Hydrogen Flame Propagation Research Articles

Hydrogen concentration measurements using spark induced breakdown spectroscopy in a real engine

This study develops spark induced breakdown spectroscopy (SIBS) for measurements of direct injected hydrogen concentration in a multi-cylinder engine operated at real conditions. To this end, calibration is performed in the air excess ratio (λ) range of 2.1 ∼ 2.9 and simultaneously, high-speed schlieren imaging of laminar hydrogen flame propagation is performed in an optical high-pressure constant volume combustion chamber (CVCC). To achieve real engine applications, the SIBS is further developed using a modified conventional spark plug for sapphire window insertion onto the positive electrode and guided arc formation via a newly designed nipple type ground electrode. Specifically, the peak spectra intensity ratio of Hα (656 nm) to O (777 nm) is used for calibration, which shows a linear correlation with λ. For the first time, the calibrated SIBS is applied to a real engine operated at 46.2 ∼ 63.9 Nm load and 2000 rpm with 3.5 MPa hydrogen direct injection. The results show lower local λ than the global λ despite early injection timing of 165 crank angles before top dead centre, indicating a significant influence of in-cylinder flow motion on the spatial distribution of hydrogen-air mixtures.

Study on explosion characteristics of hydrogen in a sudden expansion pipe

ABSTRACT As a promising green energy source, the risk of hydrogen explosion cannot be ignored. In the industrial environment, the connection between factories and different regions may form a mutated structural space. This structure may have a significant impact on the consequences of hydrogen accidents, which is worthy of study. In this research, the simulation method is used to analyze the explosion and combustion process of hydrogen flame propagation in a sudden expansion pipe, which focuses on the impact of the cross-section ratio. The results indicate that due to the influence of Rayleigh-Taylor instability, the flow field in the dead zone near the sudden expansion structure can be unstable and generate vortices, which will significantly change the development of gas combustion and explosion. Under the extrusion and entrainment of vortices, the flame front structure is changed from finger-like flame to mushroom-like flame. When the flame enters the downstream pipe section through the expansion port, the impact of vortices on the flame propagation velocity is changed from suppression to excitation, and the excitation effect is closely related to the cross-section ratio. At the cross-section ratio μ = 3, the maximum flame propagation velocity and maximum pressure growth rate of the downstream pipe are 198.7 m/s and 53.12 MPa/s, respectively, which are 68.8% and 151.4% times higher than those in the case of the cross-section ratio μ = 1.5. There is a remarkable impact of sudden expansion structure on flame propagation and explosion overpressure.

Direct numerical simulations of methane, ammonia-hydrogen and hydrogen turbulent premixed flames

Three turbulent premixed flames with the same unstretched laminar flame speeds and thicknesses are analyzed and compared using 3D Direct Numerical Simulation (DNS) in a slot burner configuration at atmospheric conditions: a CH4/air flame at ϕ=1, an NH3−H2/air (46% vol. H2) at ϕ=1 and an H2/air flame at ϕ=0.45.While both stoichiometric methane and ammonia-hydrogen flames behave similarly, the lean hydrogen flame brush is twice as short with less flame surface, and exhibits significant alteration of its local flame structure: for the H2 flame, the thickness of flamelets decreases significantly while burning rates increase drastically. This is observed for flame elements convexly curved with respect to fresh reactants (positive curvature) because of preferential diffusion, where thermo-diffusive instabilities generate long-tail structures that continuously head toward the fresh gases, and also for near-flat flame elements because of local strain effects. Opposite behaviors are observed for flame elements concavely curved (negative curvature) where fuel depletion caused by unfocusing of hydrogen and strain effects are unable to increase the consumption, even leading to near-extinction of the flame. The turbulent methane and ammonia-hydrogen flames studied in this work do not exhibit the instabilities seen in the pure hydrogen flame. However, local flame-flame interactions are observed in the cusp regions of all three flames, which significantly increase the displacement speed as well as flame surface destruction.A 1D-3D comparison indicates that strain effects in regions of low curvature and preferential diffusion effects in regions of positive curvature can be modeled using counterflow flamelets to account for stretch effects on the turbulent hydrogen flame propagation in addition to turbulent wrinkling.

