Large eddy simulation of premixed hydrogen/methane/air flame propagation in a closed duct

In this paper, large eddy simulation (LES) is performed to investigate the propagation characteristics of premixed hydrogen/methane/air flames in a closed duct. In LES, three stoichiometric hydrogen/methane/air mixtures with hydrogen fractions (volume fractions) of 0, 50% and 100% are used. The numerical results have been verified by comparison with experimental data. All stages of flame propagation that occurred in the experiment are reproduced qualitatively in LES. For fuel/air mixtures with hydrogen fractions of 0 and 50%, only four stages of “tulip” flame formation are observed, but when the hydrogen fraction is 100%, the distorted “tulip” flame appears after flame front inversion. In the acceleration stage, the LES and experimental flame speed and pressure dynamic coincide with each other, except for a hydrogen fraction of 0. After “tulip” flame formation, all LES and experimental flame propagation speeds and pressure dynamics exhibit the same trends for hydrogen fractions of 0 and 100%. However, when the hydrogen fraction is 50%, a slight periodic oscillation appears only in the experiment. In general, the different structures displayed in the flame front during flame propagation can be attributed to the interaction between the flame front, the vortex and the reverse flow formed in the unburned and burned zones.     

Dynamics of premixed hydrogen/air flame in a closed combustion vessel

The dynamics of a premixed hydrogen/air flame propagating in a closed vessel is investigated using high-speed schlieren cinematography, pressure measurement and numerical simulation. A dynamically thickened flame approach with a 19-step detailed chemistry is employed in the numerical simulation to model the premixed combustion. The schlieren photographs show that a remarkable distorted tulip flame is initiated after a classical tulip flame has been fully produced. A second distorted tulip flame is generated with a cascade of indentations created in succession before the vanishing of the first one. The flame dynamics observed in the experiments is well reproduced in the numerical simulation. The burnt region near the flame front is entirely dominated by a reverse flow during the formation of the distorted tulip flame. The distorted tulip flame can be formed in the absence of vortex motion. The pressure wave leads to periodic flame deceleration and plays an essential role in the distorted tulip formation. The numerical results corroborate the mechanism that the distorted tulip flame formation is a manifestation of Taylor instability.     

Experimental and numerical investigation of premixed flame propagation with distorted tulip shape in a closed duct

High-speed schlieren photography, pressure records and large eddy simulation (LES) model are used to study the shape changes, dynamics of premixed flame propagation and pressure build up in a closed duct. The study provides further understanding of the interaction between flame front, pressure wave and combustion-generated flow, especially when the flame acquires a “distorted tulip” shape. The Ulster multi-phenomena LES premixed combustion model is applied to gain an insight into the phenomenon of “distorted tulip” flame and explain the experimental observations. The model accounts for the effects of flow turbulence, turbulence generated by flame front itself, selective diffusion, and transient pressure and temperature on the turbulent burning velocity. The schlieren images show that the flame exhibits a salient “distorted tulip” shape with two secondary cusps superimposed onto the two original tulip lips. This curious flame shape appears after a well-pronounced classical tulip flame is formed. The dynamics of “distorted tulip” flame observed in the experiment is well reproduced by LES. The numerical simulations show that large-scale vortices are generated in the burnt gas after the formation of a classical tulip flame. The vortices remain in the proximity of the flame front and modify the flow field around the flame front. As a result, the flame front in the original cusp and near the sidewalls propagates faster than that close to the centre of the original tulip lips. The discrepancy in the flame propagation rate finally leads to the formation of the “distorted tulip” flame. The LES model validated previously against large-scale hydrogen/air deflagrations is successfully applied in this study to reproduce the dynamics of flame propagation and pressure build up in the small-scale duct. It is confirmed that grid resolution has an influence to a certain extent on the simulated combustion dynamics after the flame inversion.     

Impacts of plate slits on flame acceleration of premixed methane/air in a closed tube

The paper aims at revealing the interaction of various numbers of premixed methane/air jet flames in a closed duct. In the experiment, a high-speed video camera and pressure transducers are used to study the flame structure and pressure dynamics. In the numerical simulations, large eddy simulation (LES) with Power-Law combustion model is employed to investigate the interaction between the moving flame and vortices induced by the thin plate. The results demonstrate that the flame propagation for all plate configurations can be divided into four typical stages, i.e. hemispherical flame, finger-shaped flame, jet flame and bidirectional propagation flame. For three plate configurations, the jet flames merge together under the effect of the vortices, and the more slits with the same blockage ratio (BR) do not mean the stronger deflagration. It is observed that the peaks of flame tip speed and pressure growth rate decrease with the increase of the number of slits. The sub-grid scale combustion model, Power-Law model, coupled with sub-grid scale viscosity model, dynamic Smagorinsky-Lilly eddy viscosity model can well reproduce the flame propagation. By analyzing the numeric flow structure, the flame propagation mechanism of premixed methane/air flame propagation in a tube with various slits can be explained in the view of pure hydrodynamics.     

