Flame Flashback Research Articles

Assessing the effect of air-throttling on ignition dynamics in a hydrogen-fueled scramjet under various fuel-injection strategies using LES

Air-throttling has been experimentally substantiated as a potent technique for augmenting the ignition of fuel jets in high-speed scramjet engines. Nonetheless, the elucidation of its combustion enhancement mechanism remains conspicuously scant in the extant literature. In this work, a comprehensive numerical investigation was conducted to assess the ignition dynamics of a hydrogen-fueled cavity-stabilized scramjet combustor under various fuel-injection and air-throttling strategies using large-eddy simulation. The results demonstrated that air-throttling could improve the ignition environment by augmenting backpressure, fuel residence time, and mixture homogeneity. Remarkably, the subsonic region was improved by over 30%, and the maximum Mach number was limited to no more than 2.25. Additionally, the enhanced ignition modes under two injection strategies were quite different. For the passive injection, the flame developed rapidly as a result of the deflagration of lean mixture in the interval of Zmix = [0.01, 0.02]. Nevertheless, the direct injection resulted in an additional deflagration-to-detonation transition phenomenon in the expansion section due to the ignition of rich mixture in the interval of Zmix = [0.0225, 0.03]. Both injection strategies exhibited severe flow separation and flame flashback. After removing air-throttling, the flame became dispersed with large-amplitude fluctuations, emphasizing the importance of precise control of air-throttling parameters and operating sequence. These findings advanced the current understanding of the intricate interactions between pilot-hydrogen and air-throttling, thereby elucidating their pivotal role in promoting ignition within high-speed environments.

Dynamic response characteristics of flame during condition transition in a cavity-based scramjet combustor

Experimental study is performed to investigate the dynamic response characteristics of the flame when global equivalence ratio increases suddenly during the condition transition in a cavity-based scramjet combustor. High-enthalpy supersonic flow enters the combustor on conditions that Mach number, stagnation temperature, and total pressure are 2.52, 1486 K, and 1.6 MPa, respectively. In this paper, more attention is paid to the characteristics of flame evolution after sudden increase of fuel injection pressure and combustion characteristic when the injection pressure gets stabilized again. Three flame modes after condition transition have been observed and analyzed according to the transient chemiluminescence image captured by high-speed photography. There are two kinds of stable flame modes after transition mission at a low injection pressure difference, which are mainly reflected in the distribution of flame and intensity of visible light. Whereas, once the injection pressure difference exceeds a critical value, intense periodic flame flashback phenomenon begins to appear in the combustor. It can clearly capture the back and forth motion of the flame in the combustor, along with the local blowout and reignition phenomenon. Furthermore, the dominant frequency of flame flashback decreases and contrast of combustion intensity becomes more obvious as the injection pressure difference increases.

Characterisation of non-premixed, swirl-stabilised, wet hydrogen/air flame using large eddy simulation

The quest for decarbonisation of industry calls for phasing out fossil fuels and in some cases replacing them with green hydrogen in line with the Global Sustainable Goals. To that end novel hydrogen burners will be developed. In this work, the non-premixed combustion of steam-diluted hydrogen combustion is studied numerically in a swirl-stabilised combustor. The flame shape and stabilisation is discussed along with the effects of flow-field, stratification and temperature distribution on the flame. The characteristics of the swirl-stabilised flame is illustrated using several time-averaged and instantaneous contours of parameters including heat-release rate, local equivalence ratio and temperature. The effect of mixing and temperature distribution in the non-premixed configuration is studied quantitatively, subsequently identifying several flame regions. The combustion modes in these regions are also reported. Importantly, neither local hotspots nor flame flashback is observed in this steam-diluted hydrogen combustion.While a typical short hydrogen flame is observed near the centreline of the combustor, a weak V-shaped flame tail also co-exists. The heat-release rate in the flame tail is about two orders of magnitude lower than the main flame. Examining the flame structure using a normalised flame index showed that the combustion in the main flame is premixed-like while that in the main flame is non-premixed. The effect of the flow-field on the transport of fuel and air seems to influence this observation in the flame tail.The high heat-release rate regions of the flame comprised lean (ϕ<1), moderately hot (T = 800 to 1400 K) mixture. The flame tail region resulted from lean, hot (T >1500 K) mixtures. A flame region with rich mixture (ϕ = 1 to 2), featuring moderate heat-release rate, was found to be trapped between an axial fuel jet and the central recirculation region. All these flame regions had flameless combustion as the dominant mode of combustion. In particular, the flame tail region burned completely in the flameless mode. The lean and dominantly flameless combustion in the main flame regions are likely to lead to lower NOx.

