Search Results (112)

This study investigated the suppression effects and mechanisms of fine water mist on methane/air explosions through large-scale roadway experiments and numerical simulations. Experiments showed that fine water mist curtains deployed at 40 m and 70 m effectively mitigate flame propagation and reduce overpressure. A coupled gas–liquid numerical model was developed to reproduce flame dynamics and droplet–flow interactions. The simulations revealed droplet breakup, transport, and coupling with the evolving explosion flow field, providing mechanistic insight into gas–liquid interactions in a confined roadway. Suppression by fine water mist is primarily driven by heat absorption and cooling, while radical chain interruption plays a secondary role. These coupled mechanisms significantly reduce flame propagation velocity and pressure rise rate, achieving complete suppression under optimized configurations. This study provides a solid foundation for the design and optimization of water mist explosion suppression systems in large-scale roadways. Full article

In a weakly constrained, confined space, four common ignition sources—electrical spark, open flame, tungsten filament, and electrochemical igniter—were employed to investigate how the ignition mode influences the overpressure and flame-propagation characteristics during the vented explosion of gasoline vapor. The results show that the explosion process can be divided into four stages, featuring three typical overpressure peaks. The flame velocity exhibits two pronounced accelerations: one upon rupture of the vent membrane and another when the flame reaches the vent opening. Among the ignition sources tested, the electric spark produced the most severe destructive effects associated with overpressure, while the electrochemical igniter yielded the fastest flame propagation, and the tungsten filament ignition generated the longest external flame, constituting the greatest external fire threat. Explosions initiated by the tungsten filament and electrochemical igniter experience flame instability at the outset, induced by disturbances from the ignition source itself. Full article

To reveal the mechanisms by which obstacle distribution affects propane–air premixed deflagration under different blockage ratios, large eddy simulation (LES) was employed to investigate flame propagation and pressure rise in a confined duct with four obstacle distributions and four blockage ratios. The coupled effects of obstacle layout and blockage ratio on flame morphology, propagation velocity, vorticity evolution, and pressure rise rate were analyzed. The results show that obstacle distribution significantly changes flame front structures: One-side obstacles produce claw-like flames, center layout obstacles generate tongue-like flames with large vortex regions at low-to-moderate blockage ratios, and both-side or around layout obstacles form mushroom-like flames. At high blockage ratios, around layout obstacles redirect the flow into a high-speed axial jet, leading to the highest flame velocity and maximum pressure rise rate. These findings indicate that the dominant flame acceleration mechanism shifts from vortex-induced flame wrinkling at low-to-moderate blockage ratios to axial-jet-driven flame acceleration at high blockage ratios, providing guidance for obstacle layout optimization and explosion risk mitigation in confined propane–air systems. Full article

The formation of Ti₂AlC was investigated by self-propagating high-temperature synthesis (SHS) from the elemental Ti-Al-C powder compacts. The compositional ratios of Ti:Al:C varied from 2:1:1 to 2:1.2:0.8 to explore the effects of deficient carbon and excess Al on the combustion kinetics and product formation. For the Ti-Al-C powder compacts, self-sustaining combustion featuring a distinct combustion wave was readily achieved upon ignition. Excess Al caused a decrease in combustion temperature and flame-front velocity, while deficient carbon showed relatively little influence. The synthesized product from the sample with an exact stoichiometry of Ti:Al:C = 2:1:1 was composed of 79.5 wt.% Ti₂AlC, 9.8 wt. Ti₃AlC₂, 10.7 wt.% TiC, and a small amount of Ti₃AlC. The addition of excess Al by 20 at.% not only increased the yield of Ti₂AlC but avoided the formation of Ti₃AlC. A reduction of carbon further improved the evolution of Ti₂AlC. The sample with an off-stoichiometric proportion of Ti:Al:C = 2:1.2:0.9 yielded the optimum product composition of 91.9 wt.% Ti₂AlC, 4.2 wt.% Ti₃AlC₂, and 3.9 wt.% TiC. This was attributed to the fact that excess Al and deficient carbon facilitated the formation of TiAl and sub-stoichiometric TiC, both of which acted as the intermediate phases to combine into Ti₂AlC. The as-synthesized Ti₂AlC grains were in the shape of thin platelets with a size of 4–8 μm and a thickness of about 1.0 μm. A laminated microstructure formed by closely stacked platelets is typical of the MAX carbide. Full article