An experimental study of premixed hydrogen/air flame propagation in a partially open duct

High-speed schlieren cinematography and pressure records are used to investigate the dynamics of premixed hydrogen/air flame propagation and pressure build up in a partially open duct with an opening located in the upper wall near the right end of the duct. This work provides basic understanding of flame behaviors and the effects of opening ratio on the combustion dynamics. The flame behaves differently under different opening conditions. The opening ratio has an important influence on the flame propagation and pressure dynamics. When the opening ratio α ≤ 0.075 a significant distorted tulip flame can be formed after the full formation of a classical tulip flame. The propagation speed of flame leading tip increases with the opening ratio. The coupling of flame front with the pressure wave is strong at low opening ratio. Both the pressure growth rate and oscillation amplitude inside the duct increases as the opening ratio decreases. The formation times of tulip and distorted tulip flames and the corresponding distances of flame front increase with the increase of the opening ratio.     

Flame characteristics of premixed H2-air mixtures explosion venting in a spherical container through a duct

The explosion venting duct can effectively reduce the hazard degree of a gas explosion and conduct the venting energy to the safe area. To investigate the flame quantitative propagation law of explosion venting with a duct, the effects of hydrogen fraction and explosion venting duct length on jet flame propagation characteristics of premixed H₂-air mixtures were analyzed through experiment and simulation. The experiment results under initial conditions of room temperature and 1 atm show that when hydrogen fraction was high enough, part of the unburned hydrogen was mixed with air again to reach an ignitable concentration, resulting in the secondary combustion was easier produced and the duration of the secondary flame increased. With the increase of venting duct length, the flame front distance and propagation velocity increased. Meanwhile, the spatial distribution of pressure field and temperature field, and the propagation process and mechanism of the flame venting with a duct were analyzed using FLUENT software. The variation of the pressure wave and the pressure reflection oscillation law in the explosion venting duct was captured. Therefore, in the industrial explosion venting design with a duct, the hazard caused by the coupling of venting pressure and venting flame under different fractions should be considered comprehensively.     

Experimental and numerical study on premixed hydrogen/air flame propagation in a horizontal rectangular closed duct

Hydrogen is a promising energy in the future, and it is desirable to characterize the combustion behavior of its blends with air. The premixed hydrogen/air flame microstructure and propagation in a horizontal rectangular closed duct were recorded using high-speed video and Schlieren device. Numerical simulation was also performed on Fluent CFD code to compare with the experimental result. A tulip flame is formed during the flame propagating, and then the tulip flame formation mechanism was proposed based on the analysis. The induced reverse flow and vortex motion were observed both in experiment and simulation. The interactions among the flame, reverse flow and vortices in the burned gas change the flame shape and ultimately it develops into a tulip flame. During the formation of the tulip flame, the tulip cusp slows down and stops moving after its slightly forward moving, and then, it starts to move backward and keeps on a longer time, after that, it moves forward again. The structure of the tulip flame is becoming less stable with its length decreasing in flame propagation direction. The flame thickness increases gradually which is due to turbulence combustion.     

Experimental study of premixed syngas/air flame deflagration in a closed duct

The propagation behaviour of a deflagration premixed syngas/air flame over a wide range of equivalence ratios is investigated experimentally in a closed rectangular duct using a high-speed camera and pressure transducer. The syngas hydrogen volume fraction, φ, ranges from 0.1 to 0.9. The flame propagation parameters such as flame structure, propagation time, velocity and overpressure are obtained from the experiment. The effects of the equivalence ratio and hydrogen fraction on flame propagation behaviour are examined. The results indicate that the hydrogen fraction in a syngas mixture greatly influences the flame propagation behaviour. When φ, the hydrogen fraction, is ≥0.5, the prominently distorted tulip flame can be formed in all equivalence ratios, and the minimum propagation time can be obtained at an equivalence ratio of 2.0. When φ < 0.5, the tulip flame distortion only occurs in a hydrogen fraction of φ = 0.3 with an equivalence ratio of 1.5 and above. The minimum flame propagation time can be acquired at an equivalence ratio of 1.5. The distortion occurs when the maximum flame propagation velocity is larger than 31.27 m s^?1. The observable oscillation and stepped rise in the overpressure trajectory indicate that the pressure wave plays an important role in the syngas/air deflagration. The initial tulip distortion time and the plane flame formation time share the same tendency in all equivalence ratios, and the time interval between them is nearly constant, 4.03 ms. This parameter is important for exploring the quantitative theory or models of distorted tulip flames.     

