Experimental study on the behaviors and shape changes of premixed hydrogen-air flames propagating in horizontal duct

The behaviors and shape changes of premixed hydrogen-air flames at various equivalence ratios propagating in half-open and closed horizontal ducts are experimentally investigated using high-speed schlieren imaging and pressure sensors. The study shows that the premixed hydrogen-air flame undergoes more complex shape changes and exhibits more distinct characteristics than that of other gaseous fuels. One of the outstanding findings is that obvious distortion happens to tulip flame after its full formation when equivalence ratio ranges from 0.84 to 4.22 in the closed duct. The salient tulip flame distortions are specially scrutinized and distinguished from the classical tulip collapse and disappearance. The dynamics of distorting tulip flame is different from that of classical tulip flame. The normal tulip flame can be reproduced after the first distortion followed by another distortion. The initiation of flame shape changes coincides with the deceleration both of pressure rise and flame front speed for flames with tulip distortions. And the formation and dynamics of tulip/distorting tulip flames depend on the mixture composition.     

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 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.     

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.     

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.     

Dynamics of premixed hydrogen-air flame propagation in the duct with pellets bed

Hydrogen, as the promising clean alternative energy in the future, is in the spotlight now all over the world. However, its flammable and explosive hazards should be highly considered during its practical application. In this study, the experiments are performed to study premixed hydrogen-air flame propagation in the duct with pellets bed, especially for fuel-rich condition. High-speed schlieren photography is employed to capture flame front development during the experiments. As well as the pressure transducer, is used to track the pressure buildup in the flame propagation process. Different diameters of pellets and different concentrations of gas mixture are considered in this experimental study. The typical evolutions about the tulip flame are similar in all cases, although the tulip flame formation time caused by the laminar flame speed are different. The flame propagation velocity is pretty enhanced in fuel-lean mixture under the effect of large diameter pellets bed, but it is significantly suppressed in fuel-rich conditions. While for the small diameter pellets (d = 3 mm), the suppression effect on flame propagation and pressure is obtained over a wider range of equivalence ratios, especially a better suppression effect is generated near the stoichiometric condition.     

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.     

Geometric influence of perforated plate on premixed hydrogen-air flame propagation

Geometrical influence of the perforated plate on flame propagation in hydrogen-air mixtures with various equivalence ratios and initial pressures was experimentally investigated in a channel with the length of 1 m and the cross-section of 7 cm × 7 cm. The perforated plate has the same cross section and three thicknesses of 40 mm, 80 mm and 120 mm. High-speed schlieren photography was employed to capture the flame shape evolution and derive the flame tip velocity. High-speed piezoelectric pressure transducers were flush-mounted upstream and downstream of the perforated plate to measure the pressure transient. It was found that, with the perforated plate in the path of flame, flame undergoes either “go”, or “quench” propagation mode. The limit between these two was dependent on the geometrical size of the perforated plate and the initial conditions of mixtures. Both velocity and pressure were effectively attenuated with the increase in the perforated plate length. Moreover, for “go” propagation mode, the flame process through the perforated plate was characterized by three obvious stages: laminar flame stage, jet flame stage and turbulent flame stage. Whereas, only laminar flame stage was observed in the “quench” mode.     

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.     

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.     

Experimental study on the premixed syngas-air explosion in duct with both ends open

Premixed flame of stoichiometric syngas-air mixture with various hydrogen volume fractions, 10% ≤ X (H₂) ≤ 90%, propagating in a duct with both ends open is experimentally investigated in this study. Two representative ignition locations, i.e., Ig-1, locating at the center of the duct, and Ig-2, locating at the right open end, are considered. Results show that the tulip flame is first attained in the duct with both ends open at 10% ≤ X (H₂) ≤ 50% as the flame is ignited at Ig-1. However, the flame maintains the convex shape with the cellular structure on the flame surface as the flame is ignited at Ig-2. The cellular structure results from Darrieus-Landau instability, but the Darrieus-Landau instability cannot invert the convex flame front. The flame tip and pressure dynamics have been examined. When the flame is ignited at Ig-1, the flame oscillates violently, and the overpressure profiles oscillate as a Helmholtz-type. When the flame is ignited at Ig-2, the left flame front propagates in an atmospheric pressure with a nearly constant speed. The prominent flame acceleration and oscillation are not observed at Ig-2 because of lacking flame acoustic interaction. What's more, the characteristic time of flame propagation has been compared. The time t_w is shorter while the time t_p is longer than the calculated value, and the time t_e has been delayed by both open ends. The flame propagation process is moderated as the flame propagates in the duct with both ends open.     

