Effects of explosion-venting interlayer height on methane explosion dynamics in utility tunnels

Firebrands are recognized as a major source of wildland fuel ignition and a critical driver of fire spread in wildland–urban interface (WUI). This study experimentally examines the ignition behaviour of two widely present vegetative fuel beds in the WUI, pine needle and eucalyptus, when exposed to glowing firebrands under both no wind and wind conditions. Key ignition parameters, including fuel consumption rate, rate of spread, and flame development, were evaluated. Pine needle beds consistently exhibited more intense burning behavior than eucalyptus, with higher fuel consumption rates, faster fire spread, and greater flame heights. For the same fuel load, the average peak values of mass loss rate, rate of spread, and flame height in pine needle fuel beds were approximately 4.5, 1.5, and 1.85 times greater, respectively, than in eucalyptus. Increasing fuel load resulted in increased mass loss and flame height by factors of approximately 1.7 and 1.3 times, respectively, while reducing the rate of spread to about 0.9 times. A notable flame separation phenomenon was also observed during spot fire, where the flame front detached and subsequently created two flame zones. These findings highlight the importance of fuel structure in determining ignition intensity and fire spread under firebrand exposure. • Fuel type, load, and wind govern ignition and fire propagation dynamics • Pine needle beds burn faster with higher consumption rates, and greater flames • Once spot ignition was initiated in the fuel, it leads to sustained fire propagation • Fires in fuel beds with lower loads spread faster but produce smaller flames • Spot fires cause flame separation, forming two flame zones

Stabilization of confined premixed flames in supersonic flows

Hydrogen-fueled internal combustion engines (H 2 -ICEs) hold strong potential as a pathway toward CO 2 -neutral propulsion. To reduce emissions, H 2 -ICEs are usually operated under fuel-lean conditions, where the flames are prone to thermo-diffusive instabilities (TDIs). These TDIs govern both local and global flame propagation, but their impact on full-scale engine combustion remains an open question. In this study, high-fidelity three-dimensional large-eddy simulations (LES) are performed at multiple mesh resolutions, with the finest grid sufficiently resolved to directly characterize flame front dynamics relevant to engine-scale combustion. The simulations reveal cellular and finger-like flame structures characteristic of TDIs throughout the entire combustion process. Analysis of the local thermo-chemical state demonstrates that differential diffusion induces pronounced mixture stratification and elevates reaction rates, resulting in super-adiabatic temperatures that strongly correlate with flame curvature. Building on these findings, the performance of the baseline artificially thickened flame (ATF) model and a recently developed thermo-diffusive (TD)-aware extension is assessed. Unlike the state-of-the-art ATF model, which suffers from grid dependence and underestimates the experimental pressure trace, the TD-aware formulation captures experimental trends more accurately and provides consistent, grid-independent integrated heat-release (IHR) traces. For the operating condition considered here, the results show that TD effects represent sub-grid-scale contributions that need to be accounted for to obtain consistent predictions of global combustion behavior under the investigated lean H 2 -ICE conditions. • LES captured cellular and finger-like thermo-diffusive flame structures. • Coarser grids suppressed fine-scale instabilities resolved at high resolution. • Local mixture stratification enhanced reactivity and caused super-adiabatic states. • ATF model showed grid bias from missing thermo-diffusive instability treatment. • Thermo-diffusive-aware ATF model reduced grid bias and improved predictive accuracy.

ABSTRACT This study uses a 1 m3 spherical explosion vessel to examine how nitrogen dilution affects the suppression of explosions in hydrogen-enriched methane – air mixtures. Tests are carried out under equivalence ratios (Φ) ranging from 0.6 to 1.4 with nitrogen volumetric fractions of 0%, 5%, 10%, 15%, 20%, and 25%, assessed via high-speed imaging and pressure analysis. Results indicate that Φ = 1.0 corresponds to the maximum explosion intensity, characterized by peak values of flame propagation velocity, maximum explosion pressure (Pmax), and maximum pressure rise rate ((dp/dt)max). Under this condition, the flame initially exhibits a nearly spherical structure typical of laminar propagation. As nitrogen concentration increases, the flame front shows increasing distortion and reduced structural coherence, accompanied by a decrease in flame propagation velocity of up to 41.88% relative to the undiluted mixture. At high hydrogen ratios (φ), nitrogen’s inhibition decreases due to enhanced free radical reactions. Nitrogen with a concentration ≥15% can reduce Pmax and (dp/dt)max by 36–50%. With φ = 0.7, 25% nitrogen delays tmax, marking it as a turning point for explosion behavior. This work helps to explain the nitrogen inhibition behavior in large-scale methane-hydrogen-air explosions and provides experimental support for the safety assessment of industrial fuel gases.