Large eddy simulation of a dual-mode scramjet with direct injection: Flame dynamics under flow separation induced by heat-release

The combustion process in the scramjet engines generally takes place in unsteady conditions. The unsteadiness of the flame can be present locally or globally in supersonic combustion, for instance, reactant mixing, ignition, flame stabilization, and blow-off. Thus, unsteady supersonic combustion is an urgent problem that needs to be solved for real scramjet engine applications. This work helps to advance the knowledge of dual-mode scramjet combustion operating with different equivalence ratios by conducting quantitative analyses that characterize flame dynamics under flow separation. A hydrogen-fueled direct-injection scheme was computed using blastFoam code via large-eddy simulation of a scramjet test rig at a stagnation temperature of 950 K and a pressure of 0.82 MPa. The computational results were in good agreement with the experimental data, and two typical flame structures (cavity shear-layer and cavity assisted jet-wake stabilized flame) corresponding to the scram/ram operating modes were reproduced. The local flame structure and flow regime were investigated using a modified flame index and filter functions. A particular focus was set on the continuous flame flashback during mode transition, which could be divided into three stages involving jet/shear-layer interaction, unsteady heat release, and the establishment of a large-area separation zone, with the decay of the cavity recirculation zone. Additionally, the unsteady jet-wake ram combustion presented three oscillation modes, giving rise to intermittent flameout of varying levels. The coherent-structures motion caused by the combined effects of low-speed separated flow and the fuel-jet was the main cause for this kind of unsteady combustion.

Characteristics of the flame flashback in a dual-mode scramjet combustor by the gliding arc plasma

A multi-channel gliding arc (MCGA) plasma with an average power of 6000 W is utilized to control the flame flashback in a dual-mode scramjet combustor with a cavity flameholder. Optical measurements, including synchronous high-speed CH* emissions and high-speed schlieren, simultaneous high-speed camera with the oblique view and high-speed CH* emission, accompanied by the three-dimensional RANS simulation, were employed to characterize the flame flashback with or without the MCGA plasma. The results show that a typical flame flashback cycle can be observed with a period of ∼2 ms in the combustor. The back pressure reduced by the dynamic heat release of the reaction zone is not sufficient to form the steady thermal choking in the mainstream in the absence of the MCGA, which results in the oblique shocks and bow shocks appearing in dual-mode scramjet combustion. When the plasma is added, the flame is strengthened near the plasma region, and the combustion oscillation is suppressed significantly. The re-ignition and attachment characteristics of the MCGA plasma provide the new flame kernels to the mainstream flame, and the leading edge of the mainstream flame follows the MCGA plasma. Due to the relatively stable heat release near the fuel jet assisted by the plasma, the thermal choking is stable presented in the combustor with bow shocks by higher back pressure, indicating that the combustion oscillation is suppressed by the MCGA plasma in the dual-mode scramjet combustor.

An Investigation into Burner Configuration Effects on Premixed Flame Characteristics for LPG Diluted with CO&lt;sub&gt;2&lt;/sub&gt;

