Premixed Hydrogen Air Mixture Research Articles

Flame propagation characteristics of non-uniform premixed hydrogen-air mixtures explosion in a pipeline

Hydrogen, as a clean and promising energy carrier, is considered a viable alternative fuel for the future. However, accidental hydrogen leakages and explosions pose considerable safety concerns in hydrogen energy applications and process industries. This study investigated the flame propagation characteristics of non-uniform premixed hydrogen-air mixtures in a rectangular closed duct with a length-to-diameter ratio of 5.78, taking into account varying equivalence ratios and diffusion times. First, numerical simulations using FLUENT were conducted to model the hydrogen diffusion process in a confined space, determining the hydrogen concentration evolution post-leakage. After approximately 200 s of diffusion, the premixed hydrogen-air mixtures remained in a state of homogeneous mixing, with the hydrogen concentration stabilizing at approximately 1.25 × 10−2 kg/m³. Subsequently, experimental observations were performed using a visual pipeline system, high-speed photography, and flame structure analysis. These experiments examined inhomogeneous hydrogen-air mixtures under seven different equivalence ratios and five different diffusion time conditions. The effects of equivalence ratios and diffusion times on flame propagation characteristics were analyzed. The results revealed that equivalence ratio significantly influenced the flame structure, with higher equivalence ratios producing more pronounced flame surface wrinkles. However, the typical evolution of the tulip flame remained consistent. At a constant equivalence ratio, flame propagation velocity exhibited an initial increase followed by a decrease over time. These findings demonstrate that turbulence intensity accelerated the flame propagation in non-uniform premixed hydrogen-air mixtures. This study underscores the importance of further research on hydrogen safety fundamentals and technologies to develop comprehensive safety standards.

Spontaneous ignition and flame propagation in hydrogen/methane wrinkled laminar flames at reheat conditions: Effect of pressure and hydrogen fraction

Direct numerical simulations (DNS) with detailed chemical kinetics and chemical explosive mode analysis (CEMA) are performed to investigate the effect of operating pressure and hydrogen–methane blending on laminar wrinkled flames at reheat combustion conditions. Building upon previous three-dimensional DNS datasets of reheat hydrogen flames, the present work considers two-dimensional configurations that render computationally-feasible simulations of high-pressure combustion and significantly more complex hydrocarbon chemistry. A geometrically-simplified representation of the reheat combustor is adopted and the thermodynamic states considered in the simulations are carefully chosen to mimic gas turbine operation conditions, which are constrained to achieve a target flame position and flame temperature. The two-dimensional DNS results are first assessed against the available three-dimensional data and then analyzed to extract the main trends exhibited by the reheat combustion process with respect to the fraction of the fuel consumed by spontaneous ignition and flame propagation, for increasing pressures and hydrogen fractions. Due to significant differences in the characteristic features of the velocity and thermal boundary layers between the three-dimensional and the two-dimensional configurations, a different global flame shape is observed (V-shaped flame vs W-shape flame) together with a ∼10% bias in the fraction of fuel consumed by spontaneous ignition. Crucially, results from the scaling studies reveal very clear trends with a significant decrease in the fuel consumption by spontaneous ignition for increasing pressures and hydrogen fractions, at the conditions investigated. Finally, this study highlights the important role that first-principle direct numerical simulations, even when conducted in computationally affordable two-dimensional simplified geometrical configurations, can play in obtaining detailed insights and quantitative trends useful for the characterization of complex reactive flows.Novelty and significanceThe reheat burners of the two-stage sequential gas turbine combustors are designed to stabilize flames primarily due to spontaneous ignition. However, prior numerical simulations revealed the presence of an additional assisted-ignition mode aided by recirculation zones. In this study, we used practically feasible two-dimensional direct numerical simulations of premixed hydrogen–air mixtures under lean conditions to quantify the roles of the two flame stabilization modes at different operating pressures. Achieving fundamental understanding of the effect of pressure and hydrogen fraction on flame stabilization is crucial to the development of two-stage sequential combustion and the present two-dimensional calculations provide important new insights. We show that at high pressures, both modes consume fuel to a similar extent. We also investigated the effect of methane blending on flame stabilization. This study also discusses how two-dimensional direct numerical simulations can be leveraged in the design optimization of reheat burners at low technology readiness levels.

