X_{{\text{H}}_2/\text{NG}}$ decreased the distance between the initial position of the auto-ignition kernels and the fuel injector, finally leading to flame stabilization. The H₂ fraction for which flame stabilization was initiated depended on jet penetration; flame stabilization occurred at lower H₂ fractions for the higher jet penetration depth (XH_2/NG$X_{{\text{H}}_2/\text{NG}}$ = 91% compared to 96%), revealing the influence of different flow fields and mixing characteristics on the flame stabilization process. It is hypothesized that the flame stabilization process is related to kernels extending over the duct height and thus altering the upstream conditions due to considerable heat release. This enabled subsequent kernels to occur close to the fuel injector until they could finally stabilize in the recirculation zone of the jet lee.     

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.     

Dynamic couplings of hydrogen/air flame morphology and explosion pressure evolution in the spherical chamber

This paper aims at exploring the dynamic couplings of flame morphology and explosion pressure evolution experimentally and theoretically. In the experiment, flame morphology and explosion pressure evolution under diffusional-thermal and hydrodynamic instability are recorded using high-speed schlieren photography and pressure transducer. In the theoretical calculation, the effects of cellular flame on the explosion pressure evolution are conducted using smooth flame, D = 2.0566, 2.1 and 7/3. The results demonstrate that the cellular flame formation of various equivalence ratios could be attributed to the fact Lewis number is less than unity on the lean side. The flame destabilization of Φ = 0.8 and 3.0 with increasing initial pressure is due to the decreasing flame thickness regardless of unchangeable thermal expansion ratio. Much smaller cells formation on the cellular flame surface as the explosion pressure rises could be attributed to the joint effect of the diffusional-thermal and hydrodynamic instability. Note that the explosion pressure evolution in spherical chamber is obviously underestimated assuming the flame surface is smooth during the hydrogen/air explosion. But the explosion overpressure is overpredicted significantly with D = 7/3. The theoretical overpressure with D = 2.1 is in satisfactory agreement with experimental results.     

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.     

Comparisons of the impact of reformer gas and hydrogen enrichment on flame stability and pollutant emissions for a kerosene/air swirled flame with an aeronautical fuel injector

Experimental studies were conducted on an actual aeronautical fuel injector, at conditions close as possible of the idle regime of the aircraft (P = 0.3 MPa and T = 500 K), to characterize the flame stability and pollutant emissions of two-phase kerosene/air flames. The objective was to investigate the effects of H₂ and reformer gases (RG containing H₂) enrichment of kerosene at constant power for the adaptation to an aircraft engine. Two different gas injection configurations have been tested (partially premixed, PP, and fully premixed, FP) to evaluate the consequences of the fuel injection mode on gas enrichment. We demonstrate the main interest and the benefits of RGs for aeronautical gas turbines. Through chemical mechanisms, they increase the flame stability and strongly reduce CO emissions without dramatically increasing NO_x emissions, in comparison with the injection of pure hydrogen. Their overall behavior is independent from the injection configuration.     

Effects of ignition location on premixed hydrogen/air flame propagation in a closed combustion tube

The dynamics of premixed hydrogen/air flame ignited at different locations in a finite-size closed tube is experimentally studied. The flame behaves differently in the experiments with different ignition positions. The ignition location exhibits an important impact on the flame behavior. When the flame is ignited at one of the tube ends, the heat losses to the end wall reduce the effective thermal expansion and moderate the flame propagation and acceleration. When the ignition source is at a short distance off one of the ends, the tulip flame dynamics closely agrees with that in the theory. And both the tulip and distorted tulip flames are more pronounced than those in the case with the ignition source placed at one of the ends. Besides, the flame–pressure wave coupling is quite strong and a second distorted tulip flame is generated. When the ignition source is in the tube center, the flame propagates in a much gentler way and the tulip flame can not be formed. The flame oscillations are weaker since the flame–pressure wave interaction is weaker.     

Experimental and modeling study on auto-ignition characteristics of methane/hydrogen blends under engine relevant pressure

Auto-ignition characteristics of methane/hydrogen mixtures with hydrogen mole fraction varying from 0 to 100% were experimentally studied using a shock tube. Test pressure is kept 1.8 MPa and temperatures behind reflected shock waves are in the range of 900–1750 K and equivalence ratios from 0.5 to 2.0. Three ignition regimes are identified according to hydrogen fraction. They are, methane chemistry dominating ignition (XH₂≤40%)$\left(X_{{\text{H}}_2}\le 40\mathrm{\%}\right)$, combined chemistry of methane and hydrogen dominating ignition (XH₂=60%)$\left(X_{{\text{H}}_2}=60\mathrm{\%}\right)$, and hydrogen chemistry dominating ignition (XH₂≥80%)$\left(X_{{\text{H}}_2}\ge 80\mathrm{\%}\right)$. Simulated ignition delays using four models including USC Mech 2.0, GRI Mech 3.0, UBC Mech 2.1 and NUI Galway Mech were compared to the experimental data. Results show that USC Mech 2.0 gives the best prediction on ignition delays and it was selected to conduct sensitivity analysis for three typical methane/hydrogen mixtures at different temperatures. The results suggest that at high temperature, ignition delay mainly is governed by chain branching reaction H + O₂ ⇔ OH + O, and thus increasing equivalence ratio inhibits ignition of methane/hydrogen mixtures. At middle-low temperature, contribution of equivalence ratio on ignition delay of methane/hydrogen mixtures is mainly due to chemistries of HO₂ and H₂O₂ radicals.     