Meeting stringent emission regulations, the demand for environmentally friendly fuels is increasing by the day. Alternative fuel must be burned alongside conventional fuel to increase the availability of such clean energy sources. The current experimental study investigates the characteristics of the premixed LPG flames with CO2 dilution in tube swirling and non-swirling burners. The study including testing the effects of equivalence ratios, φ, (0.8, 1, 1.2, &amp; 1.4), CO2 dilution ratios (0%, 5%, 7.5%, &amp; 10%), and aspect ratio of the non-swirling burner (2, 4, 6, 8, &amp; 10). Two swirling burners with swirl number was tested, namely 0.78 &amp; 0.48. The dilution of CO2 has been observed lengthens the flame, particularly at higher equivalence ratios and/or flow rates since there is more than one influence, they all agree on a similar influence on flame height. The flame shortens clearly when using a swirling burner. Besides, when increasing the swirl number, the flame height increases slightly. Also, the swirling burner divided the flame's inner core into segments equal to the number of swirl vanes, and a flower-shaped flame was generated at low flow rates. The burner’s aspect ratio affects flame height insignificantly. Flame stability limits increase for a higher equivalence ratio and it enhances due to CO2 addition. The LPG-CO2/air mixture has an improved reply to beat flame flashback. The addition of CO2 expands the flow rate of stable flame by about 40% and 25% for φ = 1 and 1.2 respectively. Utilizing a swirling burner improves flame stability greatly. The limit between flashback and blowout increased by about three times as a result of using a swirling burner.

Flashback of H2-enriched premixed flames in perforated burners: Numerical prediction of critical velocity

Hydrogen has been identified as a potential tool for decarbonizing various industrial sectors by replacing natural gas while maintaining the combustion-based conversion system. However, due to its high reactivity, hydrogen is prone to flashback, making it essential to implement safety measures for its efficient use in practical appliances. This study investigates the flashback of different H2/CH4 mixtures at an equivalence ratio φ = 0.8 and proposes a numerical model to predict the critical flashback velocity for laminar premixed flames. The model considers the conjugate heat transfer between the gas phase and the burner plate. To overcome the computational cost needed to resolve both gas and burner heating, different time steps are employed for the fluid and the solid domains. Two flashback regimes are identified depending on the H2 content. The results suggest that using the laminar flame speed alone is inadequate for designing the safe operation of practical premixed burners.

Numerical study on the mixing and combustion characteristics of a liquid kerosene jet in a scramjet combustor

We numerically investigate the mixing and combustion process of a kerosene jet injected into a scramjet combustor with dual-cavity at different global fuel equivalence ratios using a compressible two-phase flow large-eddy simulation based on the Eulerian–Lagrangian approach. The results indicate that the calculated wall pressure distribution is basically consistent with the experimental result. The high-temperature zone and intermediate product CO distribution zone shown by the numerical simulation agree with the flame position observed by the experiment. When the global equivalent ratio is 0.149, the combustion is in the weak combustion mode. The flame only exists in the downstream cavity, which is mostly the premixed flame. The combustion efficiency is low, and the corresponding total pressure loss is small. When the global equivalent ratio is 0.187, the combustion is in the intense combustion mode, and the flame has a strong instability characteristic. The numerical simulation reproduces the flame flashback phenomenon that the flame backpropagation from the downstream region of cavity C1 to the upstream region of the liquid jet. During the flame flashback process, the flame gradually changes from premixed flame dominated to premixed flame and diffusion flame jointly dominated. In the weak combustion mode, the interaction between flame and droplet is weak, and droplet evaporation is less affected by combustion. While in the intense combustion mode, droplets evaporate more violently, the spray penetration depth increases, and some liquid droplets are transported upstream of the jet by the wall reflux zone.

Improving Ecological Efficiency of Gas Turbine Power System by Combusting Hydrogen and Hydrogen-Natural Gas Mixtures