Experimental Studies on the Explosion Behaviors of Premixed Hydrogen-Air Mixtures in a Narrow Channel.

Premixed hydrogen–air explosion experiments were carried out in a 1000 mm × 50 mm × 10 mm half-open narrow channel, concerning with the influences of equivalence ratio and ignition position on explosion behaviors. Experimental phenomena were different from explosion in large space. The results indicated that when ignited at the closed end of the channel, three overpressure peaks appeared, caused by the rupture of the film, Helmholtz Oscillation, and the flame-acoustic interaction, respectively. As the equivalence ratio of the hydrogen–air mixtures varied from 0.6 to 1.6, the peak overpressure first increased and then decreased. The maximum peak overpressure occurred at ϕ = 1.2. The hydrogen flame would develop into the plane tulip structure without the influence of the end wall. With the ignition position moved to the open end, overpressure wave and flame oscillated significantly. Compared with other ignition positions, the minimum value of Pmax was obtained at IP950. Based on the explosion behaviors in the narrow channel, it was concluded that the closer the ignition was to the open end, the easier the oscillation was to be formed, the smaller the explosion hazard was.

Oblique Detonation Wave Control with O3 and H2O2 Sensitization in Hypersonic Flow

This numerical study investigates the effects of adding a small amount of ignition promoters for controlling the wedge-induced oblique shock wave (OSW) to oblique detonation wave (ODW) transition in a premixed hydrogen–air mixture at hypersonic speeds. The time-dependent two-dimensional compressible Euler equations for multiple thermally perfect species with a reactive source term are solved using adaptive mesh refinement and detailed chemical kinetics. The wedge with a fixed angle of 26° exhibits abrupt to smooth transitions for freestream Mach numbers 7–9 (speeds 2.8–3.2 km/s) at a pressure of 20 kPa and a temperature of 300 K. The small amount (1000 PPM by vol.) of H2O2 and O3 is found to be effective at significantly reducing the initiation length for the oblique detonation transition for all Mach numbers, which suggests a practical approach to increase the operating flight range for oblique detonation wave engine with a finite length wedge. At Mach number 8, the abrupt OSW to ODW transition turns towards a smooth transition with a small amount of H2O2 and O3 addition. Comparatively, O3 addition was found to be effective in reducing the ODW initiation length by promoting reactivity behind even a weaker oblique shock at low Mach number 7, for abrupt transition, while H2O2 addition was more effective than O3 at high Mach numbers 8 and 9, during a smooth transition. The maximum 73% and 80% reduction in initiation length of ODW was observed with 10,000 PPM H2O2 and O3 addition, respectively, during an abrupt OSW to ODW transition at Mach 7.

Explosion venting hazards of temperature effects and pressure characteristics for premixed hydrogen-air mixtures in a spherical container

Explosion venting can effectively reduce the damage effect caused by accidental gas explosions during industrial production. Determining how to eliminate secondary disasters during an explosion is therefore a challenge that can be addressed through the application of explosion venting. This paper discussed the temperature effects and pressure characteristics of premixed hydrogen-air mixtures (PHAMs) in the safety design of vent devices. Tests were designed for the behaviours of hydrogen explosion venting under different bursting pressures. The variation in pressure was controlled by a pressure sensor on the inner wall of a spherical container, and the safety design was assessed theoretically. A high-speed photography device and infrared thermal imaging device were employed to capture the real-time flame field and temperature field during the hydrogen explosion venting process. A comparison of the dimensions of both fields revealed that the high-temperature area was smaller than the flame area. An evaluation of the dynamic hazard of hydrogen explosion venting was then performed based upon pressure characteristics and temperature effects. It was concluded that the prediction results of explosion venting diameter is crucial for industrial safety design.