Simulation of Hydrogen-Air-Diluents Mixture Combustion in an Acceleration Tube with FlameFoam Solver

During a severe accident in a nuclear power plant, hydrogen can be generated, leading to risks of possible deflagration and containment integrity failure. To manage severe accidents, great experimental, analytical, and benchmarking efforts are being made to understand combustible gas distribution, deflagration, and detonation processes. In one of the benchmarks—SARNET H2—flame acceleration due to obstacle-induced turbulence was investigated in the ENACCEF facility. The turbulent combustion problem is overly complex because it involves coupling between fluid dynamics, mass/heat transfer, and chemistry. There are still unknowns in understanding the mechanisms of turbulent flame propagation, therefore many methods in interpreting combustion and turbulent speed are present. Based on SARNET H2 benchmark results, a two-dimensional computational fluid dynamics simulation of turbulent hydrogen flame propagation in the ENACCEF facility was performed. Four combustible mixtures with different diluents concentrations were considered—13% H2 and 0%/10%/20%/30% of diluents in air. The aim of this numerical simulation was to validate the custom-built turbulent combustion OpenFOAM solver based on the progress variable model—flameFoam. Furthermore, another objective was to perform parametric analysis in relation to turbulent speed correlations and turbulence models and interpret the k-ω SST model blending function F1 behavior during the combustion process. The obtained results show that in the simulated case all three turbulent speed correlations behave similarly and can be used to reproduce observable flame speed; also, the k-ε model provides more accurate results than the k-ω SST turbulence model. It is shown in the paper that the k-ω SST model misinterprets the sudden parameter gradients resulting from turbulent combustion.

Flame structures and ignition thresholds of hydrogen jets containing sodium mist under various gas concentrations

ABSTRACT Non-premixed combustion of hydrogen jets containing sodium vapor and mist reduces threats to reactor containment integrity in sodium-cooled fast reactors (SFRs) because it gradually consumes hydrogen gas generated mainly by a reaction between sodium and concrete. Previous studies have been limited to experimentally determining ignition thresholds on the jet temperature and the sodium concentration under specific gas concentrations. In this study, ignition experiments on hydrogen jets containing sodium mist were carried out at a specific jet temperature and sodium concentration under various gas concentration conditions (1–15 vol% hydrogen and 3–21 vol% oxygen). As a result, a stable sodium flame was observed in the jet and then formed a lifted hydrogen flame from a fuel nozzle outlet. An attached hydrogen flame on the outlet was also formed under high hydrogen concentration conditions. These flame structures seemed to be attributed to hydrogen flame propagation, which depends on the hydrogen concentration, jet temperature, and jet velocity. Additionally, the experimental results revealed ignition thresholds on the gas concentration and indicated a flammable region where the hydrogen-sodium jet combustion was more advantageous than an explosive premixed hydrogen combustion. Our study will enable the advancement of safety assessment technology in SRFs.

Effect of channel geometry and porous coverage on flame acceleration in hydrogen–air mixture

Flame propagation in a hydrogen–air mixture in the presence of porous materials was investigated experimentally in channels with different dimensions and cross–sections. In this study, experiments were performed in a rectangular channel with one or two walls covered with porous material to study flame propagation in stoichiometric hydrogen–air mixtures at room temperature and atmospheric pressure. Depending on the channel configuration, the porous coating of the internal walls ranged from 1/4 to 1/2 of the channel area. Four types of polyurethane foam with a number of pores per inch (PPI) ranging from 10 to 80 were used to cover the channel walls. Flame propagation was visualized using a Schlieren device and high-speed camera. The largest flame acceleration in the porous channel relative to the solid channel was observed in the 20×20mm channel. The ratio of the velocities in the porous channel to the velocity in the solid channels was 6–7 for the porous material with the largest (2.5mm) pores. In the case of a 10×10mm channel, the flame velocity in the porous channel was higher than the flame velocity in the solid channel after 350mm only when using porous coatings with 2.5mm and 1.3mm pores. When using a porous coating with smaller pores the flame velocity was lower than in the solid channel. Schlieren images show different stages of flame propagation from a turbulent flame to a supersonic flame with shock waves.