Currently, the issue of creating decarbonized energy systems in various spheres of life is acute. Therefore, for gas turbine power systems including hybrid power plants with fuel cells, it is relevant to transfer the existing engines to pure hydrogen or mixtures of hydrogen with natural gas. However, significant problems arise associated with the possibility of the appearance of flashback zones and acoustic instability of combustion, an increase in the temperature of the walls of the flame tubes, and an increase in the emission of nitrogen oxides, in some cases. This work is devoted to improving the efficiency of gas turbine power systems by combusting pure hydrogen and mixtures of natural gas with hydrogen. The organization of working processes in the premixed combustion chamber and the combustion chamber with a sequential injection of ecological and energy steam for the “Aquarius” type power plant is considered. The conducted studies of the basic aerodynamic and energy parameters of a gas turbine combustor working on hydrogen-containing gases are based on solving the equations of conservation and transfer in a multicomponent reacting system. A four-stage chemical scheme for the burning of a mixture of natural gas and hydrogen was used, which allows for the rational parameters of environmentally friendly fuel burning devices to be calculated. The premixed combustion chamber can only be recommended for operations on mixtures of natural gas with hydrogen, with a hydrogen content not exceeding 20% (by volume). An increase in the content of hydrogen leads to the appearance of flashback zones and fuel combustion inside the channels of the swirlers. For the combustion chamber of the combined-cycle power plant “Vodoley”, when operating on pure hydrogen, the formation of flame flashback zones does not occur.

Numerical study of periodic flame flashback in a cavity-based scramjet combustor

The periodic flame flashback phenomenon in an ethylene-fueled cavity-based scramjet combustor was numerically investigated by a three-dimensional unsteady Reynolds-averaged Navier–Stokes solver with two-step kinetics. The air inflow stagnation temperature is 1225 K, and its Mach number is 2.6. Spectral analyses revealed the combustion oscillations with flame flashbacks maintained in the separated scramjet mode with the establishment/vanishment of flow separation near the fuel injector, differing from previous studies of flame flashbacks connected to the ramjet/scramjet mode transitions. A mechanism with four evolution stages was proposed to elucidate the flow-flame interaction. In stage I, a rapid flame flashback upstream and shock-train extension were caused by the high-temperature induced auto-ignition tendency of well-mixed unburned gas in the near-sidewall low-speed region. In stage II, the combustion-induced back pressure and shock train gradually achieved an aerodynamic balance. The combustion flow barely changed in stage III. Meanwhile, a simplified model suggested that the gradual temperature rises occurring upstream of the cavity and away from the sidewall were caused by spanwise heat conduction. The higher temperatures would cause upstream flame propagation with enhanced heat release due to auto-ignition. However, the enhanced heat release occurred mostly in the subsonic flow, resulting in pressure decreases according to one-dimensional flow equations. A smaller near-sidewall separation was produced by the lower back-pressures, which prompted the rapid flame recession downstream in stage IV. Moreover, a simplified flame-spreading model was proposed to illuminate the flame propagation nature. The comparison of flame speeds with theoretical estimations indicated that the current flame was in the regime of turbulent flame propagation, rather than the C–J detonation or deflagration speculated in previous studies.

Evolution of the ignition and combustion process at high velocity at the channel entrance

The article deals with the non-stationary self-ignition of the non-prepared in hydrogen-air mixture in supersonic flow for understanding the influence of the inlet flow velocity on the ignition mode. A series of experiments was carried out at significant change in the inlet velocity into the model channel at the identical flow parameter and equivalence ratio. At the velocity increase, the ignition region shifts downstream due to the change in the wave structure of the flow. Simultaneously, the significant increase of the propagation time of the flame front over the channel occurs that results in the growth of time to reach the stationary combustion regime. It is shown that a flame flashback is a necessary condition for the implementation of combustion at the supersonic flow velocity in the channel, regardless of the inlet velocity. Under identical inlet condition the increase of the Mach number leads to an acceleration of the transition to the subsonic combustion and channel choking.

Confined Boundary-Layer Flashback Flame Dynamics in a Turbulent Swirling Flow

Understanding boundary-layer flashback is critical to the design of safe and efficient gas turbines, especially as the addition of highly reactive hydrogen to these devices becomes a prevailing trend for reduction of carbon dioxide emissions. In this work, the boundary-layer flashback of lean hydrogen–air mixtures is studied using large-eddy simulations based on a generic turbulent swirl burner previously investigated experimentally. Simulations at increasing equivalence ratios are used to identify the flashback limits, showing reasonable qualitative agreement with experimental measurements. Flame dynamics during flashback are compared to previous experimental studies, indicating that important physics are captured in the simulations even if the exact flashback limits are not. In particular, a change in the swirl vane angle is shown to dramatically change the flame dynamics, with flame propagation occurring in the direction of swirl at high angles and against the direction of swirl at low angles, consistent with experimental observations. The near-wall nonreacting mean velocity field is shown to be controlled by a temporally stationary flow instability, which is directly responsible for the counterswirl flame flashback exhibited at low swirl angles. Finally, the interaction of local axial flow velocity and flame propagation speed of the leading flame point is investigated with varying swirl angles both before and after the onset of flashback, elucidating the differences in flame dynamics in all four of these cases. In particular, the importance of local flame extinction on flashback limits is emphasized.