The effect of hydrogen addition on biogas non-premixed jet flame stability in a co-flowing air stream

An investigation of the stability limits of biogas jet non-premixed (diffusion) flames in a co-flowing air stream was conducted. The stability limits were determined experimentally for two different methane–carbon dioxide mixtures that represent the typical biogas composition. Moreover, the effect of jet nozzle diameter was also investigated. It was found that with the presence of a significant amount of CO₂ in the fuel, the stability limits were very low and the flames can only be stabilized over a very small range of co-flowing air velocities. As expected, an increase in carbon dioxide concentration resulted in the narrowing of the region for stable flames. However, it was shown that the flame stability of such mixtures can be enhanced very significantly over a much wider range of co-flowing air velocities by introducing a small amount of hydrogen into the fuel. Results obtained in the current experimental setup indicate that an increase in the stability limits by approximately four-fold when 10% (by vol.) of hydrogen is added under the same operating conditions. The effect of the addition of hydrogen on the enhancement of biogas stability is most significant with a 10% initial addition. The degree of enhancement diminishes with further increases in hydrogen addition from 10% to 30%.     

An experimental study of premixed hydrogen/air flame propagation in a partially open duct

High-speed schlieren cinematography and pressure records are used to investigate the dynamics of premixed hydrogen/air flame propagation and pressure build up in a partially open duct with an opening located in the upper wall near the right end of the duct. This work provides basic understanding of flame behaviors and the effects of opening ratio on the combustion dynamics. The flame behaves differently under different opening conditions. The opening ratio has an important influence on the flame propagation and pressure dynamics. When the opening ratio α ≤ 0.075 a significant distorted tulip flame can be formed after the full formation of a classical tulip flame. The propagation speed of flame leading tip increases with the opening ratio. The coupling of flame front with the pressure wave is strong at low opening ratio. Both the pressure growth rate and oscillation amplitude inside the duct increases as the opening ratio decreases. The formation times of tulip and distorted tulip flames and the corresponding distances of flame front increase with the increase of the opening ratio.     

Experimental and numerical study of the effect of initial temperature on the combustion characteristics of premixed syngas/air flame

The effects of different initial temperatures (T = 300–500 K) and different hydrogen volume fractions (5%–20%) on the combustion characteristics of premixed syngas/air flames in rectangular tubes were investigated experimentally. A high-speed camera and pressure sensor were used to obtain flame propagation images and overpressure dynamics. The CHEMKIN-PRO model and GRI Mech 3.0 mechanism were used for simulation. The results show that the flame propagation speed increases with the initial temperature before the flame touches the wall, while the opposite is true after the flame touches the wall. The increase in initial temperature leads to the increase in overpressure rise rate in the early flame propagation process, but the peak overpressure is reduced. The laminar burning velocity (LBV) and adiabatic flame temperature (AFT) increase with increasing initial temperature. The increase in initial temperature makes the peaks of H, O, and OH radicals increase.     

Numerical and experimental study on contact face and shock wave motion in the receiving tube of gas wave refrigerator

The contact face and shock wave motion in an open ends receiving tube of gas waverefrigerator are investigated numerically and experimentally.The results show that,velocity of the contact face rises rapidly as gas is injected into the receiving tube,anddrops sharply after a steady propagation.However,velocity of the shock wave in thetube is almost linear.With increasing of inlet pressure,velocity of the shock waveand steady velocity of contact face also increase.In addition, time and distance ofcontact face propagation in the receiving tube become longer.     

Effect of bluff body shape on the blow-off limit of hydrogen/air flame in a planar micro-combustor

We recently developed a micro-combustor with a triangular bluff body, which has a demonstrated 5-time extension in the blow-off limit compared to straight channel. In the present work, the effect of bluff body shape on the blow-off limit was investigated with a detailed H₂/air reaction mechanism. The results show that the blow-off limits for the triangular and semicircular bluff bodies are 36 and 43 m/s respectively at the same equivalence ratio of 0.5. Analyses reveal that flame blowout occurs due to the stretching effect in the shear layers for both the triangular and semicircular bluff bodies. Moreover, it is found that the triangular bluff body has a smaller blow-off limit because of the stronger flame stretching as compared with the semicircular case. Calculations indicate that the two cases have negligible differences in heat losses because the reaction zones and high temperature regions are located in the combustor centers. Therefore, the heat losses have a negligible effect on the difference in the blow-off limit of the two micro-combustors.     