К 125-летию со дня рождения лауреата Нобелевской премии академика Николая Николаевича Семенова Цепной механизм воздействия добавок дихлордифторметана на горение водорода и метана в кислороде и воздухе

The effect of difluorodichloromethane additives on spark initiated combustion of hydrogen and methane in air and oxygen at atmospheric and reduced pressures was investigated. It has been found that the ignition concentration limit of the premixed hydrogen-air mixture in the presence of difluorodichloromethane at 1 atm exceeds 10%, while it has been shown for the first time that the ignition limit of the premixed methane-air mixture is 1% of difluorodichloromethane, which is thereby the most effective methane combustion inhibitor. This also means that the active combustion centers of hydrogen and methane, which determine the development of combustion, have a different chemical nature. Thus, the reaction including a difluorodichloromethane molecule resulting in the formation of HF (v = 2.3) during methane combustion should include a step involving the active methane combustion intermediate. Using hyperspectrometers of the visible and near-infrared ranges in the products of the oxidation reactions of hydrogen and methane in the presence of difluorodichloromethane, vibrationally excited HF molecules (v = 2.3) were first discovered. For the first time, it was found that HF molecules (v = 3) during methane combustion are formed at the moment when the maximum rate of chemical conversion is achieved, that is, reactions involving inhibitor molecules compete with the process of development of reaction chains. Keywords: chain burning, inhibition, methane, hydrogen, dichlorodifluoromethane, hyperspectrometer, high-speed color filming, radicals, excited particles.

Numerical investigation of the operating process in an annular detonation chamber of low diameter

Burning in unsteady rotating detonation waves is one of the ways of operating process organization in future aircrafts engines. However, a minimum combustion chamber diameter in which the unsteady rotating detonation process can be actualized is a significant limitation of such engines. A possibility of hydrogen-air mixture burning in an annular chamber with a diameter of 78 mm is shown in this paper using methods of three-dimensional computational fluid dynamics. The geometry of the chamber and the modes of its operation were selected, and the influence of the total pressure of air at the inlet was studied. At the pressure of 32 bar, the mass flux in the critical section of the aerodynamic nozzle was 7349.2 kg/(s·m2). It is shown that in a rotating reference frame in which the detonation wave is at rest, streamlines come to the wavefront almost at right angles. This fact allows making simple estimates of the fresh mixture parameters in front of the wave. Comparison with the detonation parameters in a premixed hydrogen-air mixture in the tube showed that the velocity of the detonation wave in the chamber is 0.65-0.74 of the Chapman-Jouguet detonation velocity, and its height is approximately equal to the characteristic size of the cellular structure of the detonation.

Simulation Study on the Explosion Characteristics of Premixed Hydrogen-air Mixtures

In order to study the explosion characteristics of hydrogen in the air and the influence of concentration on the explosion characteristics of hydrogen, the fire and explosion of small-scale hydrogen cloud with different concentration are simulated. The distribution of explosion characteristic parameters, such as overpressure, temperature and velocity, as well as the change rule with time, are obtained. The explosion characteristic parameters in horizontal and vertical directions are discussed The results show that under the influence of the ground and gravity, oblate elliptical fireballs with horizontal direction larger than vertical direction are formed in the early stage of ignition development. With the continuous combustion, they develop into hemispherical fireballs with the diameter of the ground and spread around. The influence of concentration on the law of fire and explosion is studied, and the maximum overpressure under different concentration is obtained, which provides support for building the fire field model of liquid hydrogen leakage. It is also of great significance for the evaluation of explosion hazards and the design of corresponding protection system.

Experimental Studies on Recuperative Micro-Combustion Chamber for 1 W Gas Turbine Engine

