Flame Front Structure Research Articles

Study on the impact of heat loss on premixed hydrogen/air deflagration in closed pipelines

This study developed a one-step chemical reaction model for hydrogen combustion and investigated the effect of heat loss on premixed hydrogen/air deflagration in closed pipelines using numerical simulations with the CFD code GASFLOW-MPI. The simulation results were compared and verified with experimental data. The numerical simulations accurately predicted the flame front structure, pressure, and flame tip position during the deflagration of premixed hydrogen/air. Furthermore, a comparison of adiabatic simulation and heat transfer simulation results revealed that large-scale vortices within the turbulent flow were closely linked to heat loss during the tulip flame stage. Simultaneously, heat loss reduced pressure and flame tip position by diminishing the flame wrinkling factor caused by thermodiffusive instability. Finally, convective heat transfer identified as the most critical factor contributing to heat loss during the deflagration of premixed hydrogen/air in closed pipelines. Specifically, during the tulip flame stage, heat loss was mainly due to convective heat transfer (73.8%∼76.3%), with radiation playing a minor role (23.7%∼26.2%). Therefore, accurate prediction of characteristic parameters during the deflagration of premixed hydrogen/air requires numerical simulations that account for both convective and radiative heat transfer.

Study on flame quenching characteristics inside the Tesla valve structure flame arrester unit

Flame arrester is a kind of safety device designed to prevent the spread of flames. In order to achieve a more efficient flame arrester unit structure, this paper proposes a flame arrester unit based on the Tesla valve structure. A comparative study of the quenching characteristics of deflagration flames of propane-air premixtures in traditional crimped ribbon and Tesla valve structure flame arrester units is conducted through numerical simulations and experiments. The results indicate that the flame arrester unit based on the Tesla valve structure exhibits more efficient flame quenching performance compared to the crimped ribbon flame arrester unit. The flame quenching process fusion image shows that the flame quenching length of Tesla valve flame arrester unit with the same cross-sectional area is only 61% of the crimped ribbon flame arrester unit. In addition to heat transfer and wall effect, the collision of airflow in the branched pipe and main pipe within the Tesla valve structure flame arrester unit also plays a significant role in hindering the further advancement of flames. The quenching process of the Tesla valve structure flame arrester unit can be divided into four stages: expansion, diversion, convergence, and contraction stages. Among the five structural parameters that make up the Tesla valve structure flame arrester unit, the angle is the most critical factor affecting quenching performance, with the angle and entrance structure having a significant impact on flow performance. Furthermore, experimental evidence confirms the typical morphological changes the flame front structure undergoes during flame propagation: spherical (hemispherical),finger-shaped and tulip-shaped. However, tulip-shaped flames do not always propagate continuously to the end of the pipeline.

Experimental and numerical study on the explosion characteristics of syngas under different venting conditions

To explore ways to mitigate the harm and damage caused by syngas explosions and provide a theoretical basis for the safe and widespread use of syngas, the explosion characteristics of syngas under different venting conditions were experimentally and numerically investigated in our work. Five venting configurations were established based on the location and size of the lateral vent. The combined effects of venting conditions and hydrogen concentrations on explosion characteristics were concerned. Results showed the lateral vent significantly impacted the flame front structure, flame propagation velocity, and overpressure. Moreover, the formation and development of tulip flame could be affected by the lateral vent and hydrogen concentration. The presence of a lateral vent reduced both the flame propagation velocity and overpressure. With increasing hydrogen concentration, the drop ratio of the maximum flame propagation velocity did not increase, whereas the drop ratio of the maximum overpressure increased for the same venting condition. Finally, the internal mechanism of syngas venting explosion under different venting conditions was revealed by numerical simulation. In short, the research results provided the theoretical basis for the safe utilization of syngas in the industrial production field.