Characterization of unconventional hydrogen flame propagation in narrow gaps.

The physical limits of the unconventional flame propagation regimes recently discovered [Veiga-Lopez et al., Phys. Rev. Lett. 124, 174501 (2020)PRLTAO0031-900710.1103/PhysRevLett.124.174501] are analyzed. These regimes appear in combustible gaseous mixtures approaching the lean quenching limit of hydrogen-air flames in narrow gaps. They are characterized by a split of the flame front into a dendritic and a bifurcating set of flame cells separated by nonburned material. A feature selection analysis utilizing dimensionless numbers is applied to reveal the most significant parameters governing the separation between unconventional and traditional flame propagation regimes. It is concluded that (a) the outbreak of unconventional propagation is mostly due to heat losses, (b) the phenomenon is governed by the Peclet number and only appears in thin channels, and (c) the Lewis number does not determine the propagation regime. Additionally, an equation describing the optimal border of the unconventional regime is derived from experiments.

Numerical Simulation of the Influence of Vent Conditions on Hydrogen Flame Propagation

ABSTRACT In order to reduce the damage caused by a gas explosion in a ventilation duct, a Large Eddy Simulation (LES) model was used to simulate the hydrogen/air explosion process in the ventilation duct under different side vent. The results show that in the process of flame propagation, the large size side vent near the ignition end produces a larger discharge effect, which causes more serious distortion and longer time for the flame front passing through the side vents. The influence mechanism of the side vent on the flame propagation is different at different stages of flame propagation. When the flame front is behind the side vent, the positive flow field traction on the flame front increases the contact area between the flame front and unburned gas, and thus accelerates the flame propagation. When the side vent is 1 m away from the ignition end, with the side vent diameter increasing from 40 mm to 80 mm, the peak flame propagation speed increases by 19.02% before the flame reaches the vent. When the flame front passes through the side vent, the exhaust of the side vent and the disturbance of the vertical flow field can restrain the flame propagation. However, when the flame front is in front of the vent, the synergistic effect between turbulent vortex and reverse flow field causes the flame propagation speed to fluctuate greatly. The influence of the side vent size on the explosion relief effect is restricted by the side vent position. When the side vent is located in the pressure rising section, the pressure relief effect of the side vents with different sizes is very great and is almost unaffected by the size of the side vent. When the side vent is 1 m away from the ignition end, the peak overpressure in the tube decreased by 50.75%, 52.88% and 55.43%, respectively, when the diameter of the side vent was 40 mm, 60 mm and 80 mm. On the contrary, when the side vent is outside the pressure rising section, the pressure relief effect of the side vent will be weakened, and vents with different sizes have a great alteration to pressure relief effect. For the side vent being 5 m from the ignition end, the peak overpressure in the tube decreased by 4.49%, 13.41%, and 35.51%, respectively, when the diameter of the side vent was 40 mm, 60 mm, and 80 mm.

Flame propagation in a straight and 90-degree bend pipe for premixed hydrogen/air and methane/air

This paper reports on the experimental and numerical analyses towards understanding the flame propagation in hydrogen and methane explosions, with the effect of pipe configurations. The experimental works were performed in a straight and 90-degree bend pipe with the length to diameter ratio, L/D is 51. FLACs code simulation was adopted to investigate the dynamic of flame behaviour in both pipe configurations. From the results, it was observed that the presence of 90-degree bend enhances the explosion severity by a factor of 1.03-3.58 as compared to that of the straight pipe. Based on the simulation analysis, the compression effect at the bending region and at the end of the pipe plays an important role to attenuate the burning rate, which results to the increase of the pressure wave. Furthermore, compression effect is the main mechanism for the flame to propagate and potentially lead to a catastrophic explosion.