Experimental investigations are carried out on recuperative micro-combustion chamber for 1 W gas turbine engine. The combustion chamber under consideration is of Swiss Roll type. The micro-combustion chamber is operated at ultra-lean premixed hydrogen air mixture. The central reaction zone volume is 3 mm × 4 mm × 5 mm, with reactant and product passages coiled around. The reaction cross-sectional passage has inlet of 0.6 mm × 5 mm, while the product cross-sectional passage is of 1.5 mm × 5 mm. The heat loss due to small size is virtually eliminated as reactant passages are coiled around product passage. The reactants are thus pre-heated by the heat loss to the walls by the product of combustion. Velocity reduction is achieved by dumping the reactants in the central reaction zone. The cross-sectional area is increased of reactant passage from inlet to central reaction zone, thereby reducing the velocity in the reaction passage. This increases the residence time and helps in achieving stable combustion. The micro-combustor is experimented by varying the equivalence ratios from very lean condition of 0.12 to near stoichiometry. The results suggest that the combustor performance deteriorates above equivalence ratio of 0.52, in terms of NOx emissions. The generation of hot spots becomes evident after equivalence ratio of 0.43. Above equivalence ratio of 0.43, the flame front extends to the product passage, giving very high temperatures, leading to reactants being heated to self-ignition temperature.

Numerical simulations of inert and reactive highly underexpanded jets

In this study, the high-resolution numerical simulations of the two-dimensional (2D) multi-component inert and reactive highly underexpanded jets are conducted to quantify the influences of the injected gas mixture properties on the flow structure. First, the gas mixture with the specified species mass fractions is imposed to exhaust into the quiescent air with a Mach number of 1.0, of which the specific heat ratios (γe) range from 1.3 to 1.6. Our results indicate that the larger γe yields a relatively shorter and thinner jet core under the same inlet pressure ratio due to the sound speed increasing. Next, we focus on the chemical reaction effects on the jets with a premixed hydrogen–air mixture injection. The results reveal that the shock-induced combustion develops into a detonation, inducing numerous vortices behind the combustion wave, while the combustion in the mixing layer cannot be preserved due to the instability of the supersonic shearing. During the detonation process, the increasing pressure accompanied by the heat release forces the Riemann wave to move upstream compared with the inert one. The violent detonation periodically propagates between the two jet triple points. The detonation collision leads to the intersection of their slip lines, which causes distinct vortex formation. In addition, the main frequencies, corresponding to the Riemann wave movement, the oscillation of the shock-induced ignition positions, the periodical propagation of the detonation, and the collision of the detonation triple points, are explored to explain the unsteady process of the reactive highly underexpanded jet.

Dynamic hazard evaluation of explosion severity for premixed hydrogen–air mixtures in a spherical pressure vessel

To evaluate dynamic explosion severity levels of premixed hydrogen–air mixtures, pressure sensors were used to test explosion pressure in a spherical pressure vessel (inner diameter: 0.34 m). ANSYS Fluent 19.0 three-dimensional software was used to simulate the explosion process. The results revealed that when the hydrogen volume fraction was 30 vol%, the explosion pressure and pressure rise rate reached maximum values at 1.0 atm. As the initial pressure increased, the explosion pressure, and pressure rise rate increased gradually. At initial pressures of 1.0, 1.2, 1.5, and 2.0 atm, the peak pressure levels were 0.85, 0.87, 0.92, and 0.99 MPa and the pressure rise rates were approximately 198, 241, 302, and 558 MPa/s, respectively. The initial pressure exerted greater effects on the pressure rise rate than did the explosion pressure. The simulation results were consistent with the experimental findings. Moreover, the simulation yielded multi-dimensional transient explosion parameters—such as turbulent kinetic energy distribution—that are difficult to obtain in experiments. The findings pertaining to the physical experiments, numerous simulations, and influence of initial pressure on the explosion reaction mechanism of premixed hydrogen–air mixtures were discussed.

Deflagration characteristics of hydrogen-air mixture beyond the original premixed fuel region

Explosions are a major type of accident resulting in casualties in the processing industry. Fires frequently occur after a gas explosion. The attenuation rule of the deflagration state beyond the original premixed gas fuel area is the basis for determining the hazard risk once a gas explosion occurs. In this study, a numerical simulation was conducted using a finite element computational code for the fluid dynamics to reveal the attenuation law of the deflagration process of hydrogen–air mixtures in a tube 10 m long with a 0.1 m × 0.1 m sized section with one end closed and the other end open. The combustion characteristics beyond the original premixed hydrogen–air mixture region were examined. The numerical method was verified and its results showed good agreement with the experimental results. It was found that the fuel region can extend and enlarge the area of the flame, and thus a flame beyond the original fuel occupied zone can clearly be seen, which can expand the area of the damage. The explosion state inside the premixed region has a significant effect on the flame area, and a higher explosion strength will shorten the explosion time and reduce the disturbance of the unburned fuel. It was also found that a fuel-rich mixture has a larger flame area beyond the original premixed hydrogen–air mixture. Under all conditions considered in this study, the flame in the hydrogen–air mixture is 1–2 times as long as the one in the hydrogen–air mixture in the initially occupied area.