Numerical Study of the Influence of the Thermal Gas Expansion on the Boundary Layer Flame Flashback in Channels with Different Wall Thermal Conditions

In recent years, boundary layer flame flashback (BLF) has re-emerged as a technological and operational issue due to the more widespread use of alternative fuels as a part of a global effort to promote carbon neutrality. While much understanding has been achieved in experiments and simulations of BLF in the past decades, the theoretical modeling of BLF still largely relies on the progress made as early as the 1940s, when the critical gradient model (CGM) for the laminar flame flashback was proposed by Lewis and von Elbe. The CGM does not account for the modification of the upstream flow by the flame, which has been recently shown to play a role in BLF. The aim of the present work is to gain additional insight into the effects of thermal gas expansion and confinement on the flame-flow interaction in laminar BLF. Two-dimensional simulations of the confined laminar BLF in a channel are performed in this work. The parametric study focuses on the channel width, the thermal gas expansion coefficient, and the heat losses to the wall. This study evaluates the influence of these factors on the critical condition for the flame flashback. By varying the channel width, it is demonstrated that at the critical condition, the incoming flow in narrow channels is modified globally by the thermal gas expansion, while in wider channels, the flow modification by the flame tends to be more local. In narrow channels, a non-monotonic dependence of the critical-condition centerline velocity on the channel width has been identified. The variation of the heat loss to the wall confirms that the wall’s thermal conditions can significantly alter the flashback limit, with the flashback propensity being larger when the thermal resistance of the wall is high. To assess the general applicability of the CGM, the flame consumption speed and the flow velocity near the wall are quantified. The results confirm that the assumption of flame having no influence on the upstream flow, employed in the CGM, is not fulfilled under confinement for a realistic thermal gas expansion. This results in a general disagreement between the simulations and the CGM, which implies that the thermal expansion effects should be accounted for when considering the confined boundary layer flashback limits. It is shown that the critical velocity gradient increases with the gas expansion coefficient for the given channel width and wall thermal condition.

Numerical Analysis of Flame Flashback and Instability in Cavity-Stabilized Supersonic Combustion

Numerical Analysis of Flame Flashback and Instability in Cavity-Stabilized Supersonic Combustion

산업용 가스터빈 연소기에서 연소 모델에 따른 수소 화염의 역화 특성 비교연구

Four different combustion models were investigated to test hydrogen flame flashback prediction capability in an industrial gas turbine combustor. The applied combustion models are (i) flamelet generated manifold -finite rate (FGM-FR) model, (ii) flamelet generated manifold - turbulent flame speed (FGM-ST) model, (iii) steady diffusion flamelet (SDF) model, and (iv) eddy dissipation concept (EDC) model. Two conditions were simulated which showed experimentally non-flashback and flashback respectively. For the experimentally non-flashback condition, FGM-ST, SDF, and EDC model simulations show flashback, while FGM-FR did not show flashback. For the experimentally flashback condition, FGM-ST, SDF, and EDC model simulations show flashback, while FGM-FR did not show flashback. To be able to predict hydrogen-flame flashback, further investigation on combustion model are needed.