Numerical Simulation on the Influence of Pipe Section Size on Hydrogen Flame Propagation Process in Closed Pipe

ABSTRACT In order to reveal the influence of pipe section size effect on hydrogen detonation characteristics, Large Eddy Simulation (LES) model was used to numerically investigate the hydrogen/air detonation process in closed space of different size. The results show that in the process of flame propagation, the reflection frequency of the shear wave produced by the small cross-section pipeline is higher, which increases the probability of the tulip flame and flame structure distortion. For a 6-m-long square-closed pipe, the influence mechanism of the cross-section size on the flame propagation is different at different stages of flame propagation. In the early stage of flame propagation (that is, in the pipe segment 0–0.8 m away from the ignition end), the internal friction caused by the friction between the flame and the tube wall increases the turbulent intensity of the flammable cloud and accelerates the flame propagation. In a confined space with the cross-section edge length of 100 mm, the maximum flame propagation speed is 176.7 m/s, which is 48.5% faster than that in the confined space with the cross-section edge length of 250 mm. In the middle of the flame propagation (that is, in the pipe segment 0.8-5m away from the ignition end), the flame area increases sharply during combustion and releases more energy. Therefore, larger vortices are formed, and the flame propagation is accelerated by turbulence efficiency. The maximum flame propagation speed can reach 500 m/s when the cross-section edge length is 250 mm, which is 140.8% faster than that when the cross-section edge length is 100 mm. In the later stage of flame propagation (that is, in the pipe segment 0.8–5 m away from the ignition end), the pressure in the pipe increases sharply, and the reflected wave from the pipe end produces great resistance to the propagation of the flame front, and the flame propagation speed decreases sharply in the pipeline of various cross-sections.

Hydrogen flame propagation inside an obstructed chamber

Hydrogen is recognized as a most dangerous gas due to high ignition rate and flame speed. In this work, numerical simulations have been performed to simulate the flow pattern and flame deflagration of hydrogen gas inside a confined chamber with different obstacles. This study tries to disclose the transient progress of flame to define the key actual factors affecting flow feature and flame propagation. In order to simulate hydrogen flame deflagration the limited obstacle channel, a three-dimensional model is established and large eddy simulation (LES) method is applied for the simulation of the model. The impacts of obstacle geometry on the pressure distribution are carefully examined. Obtained results show that overpressure inside the confined channel significantly increase within 0.3–0.6 ms. In this work, comprehensive reliable correlation for prediction of the pressure inside the confined channel is present. Our findings clearly demonstrates that velocity-density coefficient plays significant role the pressure distribution inside the model.

Expanding hydrogen–air flames over the heat absorbing substrate

Acceleration of hemispherical gas flames depends both on the composition of the gas mixture and on the boundary conditions. Experimental data on the hemispherical flame propagation in hydrogen–air mixtures with a hydrogen content of 15% were obtained at the ignition with an energy of 5 J. The flame propagates at atmospheric pressure over a solid aluminium wall or a layer of steel wool. To visualize the flame propagation, an ir InfraTec ImageIR 8320 camera with spectral range 2–5.7 µm was used. It was found that the flame over the layer of steel wool propagates 1.8 to 2.5 times more slowly than that over the surface of an aluminium wall. Calculation of heat absorption in the steel wool layer shows that the heat losses due to the absorption are the main phenomenon causing the flame front speed reduction, which was observed in the experiments. Additionally, the speed of the flame was affected by the absorption of oxygen and the release of heat during the oxidation of the steel wool, as well as by the roughness of the layer of steel wool.

Prediction of hydrogen flame propagation in a channel with exit contraction

The propagation of a flame front in a homogeneous and initially quiescent hydrogen-air mixture in a channel with exit contraction is numerically analyzed by means of Computational Fluid Dynamics. For the given configuration, the compressibility effects are important, the average pressure increases in time due to the exit contraction, and pressure waves occur, which affect the flame propagation. Flowturbulence is modelled by the Realizable k-e model. In modelling combustion, turbulence-chemistry interactions are neglected. Predictions are compared with the measurements for evolution of the flame shape, propagation speed and pressure. It is observed that the flame propagation speed, and, thus, the rate of pressure increase are over-predicted by the present approach. Still, a fair qualitative agreementto measurements is observed.