This article presents the simulation methodology and results of dual-fuel combustion for internal combustion engines (ICE). Simulations were performed in ANSYS Forte^®, which modeled flame propagation using the G-equation model, and results were validated against experimental data. The article also presents results from simulations performed in Converge CFD^®, which used the SAGE combustion model, presented in previous work. Typical combustion modelling challenges in such ICE simulations are discussed, and the applied methodology is described. The range of methane-air equivalence ratio was 0.47 ≤ ϕ ≤ 0.57 across four load conditions with a rotational velocity range of 1228 ≤ RPM ≤ 1800. The methane-air combustion at these low equivalence ratios led to the required tuning of the stretch factor coefficient used in the flame speed model in ANSYS Forte^® due to methane’s thermo-diffusive effects at lean equivalence ratios. As a result, the flame stretch factor coefficient was found to increase with decreasing equivalence ratio. The study thus demonstrates the importance of flame stretch sensitivity and thermo-diffusive instabilities in ICE combustion through CFD combustion simulations. Full article

This study investigated premixed NH₃ combustion in a closed circular duct using two-dimensional numerical simulations. By varying the equivalence ratio and the oxygen volume fraction from 21% to 30%, the evolution of flame morphology, flame propagation velocity, flame surface area, as well as the temporal variations in duct-averaged temperature and pressure were systematically examined. In addition, sensitivity analysis and reaction-pathway analysis based on a detailed chemical kinetic mechanism were performed to clarify the coupling between local chemical reactions and global flow dynamics. The results showed that the flame generally evolves through a sequence of hemispherical, finger-shaped, wall-attached skirt, and planar finger- and tulip-shaped structures. Well-developed tulip flames are mainly observed under conditions close to stoichiometric composition with moderate to elevated oxygen enrichment, corresponding to an intermediate overall reactivity. As the oxygen volume fraction increases from 21% to 30%, flame propagation becomes markedly faster. The tube-averaged temperature and the peak overpressure show an overall increasing trend. This increase in overpressure is most pronounced at equivalence ratios of 1.0–1.2. This study identifies hazardous parameter ranges in oxygen-enriched NH₃ combustion that are prone to producing strong tulip flames and high overpressure, providing useful guidance for explosion risk assessment and safety-oriented design of NH₃-fueled combustion systems. Full article

Modern high-speed train compartments contain intricate internal configurations. In the event of a fire emergency, the propagation velocity of flames through the passenger cabin is determined by multiple factors, including compartment design, ignition source characteristics, and airflow conditions. This study employed computational fluid dynamics (CFD) and large eddy simulation (LES) to investigate the effects of fire source power, fire source location, and longitudinal ventilation velocity on the rate of flame progression. Unlike simplified homogeneous fuel models, this study incorporates the specific heterogeneous material layout of the CR400AF to capture realistic flame spread dynamics. The simulation results reveal that, under forward ventilation conditions, the magnitude of fire power has a minimal influence on flame propagation speed. However, stronger fire sources lead to earlier initiation of flame spread along the carriage. Central positioning of the ignition source results in bidirectional flame movement toward both ends of the carriage, with faster propagation rates than those of fires originating at the extremities. Longitudinal airflow patterns significantly influence the fire dynamics. When the airflow speed within the tunnel remains below 3 m/s, the impact of longitudinal ventilation on fire propagation speed in the train is minimal under forward ventilation conditions. Conversely, in reverse-ventilation scenarios, the rate of flame advancement shows a positive correlation with increasing ventilation speed. Nevertheless, once tunnel ventilation velocities exceed 3 m/s, combustion propagation within high-speed rail carriages becomes impossible due to intact windows, which create oxygen-deficient conditions that prevent the development of fire. This paper investigates the heat release rate and spread process of vehicle fires. It comprehensively considers the effects of fire source power, fire source location, and longitudinal ventilation rate on the rate of spread. The research results provide data support for the fire-resistant design of rail transit vehicles and for the formulation of emergency evacuation strategies for different fire scenarios, which are vital for enhancing rail vehicle fire safety and ensuring personnel evacuation safety. Full article

Velocity, pressure, and density fields computed in earlier three-dimensional direct numerical simulations of a statistically stationary, planar, one-dimensional, low-Mach-number hydrogen–air flame propagating in small-scale, moderately intense, spatially decaying turbulence are filtered out using top-hat filters of four different widths. Certain source/sink filtered terms in the transport equations for resolved and subfilter-scale kinetic energies are analyzed. These are (i) the rate of inertial transfer of kinetic energy between resolved and subfilter scales, (ii) baropycnal work, (iii) subfilter-scale velocity–pressure–gradient term, and (iv) subfilter-scale pressure–dilatation term. These filtered terms are averaged over transverse planes and time or conditioned to the filtered combustion progress variable. Results show that terms (i) and (ii) work to transfer kinetic energy from smaller to larger scales (backscatter) and from larger to smaller scales, respectively, with the baropycnal work dominating the former term. These trends are observed for mean and conditional terms. The mean velocity–pressure–gradient term is positive and works to increase subfilter-scale kinetic energy due to combustion-induced thermal expansion. The pressure–dilatation term changes its sign from negative to positive at the leading and trailing edges, respectively, of the turbulent flame brush. Under conditions of the present study, the magnitudes of the mean velocity–pressure–gradient and pressure–dilatation terms are smaller when compared to the baropycnal work. Probability Density Functions (PDFs) for the explored filtered terms exhibit long tails, are highly skewed, and are characterized by a large kurtosis, thus implying significant intermittency of inter-scale energy transfer and energy exchange between internal and kinetic energy in the flame. These PDFs indicate that the intermittency of the inter-scale energy transfer and energy exchange depends substantially on mechanisms and scales of energy injection. Full article