Relaxational oscillations of burner-stabilized premixed methane–air flames

In this paper, the dynamics of diffusion-thermal oscillations of burner-stabilized methane–air flames are numerically and experimentally investigated. High speed imaging and laser induced fluorescence of OH radicals are used as experimental techniques, while a hierarchy of models is employed in numerical simulations: a system of ordinary differential equations describing dynamics of flame location and temperature, models with one-step and detailed reaction mechanisms. It is demonstrated that for small (near the neutral stability boundary) and moderately relaxational oscillations, all three approaches are able to describe the flame dynamics however with different levels of detailization. This is due to the fact that in these regimes the flame front structure remains similar, although perturbed, to that observed in freely propagating combustion waves. However, this flame structure breaks in the regime of highly relaxational oscillations, which can only be described within the detailed reaction mechanisms. The combustion front splits into the high and low-temperature reaction zones, which are shown to be highly and weakly sensitive to the diffusive-thermal pulsations. The high temperature reaction zone is blown away and extinguishes in the course of flame pulsation, and the low temperature zone remains near the burner surface and causes re-ignition of the combustion front. Prospects of further investigations are discussed.

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.

Adaptive simulations of flame acceleration and detonation transition in subsonic and supersonic mixtures

Two-dimensional simulations were carried out to investigate the flame acceleration and deflagration-to-detonation transition (DDT) in a combustion chamber filled with a subsonic or supersonic mixture by employing Navier-Stokes equations together with a detailed chemistry reaction mechanism of 11 species and 27 steps under adaptive mesh refinement. The effects of the initial mixture Mach number and mesh resolution on the flame acceleration and DDT were studied in detail, and the entire processes of the flame acceleration, DDT and detonation propagation were revealed. Two DDT mechanisms are obtained in a chamber having the same low blockage ratio but with different initial velocities of the mixture. Regime I: multiple shock wave collisions, shock focusing and shock reflection result in a rapid energy deposition in a small region; a direct detonation subsequently occurs in the boundary wall for the subsonic mixture. Regime II: the classic hot-spot mechanism due to the reactive gradient mechanism is responsible for the detonation transition in the supersonic mixture when the intense leading shock wave impacts and reflects on the solid surface. By increasing the initial mixture Mach number, the run-up time and distance to DDT are dramatically reduced. The flame front structure and propagation in the supersonic flow demonstrate that the detonation cell size rapidly increases when propagating into the smooth region due to detonation attenuation and resulting pressure decrease. Additionally, a much higher combustion temperature occurs in the upper and lower walls because of the Mach stem. In comparison, the results show that the detonation overdrive degree and pressure gain ratio in the subsonic mixture are higher than in the supersonic mixture. Moreover, flame propagation upstream also suggests that increased pressure and temperature occur in the inlet isolation section, even forming a localized explosion point.

Flame front reconstruction and volume estimation through computational geometry: a case study on machine vision applied to combustion systems

A computationally-supported experimental procedure to estimate the primary dimensions of diffusion flames, using volume reconstruction from thermal imagery, is presented. The experimental setup uses a 4 × 16.94 mm radial distribution gas-burner, with a 0.8 mm nozzle diameter, a thermal imaging camera and a proprietary image processing algorithm. Flame thermal imagery was captured, using four different fuel loads, 350, 650, 950 and 1200 cc/min, from two different visualisation planes, 0° and 90°. The images were visually and qualitatively processed leaving aside the temperature measurement and favouring instead a non-dimensional temperature gradient, . Corresponding flame front structures were estimated and reconstructed employing computational geometry. The height and diameter magnitudes were measured indirectly through a reference length. The results show that at the flame front structure separates itself from the background noise. Furthermore, when compared against available benchmarks, at and , the resulting flame coincides with the luminous and continuous flame heights, respectively. This approach yields maximum relative error of 36.54% and 18.91% for both compared geometries. When compared to image convolution and spatial density clustering procedures, this approach reduces the maximum error obtained by 47%. Based on this information, the methodology presented is considered suitable for dimensioning diffusion flames, thus, proposed as an estimation tool for the design and manufacturing of gas-fuelled appliances/devices.

