Flame Evolution Research Articles

Laser-induced plasma-ignited hydrogen jet combustion in engine-relevant conditions

This work aims to investigate the mechanisms governing H2 jet flame evolution and stabilisation by precisely controlling the jet ignition location using a laser-induced plasma in compression-ignition engine-relevant conditions. The experiments examine the flame evolution with high-speed schlieren imaging and pressure trace measurements in an optically-accessible constant-volume combustion chamber over a range of ambient O2 concentration (10–21 vol%) and temperature (600–800 K) conditions. Optical results reveal that in most cases, a localised flame kernel forms at the time and location of the laser-induced plasma and grows in connected regions to engulf the entire upstream and downstream jet volume of the ignition spot. The images also reveal that the flame lift-off is sensitive to ambient O2 and temperature changes. The flame appears attached to the nozzle at 21 vol% O2, but becomes lifted at a lower ambient O2 concentration and a colder temperature. A simplified numerical analysis suggests that edge-flame deflagration into stratified premixed fuel-ambient reactant streams explains the lift-off response to ambient O2 and temperature changes. Furthermore, at the lowest tested O2 concentration, the flame stabilisation occurs in leaner mixtures at the jet peripheral where it is likely exposed to stronger turbulence-chemistry interactions.

Effects of the partially open inlet on shock waves and spontaneous ignition during the leakage of hydrogen

Unexpected spontaneous combustion and shock waves can occur when high-pressure hydrogen leaks into pipelines, whilst irregular leakage ports affect their features. In this paper, shock waves and self-ignition are experimentally and numerically studied after the pressurized hydrogen is released through the partially open inlet. And the effects of tube length and release pressure are investigated. Pressure signals, light signals, and flame images are used to characterize the shockwave, self-ignition, and flame propagation. Results show that the shock-affected region can be formed near the partially open inlet. It is accompanied by complex wave structures, shock wave interactions, and shock wave focusing. The contact surface is distorted and deformed. The flow field parameters near the inlet change dramatically and are unevenly distributed, which affect the overpressure characteristics recorded by the pressure sensors. The initial intensity of the shock wave is lower than that in tubes with the fully open inlet at the early stage of the leakage. In addition, the partially open inlet influences the critical pressure at which spontaneous ignition occurs and the flame evolution inside or outside the tube. It has an inhibition effect on spontaneous ignition, but this inhibition effect weakens with increasing tube length.

Burning rate estimation based on flame evolution in a channel

Non-stationary modes of gaseous fuel combustion are of paramount importance for their efficient and safe usage in the space industry. The paper analyzes experimental and numerical data on the early stages of flame propagation in channels of different widths. It is demonstrated that there is a self-similar behavior of flame development at least at the considered stages. Qualitatively flame acceleration proceeds exponentially in channels of different widths, though the particular acceleration increment is inversely proportional to the width of the channel. Exponential acceleration is related to the positive feedback between the flame and flow dynamics, while the previous stage is associated mainly with the flame propagation induced by the expansion of combustion products. Based on the analysis of experimental data, one can estimate both the time instant at which the exponential stage starts and the increment of exponential acceleration, which in turn depends unambiguously on the normal burning rate. As a result, a novel technique for normal burning rate determination with the use of a channel reactor can be proposed.

Flame acceleration and onset of detonation in inhomogeneous mixture of hydrogen-air in an obstructed channel

Flame acceleration (FA) and deflagration-to-detonation transition (DDT) in combustible premixed gases is a complex process involving the flow and combustion phenomena. Practically, the combustible premixed gases are often inhomogeneous. The numerical study aims to explore the effects of different solid obstacle distribution patterns on the FA and DDT process in the inhomogeneous concentration field. Results show that the shape of detonation wave is arc-shaped due to the inhomogeneous hydrogen concentration, and the detonation initiation process can be classified into three types: i) detonation induced by the coupling of the flame surface and the high-pressure zone; ii) detonation induced by the interaction between flame surface and reflected shock wave; iii) detonation induced by the interaction between the oblique shock wave or Mach stem and obstacles in the flow field. Also, this study finds that the flame evolutions have experienced four velocity augmentation regions before the detonation initiation. Further, the rich hydrogen content in the aperture and the higher unilateral blockage ratio can both promote flame acceleration, and further shorten the run-up time of the DDT.

