Hydrogen-air Flames Research Articles

A comparative study on premixed hydrogen–air and propane–air flame propagations with tulip distortion in a closed duct

The premixed hydrogen–air and propane–air flames in a closed duct were experimentally studied for a deep insight into the tulip distortion phenomenon. High speed schlieren photography and pressure sensors recorded the flame images and pressure dynamics at different equivalence ratios for detailed analyses and comparisons. The distorted tulip shape which was first scrutinized and distinguished as an exclusive feature in premixed hydrogen–air flame, was also observed in stoichiometric premixed propane–air flame with a similar behavior on flame shape changes. Tulip distortions are always accompanied by remarkable flame tip velocity fluctuations with near-constant amplitudes. But the pressure dynamics is totally different. There are no observable oscillation and stepped rise in pressure trajectory of stoichiometric premixed propane–air flame. The pressure wave effect is definitely not the incitation factor, but an additional enhancement of flame acceleration, deceleration and deformation.

Numerical investigations of heat losses to confinement structures from hydrogen-air turbulent flames in ENACCEF facility

In hydrogen safety analysis, structure response due to the pressure and thermal loads from the combustion is of great concern. It is of high significance to understand not only the combustion process itself, but also the heat losses from the combustion products to the solid structures which may have strong impacts on the pressure and temperature decays. In many previous numerical simulations, heat losses from turbulent hydrogen flames to the confinement structures were usually considered to be negligible or less important. However, it has been revealed by many experimental studies that modeling of heat losses from the combustion products is important for accurate predictions. Our objectives are to study the importance of various heat transfer mechanisms and their relative contributions to the total energy losses. Numerical investigations on the mechanisms of heat losses caused by propagating turbulent flames were performed using a semi-implicit pressure-based all-speed CFD code GASFLOW-MPI. Heat losses from turbulent sonic flames to the structures of the ENACCEF facility at IRSN were studied. It appears that the effect of heat losses on the flame propagation properties is not significant. However, the impacts of heat losses on the pressure peak and pressure decay after hydrogen combustions should not be neglected. It indicates from our simulation results that the convective heat transfer and thermal radiative heat transfer are the main contributors of the total energy losses to the structures of ENACCEF. In our cases, the effect of steam condensation heat transfer is relatively small but not negligible. The relative contributions of various heat transfer mechanisms could be different in other experimental facilities with various geometrical configurations, various internal structures, and various optical and thermal characteristics of the burnable gas mixtures. In general, it is suggested to include the heat transfer mechanisms in order to improve the reliability and accuracy of numerical analyses of hydrogen safety issues.

An a priori DNS subgrid analysis of the presumed β-PDF model

A common way of carrying out LES or RANS of premixed and partially premixed turbulent flames with tabulated combustion chemistry consists of using a presumed shape for the probability density function (PDF) of progress variable and mixture fraction in order to compute the reaction rates. Commonly utilized for this purpose is the β-function PDF. To the aim of clarifying the applicability of the presumed β-PDF approach to the modeling of methane and hydrogen turbulent premixed flames, in this paper an investigation of the probability density distribution is performed by processing three-dimensional DNS computational results performed with detailed chemistry. This analysis is performed by means of a detailed comparison between the DNS data and the corresponding a priori LES obtained with top-hat filters of various sizes. The analysis is conducted for hydrogen and methane turbulent flames, for comparison. In particular, it is assessed whether a lean premixed turbulent hydrogen-air flame can be well-represented in LES by a β-PDF approach as traditionally applied for methane in literature. It is shown that the presumed β-PDF model performs rather well for both hydrogen and methane. The total error between the real distribution and the presumed β-PDF is of comparable amount for the two fuels. However, the error shows a more consistent profile in methane flames. In addition, it is shown that mean and variance are not sufficient as control parameters for an improved modeling of hydrogen flames by means of presumed PDF, plausibly because of its strong differentially diffusive effects.