Regimes of High-Speed Hydrogen Flame Propagation in Channels: Classification and Criteria of Realization

ABSTRACTThe paper discusses high-speed combustion regimes inside confined volumes on the example of hydrogen-containing gaseous mixtures. A unified description of high-speed combustion regimes is proposed on the basis of systematic analysis of flame evolution. The stage of the so-called chocked flame is shown to be the necessary step on the way to the formation of high-speed flames and detonation. Further flame evolution could proceed in different ways including quasi-steady transonic flame, deflagration-to-detonation transition or sequential auto-ignitions ahead of the primary flame front. The paper proposes a method for estimation of probability of different devastating high-speed combustion regimes, including detonation, relying only on the mixture composition and its initial thermodynamic state. Herewith, since in the process of flame acceleration inside confined volume the compression is yielded by the series of compression waves, the initial state and the state corresponded to the high-speed stage of flame propagation could be related via Hugoniot adiabat. The conducted parametric study of hydrogen flames allowed obtaining criteria according to which the possibility of one or another regime of high-speed combustion can be estimated. The obtained results correlate well with the experimental data on the different high-speed regimes taking place in hydrogen–air mixtures, stoichiometric hydrogen–oxygen, and equimolar hydrogen–oxygen mixtures. It is shown that to assess the probability of detonation onset in obstructed channels, the proposed technique is not enough and one should apply it together with the geometrical criteria. Results obtained with combined technique agree well with the available data on the criteria of deflagration-to-detonation transition in hydrogen–air mixtures of different compounds.

Propagation of a hemispherical flame over a heat-absorbing surface

This paper presents an experimental study of hydrogen–air flame front dynamics and propagation over a heat-absorbing surface. Experimental data on the hemispherical flame propagation in hydrogen–air mixture with hydrogen content of 15% in 4.5 m3 cylindrical volume were obtained at the ignition at the centre of the bottom with energy of 5 J. The flame propagates at atmospheric pressure over a solid aluminium wall or a layer of steel wool. The flame acceleration dynamics were compared at hemispherical and finger flame stages. It was found that in a mixture with a hydrogen content of 15% the flame over the layer of steel wool propagates 2.5 times more slowly than that over the surface of an aluminium wall. Calculation of heat absorption in the steel wool layer shows that the heat losses due to the absorption are the main phenomenon causing the suppression of Darrieus–Landau instability and flame front speed reduction, which was observed in the experiments. Experimental results are compared with analytical model of finger flame propagation from literature.

Detonation suppression in hydrogen–air mixtures using porous coatings on the walls

We considered the problem of detonation suppression and weakening of blast wave effects occurring during the combustion of hydrogen–air mixtures in confined spaces. The gasdynamic processes during combustion of hydrogen, an alternative environmentally friendly fuel, were also considered. Detonation decay and flame propagation in hydrogen–air mixtures were experimentally investigated in rectangular cross-section channels with solid walls and two types of porous coatings: steel wool and polyurethane foam. Shock wave pressure dynamics inside the section with porous coating were studied using pressure sensors; flame front propagation was studied using photodiodes and high-speed camera visualization. For all mixtures, the detonation wave formed before entering the section with porous coating. For both porous materials, the steady detonation wave decoupled in the porous section of the channel into a shock wave and flame front propagating with a velocity around the Chapman–Jouguet acoustic velocity. By the end of the porous section, shock wave pressure reductions of 70 and 85% were achieved for the polyurethane foam and steel wool, respectively. The dependence of the flame velocity on the mixture composition (equivalence ratio) is presented.

