Flame Propagation Characteristics Research Articles

Flame propagation characteristics and reaction kinetics of electrolyte solvent vapor (ethylene carbonate) released from lithium-ion batteries

Flame propagation characteristics and reaction kinetics of electrolyte solvent vapor (ethylene carbonate) released from lithium-ion batteries

Numerical simulation study of hydrogen/air flame propagation and detonation characteristics in an annular cross section of gas turbine combustion chamber

The trends and future directions of hydrogen safety research cannot be separated from the thermodynamic behavior of combustion and explosion, hydrogen spontaneous combustion, flame propagation behavior, thermodynamic mechanisms, and other related topics. In this paper, through the method of numerical simulation, considering the hydrogen flame propagation and detonation characteristics in the annular section of the combustion chamber commonly used in gas turbines, the form of detonation and detonation impact in the channel are evaluated. By discussing the deflagration to detonation transition of hydrogen/air premixed gas and premixed gas under different working conditions, it is found that the flame in the annular channel propagates close to the inner wall and forms a strong expansion and turbulence between the outer wall and the outer wall of the flame. The flame surface and the airflow shear accelerate the detonation of hydrogen. The area close to the wall on the outer side of the flame surface and the tip of the flame surface are prone to set off detonation. The high-pressure area after the detonation mainly acts on the symmetrical end face of the outer wall surface and ignition area. There is a critical working temperature to make the impact strength strongest when the detonation occurs. Reducing the equivalence ratio of the filling gas can significantly reduce the reaction speed and weaken the impact strength of the wall. When the equivalence ratio is less than a certain value, the filling gas is completely consumed in the form of deflagration.

Flame Propagation and Explosion Characteristics of Multi-component Natural Gas in a Low-pressure Hydrogen-doped Environment

Abstract Hydrogen is considered to be the most promising clean energy of the 21st century, and its development and application are conformed to ‘carbon peaking and carbon neutrality goals’. Hydrogen-doped natural gas pipelines, as the main means of transporting hydrogen energy, are normally laid through low-pressure areas. The influence of the low-pressure hydrogen-doped environment on the flame propagation and explosion characteristics of hydrogen-doped natural gas is not yet clear. To solve this problem, this paper configures real multi-component natural gas and experimentally obtains flame propagation as well as explosion hazard parameters for natural gas/H2/air mixtures. The tensile flame propagation velocity changes from a gradual decrease to a gradual increase with increasing tensile rate for increasing hydrogen content. The laminar burning velocity increases from 0.44 m/s to 0.66 m/s at 0.75 bar and from 0.49 m/s to 0.69 m/s at 0.50 bar. The increase in laminar burning velocity becomes more and more significant after the hydrogen content exceeds 20%. As the hydrogen content increases, the flame surface goes from smooth to cracked and gradually increases. Lewis number, flame thickness, Markstein length and critical destabilisation radius decrease with increasing hydrogen and increase with decreasing pressure. The flame stability shows the same trend. The explosion hazard of natural gas/H2/air is reinforced as the hydrogen content rises, and the reduction in pressure reduces the explosion overpressure by 70% and the flame temperature by 10%. The results of the current research could provide a scientific reference for the safe delivery of hydrogen-doped natural gas.

An experimental study on flame propagation and combustion characteristics of multiple emulsified diesel and biodiesel fuel droplets

To unravel the interaction mechanism between droplets that is relevant for practical liquid fueled burners, the flame propagation characteristics of multi-droplet of emulsified diesel and biodiesel (FAME) fuels were studied using a suspended line droplet array. It was found that when S/d0 ≤ 3 (ratio of droplet spacing to initial droplet diameter), the double droplets were ignited simultaneously and surrounded by a single flame, and as the droplet spacing increased, two independent flames were formed around each droplet. The highest flame propagation rate was attained at S/d0 = 3, which was attributed to the consistent values of flame standoff ratio limited by the dominant heat conduction and finite evaporating rate. The droplet interaction, such as oxygen competition effect, enhanced with smaller droplet distance, can result in the reduction in the droplet combustion rate and flame propagation rate; the latter one, however, showed an increasing trend when small droplet or the size non-uniformity of droplet cloud was considered. In addition, the child droplet ejected from the emulsified droplets showed three burning modes and was an important factor promoting the flame spread, and the results showed that compared with the FAME emulsified fuels, the child droplets in diesel cases were more prone to ignition, expanding the burning regime.