Combined experimental and numerical studies on soot characteristics of diesel sprays with split injection strategies

Experimental and numerical studies on the soot characteristics of Spray A flames were conducted with a focus on the effects of different split injection strategies in high-pressure high-temperature vessels within the engine combustion network. Three high-speed imaging techniques were used to obtain the transient information of flame evolution. Schlieren was applied to identify the spray boundaries and spray penetration. Moreover, a combined extinction and radiation technique was applied to quantify the spatial soot and temperature fields simultaneously. Finally, high-speed OH* chemiluminescence was applied at the baseline condition to measure the temporal high-temperature flame evolution. To substantiate the analysis, the experimental results were compared with the computational fluid dynamics (CFD) predictions from a Reynolds-averaged Navier Stokes approach with Unsteady Flamelet Progress Variable combustion and a two-equation soot-in-flamelet models, which qualitatively reproduced the experimental results.According to the chosen split injection strategy, dwell and first-injection pulse durations were varied. For all cases, the ignition delay (ID) and soot production of the second-injection pulse were shorter and higher than those of the first one, respectively. A slight change in the experimental soot production was observed when the dwell time was changed. The CFD calculations revealed a small reduction in the ID of the second pulse with long dwell times, which was due to variations in the interaction between the newly injected fuel and combustion recession. However, the effect of dwell variation was extremely small to produce significant differences in the evolution of the second pulse, neither in combustion nor in soot production. With regard to the duration of the first injection, no effect on the experimental soot production of the second pulse was observed despite noticeable differences due to the first pulse. The CFD results indicated that because of the quasi-steady flow and combustion process near the nozzle along the first injection, the evolution of the spray in the region close to the nozzle was identical in all first-pulse-duration cases, with differences only occurring at the tip of the first pulse jet. Thus, the second pulse evolution occurred under identical conditions for ignition and soot production. Although the sensitivity to both dwell and first-pulse durations was low, the CFD analysis highlighted the important effects of both end-of-injection wave propagation and combustion recession on the ignition and soot production of the second pulse.

Effects of CO2 addition on deflagration characteristics of syngas-air premixed mixtures in T-pipeline

With the industrial application of syngas, the explosion accident caused by it has gradually become a topic of concern for researchers. In this paper, the effects of CO2 addition on the deflagration characteristics of syngas-air premixed mixtures were investigated through experiments and numerical simulations. Experiments were carried out inside a T-pipeline, using a high-speed camera and a pressure sensor to simultaneously record the flame evolution and pressure dynamics during deflagration. Simulations were calculated using the GRI 3.0 mechanism by Chemkin Premix Code. The results show that the addition of CO2 has a certain inhibitory effect on the flame propagation, which can make the finger flame in the vertical pipe evolve into a “tulip” flame. And under the inhibition of CO2, the deflagration overpressure of the mixture is reduced, and the number of H, O, OH radicals is also greatly reduced, and the chemical reaction rate is correspondingly slowed down.

Experimental study on the flame height evolution of two adjacent hydrocarbon pool fires under transverse air flow

Hydrocarbon energy storage has a wide range of applications in modern production and life. Regarded as an important form of energy dissipation, combustion plays an important role in energy storage failure. The possible ignition of adjacent leaked fuel might induce multiple fires burning simultaneous. The present work experimentally characterizes the flame evolution of two adjacent pool fires with square and line burners under transverse air flow. As for the effect of transverse air flow, the flame height evolution was recorded and measured involving six different transverse air flow speeds, as the transverse air flow speed increased up to 3 m/s, the flame height gradually decreased. When the transverse air flow increased from 3 to 3.7 m/s, the flame height only changed slightly on both types of burners. A new correlation was proposed that which correlates well the flame height as a function of the ratio of air entrainment caused by transverse air flow to that induced by the flame buoyancy itself in quiescent condition, including the fuel of heptane and ethanol, and the Froude number representing flame tilting caused by transverse air flow. The experimental data and proposed correlations in the present study provide an essential base for quantifying two adjacent pool fires characteristics under transverse air flow.