This study examines the influence of ignition position on explosion characteristics in linked vessels with a methane concentration gradient, aiming to support the safety of industrial combustible gas storage systems. A numerical simulation method was adopted, using a vessel-pipe-vessel linked device. Explosion parameters including pressure, pressure rise rate, temperature, and flame propagation speed were analyzed, with mechanism insights drawn from methane consumption rate and Reynolds number. Results indicate that maximum explosion pressure always occurs in the small vessel, decaying exponentially with increased dimensionless length of the ignition position, and ignition in the large vessel results in significantly higher pressure. The maximum pressure rise rate, maximum temperature rise rate, maximum flame speed, and maximum methane consumption rate each follow a quadratic trend, first decreasing and then increasing with the dimensionless length of the ignition position. Flame propagation is dominated by pipe acceleration, peaking at one end of the pipe near the small vessel at velocities up to 600 m/s. Turbulence intensity increases linearly with the dimensionless length of the ignition position and is highest when igniting in the small vessel. This research clarifies the influence mechanism of ignition position and provides theoretical support for the explosion prevention and control of linked vessel systems with concentration gradients. Full article

To investigate the influence of injector recess depth on the combustion characteristics of air heaters, high-speed shadowgraph imaging technology combined with numerical simulation was employed. Targeting a tripropellant coaxial direct-flow single injector, three test cases with recess depths of 0 mm, 5 mm, and 10 mm were designed to systematically study the ignition process, flame propagation characteristics, quasi-steady combustion, and flow field evolution mechanisms. Experimental results indicate that the recessed structure can expand the liquid mist distribution range before ignition: the dimensionless spray width ratios of the 5 mm and 10 mm recess cases are increased by 57.5% and 64.9% respectively compared to the non-recessed case, with an obvious “saturation effect” observed. Injectors with recess exhibit the characteristic of “jet head priority ignition”, which shortens the ignition time and improves ignition efficiency. The 5 mm shallow recess case achieves the optimal combustion stability with the smallest chamber pressure fluctuation (±0.1 MPa). Although the 10 mm deep recess enhances near-field mixing and combustion intensity, it tends to induce flame oscillation and combustion instability. Simulation results verify the experimental observations: the recess depth regulates droplet atomization, component mixing, and combustion heat release processes by altering the recirculation zone range, velocity gradient, and gas–liquid momentum exchange efficiency. This research provides experimental and theoretical support for the structural optimization of injectors in combustion-type air heaters. Full article

As sustainable green fuels for heavy-duty engines, using hydrogen doping with ammonia helps to mitigate greenhouse gas emissions. Based on the background of hydrogen production from ammonia reforming, the combustion and emission characteristics of hydrogen-doped ammonia engines are studied. By employing 3D-CFD numerical simulation, this study systematically explores the combined effects of the ignition timing, hydrogen energy ratio (HER), and equivalence ratio (Φ) on the premixed combustion and emission performances of ammonia–hydrogen blends. The findings indicate that at the operating conditions of HER = 4% and Φ = 1.0, the indicated mean effective pressure (IMEP) reaches its maximum at −40 °CA aTDC, with the indicated thermal efficiency (ITE) reaching 48.2%. However, to mitigate knock hazards, the ignition timing should be adjusted to −37.5 °CA aTDC. With HER increasing from 4% to 25%, the flame propagation velocity is markedly improved, and the combustion duration is notably reduced. As the equivalence ratio rises from 0.8 to 1.0, the combustion intensity is strengthened while the proportion of indicated work declines. Notably, the lean burn condition (Φ = 0.8) exhibits no knock risk and achieves the highest ITE (49.2%). In terms of emission characteristics, advanced ignition timing, higher HER, and lower equivalence ratio all promote NO_X formation. In contrast, N₂O emissions decrease as the combustion temperature rises and the combustion duration shortens. Unburned NH₃ is mainly distributed in the low-temperature areas inside the cylinder, and its emission amount decreases with the improvement of combustion completeness. Full article