Effect of the Wave Structure of the Flow in a Supersonic Combustor on Ignition and Flame Stabilization

Results of numerical and experimental investigations of a high-velocity flow in a plane channel with sudden expansion in the form of a backward-facing step, which is used for flame stabilization in a supersonic flow, are presented. The experiments are performed in the IT-302M high-enthalpy short-duration wind tunnel under the following test conditions: Mach number at the combustor entrance 2.8, Reynolds number 30 · 106 m−1, and total temperature T0 = 2000 K, i.e., close to flight conditions at M = 6. The numerical simulations are performed by solving full unsteady Reynolds-averaged Navier–Stokes equations supplemented with the k–ω SST turbulence model and a system of chemical kinetics including 38 forward and backward reactions of combustion of a hydrogen–air mixture. Three configurations of the backward-facing step are considered: straight step without preliminary actions on the flow, with preliminary compression, and with preliminary expansion of the flow. It is demonstrated that the backward-facing step configuration exerts a significant effect on the separation region size, pressure distribution, and temperature in the channel behind the step, which are the parameters determining self-ignition of the mixture. The computed results show that preliminary compression of the flow creates conditions for effective ignition of the mixture. As a result, it is possible to obtain ignition of a premixed hydrogen–air mixture and its stable combustion over the entire channel height.

Reproducing curvature effects due to differential diffusion in tabulated chemistry for premixed flames

Tabulated chemistry reduces the cost of modeling reacting flows by transporting fewer scalars. It also has the benefit of introducing fewer terms that must be closed for Reynolds-averaged Navier-Stokes and large eddy simulation modeling. In this study of premixed lean hydrogen–air flames, an extension to an existing tabulated chemistry model was developed by including differential diffusion and thermal diffusion of multiple species, using a mixture-averaged diffusion model. The transport equations for the tabulation variables (a progress variable and a mixture fraction-like variable) have been formulated, and curvature effects are intrinsic to the transport equations derived from first principles. The mathematical derivation is general, such that other fuel/air mixtures could be examined. The new tabulated chemistry model was then validated using a range of configurations, including flames with curvature, extinction, and hot spots. The model validation indicated excellent agreement with detailed chemistry simulations for lean premixed hydrogen–air mixtures. Conditional means of the progress variable source term were shown to agree well with detailed chemistry simulations of two-dimensional freely propagating flames and three-dimensional turbulent flames. Finally, progress variable source terms three times higher than one-dimensional flat flame results were identified and validated with detailed chemistry, and super-adiabatic hot spots were also predicted in the turbulent flame configuration.

Multi-stage nozzle-shape optimization for pulsed hydrogen–air detonation combustor

Thermal engines based on pressure gain combustion offer new opportunities to generate thrust with enhanced efficiency and relatively simple machinery. The sudden expansion of detonation products from a single-opening tube yields thrust, although this is suboptimal. In this article, we present the complete design optimization strategy for nozzles exposed to detonation pulses, combining unsteady Reynolds-averaged Navier–Stokes solvers with the accurate modeling of the combustion process. The parameterized shape of the nozzle is optimized using a differential evolution algorithm to maximize the force at the nozzle exhaust. The design of experiments begins with a first optimization considering steady-flow conditions, subsequently followed by a refined optimization for transient supersonic flow pulse. Finally, the optimized nozzle performance is assessed in three dimensions with unsteady Reynolds-averaged Navier–Stokes capturing the deflagration-to-detonation transition of a stoichiometric, premixed hydrogen–air mixture. The optimized nozzle can deliver 80% more thrust than a standard detonation tube and about 2% more than the optimized results assuming steady-flow operation. This study proposes a new multi-fidelity approach to optimize the design of nozzles exposed to transient operation, instead of the traditional methods proposed for steady-flow operation.