Flame propagation and NO formation characteristics of hydrogen-air mixture combustion due to dilution effect

Flame propagation and NO formation characteristics of hydrogen-air mixture combustion due to dilution effect

A preliminary investigation on a novel vortex-controlled flameholder for aircraft engine combustor

This study proposes a novel vortex-controlled flameholder (VCF) to enhance the combustion performance, particularly the lean ignition and blowout characteristics of afterburners in advanced aircraft engines across a wide range of operating conditions. Additionally, both experimental and numerical investigations were conducted to explore the effects of the cavity structure on the flow field, lean ignition, lean blowout, flame propagation characteristics, and outlet temperature rise distribution of the flameholder. Two distinct cavity structures designated closed-cavity (case-1) and open-cavity (case-2) were examined, the findings indicating that both case-1 and case-2 can generate large-scale vortex flow structures within the cavity, contributing to achieving excellent combustion stability. Case-2 demonstrated better lean ignition and blowout performance compared to case-1. Furthermore, both case-1 and case-2 exhibited the same ignition process, which comprised four distinct phases: Phase 1 involved the formation of an effective flame kernel; Phase 2 pertained to the ignition of the entire cavity by the flame kernel; Phase 3 represented the full development of the flame downstream of the cavity; Phase 4 described the formation of a fire tornado that anchors the flame front. Interestingly, the outlet temperature rise in case-1 is lower than that in case-2 at low fuel-to-air ratio (FAR) conditions. As the FAR increases, the difference in the outlet temperature rises between the two cases gradually narrows.

Research on jet ignition phases and control parameters in an active pre-chamber optical engine under ultra-lean combustion conditions

The active pre-chamber (APC) jet ignition system is one of the primary technologies for achieving ultra-lean combustion and high thermal efficiency in engines. The impact of the jet process within the engine, as well as its control parameters, on emission pollutants (particularly soot particles) remains unclear. This study focuses on examining the effects of various low-flow injection control strategies on engine combustion and emissions by using optical experiments and numerical simulations. It also explores the impact of different ignition advance angles (ignition timings) based on a short pre-chamber mixing interval to observe ignition combustion, flame propagation, and emission characteristics under ultra-lean conditions (λ = 2.0). The main conclusions are as follows. Appropriately increasing the injection mass can enhance engine load. However, further increases in injection mass significantly raise particulate emissions, resulting in an increase in particle number by up to more than 37-fold, especially for small particles in the 5–––10 nm size range. The chemical reaction between the luminous jet flame and the bright incandescent wake jet flame, as captured by high-speed photography, effectively characterizes the various stages of jet ignition. To reduce particulate matter emissions, it is crucial to avoid the foreseeable wake jet flame caused by the enrichment of the jet mixture in the pre-chamber. The low-flow injection timing should neither be too early nor close to the ignition spark timing. The early injection causes fuel to accumulate at the top of the pre-chamber, hindering the jet flame’s propagation into the main combustion chamber. Late low-flow injection leads to fuel enrichment, resulting in uneven mixing and poor atomization and diffusion due to short mixing times. Ignition after a short mixing time interval tends to increase knocking, resulting in intense combustion and an advanced phase, which slightly reduces load and combustion stability. Regarding the heat release ratio, the total heat release from the two combustion stages in the pre-chamber should ideally account for about 5–6 % of the heat release in the main chamber.