LES flamelet modeling of hydrogen combustion considering preferential diffusion effect

A flamelet-generated manifold (FGM) method that explicitly considers the preferential diffusion effect, referred to as FGM-PD method, is employed for large-eddy simulations (LESs) of a lean-premixed H2/air low-swirl lifted flame, and the validity is examined by comparing with the experiment. First, the applicability of the FGM-PD method is investigated by one-dimensional numerical simulations of planar laminar premixed H2/air flames. Next, LESs of a lean-premixed H2/air low-swirl lifted flame are performed employing the FGM-PD and conventional FGM methods. Results of the one-dimensional numerical simulations show the importance of considering preferential diffusion to accurately predict species concentrations near the flame front. The FGM-PD method accurately predicts this, and therefore, reproduces the laminar burning velocity and spatial distributions of temperature and mixture fraction. Three-dimensional LES results confirm that the prediction accuracy of the velocities near the flame front is improved by employing this FGM-PD method. Additionally, the OH mass fraction distribution predicted by the FGM-PD method exhibits the inhomogeneous finger-like structure, which has been observed in previous experiments. This inhomogeneity of OH mass fraction distribution, which corresponds to that of the reaction rate, predicted by the FGM-PD method, strongly affects the flame front structure.

Deep Neural Network-Based Generation of Planar CH Distribution through Flame Chemiluminescence in Premixed Turbulent Flame

Flame front structure is one of the most fundamental characteristics and, hence, vital for understanding combustion processes. Measuring flame front structure in turbulent flames usually needs laser-based diagnostic techniques, mostly planar laser-induced fluorescence (PLIF). The equipment of PLIF, burdened with lasers, is often too sophisticated to be configured in harsh environments. Here, to shed the burden, we propose a deep neural network-based method to generate the structures of flame fronts using line-of-sight CH* chemiluminescence that can be obtained without the use of lasers. A conditional generative adversarial network (C-GAN) was trained by simultaneously recording CH-PLIF and chemiluminescence images of turbulent premixed methane/air flames. Two distinct generators of the C-GAN, namely Resnet and U-net, were evaluated. The former net performs better in this study in terms of both generating snap-shot images and statistics over multiple images. For chemiluminescence imaging, the selection of the camera's gate width produces a trade-off between the signal-to-noise (SNR) ratio and the temporal resolution. The trained C-GAN model can generate CH-PLIF images from the chemiluminescence images with an accuracy of over 91% at a Reynolds number of 5000, and the flame surface density at a higher Reynolds number of 10,000 can also be effectively estimated by the model. This new method has the potential to achieve the flame characteristics without the use of laser and significantly simplify the diagnosing system, also with the potential for high-speed flame diagnostics.

Flame front dynamics, shape and structure on acceleration and deflagration-to-detonation transition

Developing a new generation of rocket engines based on detonation principles needs methods which ensure stability of operation including the onset of detonation wave after mild ignition. This aspect is crucial for ensuring safety of such engines. In the present paper the transition processes associated with flame acceleration and the onset of a detonation mode are under investigation. High-speed flame front photography has been performed during the deflagration of a premixed gas mixture in a long smooth tube. The evolution of flame front structure and shape has been determined, starting from the moment of deflagration initiation and ending with the occurrence of a local explosion leading to the formation of a detonation wave. Four characteristic flame propagation stages have been identified: the initial flame acceleration stage, followed by the apparent velocity deceleration stage, followed by the nearly constant propagation velocity stage, and finally the second acceleration stage, during which detonation is formed. The changes in process dynamics with a change in the initial pressure have been demonstrated. The behavior of flame front at the early stage of acceleration has been identified. The experimental findings are compared with the theoretical models available in the literature. At the initial stage of the accelerated propagation of flame extended along the channel walls — until deceleration — the velocity of the leading tip of the front is effectively described by the exponential relationship proposed by C. Clanet and G. Searby (1996). The most interesting and poorly studied, in the authors' opinion, stage of the deflagration-to-detonation transition process, the intensive repeated acceleration stage, during which flame shape changes abruptly, has been described in detail. The laminar burning velocity of the stoichiometric acetylene–oxygen mixture diluted with either nitrogen or argon at a reduced initial pressure (8–21 kPa) has been identified.