DNS study of the propagation and flashback conditions of lean hydrogen-air flames in narrow channels: Symmetric and non-symmetric solutions

In this paper we present results of direct numerical simulations of lean hydrogen-air flames freely propagating in planar narrow adiabatic channels, using detailed chemistry and transport. Our simulations show that double solutions, symmetric and non-symmetric, can coexist for the same set of parameters. The symmetric solutions were calculated imposing symmetric boundary conditions in the channel midplane and when this restriction was relaxed non-symmetric solutions developed. This indicates that the symmetric solutions are unstable to non-symmetric perturbations, as predicted before within the context of a thermo-diffusive model and simplified chemistry. Moreover, the propagation speeds of the symmetric and the non-symmetric flames are found to be very different, and therefore the flashback critical conditions also differ significantly. This analysis seems to have received no attention in the literature, even if it can have important safety implications in micro-scale combustion devices burning hydrogen in a lean premixed way.

Dynamics of Premixed Hydrogen-Air Flames in Microchannels with a Wall Temperature Gradient

Two-dimensional numerical investigations on flame dynamics in a microchannel have been carried out for premixed hydrogen-air mixtures with detailed chemistry. Detailed studies on the formation of flames with repetitive extinction and ignition (FREI) mode have been carried out for a 0.75-mm diameter tube with fixed conditions of flow velocity of 10 cm/s and Φ = 0.5–1.0 with wall temperature linearly varying from 300 K to 960 K. An unsteady flame propagation behavior similar to FREI has been observed to appear for a range of mixture equivalence ratios and channel diameters. FREI was observed to occur for 0.5 < Ф < 0.8 for 0.75-mm diameter channel and disappears for higher mixture equivalence ratios. The effect of tube diameter has also been analyzed for 10 cm/s inlet velocity of mixture at 300–1050 K wall temperature for diameters of 0.6 mm, 0.75 mm, and 1.0 mm. As the tube diameter increased, the frequency of the FREI process decreased, which hints to the contribution of disappearance of FREI phenomenon.

Experimental studies of autoignition events in unsteady hydrogen–air flames

An experimental study is presented of unsteady N2-in-H2 jet flames in a co-flow of hot combustion products from lean premixed hydrogen combustion for investigation of the statistical likelihood of autoignition events in the mixing region. The unsteady jet flame is characterized by rapid ignition followed by a gradual blowout of the flame. Audio recordings and Schlieren imaging high speed videos are used in investigating the unsteady flame. The frequency of the blowout re-ignition event is investigated as a function of nitrogen dilution mole fraction (YN2=0.180–0.566), co-flow equivalence ratio (Φcf=0.20–0.27) and jet velocity (Vjet=300–500m/s). The results from the audio recordings and Schlieren imaging indicate that autoignition dominates the re-ignition. The frequency of ignition increase with increasing nitrogen dilution until a maximum is reached after which it decreases with further nitrogen dilution. For increasing equivalence ratios a higher nitrogen dilution is needed in the jet for the flame to become unsteady. The effect of the nitrogen dilution is explained primarily through a reduction in reaction rates and increased jet momentum. Furthermore, the results suggest that the re-ignition rates are controlled by both chemistry and turbulent mixing. The results from the audio recordings and the Schlieren imaging videos correspond well which validates the use of audio recordings as a diagnostic for studying of unsteady hydrogen jet flames.

Some aspects of stabilization and structure of laminar premixed hydrogen-air flames in a microchannel

In the present paper, flame stabilization and structure are investigated numerically for non-adiabatic hydrogen-air flames at different equivalence ratios and inlet velocities. A cylindrical microcombustor in which combustion occurs in the annular region between two concentric tubes is investigated. The inner hollow tube contains static nitrogen gas and this combination acts as a thermal reservoir that stores and recirculates heat to the incoming mixture. Investigations are carried out using detailed numerical model incorporating two-dimensional transport, thermal radiation, multi-step kinetics, and conjugate heat transfer. Flame is sustained by the continuous ignition mechanism activated by an uninterrupted temperature field between gas mixture and wall developing at steady state. Heat losses from flame resulted in a crossover temperature higher than that of the lean-limit and stoichiometric free flames due to slow radical build-up. Thermal diffusion of hydrogen is shown to be responsible for enhancing the burning intensity of leading edge. Diffusion and reaction kinetics at the flame tip result in twin-peaked heat release rate distribution, most prominently for fuel rich flame (ϕ = 1.7).