Investigation of flame evolution and stability characteristics of H2-enriched natural gas fuel in an industrial gas turbine combustor

Enhancing the H2 blending ratio in industrial gas turbines is essential to reduce fossil energy consumption and achieve carbon neutrality. However, several uncertainties still exist in their safe operation with H2 addition. In this work, a numerical investigation is conducted to evaluate the behavior and combustion stability of an H2-enriched natural gas fuel premixed swirl flame in an industrial gas turbine. The results indicate that the flame exhibits three types of behaviors upon increasing the H2 blending ratio, i.e., stable combustion, central flashback, and boundary layer flashback. The flame structure is highly sensitive to the inlet air temperature. The combustion reaction zone moves to the swirler as the inlet air temperature decreases, which in turn aggravates the central flashback. Injecting recirculated flue gas into the inlet air is proposed to restrain the central flashback. It is found that the flashback and blow-out limits exhibit quadratic polynomial and linear relationships with the recirculated flue gas ratio, respectively. The laminar flame speed of H2-enriched natural gas premixed flames with recirculated flue gas dilution at the flashback limit is 35.8 ± 0.5 cm/s. It is recommended to design or control H2-blended gas turbine burners with a laminar flame speed to ensure operational safety.

Flame and shock wave evolution characteristics of methane explosion in a closed horizontal pipeline filled with a three-dimensional mesh porous material

In the energy security field, porous materials are regarded as superb explosive attenuators that quench flames and dissipate explosion energy waves. Traditional porous materials based on polymer have been extensively explored for their flame and explosion-proof capabilities, but their applications are limited due to the lack of indispensable physical properties such as flame-retardant and antistatic properties. The polymer composites may contaminate the oil medium, as does the deterioration of their properties. By designing the material's structure and formula, a new three-dimensional mesh porous flame and explosion-proof material (TFEP) is developed through melt blending—extrusion spinning technology and material's explosion-proof performance will also be enhanced. This porous material integrates flame-retardant, antistatic, and oil-resistant performances. The TFEP's suppression effect on 9.5 vol% CH4/air premixed gas explosion was investigated under various filling conditions, and the optimal filling condition of TFEP was explored. Under these conditions, TFEP completely quenches the explosion flame, and the maximum explosion overpressure decay ratio for the upstream and downstream of the pipeline are 87.5% and 82.5%, respectively. TFEP eliminates the electrostatic risk generated by long-term friction of traditional polymer materials. It has moderate oil resistance and may be a candidate for explosion-proof materials in the fuel oil field.

The evolution and structure of ignited high-pressure cryogenic hydrogen jets

The anticipated upscaling of hydrogen energy applications will involve the storage and transport of hydrogen at cryogenic conditions. Understanding the potential hazard arising from leaks in high-pressure cryogenic storage is needed to improve hydrogen safety. The manuscript reports a series of numerical simulations with detailed chemistry for the transient evolution of ignited high-pressure cryogenic hydrogen jets. The study aims to gain insight of the ignition processes, flame structures and dynamics associated with the transient flame evolution. Numerical simulations were firstly conducted for an unignited jet released under the same cryogenic temperature of 80 K and pressure of 200 bar as the considered ignited jets. The predicted hydrogen concentrations were found to be in good agreement with the experimental measurements. The results informed the subsequent simulations of the ignited jets involving four different ignition locations. The predicted time series snapshots of temperature, hydrogen mass fraction and the flame index are analyzed to study the transient evolution and structure of the flame. The results show that a diffusion combustion layer is developed along the outer boundary of the jet and a side diffusion flame is formed for the near-field ignition. For the far-field ignition, an envelope flame is observed. The flame structure contains a diffusion flame on the outer edge and a premixed flame inside the jet. Due to the complex interactions between turbulence, fuel-air mixing at cryogenic temperature and chemical reactions, localized spontaneous ignition and transient flame extinguishment are observed. The predictions also captured the experimentally observed deflagration waves in the far-field ignited jets.

