Fast Flame Research Articles

A Comprehensive Review on Recent Advancements in Thermochemical Processes for Clean Hydrogen Production to Decarbonize the Energy Sector

Hydrogen is a source of clean energy as it can produce electricity and heat with water as a by-product and no carbon content is emitted when hydrogen is used as burning fuel in a fuel cell. Hydrogen is a potential energy carrier and powerful fuel as it has high flammability, fast flame speed, no carbon content, and no emission of pollutants. Hydrogen production is possible through different technologies by utilizing several feedstock materials, but the main concern in recent years is to reduce the emission of carbon dioxide and other greenhouse gases from energy sectors. Hydrogen production by thermochemical conversion of biomass and greenhouse gases has achieved much attention as researchers have developed several novel thermochemical methods which can be operated with low cost and high efficiency in an environmentally friendly way. This review explained the novel technologies which are being developed for thermochemical hydrogen production with minimum or zero carbon emission. The main concern of this paper was to review the advancements in hydrogen production technologies and to discuss different novel catalysts and novel CO2-absorbent materials which can enhance the hydrogen production rate with zero carbon emission. Recent developments in thermochemical hydrogen production technologies were discussed in this paper. Biomass gasification and pyrolysis, steam methane reforming, and thermal plasma are promising thermochemical processes which can be further enhanced by using catalysts and sorbents. This paper also reviewed the developments and influences of different catalysts and sorbents to understand their suitability for continuous clean industrial hydrogen production.

Rotational symmetry-driven modal dynamics of high-frequency transverse instabilities in a lean-premixed multislit hydrogen combustor

A cluster of small-scale multislit injectors, with aperture dimension of the same order of magnitude as the characteristic thickness of hydrogen flames, effectively mitigates flashback risk in extremely fast lean-premixed hydrogen flames. We recently reported that this approach is prone to triggering of high-frequency transverse combustion instabilities between 3.1 and 3.5 kHz even under typical laboratory-scale near-atmospheric pressure conditions. With the aim of testing the possibility that the high-frequency transverse mode can be controlled by manipulating multislit pattern-driven hydrogen flame interactions, here we investigate self-excited instabilities of four different multislit configurations over a broad range of operating conditions at 30–100 kW thermal power. Experimentally, we demonstrate that high-frequency spinning and standing modes are generated in connection with rotational symmetry, but the resulting modal dynamics and associated thermoacoustic properties vary considerably depending on the injector's spatial arrangement. While multislit injectors consisting of rotationally symmetric circumferential segments tend to promote the development of standing modes with noticeable amplitude modulations, the assembly consisting of rotationally symmetric radial segments prevents the nodal line from strongly locking at a single angular position, therefore undergoing intermittent transitions between standing and spinning modes. Using several probabilistic approaches in conjunction with wave decomposition and phase space analyses, we show that the preferred orientation angle of the (anti-)nodal line is determined by the existence of slight asymmetry and local deformation. When the breaking of rotational symmetry is large enough to produce conspicuously irregular patterns, the growth of symmetry-enforced multislit hydrogen flame dynamics can be completely suppressed. These results provide important insights into the nature of the modal dynamics of lean fully-premixed multislit hydrogen-air flames, and suggest that the observed correlations might be applied to the exploration of the high-frequency transverse instabilities frequently observed in high-pressure combustion systems.

Experimental study of gasoline engine with EGR dilution based on reactivity controlled turbulent jet ignition (RCTJI)