Flame propagation analyses of aluminium coated with various concentrations of stearic acid based upon elementary reaction simulation

Coated aluminium, a novel material, has been employed in a multitude of applications. However, the thermal physicochemical properties of certain coated materials can elevate the ignition sensitivity and exacerbate explosion hazard of coated aluminium powder, posing serious thermal risks throughout the production, processing and storage. To render a theoretical basis for the prevention and mitigation of stearic acid-aluminium (SA-Al) dust explosion accidents, the flame propagation and elementary reaction sensitivity characteristics of SA-Al dust with various stearic acid concentrations were analysed by experiment and numerical simulation. Results showed that as the SA coating concentration increased from 0 % to 20 %, the flame propagation behaviour changed from dust-driven to gas-driven combustion, the flame luminous intensity and temperature abated gradually, and the average flame propagation velocities and free radical accumulation tended to first increase and then decrease, reaching maximum value in the 10 % SA-Al explosion. There may be synergistic combustion effect during Al cores and SA coating, which reflected in that a series of branch chain reactions of H and OH of SA in the initial combustion stage resulted in the accelerating melting and the forward explosion reaction kinetics of Al cores. Concurrently, the accelerated combustion of Al intensified the pyrolysis and oxidation of unburned SA-Al particles.

Numerical simulation of flame propagation characteristics of NH3/Air flames assisted by non-equilibrium plasma discharge

Nanosecond non-equilibrium plasma-assisted combustion technology emerges as a reliable novel approach to enhance the flame propagation speed of NH3. In this study, we developed a zero-dimensional + one-dimensional (0-D+1-D) non-equilibrium plasma-assisted combustion model to investigate the impact of nanosecond pulse discharge on the freely propagating flame speed of NH3/Air mixture. The results reveal that due to the plasma discharge, abundant intermediate species (N2H4, N2H3, NO, H2O2) are formed at the inlet and are subsequently transported downstream, facilitating flame propagation. As a result, the speed of the 1-D freely propagating flame increases, and the flame front is closer to the inlet compared to the non-plasma condition. The transport effect of H2 is also evident, with high concentrations of H2 from the inlet providing the basis for reactions at the flame front that promote combustion. Furthermore, after the initial mixture flows into the flame front, a slight increase in heat release is observed, but this increase occurs within a very limited distance. Notably, in the case of plasma, a stronger heat release is evident at the flame front. Moreover, with plasma, the peaks of OH, H, O, NH2, and HO2 are higher and earlier than those of the non-plasma case due to the transport and kinetic effects of plasma. Pathway flux analyses reflect significant changes in the production and consumption paths of the three components OH, H, and O, which are most important for consuming NH3 due to plasma addition. The higher OH mass fraction promotes the chain reactions that consume NH3, effectively enhancing the flame propagation speed. Novelty and significance statementThis study introduces a novel 0-D+1-D nanosecond non-equilibrium plasma-assisted combustion model to examine the impact of nanosecond pulse discharge on NH3/Air flame propagation. It uniquely analyzes the interaction between species at the inlet and flame front, highlighting the transport effects of plasma-generated intermediates (N2H4, N2H3, NO, H2O2, H2) that enhance flame speed, with a detailed pathway analysis of key species (O, OH, H).

Investigation on decelerated propagation of hydrogen-air premixed flames in confined space

This study investigates the flame propagation characteristics of hydrogen-air mixture explosion in confined spaces through a small-scale experiment and simulations, in which the experiment were conducted at normal temperature and pressure, covering a wide range of equivalence ratios (0.8–3.5) to obtain flame evolution and propagation velocity. The results indicate that enhanced thermal-diffusive instability leads to an increased cellular structure in hydrogen-air flames, with flame propagation speed increasing with radius enlargement; the numerical simulation results demonstrate that flame propagation speed is generally divided into acceleration, deceleration, and weak rebound phases; flame deceleration is mainly caused by increased pressure and wall constraints. The flame deceleration critical radius (Rcr) exhibits a trend of initially decreasing and then increasing with increasing equivalence ratio, primarily due to the influence of the pressure rise rate, while the magnitude of the deceleration factor is not directly related to the equivalence ratio. Additionally, an empirical formula summarizing the relationship between flame propagation speed during the deceleration phase, laminar burning velocity, and pressure is as follows: v=2.7∙SL0∙(P/P0)-0.48.

Research on Gas Explosion Pressure and Flame Propagation Characteristics in Turning Pipelines.