Experimental Observation of Premixed Methane/Air Flame Propagation in an Obstructed Tube With Increasing Blockage Ratios

Abstract Methane explosion is one of the major hazards in the mechanical industry. In this paper, a series of methane/air premixed combustion experiments were carried out to study the influence of blockage ratio on flame propagation. The blocking ratio (BR) is referred as the cross-sectional area of occlusion divided by the total area. Flame shape and pressure response were measured and analyzed. The results showed that five typical stages were experienced for all configurations except for BR = 0.999, particularly, a diamond-shaped flame and a mushroom-shaped flame front were observed for some lower BRs. The largest length of jet flames was selected to characterize flame–obstacle interaction rose smoothly until BR = 0.9 and then declines rapidly. The flame evolution process was simulated by a power-law flame wrinkling model. Flame behaviors and pressure dynamics for each configuration were investigated, and the maximum flame speed, explosion pressure, and growth rate coincide exactly with the largest length of jet flames in trend. It can be seen that BR was of great significance to flame propagation and BR = 0.9 was a limit or a watershed. In addition, the pressure growth rate was positively correlated with the flame tip speed.

Formation and evolution characteristics of hydrothermal flames inside a transpiring wall reactor: A transient numerical investigation

Transpiring-wall reactors with hydrothermal flames are effective in preventing corrosion and salt deposition in supercritical water oxidation systems. To uncover the not-well-understood evolution characteristics of the hydrothermal flame surrounded by a transpiring water film, a 2-D unsteady simulation is firstly conducted in this study. The transient results show that the variations of flow fields and flame shapes are attributed to the competition between inertial forces and buoyant forces. And the quasi-steady results present that high feed concentration may damage transpiring wall and delay the evolution of hydrothermal flames. In addition, a low feed temperature or a big flow area of jet can lead to open hydrothermal flames, which adversely affect the reactor. Furthermore, Froude Number, Fr, shows effectiveness in predicting quasi-steady flame shapes. For large-Fr conditions, the inertial force outweighs the buoyant force and hence closed flames are developed; while for low-Fr cases, open flames are developed.

Non-premixed flame propagation inside and outside the different three-way tubes after the self-ignition of pressurized hydrogen

Accidental leakage of pressurized hydrogen into the pipeline is often accompanied by the non-premixed self-ignition flame, whilst the complex structure of pipeline affects the flame propagation. In this study, the propagation of self-ignition hydrogen flame in different three-way tubes is studied and the flame evolution near the two exits is compared. Effects of the tube length and the location of bifurcation points are investigated. Results show that the bifurcation structure has the attenuation effect on the intensity of flame and shock wave, and they attenuate more sharply in the branch pipeline. But the flame intensity can increase again downstream the bifurcation point in the trunk pipeline, and this increase is not continuous and decrease again as the tube length increases. In addition, the flame evolution can be different near two exits of the tube. As the bifurcation point moves downstream, the number of Mach disk and the time required to form the stable flame will increase near the exit of branch pipeline. Despite different flame evolution, the flame morphology and propagation are all controlled by the shock wave structures before the formation of the vortex, but the flame changes dramatically and grows rapidly to form the stable lift-off flame after the formation of the vortex.

Study on the Influence of Vent Shape and Blockage Ratio on the Premixed Gas Explosion in the Chamber with a Small Aspect Ratio.

Given the current situation of deflagration caused by gas leakage for domestic use and the study of the single shape of the vent, the differential change law of vent parameters on the small aspect ratio of the chamber explosion flame evolution and explosion overpressure and other effects were experimentally investigated. Based on the theory of geometric similarity, the experimental platform of a small length–diameter ratio explosion chamber was designed and built, and the premixed gas explosion experiment was carried out by changing the shape (square and rectangle) and blockage ratio (0.1, 0.3, 0.5, 0.7, and 0.9). The explosion flame structure, flame front position, flame propagation speed, explosion pressure waveform, overpressure peak value, and so on were tested and analyzed. The results showed that the blockage ratio had the most significant effect on flame propagation and explosion flame evolution. With the increase of blockage ratio, the stretching degree of the flame became more and more obvious, the flame front became sharper and sharper, and the sharp flame changed from the lower part to the upper part. The position of the flame front increased rapidly, and the steepness and peak value of explosion overpressure became larger. The oscillation of the flame propagation velocity was more violent after the turn, and the speed of propagation to the outside of the chamber gradually accelerated. In the same blockage ratio, there was a difference in the time point at which the flame propagation velocity turned under the rectangular and square shape of the vent. In blockage ratios of 0.1, 0.3, 0.5, and 0.9, the peak overpressure reduction under the shape of the blast square relative to the rectangle was 41.3, 47.9, 1.03, and 27.6%. This indicated that the explosion relief effect of the square was better than that of a rectangle. The research results can provide a reference and basis for the reasonable deployment of explosion venting and explosion decompression work.