The exhaust gas recirculation (EGR) method can suppress knock and improve the thermal efficiency of engines, but it also deteriorates the combustion stability and engine power. Turbulent jet ignition (TJI) is a reliable ignition way for improving ignition stability and burning rate. However, the EGR in the pre-chamber will worsen the reactivity of the fuel-air mixture in the pre-chamber and subsequently lower the intensity of the TJI. To this end, the reactivity controlled turbulent jet ignition (RCTJI) based on a newly air-assisted pre-chamber turbulent jet ignition with a scavenging function has been proposed to improve the reactivity in the pre-chamber for the first time. The combustion characteristics, pressure oscillation, and combustion stability of a single-cylinder spark-ignition engine under four ignition modes of spark ignition (SI), passive TJI, active TJI, and scavenged TJI have been investigated. The results show that three TJI modes can obviously shorten the combustion duration due to the fast jet flame. Compared to the SI mode, the spark timing of TJI is not sensitive to the change in EGR rate. In particular, the scavenged TJI could effectively promote the chemical reactivity in the pre-chamber, and subsequently improves the intensity of the turbulent jet flame and shortens the ignition delay and combustion duration. As a result, using the scavenged TJI can effectively reduce cycle variation and achieve the lowest indicated specific fuel consumption (ISFC) at high EGR rates. However, with the increase in EGR rate, the SI combustion gradually becomes unstable. In contrast, the scavenged TJI can effectively extend the EGR tolerance. In addition, the present work further has confirmed the three main combustion stages of jet ignition in terms of the heat release rate.

Trade-off study on environmental-economical aspects of a CRDI engine using hydrogen as dual fuel mode powered with different low viscous alcohol additives

Fuelling a diesel engine with a hydrogen-powered CRDI system is the primary goal of this research. The test was carried out using a water-cooled engine with a brake power of 3.5 kW at a steady speed. Diesel ignited premature hydrogen and air, and two different LVAA fuels were pumped into the combustion chamber at a 23° degree bTDC crankshaft angle. At a rate of 10–20% HES, hydrogen is fed into the reactor. The in-house DAQ program (PC-based) developed in a data factory was used to control the timed manifold injection approach, which was used to mix the hydrogen and air homogeneously. During the suction stroke, the electronic control unit aids in hydrogen induction for a total of 211° degree CA. The findings of combustion, emission, performance, and experiments show that 20%HES with diesel and LVAA as pilot fuel is the most effective level of hydrogen injection. Furthermore, BTE improved by 36.83%, while the BSEC decreased by 19.48%. This also reduced harmful pollutants like smoke, UHC, CO, and CO2 by 55.86%, 30%, 52.33%, and 46.3%, respectively. However, compared to diesel, the level of nitrogen oxides in the air is primarily similar at its highest power output. Because hydrogen burns with a faster flame, less time is consumed during combustion. As a result, the researchers concluded that adding hydrogen to the CRDI engine resulted in lower emissions and better performance when running at the recommended 20% hydrogen with ethanol and diesel blend as pilot fuel. Mainly, DE + HES (20%) fuel strategy was found to be the most suitable dual-fuel mode combination in a conventional CRDI-powered diesel engine in terms of their quality performance and control emissions along with the best economy.

Strong flame acceleration and detonation limit of hydrogen-oxygen mixture at cryogenic temperature

A series of experiments were carried out in a closed tube at cryogenic temperature (77 K) for hydrogen-oxygen mixtures. Flame propagation speed and overpressure were measured by optical fibers and pressure sensors, respectively. The first and second shock waves were captured in the cryogenic experiments, although the shock waves always precede the flames in all cases indicating the absence of stable detonation. However, strong flame acceleration was observed for all situations, which is consistent with the prediction by expansion ratio and Zeldovich number. Besides, the tube diameter and length are also critical for flame acceleration to supersonic. All the flames in this work accelerate drastically reaching the C-J deflagration state. But at 0.4 atm, only fast flame is formed, while at higher initial pressures, the flame further accelerates to a galloping mode manifesting a near-limit detonation, which could be indicated by the stability parameter χ.