Hydrogen-air deflagrations in open atmosphere: Large eddy simulation analysis of experimental data

The largest known experiment on hydrogen-air deflagration in the open atmosphere has been analysed by means of the large eddy simulation (LES). The combustion model is based on the progress variable equation to simulate a premixed flame front propagation and the gradient method to decouple the physical combustion rate from numerical peculiarities. The hydrodynamic instability has been partially resolved by LES and unresolved effects have been modelled by Yakhot's turbulent premixed combustion model. The main contributor to high flame propagation velocity is the additional turbulence generated by the flame front itself. It has been modelled based on the maximum flame wrinkling factor predicted by Karlovitz et al. theory and the transitional distance reported by Gostintsev with colleagues. Simulations are in a good agreement with experimental data on flame propagation dynamics, flame shape, and outgoing pressure wave peaks and structure. The model is built from the first principles and no adjustable parameters were applied to get agreement with the experiment.     

Effects of ignition location on premixed hydrogen/air flame propagation in a closed combustion tube

The dynamics of premixed hydrogen/air flame ignited at different locations in a finite-size closed tube is experimentally studied. The flame behaves differently in the experiments with different ignition positions. The ignition location exhibits an important impact on the flame behavior. When the flame is ignited at one of the tube ends, the heat losses to the end wall reduce the effective thermal expansion and moderate the flame propagation and acceleration. When the ignition source is at a short distance off one of the ends, the tulip flame dynamics closely agrees with that in the theory. And both the tulip and distorted tulip flames are more pronounced than those in the case with the ignition source placed at one of the ends. Besides, the flame–pressure wave coupling is quite strong and a second distorted tulip flame is generated. When the ignition source is in the tube center, the flame propagates in a much gentler way and the tulip flame can not be formed. The flame oscillations are weaker since the flame–pressure wave interaction is weaker.     

Effect of the initial pressures on evolution of intrinsically unstable hydrogen/air premixed flame fronts

Laminar premixed flame front may be wrinkled due to the hydrodynamic and diffusive-thermal instabilities. This may lead to the occurrence of the cellular structure and the self-acceleration. The lean unstable hydrogen/air premixed flame at various initial pressures are studied to clarify the effect of the initial pressure on the evolution of the unstable laminar flame. Linear and nonlinear development stages of the unstable flame are simulated and investigated separately. In the linear stage, the initial sinusoidal wave disturbance on the flame front will still keep its initial configuration. The growth rate increases firstly and then decreases with the increase of the wavenumbers. The effect of the self-acceleration on the unstable flame front will be stronger in the linear stage at the higher initial pressure, since there are larger thermal expansion and constant Lewis number for hydrogen/air premixed flame at higher pressure. There are little discrepancies for the calculated growth rates with those predicted by the revised dispersion relation. The nonlinear stage of the unstable flame propagation could be divided into two stages, the transitional and the stable nonlinear stages. In the transitional stage, the flame front cells splits, merges and moves all the time and the initial wavenumber has a great influence on the cell evolution process. With the evolution of the cell on the flame front, the cellular structure on the flame front will not change greatly with the initial wavenumbers in the stable nonlinear stage. The effect of self-acceleration due to the wrinkling of the flame front at this stage is weakened with the increase of the initial pressure. At the higher pressure, more wrinkled structures with smaller mean curvature are distributed on the flame front. At last, results show that the flame front will propagate faster for the larger computation domain. Based on the fractal theory, the fractal dimension of lean hydrogen/air premixed flame with the equivalence ratio of 0.6 at 0.5 MPa in the 2D domain is obtained and around 1.26.     