Interaction between heterogeneous and homogeneous reaction of premixed hydrogen–air mixture in a planar catalytic micro-combustor

Hetero-/homogeneous combustion of hydrogen–air mixture in a platinum-coated micro channel was studied by means of three-dimensional (3D) Computational Fluid Dynamics (CFD) model using detailed reaction mechanisms for homogeneous (gas phase) and heterogeneous (catalytic) reactions. The influence of heterogeneous reaction on homogeneous reaction was obtained by discussing the hetero-/homogeneous reaction characteristics, the process and competitiveness of heterogeneous reaction. Intense depletion of fuel and product formation near the inlet has shown that fuel absorption reaction was predominant whiles there was no homogeneous reaction. The free radicals from homogeneous reaction near the catalytic surface were drained, resulting in the interruption of chain reaction. The desorbed product as the third-body promoted the chain terminating reaction to deactivate free radical. The competitiveness value of heterogeneous reaction for capturing fuel was 14.85%, indicating a weak reaction intensity in the catalytic combustor. The heterogeneous reaction presented an inhibition effect on homogeneous reaction but greatly enhanced the combustion efficiency in the micro channel.

Prediction of Confined Flame Flashback Limits Using Boundary Layer Separation Theory

Premixed combustion is a common technology applied in modern gas turbine combustors to minimize nitrogen oxide emissions. However, early mixing of fuel and oxidizer opens up the possibility of flame flashback into the premixing section upstream of the combustion chamber. Especially, for highly reactive fuels, boundary layer flashback (BLF) is a serious challenge. For high preheating and burner surface temperatures, boundary layer flashback limits for burner stabilized flames converge to those of so-called confined flames, where the flame is stabilized inside the burner duct. Hence, the prediction of confined flashback limits is a highly technically relevant task. In this study, a predictive model for flashback limits of confined flames is developed for premixed hydrogen–air mixtures. As shown in earlier studies, confined flashback is initiated by boundary layer separation upstream of the flame tip. Hence, the flashback limit can be predicted identifying the minimum pressure rise upstream of a confined flame causing boundary layer separation. For this purpose, the criterion of Stratford is chosen which was originally developed for boundary layer separation in mere aerodynamic phenomena. It is shown in this paper that it can also be applied to near-wall combustion processes if the pressure rise upstream of the flame tip is modeled correctly. In order to determine the pressure rise, an expression for the turbulent burning velocity is derived including the effects of flame stretch and turbulence. A comparison of the predicted flashback limits and experimental data shows high prediction accuracy and wide applicability of the developed model.

Combined effects of obstacle position and equivalence ratio on overpressure of premixed hydrogen–air explosion

To investigate the combined effects of obstacle position and equivalence ratio on overpressure of premixed hydrogen–air explosion, an experimental study was performed in a 5 L duct with premixed hydrogen–air mixtures over a wide range of equivalence ratios, and along with the variation of a single obstacle position. In this paper, the equivalence ratios of the hydrogen–air mixtures were varied from 0.6 to 1.4, the obstacle was centrally located and was respectively 100, 200, 300 mm from the bottom end of the duct, the experiment without an obstacle was designed as a control experiment. For brevity, four configurations were defined according to the variation of the single obstacle position. The results indicated that the overpressure of premixed hydrogen–air explosion was closely related to the flame structure. The rise of dp/dt for hydrogen–air mixtures occurred when the flame started to feel the presence of the obstacle. The maximum overpressure could be observed at the moment that the two flames, generated from the gap between the obstacle and the inwall of the duct, just started to merge in the downstream region of the obstacle. It was also found that the venting overpressure might be barely observed under the combined effects of obstacle position and equivalence ratio. The rise time of dp/dt in a given configuration was gradually shortened with increasing equivalence ratio. Additionally, it was not simply a synergistic effect of obstacle position and equivalence ratio on peak overpressure. The peak overpressure increased with increasing obstacle position for lean hydrogen–air mixtures, and the maximum peak overpressure occurred in the downstream region of the farthest obstacle position. Interestingly, for the stoichiometric and rich mixtures, the peak overpressure reached the maximal when the obstacle was at the middle position. The occurrence of the maximum peak overpressure mainly depended on the maximum flame surface area within the duct.