Ultrafast flame coating of carbon and chemical vapor deposition of graphene on NiTi alloy to enhance its corrosion resistance

To study their surface protection performance, both graphene and carbon layers have been deposited onto the surface of NiTi alloy via the common chemical vapor deposition (CVD) and a facile flame coating technique, respectively. The latter can be accomplished within tens of seconds with the combustion flame itself as the carbon source and alloy substrate as catalyst. The carbon layer not only inhibits effectively the Ni2+ releasing from NiTi alloy, but also displays lower surface roughness and higher corrosion resistance than that of the graphene coating. The effect of the flame coating time on the performance of resulted carbon layer has been studied comprehensively. The lowest corrosion current density (1.6 μA/cm2) of the flame coated alloy in simulated body fluid has been achieved by adjusting the flame time to be just 30s. This value is much less than that of graphene coating (2.4 μA/cm2) in the same test conditions. And it gets even less in 3.5 wt% NaCl solution (1.3 μA/cm2), implying the potential application of the flame coating method for more corrosive marine environments. Also, the Ni2+ releasing concentration of NiTi/C-30s is decreased to be around one fourth that of bare NiTi. Moreover, the excellent scratch resistance of the carbon layer and its strong adhesion to NiTi substrate have been also confirmed by both the cross-cut and scratching tests. Furthermore, both the elastic modulus and hardness of the alloy decreased marginally after the fast and mild-temperature flame coating process according to the nano-indentation test results. Compared with traditional surface engineering approaches, the flame coating method has many advantages, such as simple and fast process, high protective performance, environmental-friendly fabrication and protection. All of these benefit its promising application in various fields.

Research on combustion and emission characteristics of a hydrous ethanol/hydrogen combined injection spark ignition engine under lean-burn conditions

Ethanol, as one of the carbon-neutral fuels for spark ignition (SI) engine, has been widely used. Dehydration and purification of ethanol during production process will lead to high energy consumption. If hydrous ethanol can be directly applied to the engine, the cost of use will be greatly reduced. Due to the high latent heat of vaporization of ethanol and water, it is necessary to consider the performance of atomization, evaporation and combustion stability when hydrous ethanol is used in engine. As a zero-carbon fuel, hydrogen has excellent characteristics such as low ignition energy, fast flame propagation speed and wide combustion limit. The combination of hydrous ethanol and hydrogen can reduce the use cost and ensure better combustion performance. Therefore, this study explores the performance of hydrous ethanol/hydrogen in SI combined injection engine. The hydrous ethanol is injected into the intake port and the hydrogen is directly injected into the cylinder during the compression stroke. In this study, we firstly analyze the optimal water blending ratio (ω) of hydrous ethanol, which including 0, 3%, 6%, 9% and 12%. The experimental results show that the hydrous ethanol with 9% water ratio has the best performance without hydrogen addition. Based on the 9% water ratio, the effects of hydrogen blending ratio (0, 5%, 10%, 15% and 20%) on the combustion and emission under different excess air ratio (λ) (1, 1.1, 1.2, 1.3, 1.4). Hydrogen addition can increase the degree of constant volume combustion, so that the maximum cylinder pressure and temperature increase with the increase of the hydrogen blending ratio (HBR). When λ = 1.3 and HBR = 20%, the maximum in-cylinder pressure can be increased by 108.64% compared to pure hydrous ethanol. Hydrogen effectively increases the indicated mean effective pressure (IMEP) and reduces the coefficient of variation of IMEP (COVIMEP). Adding hydrogen can reduce CO and HC emissions, while NOx emissions will increase. When λ = 1.2 and HBR increasing from 0 to 20%, the NOx emissions increase by 106.75%, but it is still less than the NOx emissions of pure hydrous ethanol at λ = 1. On the whole, hydrogen direct injection can improve the combustion performance of hydrous ethanol and achieve stable combustion under lean-burn conditions.