Research on flame prediction in a scramjet combustor using a data-driven model

Flame prediction using deep learning technology could promote the research and development of flame propagation in scramjet combustors. A data-driven prediction model is proposed to effectively predict a future flame based on the flame sequence at the previous moments. A convolutional neural network is used to construct the prediction model, and the network training is performed using an experimental dataset. Ground experiments are conducted in a scramjet combustor using different equivalence ratio variation laws, and the flame evolution in the experiments is recorded and processed into a dataset. The flame prediction accuracy of the proposed model under different equivalence ratio variation laws is analyzed in detail. Moreover, both subjective and objective analysis results show that the flame prediction well agrees with the experimental result, and the flame boundary and area are accurately predicted to a certain extent. The influence of the prediction span on the flame prediction accuracy is also discussed.

Characterizing and predicting 21700 NMC lithium-ion battery thermal runaway induced by nail penetration

• Pioneering CFD predictions of nail penetration induced TR in 21700 cylindrical cells. • Internal heat generation is coupled with external convective and radiative heat exchange. • New tests to establish the differences in radial and axial penetration of cylindrical cells. • Fill experimental gaps by quantify the effects of contact resistance and external heat transfer. • The generic nature of the CFD based predictive tool can be easily adapted for other cell types. Combined numerical and experimental studies are conducted to characterise 21,700 cylindrical lithium-ion battery (LIB) thermal runaway (TR) induced by nail penetration. Both radial and axial penetrations are considered for 4.8 Ah 21,700 NMC cell under 100% state of charge. Heat generation from the decomposition of the cell component materials are analysed. The maximum cell surface temperature rise and time to reach it in both types of penetration tests are compared. Snapshots from the video footages captured by three high definition and one high speed cameras shade light on the dynamic processes of spark ejection and flame evolution. A generic predictive tool is developed within the frame of the in-house version of open-source computational fluid dynamics code OpenFOAM for nail induced TR. The code treats the cell as a lumped block with anisotropic thermal conductivities and considers heat generation due to nail induced internal short circuit resistance, exothermic decomposition reactions and heat dissipation through convective and radiative heat transfer. Validation with the current measurements shows promising agreement. The predictions also provide insight on the magnitudes of heat generation due to internal short circuit resistance, decompositions of solid electrolyte interphase layer (SEI), anode, cathode and electrolyte. Parametric studies further quantify the effects of cell internal short circuit resistance, contact resistance between the nail and cell, convective heat transfer coefficient and cell surface emissivity on TR evolution.

A parametric study of autoigniting hydrogen jets under compression-ignition engine conditions

This study examines the flame evolution of autoigniting H2 jets with high-speed schlieren and OH∗ chemiluminescence optical methods in a constant-volume combustion chamber over a wide range of simulated compression-ignition engine conditions. Parametric variations include the injector nozzle orifice diameter (0.31–0.83 mm), injection reservoir pressure (100–200 bar), ambient temperature (1000–1140 K), density (12.5–24 kg/m3) and O2 concentration (10–21 vol.%). The jet ignition delay was found to be highly sensitive to changes in ambient temperature while all other parameter variations resulted in minor ignition delay changes. Optical imaging reveals that in most cases, the reaction front of the H2 jet initiates from a localised kernel, before engulfing the entire jet volume downstream and recessing towards the nozzle. The flames attach to the nozzle, except at the lowest ambient oxygen condition of 10 vol.% O2 for which a lifted flame is observed. The H2 diffusion flame length shows a dependence on both the mass flow rate and the level of O2 entrainment that follows the same correlations as previously established for atmospheric H2 jet flames.