Pressure dependence of combustion instability for premixed syngas/air in a closed channel

Syngas is a promising alternative energy carrier with low carbon and pollutants emissions, which has huge application potential in internal combustion engines and gas turbines. The combustion characteristics of stoichiometric syngas/air mixtures with varied initial pressure (0.5–2.5 atm) and hydrogen content (10%–90% v/v) were examined through experiments in a rectangular closed channel. The flame images and overpressure dynamics were captured by high-speed schlieren photography and pressure sensors. The flame instabilities and the flame-acoustic wave interaction were explored. The results showed that the flame morphology and dynamics were enhanced with increasing hydrogen content and initial pressure. The Darrius-Landau instability has an impact on flame deformation, which was strengthened with growing initial pressure, while the thermo-diffusive instability could be neglected during combustion. At later stages after tulip inversion, the flame-acoustic wave interaction was relatively outstanding inducing wrinkled flame front and cellular structures on the flame surface.

Effect of porous materials on explosion characteristics of low ratio hydrogen/methane mixture in barrier tube

The effect of porous materials on the explosion characteristics of low ratio hydrogen/methane mixture in barrier tube was studied. The hydrogen ratio in the mixture is 0%–20%. The porosity of porous material is 20 PPI, 40 PPI and 60 PPI. The flame front structure, flame front velocity and overpressure data of the mixture were obtained by using high-speed camera and pressure sensor. The experimental results show that porous materials with the same porosity have similar quenching effects on pure methane explosion and low ratio hydrogen/methane premixed explosion. The greater the porosity, the better the quenching and decompression effect. The porous material with porosity of 60 PPI can meet its ability to attenuate flame front velocity and pressure wave even when adding low ratio hydrogen. The incremental effects of flame velocity and overpressure caused by the explosion of mixed gas with low ratio hydrogen under porous materials in the barrier tube are kept in a safe and controllable range. The research results provide a reference for the subsequent hydrogen mixed natural gas pipeline project.

Planar laser-induced imaging of CH3 for high resolution single-shot reaction-zone visualization in premixed methane/air flames over broad stoichiometric ratios

We report a novel approach for single-shot planar imaging of CH3 radicals in premixed methane and air flames. A 213 nm beam from the 5th harmonic of an Nd:YAG laser was resonantly absorbed by the CH3 radicals, which were excited to the highly pre-dissociative upper level and dissociated to H2 and CH (X), as the main dissociation channel. The CH radicals were consequently excited by a 388 nm beam from an alexandrite laser, and the fluorescence from the excited CH radicals was collected off-resonant at 431 nm. Using this Photo-Fragmentation Planar Laser-Induced Fluorescence (PF-PLIF) technique, instantaneous flame front structures, represented by CH3 radicals, can be visualized with high spatial resolution over a broad range of stoichiometric ratios. Signal-to-noise ratios up to 50 were observed for premixed methane/air flame with stoichiometric ratio as low as 0.26. The CH radicals naturally presented in flame front are more than 400 times lower in concentration than the CH3 radicals in premixed methane/air flames even at the conditions close to stoichiometric or slightly fuel rich cases where the highest CH concentrations exist, and the CH3/CH concentration ratios increase dramatically moving towards fuel lean conditions. By adopting a structured illumination of the 213 nm pump beam, the naturally presented CH radicals were visualized simultaneously with CH3 at slightly fuel rich laminar flames, where the CH signal intensity was 5 times lower than that from CH3. The results indicate that the CH3 PF-PLIF technique can provide much stronger signal than the CH PLIF and presented a much promising potential for applications in fuel-lean flames. Finally, the CH3 PF-PLIF was performed in premixed turbulent flames to demonstrate the feasibility of this technique for flame front visualization in turbulent premixed flames.

Flame Propagation Characteristics of Syngas-Air in the Hele-Shaw Duct with Different Equivalence Ratios and Ignition Positions.