Demonstration of Natural Gas and Hydrogen Cocombustion in an Industrial Gas Turbine

Abstract Hydrogen cofiring in a gas turbine is believed to cover an energy transition pathway with green hydrogen as a driver to lower the carbon footprint of existing thermal power generation or cogeneration plants through the gradual increase of hydrogen injection in the existing natural gas grid. Today there is limited operational experience on cocombustion of hydrogen and natural gas in an existing gas turbine in an industrial environment. The ENGIE owned Siemens SGT-600 (Alstom legacy GT10B) 24 MW industrial gas turbine in the port of Antwerp (Belgium) was selected as a demonstrator for cofiring natural gas with hydrogen as it enables ENGIE to perform tests at higher H2 contents (up to 25 vol. %) on a representative turbine with limited hydrogen volume flow (one truck load at a time). Several challenges like increasing risk of flame flashback due to the enhanced turbulent flame speed, avoiding higher NOx emissions due to an increase of local flame temperature, supply and homogeneous mixing of hydrogen with natural gas as well as safety aspects have to be addressed when dealing with hydrogen fuel blends. In order to limit the risks of the industrial gas turbine testing a dual step approach was taken. ENGIE teamed up with the German Aerospace Center (DLR), Institute of Combustion Technology in Stuttgart to perform in a first step scaled-burner tests at gas turbine relevant operating conditions in their high-pressure combustor rig. In these tests the onset of flashback as well as the combustor characteristics with respect to burner wall temperatures, emissions and combustion dynamics were investigated for base load and part load conditions, both for pure natural gas and various natural gas and hydrogen blends. For H2 &amp;lt; 30 vol. % only minor effects on flame position and flame shape (analyzed based on OH* chemiluminescence images) and NOx emission were found. For higher hydrogen contents the flame position moved upstream and a more compact shape was observed. For the investigated H2 contents no flashback event was observed. However, thermo acoustics are strongly affected by hydrogen addition. In general, the scaled-burner tests were encouraging and enabled the second step, the exploration of hydrogen limits of the second generation dry low emissions burner installed in the engine in Antwerp. To inject the hydrogen into the industrial gas turbine, a hydrogen supply line was developed and installed next to the gas turbine. All tests were performed on the existing gas turbine hardware without any modification. A test campaign of several operational tests at base and part load with hydrogen variation up to 25 vol. % has been successfully performed where the gas composition, emissions, combustion dynamics and operational parameters are actively monitored in order to assess the impact of hydrogen on performance. Moreover, the impact of the hydrogen addition on the flame stability has been further assessed through the combustion tuning. The whole test campaign has been executed while the gas turbine stayed online, with no impact to the industrial steam customer. It has been proved that cofiring of up to 10% could be achieved with no adverse effects on the performance of the machine. Stable operation has been observed up to 25 vol. % hydrogen cocombustion, but with trespassing the local emission limits.

Effect of fuel reactivity on flame properties of a low-swirl burner

The present paper examines the capability of low swirl flows in stabilizing low calorific value fuels. To such an aim, CO2 gas is used to adjust the reactivity of CH4/air mixtures. The results show that increases in CO2 percentage in the fresh mixture result in shrinking the burner stability map by moving the lean and rich blowout limits toward the stoichiometric condition. The effects of CO2 addition on rich blowout limits are more intense than the lean blowout limits. For instance, the equivalence ratio associated with the lean blowout increases by 0.23, when the discharge flow velocity from the burner rises from 1.5 to 5 m/s in the 40 % CO2 diluted case. Under a similar augmentation in the burner discharge velocity, the rich blowout limit reduces by more than 0.5 in the 40 % CO2 diluted case. Unlike the lean blowout, the rich blowout limit responds non-monotonically to the discharge flow velocity from the burner for both undiluted and diluted mixtures. For the 50 % CO2 diluted case, the low swirl burner operates efficiently at low discharge velocities (1.5 to 3.5 m/s), but it blows out at high discharge velocities (3.5 to 9 m/s). Furthermore, unsteady features of low swirl flame near blowout limits are studied in detail. Moreover, strong curvy structures appear at the flame boundary prior to the flame flashback. However, such structures diminish occasionally and specifically as the flame approaches the flashback. Some deterministic features are more obvious before the flame blowout than those before the flame flashback.