Effects of combustor size and filling condition on stability limits of premixed H2‐air flames in planar microcombustors

An experimental study on stability limits of premixed hydrogen‐air flames in planar microcombustors (H = 1 mm and 1.5 mm) partially filled with porous medium is carried out, focusing on the effects of combustor sizes and filling conditions. Critical conditions for blow‐off, flashback, and breaking through the porous medium are experimentally measured. The blow‐off limits are nearly independent of combustor sizes and filling conditions, while the flashback limits are strongly influenced by the combustor size and the filling conditions. Critical values for breaking through are identified with two different methods, and it is shown that standing combustion waves are settled over a range of velocities, instead of a fixed value of filtration velocity, which is considered an important characteristic of microcombustion. Most results can be explained by the classic boundary velocity gradient theory by von Elbe and Lewis, and thus the validity of the theory to the present channel spacings is confirmed. © 2015 American Institute of Chemical Engineers AIChE J, 61: 2571–2580, 2015

Experimental assessment of various methods of determination of laminar flame speed in experiments with expanding spherical flames with positive Markstein lengths

Experimental data obtained from lean propane–air expanding spherical laminar flames at pressures of 1 and 0.5atm, with all other things being equal, are analyzed using four different extrapolation equations available in the literature and a unified processing algorithm developed in the present work. For comparison, experimental data obtained recently by Zamashchikov et al. (2014) from a very rich hydrogen–air flame are also processed using the same algorithm. Dependencies of (i) rms differences between measured and calculated flame radii, (ii) laminar flame speeds, and (iii) Markstein lengths on the boundaries of the analyzed range of flame radii are studied in order to (i) investigate the sensitivity of processing results to these boundaries, (ii) determine the proper radius range for each extrapolation equation, and (iii) to find an equation that fits the measured data in the widest radius range. For the lean propane–air mixture under room conditions, which is associated with a relatively weak effect of flame stretching on the flame kernel growth, all four equations yield approximately equal flame speeds, but the associated Markstein lengths are different. Moreover, the flame speeds and Markstein lengths are sensitive to the processed range of flame radii and the same measured curve can be fitted with the same equation using substantially different pairs of Markstein length and flame speed. Due to these two sources of errors, choice of a proper processing method does not seem to be a critical issue when analyzing weakly non-linear dependencies of flame radius on time. To the contrary, for two other flames, which are associated with stronger stretch effects, the two aforementioned sources of errors play a minor role, i.e. flame speeds and Markstein lengths are weakly sensitive to the processed range of flame radii provided that it is properly selected, while the rms differences between measured and calculated flame radii are sufficiently sensitive to variations in Markstein length and flame speed. Accordingly, a proper choice of a processing method is of paramount importance when analyzing substantially non-linear dependencies of flame radius on time. Results of present work indicate that the phenomenological Markstein equation, which is linear with respect to flame curvature, fits the experimental data well in a wider range when compared to three other equations. Because, close results were also obtained using a non-linear equation derived theoretically by Kelley et al. (2012), the latter equation is worth being highlighted from the basic standpoint.