In order to investigate the overpressure and flame propagation characteristics of gas explosions in turning pipelines, this study designed a transparent organic glass pipeline test system with different turning angles (30°, 60°, 90°, 120°, and 150°) and conducted a series of experimental studies to analyze the explosion shock wave overpressure and flame propagation behavior. The experimental results show that with the increase of the turning angle of the pipeline, the overpressure of the explosion shock wave significantly increases. In terms of flame propagation characteristics, when the turning angle is small, the flame can adhere to the outer wall of the pipeline corner and gradually fill the entire pipeline section. When the turning angle increases, the flame forms a blank area near the outer wall of the corner, and the blank area expands with the increase in the corner. In addition, the increase in the turning angle promotes the increase in the velocity of the explosion flame front. The research results of this review are of great significance for a deeper understanding of the mechanism of gas explosions in turning pipelines and evaluating their potential hazards.

Experimental Study on the Combustion Characteristics of Ultralow-Concentration Methane in Divergent Porous Media Burner

ABSTRACT To improve the stable combustion characteristics of ultralow-concentration methane (Concentration of 3.5%-5.0%), a series of combustion experiments were carried out by filling alumina spheres with different particle sizes (i.e. 5 mm, 10 mm and 15 mm) in a divergent burner, and the effects of divergent angle (i.e. 10°, 20° and 30°) and pore distribution on the flame propagation and stable combustion characteristics of ultralow-concentration methane were revealed. The results show that larger particle sizes and methane flow rates lead to the double flame surface phenomenon, and increasing angle and decreasing particle size can help to improve the preheating efficiency of the burner. Moreover, the inlet flow rates corresponding to the steady combustion of ultra-low concentrations methane are between 0.225 m/s and 0.308 m/s, the increase in inlet velocity, particle size, and angle will reduce the stability of the flame and make the shape of the flame surface developed in the order of “V→W→∧.” The burner with 10 mm particles and 30° angle has a wider stable combustion range, and the burner filled with smaller pore distribution help to improve the utilization of the lower concentration methane premixed.

Comparative study of flame propagation characteristics of methane explosion in pipeline with different venting structures at different ignition positions

The effect of central ignition and four types of explosion vents (circular vents with areas of 22.89, 15.20, and 9.07 cm2 and a cross-shaped vent with an area of 15.20 cm2) on the flame propagation of methane explosions was investigated by using a rectangular blast pipe system and analyzed in comparison with the closed-end ignition condition. Central ignition shows that the flame propagation velocity on both sides of the pipe increases and decreases. The peak pressure decreases as the vent area increases. Specifically, Pm (9.07 cm2) > Pm (15.20 cm2) > Pm (22.89 cm2). The flame temperature at the closed end was consistently higher than that at the vented end. The vm values in the closed-end ignition pipe increased by 20.0 %, 41.8 %, 45.8 %, 40.9 %, 31.5 %, and 19.0 %, respectively, over the central ignition position when the methane concentration was 8–13 %. When the methane concentration deviates from the stoichiometric ratio, the difference in peak pressure between closed-end and central ignition can be up to 0.15 MPa, which is 26.7 % of that of central ignition. The peak temperatures observed at points 1–2 in the closed-end ignition were consistently higher than the central ignition.

Study on the hydrogen-air premixed flame propagation characteristics in semi-open space with obstacle

In semi-open space, obstacles have the potential to accelerate flame propagation and increase hydrogen-air deflagration pressure. Therefore, this paper is dedicated to exploring the influence of obstacles on the premixed hydrogen deflagration, which is crucial for enhancing the safety of industrial production and energy utilization. By using Large Eddy Simulation (LES) model in OpenFOAM, this study investigates the deflagration characteristics of premixed hydrogen in the presence of three different shaped obstacles. The analysis results reveal that under obstacle conditions, the flame shape can be categorized into four phases: the hemispherical phase, finger-shaped phase, jet phase, and vortex phase. The velocity of the flame front is nearly same for elliptical and rectangular obstacle condition, but it is 36% higher compared to triangular obstacle condition. The impact of triangular and rectangular obstacles on explosion overpressure is less than that of triangular obstacles on explosion overpressure, but it is 16% higher than that of elliptical obstacle. Analyzing the vorticity generated by different obstacle reveals that the vorticity produced by rectangular obstacle is twice as much as that produced by elliptical obstacle, whereas the vorticity produced by triangular obstacle is 2.4 times greater than that produced by elliptical obstacle. The acceleration of hydrogen-air explosion process occurs due to the narrow space created by obstacle and pipeline walls, and the shape of obstacle significantly influences this acceleration.