Explosion Characteristics of Hydrogen Gas in Varying Ship Ventilation Tunnel Geometries: An Experimental Study

Hydrogen is widely regarded as a key element of prospective energy solutions for alleviating environmental emission problems. However, hydrogen is classified as a high-risk gas because of its wide explosive range, high overpressure, low ignition energy, and fast flame propagation speed compared with those of hydrocarbon-based gases. In addition, deflagration can develop into detonation in ventilation or explosion guide tunnels if explosion overpressure occurs, leading to the explosion of all combustible gases. However, quantitative evidence of an increase in the explosion overpressure of ventilation tunnels is unavailable because the explosive characteristics of hydrogen gas are insufficiently understood. Therefore, this study investigated an explosion chamber with the shape of a ventilation pipe in a ship compartment. The effect of tunnel length on explosion overpressure was examined experimentally. For quantitative verification, the size of the hydrogen gas explosion overpressure was analyzed and compared with experimental values of hydrocarbon-based combustible gases (butane and LPG (propane 98%)). The experimental database can be used for explosion risk analyses of ships when designing ventilation holes and piping systems and developing new safety guidelines for hydrogen carriers and hydrogen-fueled ships.

Epoxy resin modified with chitosan derivatives and DOPO: Improved flame retardancy, mechanical properties and transparency

Improving the flame-retardant efficiency while maintaining the good properties of epoxy resin has become increasing important for industry applications. In this work, a fast char-forming flame retardant (nitro-containing chitosan-derivate, named CSN) derived from a one-pot reaction between chitosan and p-nitrobenzaldehyde was synthesized. When the total addition amount of CSN and 9,10-dihydro-9-oxa-10-phospha-phenanthrene-10-oxide (DOPO) in epoxy resin (EP) is only 5wt%, the cured EP passed the UL-94 V-0 rating and the limit oxygen index (LOI) reached to 34.0%. The total heat release (THR) and total smoke production (TSP) of the modified EP decreased by 23.5% and 77.2%, correspondingly, and the char-forming ability was significantly improved when compared with pure EP. Meanwhile, the CSN and DOPO modified EP maintained comparable tensile strength, flexural strength and transparency with pure EP, which is closely related to the abundant hydroxyl and amine groups as well as rigid structure in CSN and DOPO. Considering that the facile preparation method and small addition amount of the flame retardant, the present work provides a convenient solution for the preparation of modified epoxy resins with good flame retardancy, heat stability, transparency and comparable mechanical properties with pure EP, which is expected to be widely used in industries.

Research on flame recognition technology based on local complex features

Traditional flame recognition methods based on image features are difficult to extract flame image features effectively, resulting in low flame recognition accuracy, while such methods mostly perform specific flame recognition for specific scenes, and when the scene, flame color and other features change, it is difficult to perform flame recognition effectively. To address this problem, this paper proposes a flame recognition scheme based on local complex features. Its main purpose is to fuse multi-scene flame data, introduce the characteristics of flame in color space through the process of extracting feature descriptors in SIFT, so as to filter the extracted feature descriptors with noise interferers, and transform the extracted feature descriptors into feature vectors by using the key point bag-of-words method, and finally a general fast flame recognition model based on the limit learning machine. In this paper, we explore the upper limit of the capability of traditional image feature representation to pave the way for the application of deep learning to the flame recognition problem.

Spontaneous runaway of fast turbulent flames for turbulence-induced deflagration-to-detonation transition

One of the fundamental mechanisms for detonation initiation is deflagration-to-detonation transition (DDT). This research experimentally explores the runaway condition for highly turbulent fast flames before DDT, which are characterized by extremely high turbulent flame speeds. Such fast turbulent flames experience increased effects of compressibility and may develop a runaway acceleration combined with a pressure buildup that leads to a turbulence-induced DDT (tDDT) mechanism that has been recently reported. The flame dynamics and the associated reacting flow field are characterized using simultaneous high-speed particle image velocimetry, OH* chemiluminescence, pressure measurements, and schlieren imaging. We study the flow-field conditions for runaway acceleration of fast turbulent flames and effects of compressibility on the evolution of these flames. The locally measured turbulent flame speed is found to be greater than that of a Chapman–Jouguet deflagration speed, which places the flame in the runaway transition regime that would eventually lead to a detonation.