Theoretical analysis for the centrifugal effect on premixed flame speed in a closed tube

A theoretical analysis is described to study the effect of centrifugal acceleration, especially high centrifugal acceleration, i.e. more than 200 times of gravity acceleration (200g), on the premixed flame speed in a rotating closed tube. Based on one-dimensional (1-D) steady adiabatic flame model, simplified governing equations are directly solved by integration method in the reaction zone. A theoretical prediction that describes the premixed flame speed in a rotating closed tube is obtained. The theoretical prediction agrees well with the experimental data obtained by Lewis & Smith. The result verifies that the flame speed accelerated by the centrifugal force is nearly proportional to the square root of the centrifugal acceleration. It is shown by theoretical analysis that the flame speed in a rotating closed tube is determined by the initial temperature, the critical ignition temperature, the adiabatic flame temperature and the thicknesses of reaction zone. The premixed flame speed in a rotating closed tube increases nearly linearly with the increasing of the initial temperature or square root of the thicknesses of reaction zone, or with decreasing of the critical ignition temperature or the adiabatic flame temperature.     

Dynamics of upstream flame propagation in a hydrogen-enriched premixed flame

An unconfined strongly swirled flow is investigated to study the effect of hydrogen addition on upstream flame propagation in a methane-air premixed flame using Large Eddy Simulation (LES) with a Thickened Flame (TF) model. A laboratory-scale swirled premixed combustor operated under atmospheric conditions for which experimental data for validation is available has been chosen for the numerical study. In the LES-TF approach, the flame front is resolved on the computational grid through artificial thickening and the individual species transport equations are directly solved with the reaction rates specified using Arrhenius chemistry. Good agreement is found when comparing predictions with the published experimental data including the predicted RMS fluctuations. Also, the results show that the initiation of upstream flame propagation is associated with balanced maintained between hydrodynamics and reaction. This process is associated with the upstream propagation of the center recirculation bubble, which pushes the flame front in the upstream mixing tube. Once the upstream movement of the flame front is initiated, the hydrogen-enriched mixture exhibits more unstable behavior; while in contrast, the CH₄ flame shows stable behavior.     

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.     

Extinction conditions of a premixed flame in a channel

A local refinement method is used to numerically predict the propagation and extinction conditions of a premixed flame in a channel considering a thermodiffusive model. A local refinement method is employed because of the numerous length scales that characterize this phenomenon. The time integration is self adaptive and the solution is based on a multigrid method using a zonal mesh refinement in the flame reaction zone. The objective is to determine the conditions of extinction which are characterized by the flame structure and its properties. We are interested in the following properties: the curvature of the flame, its maximum temperature, its speed of propagation and the distance separating the flame from the wall. We analyze the influence of heat losses at the wall through the thermal conductivity of the wall and the nature of the fuel characterized by the Lewis number of the mixture. This investigation allows us to identify three propagation regimes according to heat losses at the wall and to the channel radius. The results show that there is an intermediate value of the radius for which the flame can bend and propagate provided that its curvature does not exceed a certain limit value. Indeed, small values of the radius will choke the flame and extinguish it. The extinction occurs if the flame curvature becomes too small. Furthermore, this study allows us to predict the limiting values of the heat loss coefficient at extinction as well as the critical value of the channel radius above which the premixed flame may propagate without extinction. A dead zone of length 2-4 times the flame thickness appears between the flame and the wall for a Lewis number (Le) between 0.8 and 2. For small values of Le, local extinctions are observed.     

Experimental investigation of unconfined spherical and cylindrical flame propagation in hydrogen-air mixtures

This paper presents results of experimental investigations on spherical and cylindrical flame propagation in pre-mixed H₂/air-mixtures in unconfined and semi-confined geometries. The experiments were performed in a facility consisting of two transparent solid walls with 1 m² area and four weak side walls made from thin plastic film. The gap size between the solid walls was varied stepwise from thin layer geometry (6 mm) to cube geometry (1 m). A wide range of H₂/air-mixtures with volumetric hydrogen concentrations from 10% to 45% H₂ was ignited between the transparent solid walls. The propagating flame front and its structure was observed with a large scale high speed shadow system. Results of spherical and cylindrical flame propagation up to a radius of 0.5 m were analyzed. The presented spherical burning velocity model is used to discuss the self-acceleration phenomena in unconfined and unobstructed pre-mixed H₂/air flames.     