To reveal the evolution characteristics of flame and shock waves in gas-coal dust coupled explosions under conditions of sudden cross-sectional changes, this paper designs and builds a transparent plexiglass pipeline test system with three different cross-sectional sizes. Comparative experiments were conducted on coupled explosions with 8.5 and 10.5% gas concentrations and 100g/m³ coal dust. By combining high-speed camera technology and a high-frequency pressure acquisition system, a systematic analysis was performed on the morphological evolution of the explosion flame, changes in flame front velocity, shock wave overpressure distribution, and its dynamic fluctuation behavior. The results indicate that gas concentration has a decisive impact on the flame propagation speed, acceleration timing, and pressure peak value of the coupled explosion. Under the condition of 10.5% gas, the time taken for the flame to develop from ignition to the outlet is shorter (31 ms vs. 42 ms), the peak velocity is higher (348.6m/s vs. 260.66m/s), and the pressure peak occurs earlier and is more intense (812.05 mbar vs. 509.97 mbar). The sudden cross-sectional change structure significantly affects the flame morphology and shock wave propagation, with small cross-sectional pipelines inducing flame stretching and acceleration, while large cross-sectional areas lead to rapid pressure decay. Under low gas concentration conditions, the shock wave exhibits stronger nonlinear fluctuations and reflection superposition characteristics. The study reveals the coupling mechanism of gas concentration and pipeline structure in regulating the propagation of coupled explosions, providing a theoretical basis for the prediction and prevention of explosion disasters under complex mine conditions.

On the spatial structure and intermittency of soot in a lab-scale gas turbine combustor: Insights from large-eddy simulations

Abstract High-speed shadowgraph imaging combined with optical flow analysis is used to obtain high resolution, time resolved velocity fields in a hydrogen fuelled valveless pulsejet engine. The experiments were conducted using a rectangular cross-section pulsejet engine, with shadowgraph images acquired at 8000 frames per second and an exposure time of 3 µs. Optical flow velocities around pulsejet engine inlet and exhaust were extracted using a physics based variational formulation and converted from image plane units to physical units using spatial calibration and the known inter-frame time. Optical flow validation using a synthetic Oseen vortex pair demonstrated good agreement with analytical solutions, yielding a normalized root mean square velocity error of 1.68%. The optical flow analysis was able to successfully capture flow features of the pulsejet engine cycle. The initial flame front propagation at the inlet reached a peak velocity of 73.2 m/s. During the exhaust phase, the maximum axial velocity at the tail exit was approximately 155 m/s, while the peak suction velocity during the intake phase was -52 m/s. A classical ring vortex was identified at the tail moving downstream with maximum velocity of 69 m/s. The results demonstrate that optical flow applied to high-speed shadowgraph imaging provides reliable quantitative velocity field in highly unsteady reacting flows. Optical flow method's ability to directly estimate vortex size, convective speed, and pulsejet engine flow features from shadowgraph images offers a practical tool for improved understanding of thermoacoustic and combustion instabilities.

ABSTRACT Numerical simulations were performed to study the evolution of a flame front interacting with two consecutive shocks. The two-dimensional (2D) fully compressible, reactive Navier–Stokes equations were solved using a high-order numerical method on a dynamically adapting mesh. Two flame configurations were examined: light/heavy (shock propagating from burned to unburned region) and heavy/light (opposite direction), with various inter-shock intervals normalized by the flame characteristic time (Δt*). The results show that the flame instability progresses through five stages during the double shock interactions: compression, perturbation growth, secondary compression, enhanced perturbation growth, and flame-tip collapse, and that the perturbation growth rates increase monotonically with Δt* for both configurations. For the light/heavy cases, the second shock enhances flame perturbation growth for all Δt* values through vorticity superposition, as both shock–flame interactions generate baroclinic vorticity with the same sign. By contrast, for the heavy/light cases, the second interaction generates vorticity with polarity opposite to that produced by the first interaction. This leads to different perturbation responses depending on Δt*. At Δt* ≥0.48, the second interaction also enhances perturbation growth. However, at shorter interval (Δt* = 0.25), the vorticity from the first interaction has not been fully dissipated before the second shock arrival, resulting in partial cancellation of the newly generated opposing vorticity from the second interaction. This vorticity cancellation leads to significantly weaker flame perturbation growth compared to the single shock case.