A Fast Video Fire Detection of Irregular Burning Feature in Fire-Flame Using in Indoor Fire Sensing Robots

Indoor fire sensing robot could become the best choice to detect irregularity feature in the complex situation with help of nascent visual technologies. This paper works on improvement computational time and accuracy of video fire detection (VFD) using on those robots. The profile of flame is first identified by flame luminance and chrominance using the proposed VFD network. A more efficient layer aggregation connection is designed to improve detection accuracy. The proposed feature pyramid layer combining ghost convolution and cross-connection improves the feature extraction ability. Improvements in re-parameterized blocks with residual connections increase inference speed. The irrigation feature of fast burning fire flame in the incipient, growth, full-developed and decay stages are fully detected with analysis of the proposed visualized Heat Release Rate (vHRR) with spark, flicker, and other dynamic disturbances involved. Compared with high-performance methods, the average detection accuracy of this method on public datasets and self-built datasets is improved by 3.5% and 2.3%. Five indoor representative scenarios illustrate that our method achieves a good balance between the detection accuracy and speed of flame trends. In the future, the model could be deployed in lightweight, fast-moving indoor sensor robots for fire detection in hazardous environments.

Carbon fibers modified with carbon nanoparticles by a facile and fast flame preparation for in-tube solid-phase microextraction

Carbon fibers (CFs) were modified with carbon nanoparticles (CNPs) by a facile and fast flame preparation method. CNPs-CFs were observed by scanning electron microscope, and the nano-scaled granular structure on the surface was found. This material was also characterized by Raman microscope and X-ray photoelectron spectroscope. It was placed into a polyetheretherketone tube to get an extraction tube, which was connected with high performance liquid chromatography for online analysis. The tube displayed the effective extraction to several polycyclic aromatic hydrocarbons (PAHs), based on the possible hydrophobic and π stacking mechanism. Under the optimized conditions (60 mL of sample volume, 2.00 mL min −1 of sample rate, 0.5% of methanol in sample, 2.0 min of desorption time), an analytical method towards these PAH targets was established. The low limits of detection (0.001–0.005 μg L −1 ), satisfactory linear ranges (0.003–5.0 μg L −1 , 0.003–10.0 μg L −1 ) and efficient enrichment factors (1012–3164) were presented. The method was applied to detect trace targets in several water samples and satisfactory results were obtained. The method provided some superiorities over other analytical methods, like online test, shorter time and better sensitivity.

Performance improvement and emission reduction of gasoline engine by adding hydrogen gas into the intake manifold

Reducing the emission pollution related to fossil fuel combustion is gaining increasing interest worldwide and the researcher did an intensive effort to reduce the dependency on fossil fuel. This work conducted an experimental study on the effect of the hydroxyl gas (HHO) produced by water electrolysis on the spark ignition engines performance. Experimental setup and procedures are performed in internal combustion laboratory at variable speeds. Different parameters like brake power, brake specific fuel consumption, thermal efficiency and emission gasses from the exhaust, were taken under consideration. It was found that brake power increased from 1059 W to 1186 W at engine speed 2600 rpm when 3 L/min of HHO added in the intake manifold and increased from 1198.2 to 1413 when the engine speed 2400 rpm. It has been observed that a significant reduction in specific fuel consumption from 968 for pure gasoline to 792 g/kW.hr at the brake power 314 W and from 506 to 365 g/kW.hr. The thermal efficiency improved from 16.93% for pure gasoline to 23.48 % by adding 3 L/min of HHO , this improvement can be attributed as a result of wide range of flame propagation and fast flame speed of hydrogen, while carbon monoxide, hydrocarbons significantly reduced. HC reduced from 240 to 215 PPM at 2600 rpm whereas CO decreased from 3.9 to 2.7 % volume. Therefore, HHO is recommended as promising substitute fuel. Since the addition of HHO gas to the fuel increased the combustion temperature, NOx emissions were slightly increased.