Experimental study on transmission of an overdriven detonation wave from propane/oxygen to propane/air

Two sets of experiments were performed to achieve a strong overdriven state in a weaker mixture by propagating an overdriven detonation wave via a deflagration-to-detonation transition (DDT) process. First, preliminary experiments with a propane/oxygen mixture were used to evaluate the attenuation of the overdriven detonation wave in the DDT process. Next, experiments were performed wherein a propane/oxygen mixture was separated from a propane/air mixture by a thin diaphragm to observe the transmission of an overdriven detonation wave. Based on the characteristic relations, a simple wave intersection model was used to calculate the state of the transmitted detonation wave. The results showed that a rarefaction effect must be included to ensure that there is no overestimate of the post-transmission wave properties when the incident detonation wave is overdriven. The strength of the incident overdriven detonation wave plays an important role in the wave transmission process. The experimental results showed that a transmitted overdriven detonation wave occurs instantaneously with a strong incident overdriven detonation wave. The near-CJ state of the incident wave leads to a transmitted shock wave, and then the transition to the overdriven detonation wave occurs downstream. The attenuation process for the overdriven detonation wave decaying to a near-CJ state occurs in all tests. After the attenuation process, an unstable detonation wave was observed in most tests. This may be attributed to the increase in the cell width in the attenuation process that exceeds the detonability cell width limit.     

A numerical study on the effect of hydrogen/reformate gas addition on flame temperature and NO formation in strained methane/air diffusion flames

This paper investigates the effects of hydrogen/reformate gas addition on flame temperature and NO formation in strained methane/air diffusion flames by numerical simulation. The results reveal that flame temperature changes due to the combined effects of adiabatic temperature, fuel Lewis number and radiation heat loss, when hydrogen/reformate gas is added to the fuel of a methane/air diffusion flame. The effect of Lewis number causes the flame temperature to increase much faster than the corresponding adiabatic equilibrium temperature when hydrogen is added, and results in a qualitatively different variation from the adiabatic equilibrium temperature as reformate gas is added. At some conditions, the addition of hydrogen results in a super-adiabatic flame temperature. The addition of hydrogen/reformate gas causes NO formation to change because of the variations in flame temperature, structure and NO formation mechanism, and the effect becomes more significant with increasing strain rate. The addition of a small amount of hydrogen or reformate gas has little effect on NO formation at low strain rates, and results in an increase in NO formation at moderate or high strain rates. However, the addition of a large amount of hydrogen increases NO formation at all strain rates, except near pure hydrogen condition. Conversely, the addition of a large amount of reformate gas results in a reduction in NO formation.     

Acoustic excitation effect on NOx reduction and flame stability in a lifted non-premixed turbulent hydrogen jet with coaxial air

The effects of acoustic excitation on the reduction in nitric oxidant (NO_x) emission were experimentally investigated in non-premixed lifted hydrogen jet flames with coaxial air. The purpose of the present work was to analyze the acoustic forcing effect on the flow field, the reaction zone, and NO_x emission, and to study the mechanisms of NO_x reduction and flame stabilization. To analyze of the flow field, a PIV method was used that incorporated two Nd-YAG lasers and a CCD camera. The reaction zone was visualized by taking OH* chemiluminescence images with a 307.1 ± 5 nm narrow band pass filter and an ICCD camera. A flow condition was carefully selected at u_F = 150, 200, 250 m/s and u_A = 12, 16, 20 m/s, which was sustainable for acoustic excitation in a lifted flame region. The frequency was swept from 150 to 1000 Hz in 5 Hz steps. From the measurements of the flow field, the reaction zone, and NO_x emission, we concluded that NO_x emission was reduced and minimized at the resonance frequency. The vortex that was generated by acoustic forcing promoted air entrainment and enhanced the fuel-air mixing rate. This premixing effect resulted in a lower flame temperature, and thus lower NO_x emissions. In addition, the liftoff height periodically fluctuated due to the stretch effect as the vortex interacted with the flame base.     

Fundamental study on accidental explosion behavior of hydrogen-air mixtures in an open space

In this study, the flame propagation behavior and the intensity of the blast wave by an accidental explosion of a hydrogen-air mixture in an open space were measured simultaneously using the soap bubble method. The results show that the flame in lean hydrogen-air mixtures propagated by spontaneous flame instabilities. The flame in rich hydrogen-air mixtures propagated smoothly in the early stage, and was intensively wrinkled and accelerated in the later stage by different type of instabilities. The flame wrinkling in the later stage of rich hydrogen flame is generated when the flame approaches the non-uniformity transition region of concentration distribution. The intensity of the blast wave of hydrogen/air mixtures is strongly affected by the acceleration of the flame propagation by these spontaneous flame disturbances.