On velocity and reactive scalar spectra in turbulent premixed flames

AbstractKinetic energy and reactive scalar spectra in turbulent premixed flames are studied from compressible three-dimensional direct numerical simulations (DNS) of a temporally evolving rectangular slot-jet premixed flame, a statistically one-dimensional configuration. The flames correspond to a lean premixed hydrogen–air mixture at an equivalence ratio of 0.7, preheated to 700 K and at 1 atm, and three DNS are considered with a fixed jet Reynolds number of 10 000 and a jet Damköhler number varying between 0.13 and 0.54. For the study of spectra, motivated by the need to account for density change, which can be locally strong in premixed flames, a new density-weighted definition for two-point velocity/scalar correlations is proposed. The density-weighted two-point correlation tensor retains the essential properties of its constant-density (incompressible) counterpart and recovers the density-weighted Reynolds stress tensor in the limit of zero separation. The density weighting also allows the derivation of balance equations for velocity and scalar spectrum functions in the wavenumber space that illuminate physics unique to combusting flows. Pressure–dilatation correlation is a source of kinetic energy at high wavenumbers and, analogously, reaction rate–scalar fluctuation correlation is a high-wavenumber source of scalar energy. These results are verified by the spectra constructed from the DNS data. The kinetic energy spectra show a distinct inertial range with a $\def \xmlpi #1{}\def \mathsfbi #1{\boldsymbol {\mathsf {#1}}}\let \le =\leqslant \let \leq =\leqslant \let \ge =\geqslant \let \geq =\geqslant \def \Pr {\mathit {Pr}}\def \Fr {\mathit {Fr}}\def \Rey {\mathit {Re}}-5/3$ scaling followed by a ‘diffusive–reactive’ range at higher wavenumbers. The exponential drop-off in this range shows a distinct inflection in the vicinity of the wavenumber corresponding to a laminar flame thickness, $\delta _L$, and this is attributed to the contribution from the pressure–dilatation term in the energy balance in wavenumber space. Likewise, a clear spike in spectra of major reactant species (hydrogen) arising from the reaction-rate term is observed at wavenumbers close to $\delta _L$. It appears that in the inertial range classical scaling laws for the spectra involving the Kolmogorov scale are applicable, but in the high-wavenumber range where chemical reactions have a strong signature the laminar flame thickness produces a better collapse. It is suggested that a full scaling should perhaps involve the Kolmogorov scale, laminar flame thickness, Damköhler number and Karlovitz number.

Studying the repetitive extinction-ignition dynamics for lean premixed hydrogen-air combustion in a heated microchannel

The mechanism of repetitive extinction-ignition dynamics for lean premixed hydrogen–air mixture is studied in a microchannel with prescribed wall temperature. In this dynamics, the reacting flow is affected by the wall temperature and leads to ignition near walls. The flame expands in both downstream and upstream directions until flame bifurcation occurs. Part of the flame which propagates towards inflow consumes all the unburned mixture along its way. As the flame reaches cold inflow mixture, it extinguishes due to the heat loss. Another part of flame consumes the unburned mixture in downstream until the flame is extinguished. Afterward, unburned mixture fills the tube again until it is reignited. The repetitive extinction-ignition dynamics can be classified in five phases, namely, initiation phase, ignition phase, propagation phase, weak reaction phase, and flowing phase. Three peaks were detected for hydrogen–air mixture combustion which all appears in propagation and weak reaction phases. In the remaining phases two peaks were present. Details of flow field indicate that bifurcation of flame is due to creation of recirculation zones formed close to the walls at the beginning of ignition phase. The recirculation zones grow and merge, until a boundary zone is created in flow field with zero velocity.