Experimental study of premixed single edge-flame in a counterflow field

A single premixed edge-flame established in a counterflow field of a combustible mixture and an inert nitrogen was experimentally investigated by using twin rectangular burners which were misaligned by a few degrees. The stretch-rate gradient was quantitatively defined as a function of the angle between the two burners and the distance from the edge of the burner. The flame weakly curved at the edge toward the stagnation plane and the shape of the flame edge did not depend on the composition of the mixture. The response of edge-flames to changes in flow conditions, such as the equivalence ratio of the mixture or the injection velocity at the burner exit, was basically determined by the Damkohler number of the mixture. The ratio of the local stretch rate at the flame edge and the extinction stretch rate for a single planar flame with the same composition was slightly smaller than unity, though the extinction stretch rate for the planar flame was about half of that for the twin planar flame due to large heat conduction to the opposing inert gas. This ratio of stretch rates for the single edge-flame was larger than that for twin edge-flames in a previous work, and this result agreed with the previous theoretical analysis by Buckmaster et al. and the previous experimental results of Ronney et al. Moreover, the effect of the stretch-rate gradient on the characteristics of the edge-flame did not appear for the single edge-flame.     

Combustion chemistry and fuel-nitrogen conversion in a laminar premixed flame of morpholine as a model biofuel

The present study has been motivated by the need to understand and predict fuel-nitrogen conversion in the combustion of biomass-derived fuels. Within that broader context, an earlier related publication (Lucassen et al., Proc. Combust. Inst. 32 (2009) 1269–1276) has investigated morpholine (C₄H₉NO, 1-oxa-4-aza-cyclohexane) as a model oxygen- and nitrogen-containing biofuel, and species identification was presented for a slightly fuel-rich Φ = 1.3 (C/O = 0.41) laminar premixed morpholine-oxygen-argon flame at 40 mbar. To attempt a more detailed insight into the flame structure and combustion mechanism, the present contribution has now combined photoionization (PI) and electron ionization (EI) molecular-beam mass spectrometry (MBMS) to determine absolute mole-fraction profiles of numerous major and intermediate species with up to 6 heavy atoms. In general, PI-MBMS and EI-MBMS results were found in good agreement. The results reveal formation of a number of intermediates that may contribute to harmful emissions, including aldehydes and several nitrogen-containing compounds in percent-level concentrations. Both NH₃ and HCN pathways are seen to contribute to NO formation.To identify reaction pathways for this detailed experimental analysis, development of a flame model was started, considering a combustion mechanism for cyclohexane and analogous fuel-breakdown reactions for morpholine by addition of necessary thermodynamic, transport and kinetic parameters. The present model captures relevant features of the morpholine flame quite well, including HCN, N₂, and NO, and it can serve as a nucleus for further development of detailed combustion models for fuel-nitrogen conversion from model biofuel compounds.     

The coupling of turbulence and chemistry in a premixed bluff-body flame as studied by LES

In previous work a subgrid scale fractal model for large eddy simulation of turbulent combustion was developed and validated. In the present article the fractal model applicability is tested by simulating a bluff-body premixed flame anchored in a straight channel. The model assumes that chemical reactions take place only at the dissipative scales of turbulence, i.e., near the so-called “fine structures” (eddy dissipation concept). The model estimates the local spatial dissipative scale η, considering also the growth effect due to heat release, and turns itself automatically off where the local spatial filter Δ equals η. The premixed burner is simulated in 2 and 3 dimensions, both for cold flow and for reacting cases. Results are compared with experimental data and show three-dimensional vortex structures periodically shortening the recirculation zone downstream of the bluff body and entraining fresh mixture into the hot recirculating region. This physical mechanism is involved in flame anchoring. The effect of assuming periodic boundary conditions in the spanwise direction, instead of solid side walls, is also investigated. The analysis shows that periodic boundary conditions cannot capture various effects of side walls, such as the shortening of the recirculation zone and the flow acceleration downstream; furthermore, it also does not allow predictions of wall heat transfer. The 2D reactive case results are also compared with those using RANS κ−? and LES-Smagorinsky models. Finally, comparing kinetic energy spectral densities in the nonreacting and reacting cases it is shown that large-scale fluctuations are damped in the latter and that fast chemical reactions cause a high-frequency energy peak.     

Experimental study of flame propagation across a perforated plate

Flame propagation across a single perforated plate was experimentally studied in a square cross-section channel. Experiments were performed in premixed hydrogen-air mixture with different equivalence ratios and initial pressures, aiming at identifying the parametric influence. High-speed schlieren photography and pressure records were used to capture the flame front and obtain the pressure build-up. Four stages for the flame front crossing the perforated plate were obtained, namely, laminar flame, jet flame, turbulent flame and secondary flame front. Following ignition, a laminar flame was obtained, which was nearly not affected by the confinement. This laminar flame was squeezed to pass through the perforated plate, producing the jet flame with a step change on velocity. Turbulent flame was generated by merging the jets, which facilitated the acceleration of the flame front. Secondary flame front induced by Rayleigh-Taylor instability was clearly observed in the process of the turbulent front moving forward. Both velocity and pressure are enhanced in this stage. Parametric studies suggested that the secondary flame front is more obvious in the stoichiometric mixture with higher initial pressure, and characterized by a faster propagation velocity and a bigger pressure rise.