Influence of a heat-absorbing surface on the propagation of a hemispherical flame

Gaseous explosions lead to severe damage. Elimination or mitigation of damage is a key problem in industrial safety. This paper presents an experimental study of hydrogen–air flame front dynamics and flame suppression by means of a surface coating. The experimental stand is a metal frame that confines the cylindrical envelope to a diameter of 1.5 m and 2.4 m in height of high-density 100 μm thick polyethylene. Experimental data on the hemispherical flame propagation in hydrogen–air mixtures with a hydrogen content of 15% were obtained at the initiation of combustion with an energy of 5 J. The flame propagates at atmospheric pressure over a solid aluminium wall or a layer of steel wool. To measure the flame propagation speed, an infrared InfraTec ImageIR 8320 camera with spectral range 2–5.7 μm was used. The flame acceleration dynamics were compared at flame radii up to 0.4 m. It was found that in a mixture with a hydrogen content of 15% the flame over the layer of steel wool propagates 2.5 times more slowly than that over the surface of an aluminium wall. The steel wool was investigated by scanning electron microscope using an energy dispersive analysis system before and after the passing of the flame front. The structure and chemical composition of the metal wool was studied. Calculation of heat absorption in the steel wool layer shows that the heat losses due to the absorption are the main phenomenon causing the flame front speed reduction, which was observed in the experiments. Additionally, the speed of the flame was affected by the absorption of oxygen and the release of heat during the oxidation of the steel wool, as well as by the roughness of the layer of steel wool.

Effectiveness of diluent gases on hydrogen flame propagation in tee pipe (part II) – Influence of tee junction position

Gas explosions in obstructed vessels have been investigated for many years. However, the flame acceleration mechanism of enriched-hydrogen fuels with diluents in the piping system has received little systematic study in the literature. This particular study aimed to analyse the flame front mechanism of hydrogen-diluents/air explosion inside the pipe by considering the influence of tee junction distance from the ignition points. The tests were performed using H2/diluents-air at different concentrations and ignition positions, in two different tee junction pipe configurations. From the results, the worst case of explosion severity was found in 95% H2–2.5% Ar–2.5% N2/air for all ignition positions. In general, if ignition happened at the tee junction, the overpressure and rate of pressure rise profiles showed almost a similar trend on both configurations. Similar trend was also observed for the flame flow characteristic analysis. Overall, it was clearly demonstrated that a shorter distance between ignition point and obstacles resulted in higher explosion severity.

Effect of inhibitor gases on hydrogen flame propagation in a confined tee pipe (Part I)

Hydrogen is a promising fuel for the future. In recent years it has been successfully utilised in industries, particularly in refineries and petrochemicals. In previous studies, the effect of inhibitors on hydrogen explosion behaviour has been investigated in different systems, yet only scarce data are available. Therefore, experimental study is carried out to investigate the effects of argon, nitrogen and carbon dioxide on hydrogen/air explosion in a branched pipe configuration. The fuel/air mixtures were ignited at three different ignition positions, A, B and D. The results show that, when ignited at the furthest distance (position A), the tee junction area is most vulnerable to the critical pressure impact of gas explosion. However, no similar trend was observed at the other ignition positions. In addition, mixtures with the compositions 95% H2–2.5% Ar–2.5% N2/air, 95% H2–5% N2/air, H2/air and 95% H2–5% Ar/air showed higher risks due to the higher diffusivity ratio and the associated rate of pressure rise. This phenomenon is highlighted in the discussion part of this paper. The results show that mixtures with CO2 lead to lower severity than other compositions (∼50% reduction), as the average recorded maximum flame speed for this particular mixture was lower at all of the ignition points. This suggests that the effectiveness of the inhibitors should be in the order of Ar<N2<CO2.

Nature of the concentration limits of flame propagation in hydrogen–air mixtures

A necessary condition associated with fuel caloricity and chain ignition temperature is formulated for flame propagation. The lower temperature limit of flammability as a function of mixture composition is obtained on the basis of a branched-chain mechanism corresponding to terms of the origin of a wave in a cold mixture. The rotary points of this limit that can be associated with the concentration limits of flame propagation are determined. This qualitative picture of wave formation is confirmed by numerical calculations of the structure of a wave during its propagation. Experimental data on the ignition of mixtures of different composition is compared to the calculated data on limits.