A study of numerical hazard prediction method of gas explosion

A 3-dimensional computational fluid dynamics (CFD) simulation of a premixed hydrogen/air explosion in a large-scale domain is performed. The main feature of the numerical model is the solution of a transport equation for the reaction progress variable using a function for turbulent burning velocity that characterizes the turbulent regime of propagation of free flames derived by introducing the fractal theory. The model enables the calculation of premixed gaseous explosion without using fine mesh of the order of micrometer, which would be necessary to resolve the details of all instability mechanisms. The value of the empirical constant contained in the function for turbulent burning velocity is evaluated by analyzing the experimental data of hydrogen/air premixed explosion. The comparison of flame behavior between the experimental result and numerical simulation shows good agreement. The effect of mesh size on simulated flame propagation velocity is also tested, showing that the numerical result agrees reasonably well with experiment when the mesh size is less than about 20 cm.     

Numerical simulation study and dimensional analysis of hydrogen explosion characteristics in a closed rectangular duct with obstacles

The influence of obstacles on hydrogen explosion is studied by numerical simulation and dimensional analysis. The numerical simulation is conducted based on the premixed model in a closed rectangular duct with rectangular obstacles, and ten variables that affect the flame propagation velocity are analyzed by dimensional analysis. Continuous acceleration of flame and collision annihilation of flame were successfully realized through triangular obstacles in simulation. The result shows that with the number of obstacles changes, the flame invariably converts to hemispherical flame, finger flame, tongue flame, quasi-plane flame, and mouth flame in turn. But the flame front is more twisted in two obstacles due to hydrodynamic instability and vortices. Through the comparative analysis of the flame and flow field in the duct during hydrogen explosion. It is found that the flame-obstacles-flow field coupling and its hydrodynamic phenomena determine the flame deformation and changes in propagation velocity. The result of the dimensional analysis shows that the drag coefficient can well depict the effects of the shape of the obstacles, and the dimensionless qualitative and quantitative model of flame propagation speed is given and verified.     

Experimental study on the influence of multi-layer wire mesh on dynamics of premixed hydrogen-air flame propagation in a closed duct

Hydrogen, which is considered to be a promising clean energy source, has been studied and applied extensively in industries. In order to improve the safety of hydrogen energy application, an experimental study on the influence of multi-layer wire mesh on dynamics of premixed hydrogen-air flame propagation in a closed duct is conducted. Four different kinds of wire mesh with 40, 45, and 50 layers are chosen in the experiments. High speed schlieren photography is applied to capture the flame shape changes and determine the flame tip speed. Pressure transducer is used to measure the pressure transient. It is found that flame quenches in the cases of adding wire mesh of 60, 80, and 100 mesh with 45 and 50 layers, while for the wire mesh of 40 mesh, 50 layers cannot even quench the flame. Moreover, the multi-layer wire mesh can effectively suppress the flame tip speed, maximum pressure, and sound waves during premixed hydrogen-air flame propagation in the duct. The attenuated maximum pressure reaches approximately 78.6% in the case of adding wire mesh of 100 mesh-50 layers.     

Propagation of premixed hydrogen-air flame initiated by a planar ignition in a closed tube

Numerical simulations were used to study the dynamics of premixed flames propagating after planar ignition in a closed tube filled with stoichiometric hydrogen-air mixture. The two-dimensional fully compressible reactive Navier–Stokes equations coupled to a calibrated chemical-diffusive model were solved using a high-order numerical method and adaptive mesh refinement. The results show that the flame evolves from an initially planar flame to a double-cusped tulip flame, subsequently to a multi-cusped tulip flame, and finally to a series of distorted tulip flames (DTFs). The DTF forms one after another until the end of combustion. The initial flame lips of the double-cusped tulip flame are produced due to the stretching effect of nonuniform flow caused by the wall friction. The multi-cusped tulip flame forms as secondary cusps are created on the leading flame tips near the sidewalls. The formation of DTFs here is thought to be closely connected to pressure waves generated in the flame propagation process. The first DTF is caused by the combined effects of the vortex motions and the Rayleigh–Taylor (RT) instability driven by pressure waves, while the subsequent DTFs form due to reverse flows and RT instability. Nevertheless, both the vortex motions and reverse flows are essentially induced by the interactions between pressure waves and flow fields. Furthermore, the numerical results were compared to that in the case with a semicircular ignition. It was found that although there are significant differences in the early flame acceleration and tulip formation stages between the two differently shaped ignitions, the dynamics of DTFs are substantially consistent.     