In this paper, the effects of different ignition positions and equivalence ratios on the explosion characteristics of syngas in a half-open Hele-Shaw duct were investigated. The ignition points are set at distances of 0 and 500 mm from the closed end. Moreover, the research range of equivalence ratio is 0.8–1.2. The experimental results indicate that different ignition positions and equivalence ratios influence the flame front structure and the dynamic characteristics of flame propagation. When the ignition position is at the closed end, the flame front undergoes several typical propagation stages before eventually reaching the open end of the duct. The time required by the flame to reach the open end decreases as the equivalence ratio increases. Meanwhile, when the ignition is in the middle of the duct, the flame simultaneously spreads to the open and closed ends. The time required to reach both sides decreases with the increase in the equivalence ratio. The flame front structure and pressure are primarily affected by the ignition position and the equivalence ratio. At the same ignition position, flame propagation velocity and maximum overpressure increase with the equivalence ratio. The pressure oscillation becomes more intense when the ignition position is close to the open end. At IP500, when the equivalence ratio is 0.8, multiple finger-shaped flame fronts emerge, accompanied by high-frequency flame oscillations. This study can provide guidance for the study of the flame propagation characteristics of syngas in millimeter-scale burners.

Experimental study on premixed combustion characteristics of methane-air under dual spark plug ignition strategy in a closed spherical chamber

Dual spark plug ignition technology can effectively improve the low heat release rate of engine caused by slow combustion of natural gas. However, the mechanism of flame stability and ion current in the combustion process of double flame are not clear. This study is based on a nearly spherical closed constant volume combustion chamber experimental platform. Experiments on premixed combustion characteristics of methane-air under different ignition strategies (single spark plug ignition, dual spark plug synchronous ignition and asynchronous ignition) were carried out. The cellular structure of flame front and the relationship between the stretch flame propagation velocity and stretch rate were analyzed. The effects of different ignition strategies on heat release rate and ion current signal were analyzed. The results show that the flame stability of single spark ignition and synchronous ignition is better than that of asynchronous ignition. Under the asynchronous ignition strategy, the flame front appears fold and cellular structure with the increase of ignition interval time. Markstein length is the smallest and flame stability is the weakest due to the action of opposite flame. The heat release rate and mean ion current of synchronous ignition are higher than those of single spark plug ignition, and are not different from those of asynchronous ignition. Dual spark ignition greatly shortens the main combustion duration and is conducive to rapid combustion. Synchronous ignition is a promising ignition strategy which can guarantee flame stability and increase combustion velocity.

On the Flow Structure and Dynamics of Methane and Syngas Lean Flames in a Model Gas-Turbine Combustor

The present paper compares the flow structure and flame dynamics during combustion of methane and syngas in a model gas-turbine swirl burner. The burner is based on a design by Turbomeca. The fuel is supplied through injection holes between the swirler blades to provide well-premixed combustion, or fed as a central jet from the swirler’s centerbody to increase flame stability via a pilot flame. The measurements of flow structure and flame front are performed by using the stereo particle image velocimetry and OH planar laser-induced fluorescence methods. The measurements are performed for the atmospheric pressure without preheating and for 2 atm with the air preheated up to 500 K. The flow Reynolds numbers for the non-reacting flows at these two conditions are 1.5 × 103 and 1.0 × 103, respectively. The flame dynamics are analyzed based on a high-speed OH* chemiluminescence imaging. It is found that the flame dynamics at elevated conditions are related with frequent events of flame lift-off and global extinction, followed by re-ignition. The analysis of flow structure via the proper orthogonal decomposition reveals the presence of two different types of coherent flow fluctuations, namely, longitudinal and transverse instability modes. The same procedure is applied to the chemiluminescence images for visualization of bulk movement of the flame front and similar spatial structures are observed. Thus, the longitudinal and transverse instability modes are found in all cases, but for the syngas at the elevated pressure and temperature the longitudinal mode is related to strong thermoacoustic fluctuations. Therefore, the present study demonstrates that a lean syngas flame can become unstable at elevated pressure and temperature conditions due to a greater flame propagation speed, which results in periodic events of flame flash-back, extinction and re-ignition. The reported data is also useful for the validation of numerical simulation codes for syngas flames.