Model experiments and theory of flame front propagation in forest fire.

ABSTRACT The current energy context raises the awareness to search for renewable energy sources to lower the dependence on the fossil fuels in our energy mixes and go toward a more sustainable future. Biogas, a renewable fuel obtained from the biological breakdown of biomass, has gained a significant interest lately due to its similarity with CH4. However, because of its high CO2 content, the combustion of this renewable fuel faces a variety of instabilities such as blow-off and liftoff. In this study, the effect of CO2 addition and burner geometry on the laminar combustion of biogas is investigated. For this purpose, the range of CO2 in the fuel was varied from 0% to 70% while maintaining the flame power at 1 kW. an ICCD camera coupled with OH* filter was used to study the chemiluminescence and the effect of the mentioned parameters on the flame front and the combustion characteristics. Four different type of burners (laboratory burners, propane/butane stove, natural gas stove, and biogas stove) are used to assess the geometry effects on the biogas combustion and to determine the range of operating conditions, CO2 in particular, of every gas stove. A gas analyzer is used to measure the NOx and CO emissions. The experiments using the laboratory burners show that the increase in CO2 in the blend enhanced flame instability and increased the liftoff height while the increase in the burner’s diameter helped stabilize the flame and delayed the liftoff. It was also found that the addition of CO2 increased CO and decreased NOx emissions in exhaust gases. The results of biogas combustion in different gas stoves demonstrated that they are not compatible for sustaining a stable flame of the biogas with a high content of CO2 as the blow-off started at 20% CO2 for propane/butane, at 40% for natural gas stove and at 70% CO2 for the biogas stove. The OH* chemiluminescence images show how the CO2 content changed the flame front and altered the combustion chemistry. These findings highlighted the effect CO2 proportion in the biogas mixture on flame behavior for different burners, it is challenging to use the commercial stoves the biogas.

To investigate the propagation characteristics of gas-coal dust coupled explosions in a confined space with varying cross-sections, this study independently constructed an explosion test pipeline system featuring expansion and contraction structures. Explosion tests were conducted at different gas concentrations (8.5% and 10.5%) with the participation of 100 g/m3 coal dust. Using high-speed photography and pressure measurement systems, the flame propagation behavior, evolution of the flame front velocity, distribution of shock wave overpressure, and dynamic pressure fluctuation characteristics were systematically analyzed. The results indicate that gas concentration significantly affects the flame's response to changes in the cross-section. Under the 10.5% gas condition, flame propagation is more continuous and the exit velocity reaches 170.15 m/s, which is approximately 42% higher than that under the 8.5% condition. The involvement of coal dust releases substantial energy during the middle and later stages of combustion, forming a multipeak pressure structure. This induces pressure reconstruction and flame acceleration in the cross-sectional contraction zone. Under the 10.5% gas condition, a significant overpressure rebound occurs in the region with abrupt cross-sectional changes, reflecting the combined modulation effect of the gas-coal dust coupled explosion and geometric discontinuity on the explosion dynamics. This study reveals the propagation mechanism of gas-coal dust coupled explosions in variable cross-section pipelines, providing a theoretical basis for explosion risk assessment and prevention in tunnels with complex structures.

The article presents the results of numerical simulation of unsteady gas-dynamic and thermal processes occurring during combustion of a stoichiometric mixture of heptane vapors with air in a semi-closed cylindrical tube simulating the gas-generating cavity of a pulse-action fire extinguishing device. The relevance of the study is determined by the need to create a reliable physical and mathematical basis for describing the working process of gas generation, which is a prerequisite for designing fire extinguishing devices with enhanced characteristics. The simulation was performed in the ANSYS Fluent 2023 R1 software package using unsteady Navier-Stokes equations for a compressible reacting multicomponent gas, a k-ε realizable turbulence model and a Species Transport combustion model with oxidation kinetics according to the Arrhenius law.Based on the calculation results, the spatial and temporal distributions of temperature and pressure at five characteristic stages of the process are obtained. It is shown that the gas temperature in the reaction zone increases from 1 653 K at initiation to 4 884 K at the stage of advanced combustion at the closed end, and then stabilizes at the level of ~3 100K by the time the mixture is completely burned out.Gorenje. The maximum pressure at the closed end reaches 4,2 atm with an increase rate of ~5,1 atm/s. It is established that the acceleration of the flame front is realized by the Shelkin mechanism due to the interaction of expanding combustion products with an unburned mixture. The velocity of hot gases escaping from the open end in the initial phase of the ejection reaches sound values. The data obtained are verified based on analytical estimates of the adiabatic gorenje temperature and the normal velocity of the laminar front and form the basic boundary conditions for subsequent calculation stages.