Effect of initial pressure on flame–shock interaction of hydrogen–air premixed flames

By utilizing a newly designed constant volume combustion bomb (CVCB), turbulent flame combustion phenomena are investigated using hydrogen–air mixture under the initial pressures of 1 bar, 2 bar and 3 bar, including flame acceleration, turbulent flame propagation and flame–shock interaction with pressure oscillations. The results show that the process of flame acceleration through perforated plate can be characterized by three stages: laminar flame, jet flame and turbulent flame. Fast turbulent flame can generate a visible shock wave ahead of the flame front, which is reflected from the end wall of combustion chamber. Subsequently, the velocity of reflected shock wave declines gradually since it is affected by the compression wave formed by flame acceleration. In return, the propagation velocity of turbulent flame front is also influenced. The intense interaction between flame front and reflected shock can be captured by high-speed schlieren photography clearly under different initial pressures. The results show that the propagation velocity of turbulent flame rises with the increase of initial pressure, while the forward shock velocities show no apparent difference. On the other hand, the reflected shock wave decays faster under higher initial pressure conditions due to the faster flame propagation. Moreover, the influence of initial pressure on pressure oscillations is also analyzed comprehensively according to the experimental results.     

Structure of hydrogen triple flames and premixed flames compared

Triple flames consisting of lean, stoichiometric, and rich reaction zones may be produced in stratified mixtures undergoing combustion. Such flames have unique characteristics that differ from premixed flames. The present work offers a direct comparison of the structure and propagation behavior between hydrogen/air triple and premixed flames through a numerical study. Important similarities and differences are highlighted. Premixed flames are generated by spark-igniting initially quiescent homogeneous mixtures of hydrogen and air in a two-dimensional domain. Triple flame results are also generated in a two-dimensional domain by spark-igniting initially quiescent hydrogen/air stratified layers. Detailed flame structure and chemical reactivity information is collected along isocontours of equivalence ratio 0.5, 1.0, and 3.0 in the triple flame for comparison with premixed flames at the same equivalence ratios. Full chemistry and effective binary diffusion coefficients are employed for all computations.     

Experimental study of hydrogen/air premixed flame propagation in a closed channel with inhibitions for safety consideration

An experimental study of hydrogen/air premixed flame propagation in a closed rectangular channel with the inhibitions (N₂ or CO₂) was conducted to investigate the inhibiting effect of N₂ and CO₂ on the flame properties during its propagation. Both Schlieren system and the pressure sensor were used to capture the evolution of flame shape and pressure changes in the channel. It was found that both N₂ and CO₂ have considerable inhibiting effect on hydrogen/air premixed flames. Compared with N₂, CO₂ has more prominent inhibition, which has been interpreted from thermal and kinetic standpoints. In all the flames, the classic tulip shape was observed. With different inhibitor concentration, the flame demonstrated three types of deformation after the classic tulip inversion. A simple theoretical analysis has also been conducted to indicate that the pressure wave generated upon the first flame-wall contact can affect the flame deformation depending on its meeting moment with the flame front. Most importantly, the meeting moment is always behind the start of tulip inversion, which suggests the non-dominant role of pressure wave on this featured phenomenon.     

Study on the dynamic process of in-duct hydrogen-air explosion flame propagation under different blocking rates

To effectively analyses the flame propagation of premixed hydrogen-air explosion, this paper carries out a numerical study on the dynamics of flame propagation during hydrogen explosions in a closed duct under different blocking rates. The study shows that flame structure is roughly the same when the flame passes through an obstacle under different blocking rates. The difference in blocking rates only shows a slight difference in the degree of flame deformation. When the flame passes through the obstacle, Rayleigh -Taylor (R-T) instability accompanies the entire flame propagation process and corresponds to each stage flame acceleration. Kelvin-Helmholtz(K-H) instability has a more prominent influence on the tip flame propagation. When the explosion flame propagates, instabilities lead to difference in density gradient and pressure gradient in the duct. Interaction between density gradient and pressure gradient leads to formation of baroclinic torque, which is the main cause of the vorticity. During the flame propagation, the vorticity at the front of the flame is roughly zero, whereas the vorticity formed at the obstacle or in the burned gases is more apparent. The larger the blocking rate, the more prominent the turbulence intensity during the flame propagation.     

有序堆积床内预混气体燃烧特性实验研究

为研究预混气体在多孔介质燃烧器中的火焰燃烧特性,设计了一种新型多孔介质燃烧器,其中多孔介质区域由氧化铝圆柱体有序堆积而成.分别研究了当量比和入口速度对甲烷/空气预混气体在多孔介质燃烧器中的火焰温度分布、火焰最高温度以及火焰传播速度的影响.结果 表明:在当量比0.162～0.324、入口速度0.287～0.860 m/s...