This research employs an integrated experimental and numerical simulation approach to investigate how varying angles of continuous elbows in a “Z”-shaped pipeline affect the deflagration behavior of hydrogen-propane-air mixtures. Findings indicate that centrifugal forces acting on the flame front as it traverses an elbow cause a distinctive “tongue-shaped” propagation along the inner wall. A cavity that generates unburned gas near the outer wall. The volume of this cavity increases significantly with the Angle of the elbow. The flame propagation is regulated by it and presents three distinct stages: the initial development section within the straight pipe section, the disturbance section when entering the first elbow, and the subsequent suppression section under the action of the cavity. The more intense turbulent combustion occurs at the 90° bend, with the highest peak flame velocity. On the contrary, the 120° and 150° elbows suppress the spread of flames. In addition, the angle of the elbow has a significant effect on the second overpressure peak, which exhibits strong non-linear growth. The value at 150° is 2.7 times greater than that at 30°. This is mainly caused by the energy focusing effect of the reflected pressure wave in the cavity magnified by the large-angle elbow. These findings provide mechanism-level understanding for the safe design of complex hydrogen pipeline systems. Full article

As a zero-carbon fuel, ammonia holds significant potential for achieving the “dual carbon” strategic goals. However, its extremely low laminar burning velocity (LBV) limits its direct application in combustion systems. This work systematically reviews the research progress on the LBV of ammonia flames, focusing on three key aspects: measurement methods, effects of combustion conditions, and reaction kinetic models. In terms of measurement methods, the principles, applicability, and limitations of the spherical outwardly propagating flame method, Bunsen-burner method, counter-flow flame method, and heat flux method are discussed in detail. It is pointed out that the heat flux method and counter-flow flame method are more suitable for the accurate measurement of ammonia flame LBV due to their low stretch rate and high stability. Regarding the effects of combustion conditions, the LBV characteristics of pure ammonia flames under ambient temperature and pressure are summarized. The influence patterns of three factors on LBV are analyzed systematically: blending high-reactivity fuels (e.g., hydrogen and methane), oxygen-enriched conditions, and variations in temperature and pressure. This analysis reveals effective approaches to improve ammonia combustion performance. Furthermore, the promoting effect of high-reactivity fuel blending on liquid ammonia combustion was also summarized. For reaction kinetic models, various chemical reaction mechanisms applicable to pure ammonia and ammonia-blended fuels (ammonia/hydrogen, ammonia/methane, etc.) are sorted out. The performance and discrepancies of each model in predicting LBV are evaluated. It is noted that current models still have significant uncertainties under specific conditions, such as high pressure and moderate blending ratios. This review aims to provide theoretical references and data support for the fundamental research and engineering application of ammonia combustion, promoting the development and application of ammonia as a clean fuel. Full article

The downsized Wankel rotary engine (WRE) fueled with methanol is a promising power source for small unmanned aerial vehicles, owing to its simple structure, high-speed capability, and clean emissions. In general, a well-designed ignition timing (IT) can drastically enhance engine combustion performance. To assess the impact of IT, a numerical simulation study was conducted on a methanol-fueled WRE, analyzing its combustion characteristics and emissions to guide performance optimization. The results indicated that advancing the IT boosted the flame propagation velocity. The peak pressure increased slightly when delaying the IT from −24 °CA to −15 °CA but dropped sharply for −12 °CA at 5000 RPM. This contrasts with the behavior at 11,000 RPM and 17,000 RPM, where peak pressure clearly rose with advanced IT. Indicated thermal efficiency (ITE) decreased with the delay of the IT at 11,000 RPM and 17,000 RPM; the maximum values reached 24.98% and 25.78%, respectively. This contrasted with the trend observed at 5000 RPM, where ITE first increased and then decreased with IT delay. The optimized IT significantly affects pollutant emissions primarily under low-speed conditions (5000 RPM), while exhibiting limited impact at high engine speeds. At 5000 RPM, strategic IT adjustment achieves maximum reductions of 2% in CO emissions and 33% in formaldehyde emissions. Full article

Zr is a common element in titanium alloys to enhance their mechanical properties; however, its role in combustion remains unknown. This study aimed to elucidate the effects of Zr on the ignition conditions and flame propagation of Ti_100−xZr_x alloys via promoted ignition-combustion (PIC) tests. Results indicated that increasing Zr content (from 30 at% to 70 at%) decreased the critical oxygen pressure, ignition temperature, and burning velocity of Ti_100−xZr_x alloys. The reduction in ignition conditions was attributed to a decrease in ignition activation energy (from 108.37 kJ/mol to 94.26 kJ/mol) and an increase in combustion heat (from 986.34 kJ/mol to 1049.84 kJ/mol) with Zr addition. Additionally, microstructural analysis indicated that the suppression of flame propagation was attributed to Zr promoting the formation of a dense oxide layer. This hindered oxygen diffusion, thereby suppressing the heat release of oxidation reactions in the oxide zone and the peritectic reaction in the melting zone. These findings provided new insights into optimizing the composition of burn-resistant titanium alloys to inhibit combustion kinetics. Full article