Influence of perforated plate thickness on supersonic combustion wave propagation mechanism in a stoichiometric H2-O2 mixture

The behaviors of a supersonic combustion wave through the perforated plate were studied based on velocity and cellular structures. The perforated plate with the thicknesses of Δ = 40 mm, 70 mm, and 100 mm was placed perpendicular to the direction of combustion wave propagation. Stoichiometric hydrogen-oxygen was used in all tests with initial pressure ranging from 11 kPa to 55 kPa. Soot foils were employed to record the triple-point trajectory of a detonation through the perforated plate. Pressure transducers were used to measure the shock time-of-arrival, based on which average velocity was derived. It was found that the fast flame, entering the perforated plate, either maintains fast flame or accelerates to detonation. For CJ detonation, the diffraction kills the detonation directly, and the decoupled detonation may re-initiate or not, which is up to the initial pressure and the geometry of the perforated plate. However, for an overdriven detonation, it can transmit the perforated plate successfully with no failure. The critical condition for a detonation go or no go requires d/λ (d is the hole diameter and λ is the cell size) to be larger than 2.29.

Predictions of flame acceleration, transition to detonation, and detonation propagation using the Chemical-Diffusive Model

This paper presents numerical simulations of the evolution of premixed flames in channels with obstacles using the Chemical-Diffusive Model (CDM) coupled to a compressible Navier-Stokes solver. The CDM is a parametric model for chemical reaction and diffusive transport, and it is calibrated to reproduce a set of properties of one-dimensional laminar flame and the Zel’dovich-Neumann-Döring (ZND) detonation. In this work, we use two different CDMs, one calibrated using the theoretical half-reaction distance of the ZND detonation and the other calibrated with the experimental detonation cell size, to simulate a stoichiometric hydrogen-air mixture and investigate flame acceleration, deflagration-to-detonation transition (DDT), and detonation propagation in obstacle-laden channels. As the channel geometry varies, both CDMs predict propagation of fast flames in the choking, quasi-detonation, and CJ-detonation regimes. When the channel width is much greater or smaller than the minimal scale requirement for the onset of DDT, both CDMs predict similar choked fast flames and CJ detonations. In cases where the channel widths are close to the critical conditions, deflagration waves are more likely to undergo DDT if the CDM is calibrated for the half-reaction distance, which produces smaller computed detonation cells than experiments. In numerical simulations of channels with the same geometries as prior experiments, using the cell size-constrained CDM improves predictions of DDT.

Scaling of an Aviation Hydrogen Micromix Injector Design for Industrial GT Combustion Applications

Decarbonising the energy grid through renewable energy requires a grid firming technology to harmonize supply and demand. Hydrogen-fired gas turbine power plants offer a closed loop by burning green hydrogen produced with excess power from renewable energy. Conventional dry low NOx (DLN) combustors have been optimized for strict emission limits. A higher flame temperature of hydrogen drives higher NOx emissions and faster flame speed alters the combustion behavior significantly. Micromix combustion offers potential for low NOx emissions and optimized conditions for hydrogen combustion. Many small channels, so-called airgates, accelerate the airflow followed by a jet-in-crossflow injection of hydrogen. This leads to short-diffusion flames following the principle of maximized mixing intensity and minimized mixing scales. This paper shows the challenges and the potential of an economical micromix application for an aero-derivative industrial gas turbine with a high-pressure ratio. A technology transfer based on the micromix combustion research in the ENABLEH2 project is carried out. The driving parameter for ground use adaption is an increased fuel orifice diameter from 0.3 mm to 1.0 mm to reduce cost and complexity. Increasing the fuel supply mass flow leads to larger flames and higher emissions. The impact was studied through RANS simulation and trends for key design parameters were shown. Increased velocity in the airgates leads to a higher pressure drop and reduced emissions through faster mixing. Altering the penetration depth shows potential for emission reduction without compromising on pressure loss. Two improved designs are found, and their performance is discussed.