An improved constant-volume combustion bomb (CVCB) with a double-layer perforated plate was employed to investigate the combustion characteristics of propane-air premixed flames at different equivalence ratios (Φ = 0.8-1.25). Combined three-dimensional simulations and high-speed Schlieren imaging were used to analyze the coupled evolution of flame, flow, and pressure fields in confined space. The simulations revealed that before reaching the plate, expansion-induced flow and geometric contraction accelerated the unburned gas through the orifices, while pressure-driven jets near the orifices stretched and wrinkled the flame front, forming a complex flow-combustion structure. Experimentally, three flame development stages were identified: primary jet propagation, secondary jet acceleration, and flame-shock wave coupling. As Φ increased, both flame velocity and combustion intensity rose, and the flame mode transitioned from turbulent to quasi-detonative. Overpressure and high-frequency (4 kHz) filtered pressure analyses showed that both parameters increased with Φ, reaching 3.16 and 0.501 MPa at Φ = 1.25. The enhanced oscillations were attributed to local autoignition near the perforations and spontaneous ignition in the end region. This study clarifies the mechanisms of secondary jet flame acceleration and local autoignition in confined double-plate systems, providing new insight into explosion dynamics and safety control in fuel storage and transportation environments.

Modern combustion systems increasingly operate under challenging conditions, including Moderate and Intense Low Oxygen Dilution (MILD) combustion, high Flue Gas Recirculation (FGR), and hydrogen-rich fuels, driven by stringent emission reduction requirements. Traditional chemistry reduction methods, such as the Flamelet Generated Manifold (FGM), rely on restrictive assumptions about flame structure and fail under these emerging operating conditions where classical flame fronts become ill-defined or entirely absent. This work presents a novel Computational Singular Perturbation (CSP)-inspired chemistry reduction framework that addresses these limitations. The methodology employs analysis of chemical timescales to identify dominant species followed by a realtime homogeneous correction method designed to compute the complete chemical species profile from the reduced species basis and local flow conditions. This strategic adaptive retention of chemical information creates an intermediate level of detail based on first principles, eliminating concerns about user fine-tuning and the definitions and tabulation of basis flamelets, while achieving significant computational savings. To further enhance efficiency, machine learning acceleration through deep neural networks is explored as an alternative implementation pathway. Validation of the framework through one-dimensional freely propagating flames demonstrates exceptional accuracy, with flame temperature prediction error of 0.08% and flame speed prediction error of 0.24% using only a third of the participating species. Two-dimensional axisymmetric combustor simulations show significant improvements in flame position and structure prediction compared to detailed chemistry simulations with the Eddy Dissipation Concept (EDC). These results are highly encouraging and warrant first-party implementations of the proposed algorithm with further investigation into more complex industrial reactive flows.

Porous medium combustion is recognized for its high thermal efficiency, low pollutant emissions, and superior flame stability. However, the pore-scale mechanisms governing convection-radiation coupling and flame morphology remain insufficiently understood. This study investigates premixed methane-air combustion within a randomly packed bed of Al 2 O 3 spheres through pore-scale numerical simulations. The k-ε turbulence model, combined with the Eddy Dissipation Concept for combustion chemistry and the Discrete Ordinates model for radiative transfer, is employed. A systematic sub-domain scaling analysis identifies the N = 3 configuration, a symmetric segment scaled to three times the particle diameter, as the optimal trade-off between predictive fidelity and computational cost. Simulations for three pore Reynolds numbers (250, 350, 450) elucidate the characteristics of convection-radiation coupling within porous media combustion. Increased Re p enhances convective transport, shifting the flame front downstream, while radiation reinforces upstream heat recirculation. Quantitatively, radiation attenuates the mean gas temperature by up to 8.1% but amplifies the peak heat flux by approximately 90% and 59% at Re p = 250 and 350, respectively. Besides, the peak heat release rate exhibits a non-monotonic trend, and the axial radiation flux intensifies substantially across the burner, with increments of 45.8% and 11.2% as Re p increases from 250 to 450. The findings offer mechanistic guidance of heat recirculation and flame stability within porous media combustion for the industrial applications of porous burners.