NOx reduction strategies for high speed hydrogen fuelled vehicles

The growing need to drastically reduce aircrafts CO2 emissions has led engineers and scientists in the last years to develop a clean, renewable and sustainable energy system Hydrogen as a green fuel in aviation is a good choice since does not emit any particulate, CO and CO2.However, its high combustion temperature is disadvantageous in terms of NOx production. Further, the higher altitudes associated with hyper/supersonic flight make the emission of NOx critical for the ozone layer. In fact, at current subsonic flight altitudes in the troposphere and lower stratosphere, NOx emissions are associated with ozone production whereas, at altitude of 20,000–30,000 m, which corresponds closely to the maximum ozone density, NOx can catalyze ozone destruction. Reduction of the NOx Emission Index (grams of emission produced per kilogram of fuel consumed) remains therefore a primary concern. This paper has the goal to investigate possible strategies to reduce NOx emissions from H2 combustion. Since these increase when burning fuel near stoichiometric air-to-fuel ratios, a strategy simulated in this study to reduce NOx production consists in operating at lean or very lean equivalence ratios (thanks to the wider flammability limits of the hydrogen-air flames compared to kerosene-air flames), or in reducing the combustor length (thanks to the higher flame speed of hydrogen compared to other fuels. In this paper, the RQL (Rich-Quench-Lean) strategy for the NOx abatement is proposed for a high speed hydrogen fuelled vehicle. This strategy has shown that the equivalence ratio in the rich strategy is a key parameter to reduce the nitrogen oxide emissions to ICAO acceptable values.

Dynamic behaviors of premixed hydrogen–air flames in a planar micro-combustor filled with porous medium

The micro-combustor is a key component of the micro-thermophotovoltaic (TPV) system, and the flame stability limits are very important to its design and operation. An experimental study on the dynamic behaviors of premixed hydrogen–air flames in an H=1mm planar micro-combustor is undertaken. The combustor is partially filled with porous medium to enhance heat transfer and flame stabilities. Critical conditions (Φ1 for blow-off, Φ2 for flashback, and Φ3 for breaking through the porous medium) are experimentally identified for different combustor configurations. It is shown that the blow-off limits are nearly independent of the combustor configuration, while the flashback limits are strongly affected by the combustor configuration as well as the parameters of the inserted porous medium, including the position (Lout), the width (W) and the porosity (ε) of the porous medium. Most of the results can be explained by the classic boundary velocity gradient theory by von Elbe and Lewis in a qualitative sense. Based on the experimental findings, widening the operation window of the planar micro-combustor partially filled with porous medium is possible when the parameters are chosen properly.

Thermal and chemical structures formed in the micro burner of miniaturized hydrogen-air jet flames

The thermal and chemical structure formed in the micro burner potentially leading to the unique stability mechanism of miniaturized jet diffusion flame via excess heat recirculation through the burner wall, is studied numerically. 2-D axis-symmetric heat and mass transport processes with chemical reactions are considered in gas phase, while the heat transport process is considered in solid phase. The burner materials and fuel ejecting velocities are considered as the main numerical parameters in order to examine the role of the burner on the thermal and chemical structure inside the burner. A skeletal reaction mechanism consists of twelve steps chemical reactions and nine species is applied to consider the radical generation/consumption, and their transport in the burner. It is found that the tiny flame is stabilized even with extra-ordinary small Reynolds number, at which an extinction is generally experienced, when the low conductivity burner is adopted. This is due to the effective usage of the transferred heat from the flame to the burner to improve the stability owing to (1) preheating the incoming fuel, (2) the least heat loss from the flame toward the burner tip, and (3) enhancing the reactivity inside the burner. In such low Reynolds number jet flow with the low conductive burner, ambient air can easily diffuse back into the burner and oxidative reactions at the vicinity of the burner tip is then promoted. Accordingly, the flame structure along the axis is dramatically modified beyond the typical 1-D flame. It is suggested that the further chemical process could be promoted in the substantially-heated micro burner by considering the catalytic reaction at the inner burner surface or using oxygenated fuel likely alcohol or ether.

Three-dimensional topology of turbulent premixed flame interaction

The topology of turbulent premixed flames is analysed using data from Direct Numerical Simulation (DNS), with emphasis on the statistical geometry of flame–flame interaction. A general method for obtaining the critical points of line, surface and volume fields is outlined, and the method is applied to isosurfaces of reaction progress variable in a DNS configuration involving a pair of freely-propagating hydrogen–air flames in a field of intense shear-generated turbulence. A complete set of possible flame-interaction topologies is derived using the eigenvalues of the scalar Hessian, and the topologies are parametrised using a pair of shape factors. The frequency of occurrence of each type of topology is evaluated from the DNS dataset for two different Damköhler numbers. Different types of flame-interaction topology are found to be favoured in various regions of the turbulent flame, and the physical significance of each interaction is discussed.