Effects of cavity parameters on flame flashback phenomenon in a supersonic crossflow with a cavity flameholder

In this paper, the cavity parameters of length-to-depth ratio, aft ramp angle, and nitrogen throttling positions are numerically studied by a Large Eddy Simulation (LES) combined with the Flamelet/Progress Variable (FPV) model, to investigate the inducing factors and formation mechanism of flame flashback. These numerical studies are validated and compared to our previous experiments. It can be observed from both the calculated and experimental flow field that the larger length-to-depth ratio, sharper aft ramp angle, and nitrogen throttling closer to the cavity strengthen the mixing of fuel wake and promote the jet penetration depth. Meanwhile, the separation of the boundary layer downstream of the cavity can be induced by the shear layer, acoustic oscillation, as well as nitrogen throttling. And the above favorable prerequisites enhance the heat release nearby, while the reverse pressure gradient causes the enlargement of the boundary layer separation in turn. Under this positive feedback, thermal choking is formed and drives the reverse propagation of the high-temperature flame. It is concluded that the downstream boundary layer separation induced by the changes of the cavity parameters is a prerequisite for the flame flashback, and the formation of the thermal choking is the main reason for the flame flashback.

Flame Flashback Critical Damköhler Number for CO$$_2$$ Diluted CH$$_4$$ and C$$_3$$H$$_8$$ Mixtures with Air

The premixed laminar flame flashback phenomenon in tubes has been known and studied for several decades. However, the effect of the CO\(_2\) dilution has not been addressed, which is relevant for assessing the safety of oil-producing facilities. Furthermore, even if numerical studies have underscored the important role played by the mixture Lewis number (Le), these lack an experimental validation. For this reason, specific studies of premixed flame flashback in laminar flows have been undertaken. The objective is to assess the flame flashback critical conditions in mixtures of hydrocarbons — methane or propane diluted by various amounts of CO\(_2\) — with air. This is effected by determining the critical Damköhler number, which is computed using both experimental and numerical data. The former leads to the flashback critical velocity gradient, whereas the latter to a characteristic chemical time scale. The effect of different flame thickness definitions on the Damköhler number (Da) is examined, evidencing than an order of magnitude discrepancy may arise depending on the definition choice. For methane/air mixtures the critical Da increases with the equivalence ratio, whereas a decrease of nearly two orders of magnitude has been obtained for propane/air mixtures. The original results show that CO\(_2\) dilution increases Da only when the fuel dilution percentage is larger than 25 and 50%, for methane and propane, respectively, situations which correspond to flame extinction after flashback. The propane/air/CO\(_2\) mixture results exhibit a \(Da \propto Le^{5.06}\) dependency which closely follows the trend computed previously, whereas the methane/air/CO\(_2\) results evidence the thermal boundary condition at the tube wall influence.

Evaluation of a Turbulent Jet Flame Flashback Correlation Applied to Annular Flow Configurations With and Without Swirl

Abstract The adaptation of high hydrogen content fuels for low emissions gas turbines represents a potential opportunity to reduce the carbon footprint of these devices. The high flame speed of hydrogen air mixtures combined with the small quenching distances poses a challenge for using these fuels in situations where a significant premixing is desired. In particular, flashback in either the core flow or along the walls (i.e., boundary layer flashback) can be exacerbated with high hydrogen content fuels. In this work, the ability of a flashback correlation previously developed for round jet flames is evaluated for its ability to predict flashback in an annular flow. As a first step, an annular flow is generated with a centerbody located at the centerline of the original round jet flame. Next, various levels of axial swirl is added to that annular flow. Additional flashback data are obtained for various mixtures of hydrogen and methane and hydrogen and carbon monoxide for all the annular flow configurations. Pressures from 3 to 8 bar are tested with mixture temperatures up to 750 K. Flashback is induced by slowly increasing the equivalence ratio. The results obtained show that the same form of the correlation developed for round jet flames can be used to correlate flashback behavior for the annular flow case with and without swirl despite the presence of the centerbody. Adjustments to some of the constants in the original model were made to obtain the best fit, but in general, the correlation is quite similar to that developed for the round jet flame. A significant difference with the annular flow configurations is the determination of the appropriate gradient at the wall, which in the present case is determined using a cold flow computational fluid dynamics simulation.