ABSTRACT Ammonia (NH3) is widely recognized as a carbon-free energy carrier and an alternative to conventional hydrocarbons for green energy applications; however, its use is often constrained by the challenge of nitrogen oxide (NOₓ) emissions. Laser-assisted ignition offers precise, non-intrusive control and is investigated in this study as an effective strategy to address the inherent ignition challenges of ammonia-based fuels. A dual-pulse laser ignition system is applied to quantify its effect on instantaneous stretched laminar flame speed (S L ) and its role in improving ignition stability and NOₓ control in ammonia combustion. Instantaneous stretched laminar flame speeds (S L ) were evaluated using quasi-direct numerical simulations (DNS) with the Stagni ammonia reaction mechanism over equivalence ratios of ϕ = 0.7–1.25 and pressure ranging from 0.1 to 0.5 MPa, while assessing the effect of hydrogen addition and the initial degree of preionization. The Damköhler (Da) and Karlovitz (Ka) numbers were calculated to quantify the correlation between chemical time scales and flow-induced stretch effects. The laminar flame speed is evaluated using the radius evolution technique, which tracks the temporal growth of the flame front. To investigate the influence of dual-pulse laser ignition on S L, two modeling approaches are implemented. The first method is the DNS analysis, where S L was estimated using the radius evolution technique; however, this method is computationally intensive. The second approach is proposed to provide an efficient way to estimate the laminar flame speed by coupling the initial DNS results obtained from the plasma solver with the detailed chemistry solver. The quasi-DNS model was validated against available experimental measurements and reference data from the literature. The findings reveal that Laser plasma ignition enhances S L and suppresses NOx emissions relative to conventional ignition process for both pure ammonia and hydrogen-blended ammonia mixtures.

Spherical turbulent flame propagation limits of ammonia–hydrogen–oxygen–nitrogen pre-mixtures by intense near-isotropic turbulence in a constant volume vessel

Abstract Ammonia (NH 3 ) has emerged as a carbon-free fuel with strong potential for longterm and easily transportable energy storage, offering a means to address the intermittency and geographical limitations of renewable energy sources. However, its low chemical reactivity requires flame enhancement strategies such as partial cracking, fuel blending, preheating, or plasma activation. Although these approaches improve ignition and stability, they can also influence NO formation, highlighting the need for a detailed understanding of flame structure, temperature distribution, and intermediate species. In this study, laminar premixed flames are stabilized on a slot burner over a range of equivalence ratios and fuel compositions. Flame enhancement is achieved using NH 3 /N 2 /H 2 mixtures that mimic partially cracked ammonia. Single-shot two-dimensional Rayleigh scattering thermometry at 266 nm is employed to measure the flame temperature fields, benefiting from enhanced Rayleigh scattering cross sections at this wavelength. The Rayleigh scattering cross section of NH 3 relative to N 2 is determined experimentally to enable accurate temperature calibration. The resulting 2D temperature maps are used to identify flame fronts, from which temperature profiles along the flame-normal direction are extracted to evaluate the thermal flame thickness. The reconstructed flame cone provides the mean flame surface area and local laminar burning speeds. Experimental results obtained for various mixtures and equivalence ratios are compared with one-dimensional flame simulations and data reported in the literature.

ABSTRACT Using a 1 m 3 spherical vessel with synchronized pressure-temperature-imaging diagnostics, we evaluated nitrogen (N2), carbon dioxide (CO2) and their mixtures against 750 g ⋅ m − 3 potato-starch dust explosions. Single-gas tests showed severity declined smoothly as inert fraction rose; at the same total inert-gas volume fraction, CO2 outperformed N2, and the highest CO2 dose markedly reduced P max toward the 0.20 MPa suppression threshold, whereas the same N2 dose did not. Mixed-gas series fixed the total inert at 12% or 18% and varied the CO2 fraction in the CO2/N2 blend; replacing N2 with CO2 monotonically reduced P max , d p / d t max and T max , darkened and fragmented flames, and lengthened burn-time, with benefits amplified at 18%. High-speed imaging revealed progressively slower flame front propagation and reduced luminosity as CO2 fraction increased. The combined metrics support a physicochemical rationale: CO2’s higher heat capacity, stronger radiative absorption, and additional radical-consuming pathways quench combustion more effectively than N2. The results delineate an operational window where blended inerting remains measurable yet strongly mitigating, providing quantitative guidance for optimizing gas-based explosion protection strategies in industrial dust-handling facilities.