A Conditional Moment Closure (CMC) formulation including differential diffusion applied to a non-premixed hydrogen–air flame

A Conditional Moment Closure (CMC) formulation of the species transport equation including differential diffusion is presented. Further, the CMC equation for conditional enthalpy is also derived. The objectives of the present study are to (1) derive the CMC equations including the differential effects for both the species and enthalpy in the context of hydrogen combustion, (2) implement the new CMC equations in RANS in two dimensions without any cross-stream averaging and (3) evaluate the benefits and drawbacks of the present CMC calculations applied to a non-premixed hydrogen–air flame. The current study is focused on improving the NO concentration near the nozzle. Predictions of the flow and mixing fields and CMC results for the temperature, major chemical species, and NO concentrations are compared with the experimental data. A budget of the terms in the CMC equations for the differentially diffusing chemical species is investigated. The model is shown to have a prominent effect on NO predictions, with an improvement in the regions close to the nozzle. Some possible sources of modeling discrepancy are identified.

Buoyant Unstable Behavior of Initially Spherical Lean Hydrogen-Air Premixed Flames

Buoyant unstable behavior in initially spherical lean hydrogen-air premixed flames within a center-ignited combustion vessel have been studied experimentally under a wide range of pressures (including reduced, normal, and elevated pressures). The experimental observations show that the flame front of lean hydrogen-air premixed flames will not give rise to the phenomenon of cellular instability when the equivalence ratio has been reduced to a certain value, which is totally different from the traditional understanding of the instability characteristics of lean hydrogen premixed flames. Accompanied by the smoothened flame front, the propagation mode of lean hydrogen premixed flames transitions from initially spherical outwardly towards upwardly when the flames expand to certain sizes. To quantitatively investigate such buoyant instability behaviors, two parameters, “float rate (ψ)” and “critical flame radius (Rcr)”, have been proposed in the present article. The quantitative results demonstrate that the influences of initial pressure (Pint) on buoyant unstable behaviors are different. Based on the effects of variation of density difference and stretch rate on the flame front, the mechanism of such buoyant unstable behaviors has been explained by the competition between the stretch force and the results of gravity and buoyancy, and lean hydrogen premixed flames will display buoyant unstable behavior when the stretch effects on the flame front are weaker than the effects of gravity and buoyancy.

A DNS study of self-accelerating cylindrical hydrogen–air flames with detailed chemistry

The self-accelerating expanding cylindrical stoichiometric hydrogen–air flames at eight atmospheres were studied via two-dimensional direct numerical simulation (DNS) of the full compressible Navier–Stokes equations with detailed chemistry. The flame morphology and propagation were finely resolved by the application of a time step of 2.5ns and a grid size of 4μm. Temporally, the intermittent propagation of the flame front is captured through examining its propagation velocity. Spatially, the flame front is found to be comprised of segments exhibiting similar propagation properties, i.e. the intermittent instantaneous propagation of the flame front is attributed to the development of cellular structures induced by hydrodynamic instability. The long-term average propagation velocity of the flame front is described by a power law, with a self-acceleration exponent of 1.22 for the flame radius with respect to time. The increase in the global flame velocity is shown to be primarily a consequence of increased flame surface area, with the local front propagation velocity remaining largely at the constant laminar flame speed for the near-unity Lewis number mixture studied herein.

Impact of chemistry models on flame–vortex interaction

In numerical simulations of turbulent combustion, accurate modeling of the fuel chemistry is a critical component of capturing the relevant physics. Various chemical models are currently being used including detailed chemistry, tabulated chemistry, one-step chemistry, and rate-controlled constrained-equilibrium. However, the differences between these models and their impacts on the fluid dynamics are not well understood. Towards that end, the interaction between a laminar premixed hydrogen-air flame and a two-dimensional monopole vortex is investigated through simulations using the four previously-mentioned chemistry models. In these simulations, the flame speed, viscosity, diffusivity, conductivity, heat capacity, density ratio, and initial vortex characteristics are virtually identical providing a comparison of the effects of each chemical model alone. The results with each model are compared by considering the evolution of the flame structure and the characteristics of the vortex. All four models predict very similar results for the interaction with a small, fast vortex. In the interaction with a large, slow vortex, results from one-step chemistry are different due to a different flame thickness. The large, fast vortex case showcases differences in the flame structure and larger discrepancies between the modeling approaches.