Numerical Simulation of the Deflagration to Detonation Transition in a Tube with Repeated Obstacles: Experimental Scale Simulation Using the Artificial Thickened Flame Method

Abstract The Artificial Thickened Flame (ATF) method, which involves artificially increasing the flame thickness so as to simulate with a coarse grid resolution, is applied to reduce the computational cost of predicting the Deflagration to Detonation Transition (DDT) in a tube with repeated obstacles. While simulation results depended on the parameter N (the number of grid points in laminar flame thickness), it was found that N values of more than 10 may be excessive. The results show that the chosen simulation method predicts the flame speed as compared to a reference experiment and captures the detail of the strong ignitions near the corner between the obstacle and the sidewall. The present simulation also captures the wrinkle flame front structure during the acceleration of flame.

Investigation of Hydrogen Content and Dilution Effect on Syngas/Air Premixed Turbulent Flame Using OH Planar Laser-Induced Fluorescence

Syngas produced by gasification, which contains a high hydrogen content, has significant potential. The variation in the hydrogen content and dilution combustion are effective means to improve the steady combustion of syngas and reduce NOx emissions. OH planar laser-induced fluorescence technology (OH-PLIF) was applied in the present investigation of the turbulence of a premixed flame of syngas with varied compositions of H2/CO. The flame front structure and turbulent flame velocities of syngas with varied compositions and turbulent intensities were analyzed and calculated. Results showed that the trend in the turbulent flame speed with different hydrogen proportions and dilutions was similar to that of the laminar flame speed of the corresponding syngas. A higher hydrogen proportion induced a higher turbulent flame speed, higher OH concentration, and a smaller flame. Dilution had the opposite effect. Increasing the Reynolds number also increased the turbulent flame speed and OH concentration. In addition, the effect of the turbulence on the combustion of syngas was independent of the composition of syngas after the analysis of the ratio between the turbulent flame speed and the corresponding laminar flame speed, for the turbulent flames under low turbulent intensity. These research results provide a theoretical basis for the practical application of syngas with a complex composition in gas turbine power generation.

Experimental study on characteristics of CH4/H2 oxy-fuel turbulent premixed flames

In this work, a systematic investigation was conducted on CH4/H2 oxyfuel turbulent premixed flames. The effects of hydrogen addition and CO2 concentration on turbulent combustion characteristics of methane oxy-fuel were studied. Based on planar laser induced fluorescence (PLIF) measurement of OH radical, the turbulent flame speed and flame front structure were derived. The probability density function (PDF) of curvature radius of hydrogen enriched turbulent premixed flames was found to be similar to that of cellular flames on flat burner, which identified the role of thermal-diffusive instability. Proper orthogonal decomposition (POD) was further applied to the multi-scale analysis of flame front structure. With increase in hydrogen fraction, the effect of small-scale wrinkles was enhanced by thermal-diffusive effect, which promoted the flame propagation and made the flame prone to instability.

Effect of DC Electric Field on Turbulent Flame Structure and Turbulent Burning Velocity

ABSTRACT Effect of electric field on turbulent premixed flame structure and turbulent burning velocity is investigated through OH-PLIF technique. The ring-plate electrode configuration is used, which the ring electrode is high potential. Flame front structure and turbulent burning velocity are derived to estimate the effect of electric field on turbulent flame. Results show that electric field has similar effects compared with turbulence to some extent in this experiment, which could increase flame volume, turbulent burning velocity and decrease the flame surface density. But there are still substantial distinctions in detail. The perturbation induced by turbulence is the vortex, while that induced by electric field is the directional flow, and it can be verified through the PDF distribution of flame curvature with/without electric field. Effect of electric field is a local effect because it is dependent on the distribution of charged species. The research indicates electric field-assisted combustion is mitigated by the turbulence to some extent, and it may hinder the utilization of electric field at practical application for combustion enhancement, but other aspects need further researches. Two reasons are proposed to explain the mitigation of turbulence. Firstly, the response of turbulent burning velocity to the same perturbation decreases with turbulence intensity. Secondly, the wrinkled structure of turbulent flame mitigates the effect of electric field.