Effect of hydrogen concentration on the vented explosion of hydrogen–air mixtures in a 5-m-long duct

A series of vented hydrogen-air explosion experiments were carried out in an end-vented rectangular tube, and the effect of hydrogen concentrations varying from 10% to 50% on the flame evolution and the maximum overpressure inside and outside the duct were investigated. Experimental results reveal that the flame behavior was strongly affected by hydrogen concentration: the flame speed at the vent exit first increased and then decreased as hydrogen concentration was increased; a tulip flame and two bright fireballs were respectively observed within and outside the duct in some tests. Two dominant pressure peaks and two types of oscillations could be distinguished in the internal pressure curves, corresponding to the rupture of the membrane, venting of the burned gas, Helmholtz, and acoustic oscillations, respectively. Under the current experimental conditions, the pressure peak with a low amplitude owing to the rupture of membrane dominated the external pressure–time histories for hydrogen concentrations less than 25%. However, when the hydrogen concentration ranged from 30% to 50%, the pressure peak caused by the second external explosion become dominant. Besides, the influence of the external explosions on the internal pressure peak structures was discussed. As hydrogen concentration was increased, the maximum internal overpressure first increased and then decreased, whereas the maximum external overpressure increased monotonically. In this study, the maximum reduced overpressures calculated by Molkov’s best-fit model agreed well with the experimental data for near-stoichiometric and rich hydrogen-air mixtures.

Effects of jet/flame interaction on deflagration-to-detonation transition by non-reactive gas jet in a methane-oxygen mixture

Jet-in-crossflow (JICF) is a potential and reliable technique that can promote flame acceleration and deflagration-to-detonation transition (DDT). Most of the previous researches utilized combustible mixture or oxygen as the medium of JICF, therefore the detonation enhancement effect may be partly attributed to the chemical properties of the jet, and the purely turbulent effect induced by JICF on DDT has not been fully understood. In our previous work, the non-reactive gas jet affected flame propagation indirectly by separating the injection and ignition in the control sequence. A competing mechanism between the positive combustion-enhancement effect caused by jet-induced turbulence and the negative non-reactive gas-inhibition effect on combustion has not been found, the non-reactive gas jet with high pressure and short duration time was also verified to be beneficial for precursor shock wave formation and flame acceleration. However, no detonation was observed in the work. In this paper, the non-reactive jet interacts with the flame propagation directly by overlapping the working time of the jet and ignitor partly, and the effect of jet parameters including injection pressure and time delay on DDT is investigated systematically. The results show that the flame affected by the non-reactive gas jet with a high-pressure ratio and a short time delay can transit to detonation in a short run-up distance successfully. The schlieren images of the flame evolution process illustrate that the flame interacted by the jet experiences four states: layered flame, wrinkled flame, cellular flamelets, and turbulent flame. Kelvin–Helmholtz (K-H) instability and Rayleigh-Taylor (R-T) instability dominate in the flame distortion. The characteristic timescales of the flow and reaction are computed in the four flame states, turning out that as the flame evolving, the turbulent intensity of the flame is enhanced. Damköhler number (Da) on the flame front eventually converges to [1,2] demonstrates the characteristic timescales of the turbulence and reaction are on the same order, and the turbulence dominates the flame evolution.

Investigation of the effect of turbulence induced by double non-reactive gas jet on the deflagration-to-detonation transition

Jet-in-crossflow (JICF) is proved to be advantageous to accelerate flame propagation and stimulate the deflagration-to-detonation transition (DDT) process in the recent decade. Studies focused on the performance of the jet of combustible mixture or reactive gas on facilitating the DDT process are carried out, and the results show that the combustible JICF can promote the flame transition to detonation efficiently. Although most investigations attribute the ability of the JICF on promoting DDT to the turbulent effect induced by the jet, the combustion-enhancement effect of the jet medium is seldom considered. In our previous work, it is proposed that the non-reactive gas jet in an optimal condition can accelerate flame propagation and promote the precursor shock wave formation. Hence, the performance of the multi-jet of non-reactive gas on facilitating the DDT process is worth investigating. In this paper, three different double jet placement methods are designed: parallel, staggered, and opposite positioned, and their performances on accelerating flame propagation and promoting the DDT process are investigated systematically. The results indicate that the staggered jet can accelerate the flame to detonation within a short run-up distance whereas the parallel jet and the opposite jet can only make the flame accelerate slightly. The schlieren images of flame evolution affected by double jet indicate that the complicated reactivity gradients generate on the flame surface while the flame is disturbed by the double jet. The interaction between the staggered jet and the flame induces local explosions, which results in a prominent increase of the flame area and a large volumetric burning rate. For the opposite jet, its negative effect of combustion-inhibition of the aggregated CO2 overwhelms the positive effect of turbulence-enhancement on the flame acceleration.