Turbulence–flame interaction and fractal characteristics of H2–air premixed flame under pressure rising condition

Direct numerical simulation (DNS) of turbulent premixed flames in a constant volume vessel is conducted to investigate turbulence–flame interaction and fractal characteristics under pressure rising condition. DNS is conducted for hydrogen–air mixture with decaying homogeneous isotropic turbulence of which the Reynolds number based on Taylor micro scale is 97.1. The detailed kinetic mechanism including 12 reactive species and 27 elementary reactions is used to represent the hydrogen–air flames. Even for the pressure rising process, the local heat release rate, the flame curvature and the tangential strain rate can be scaled by the maximum laminar heat release rate corresponding to instantaneous mean pressure, the Kolmogorov length scale and the ratio of the turbulent intensity to the Taylor micro scale in the unburned mixture. These facts show a rapid response of flame characteristics to the pressure change. In the propagation process, flame displacement speed is very high compared to open flames due to the expansion of the burned gas. The compression of the unburned mixture due to the high displacement speed increases pressure in the vessel and induces slow decaying or increasing in the turbulent Reynolds number even for no mean flow. It is clarified that the fractal dimension of the flame surface does not show any dependence on pressure increase. The inner cutoff obtained in the pressure rising condition is in good agreement with the expression proposed in our previous study (Shim et al., 2011) [6] where the ratio of the diameter of coherent fine scale eddy, which is a universal fine scale structure of turbulence, to the laminar flame thickness is used as an important parameter.

Simulation of turbulent explosion of hydrogen–air mixtures

Spherically expanding turbulent premixed hydrogen–air flames are computed using the Reynolds-Averaged Navier Stokes (RANS) approach. The mean reaction rate is modelled using unstrained and strained flamelets, and an algebraic model. Since the temperature and mass fraction evolve differently in hydrogen flames because of non-unity Lewis numbers, two reaction progress variables are used in the calculations. The computed turbulent burning velocity is compared to measured values to validate the computational models. The strained flamelets model captured the experimental variations quite well while the other two models overestimated the burning velocity of stoichiometric hydrogen–air flames. All of these models underestimated the burning velocity for a lean flame, which is thermo-diffusively unstable, indicating a need to include these instability effects in turbulent combustion modelling. A comparative analyses of stoichiometric hydrogen– and methane–air flames are also performed to understand their relative behaviour for a given turbulent combustion condition signified by u′/sL0 and Λ/δ. Although the gross behaviour is observed to be similar, there is a significant difference in the normalised turbulent flame speeds, st/sL0, of these flames. A detailed analysis showed that the mean reaction rate increases very sharply, at least by two orders of magnitude, near the leading edge for the hydrogen flames compared to the methane flames. Since this behaviour is controlled not only by turbulence and its interaction with flame but also by chemical kinetics, one must also consider chemical kinetics parameters such as activation temperature, in addition to u′/sL0 and Λ/δ to characterise st/sL0.

Intensity calibrated hydrogen flame spectrum

The detection of hydrogen fires is important to the aerospace community. The National Aeronautics and Space Administration (NASA) has devoted significant effort to the development, testing, and installation of hydrogen fire detectors based on ultraviolet, near-infrared, mid-infrared, and/or far-infrared flame emission bands. Yet, there is no intensity calibrated hydrogen-air flame spectrum over this range in the literature and consequently, it can be difficult to compare the merits of different radiation-based hydrogen fire detectors. In this paper we present an intensity calibrated irradiance spectrum for a low pressure hydrogen flame burning in air from 200 nm to 13.5 microns that varies by more than six orders of magnitude. The results resolve relative intensity errors between spectral bands that appear within the literature. The impact of the measured spectrum on the choice of radiation-based hydrogen fire detectors is discussed.