A revisit to the validity of flamelet assumptions in turbulent premixed combustion and implications for future research

The flamelet assumption has been widely used for analysis and modeling of turbulent premixed flames over the past decades. Despite its popularity, validity of this assumption has been challenged originally by Borghi [1] in 1985. After Borghi [1], results of several research groups from, for example, the Universities of Toronto, Sydney, British Columbia, and Lund University showed that the reaction zone of turbulent premixed flames can in fact become relatively thick, disagreeing with the flamelet assumption. In contrast with the results of these research groups, those from, for example, the University of Michigan disagree and suggest the premixed flames reaction zone remains relatively thin. In this article, first, we compile the results from several different research groups and argue why turbulent premixed flames may in fact feature thick reaction zones, and what may be the underlying reason for existence of such discrepancy in the literature. Then, we aim to review two ongoing questions related to how fast turbulent premixed flames burn, and how this is related to the flamelet/non-flamelet behavior. It is reviewed that relaxing the flamelet assumption and considering that premixed flames can thicken allow to reconcile the difference between the local and global consumption speeds of turbulent premixed flames as well as to understand/model a premixed flame behavior, which is referred to as the bending behavior in the literature.

The evolution of pressure gain in turbulent fast flames

This research quantifies the evolution of pressure for fast burning regimes characterized by various degrees of compressibility and involving turbulent flames and shocks. The experimental exploration is conducted in a Turbulent Shock Tube facility, where the level of flame compressibility is controlled by varying the equivalence ratio of the hydrogen-air mixture. High-speed particle image velocimetry, chemiluminescence, schlieren, and pressure measurements are simultaneously acquired to capture the rise in stagnation pressure for various regimes from fast flames to shock-flame complexes. The pressure and velocity measurements are used to analyze combustion regimes on the Rankine-Hugoniot diagram that shows the flame-driven compression for a range of fast flame conditions evolving toward detonation onset. Various levels of compression are dependent on the level of shock-flame coupling and flame velocities. Lower degrees of compressibility show 52% efficiency of an ideal ZND cycle with 40% thermal efficiency, while shock-flame complexes are shown to produce 81% of the work produced by an ideal ZND cycle with 53% thermal efficiency.

Combustion dynamics of multi-element lean-premixed hydrogen-air flame ensemble

The crux of the technical issues facing the development of hydrogen gas turbine engines is the problem of how to enable low NOx, low dynamics combustor operations without detrimental flashback events in extremely fast lean-premixed hydrogen flames. While flashback mechanisms and nitrogen oxides emissions have been investigated extensively for high hydrogen content flames, the self-excited dynamics of lean-premixed pure hydrogen flame ensembles remain unclear. Here we use phase-resolved OH* chemiluminescence and OH PLIF imaging in a series of experiments in a multi-element injector configuration consisting of 293 small-scale injectors with an inner diameter of 3.0 mm. We collect a large experimental dataset to explore a variety of collective phenomena of the lean-premixed hydrogen-air flame ensemble, and perform systematic investigations of self-induced combustion instabilities. Our observations demonstrate that ultra-compact pure hydrogen flames generate high-amplitude pressure perturbations over a broad range of characteristic frequencies between 400 and 1800 Hz, corresponding to the third to tenth order eigenmodes. Low-frequency flame dynamics developed under relatively low equivalence ratio conditions involve a complex balance among several coexisting phenomena, including strong vortex interactions and periodic extinction-reignition processes, giving rise to large-scale asymmetric oscillations of the entire reaction zone. By contrast, the flame surface dynamics at an intermediate frequency of ~600 Hz exhibit prominent symmetric oscillations accompanied by merging and pinch-off of the constituent flames. Unexpectedly, high-frequency instabilities at approximately 1720 Hz (screech tones) are not influenced by such structurally complex flame dynamics, but by exceptionally simple, seemingly linear, flame surface motion without sudden flame area annihilation events like those observed for lower frequency cases. Despite the extremely low level of heat release rate fluctuations, on the order of less than 1%, the clustered premixed hydrogen flames are capable of producing disproportionately large pressure perturbations in excess of 12 kPa, originating from the synchronous phase dynamics of acoustic pressure and clustered flames’ heat release rate oscillations. These findings provide new insight into the driving mechanisms underlying high-frequency combustion dynamics of densely arranged pure hydrogen-air flames.