Flame propagation characteristics of non-uniform premixed hydrogen-air mixtures explosion in a pipeline

Hydrogen, as a clean and promising energy carrier, is considered a viable alternative fuel for the future. However, accidental hydrogen leakages and explosions pose considerable safety concerns in hydrogen energy applications and process industries. This study investigated the flame propagation characteristics of non-uniform premixed hydrogen-air mixtures in a rectangular closed duct with a length-to-diameter ratio of 5.78, taking into account varying equivalence ratios and diffusion times. First, numerical simulations using FLUENT were conducted to model the hydrogen diffusion process in a confined space, determining the hydrogen concentration evolution post-leakage. After approximately 200 s of diffusion, the premixed hydrogen-air mixtures remained in a state of homogeneous mixing, with the hydrogen concentration stabilizing at approximately 1.25 × 10−2 kg/m³. Subsequently, experimental observations were performed using a visual pipeline system, high-speed photography, and flame structure analysis. These experiments examined inhomogeneous hydrogen-air mixtures under seven different equivalence ratios and five different diffusion time conditions. The effects of equivalence ratios and diffusion times on flame propagation characteristics were analyzed. The results revealed that equivalence ratio significantly influenced the flame structure, with higher equivalence ratios producing more pronounced flame surface wrinkles. However, the typical evolution of the tulip flame remained consistent. At a constant equivalence ratio, flame propagation velocity exhibited an initial increase followed by a decrease over time. These findings demonstrate that turbulence intensity accelerated the flame propagation in non-uniform premixed hydrogen-air mixtures. This study underscores the importance of further research on hydrogen safety fundamentals and technologies to develop comprehensive safety standards.

Effects of Water Mist on the Initial Evolution of Turbulent Premixed Hydrogen/Air Flame Kernels

In this study, a series of carrier-phase direct numerical simulations are conducted on spherical expanding premixed hydrogen/air flames with liquid water addition. An Eulerian–Lagrangian approach with two-way coupling is employed to describe the liquid–gas interaction. The impacts of preferential diffusion, the equivalence ratio, water loading, and the initial diameter of the water droplets are examined and analyzed in terms of flame evolution. It is observed that liquid water has the potential to influence flame propagation characteristics by reducing the total burning rate, flame area, and burning rate per unit area, attributed to flame cooling effects. However, these effects become discernible only under conditions where water evaporation is sufficiently intense. For the conditions investigated, the influence of preferential diffusion on flame evolution is found to be more significant than the interaction with liquid water. The results suggest that due to the slow evaporation rate of water, which is a result of its high latent heat of evaporation, the water droplets do not disturb the initial flame kernel growth significantly. This has implications for water injection concepts in internal combustion engines and for explosion mitigation.

Flame propagation, explosion hazard, and pollutant emission characterization of typical clean fuels leaks in plateau low-pressure region

To accelerate energy transformation and sustainable development, various countries are rushing to lay hydrogen doped natural gas pipelines. However, the installation of pipelines will pass through plateau low-pressure region, and the impact of low-pressure environments on the flame propagation, explosion hazards, and pollutant emission characteristics of hydrogen doped natural gas is not yet clear. To solve this problem, a combination of constant volume combustion experiment and numerical simulations is used. The characterization parameters of flame, explosion and pollutant concentration of natural gas/H2/air under low-pressure environment are obtained, and their chemical reaction kinetics are analyzed. When the pressure drops, the laminar burning velocity (LBV) of them increases, and the Lewis number (Le) gradually increases by more than 1. Thermal diffusion is stronger than mass diffusion, and the growth rate of flame thickness exceeds 26 %. The effect of hydrodynamic instability enhances, causing an increment in the average size of flame cells, the critical instability radius, and the Markstein length (Lu). At the equivalent ratio (φ) of 1, there is a change from negative to positive in the Lu, and flame stability continuously strengthens. The explosion overpressure is reduced by 78 % to 90 %, the explosion temperature by 16 % to 20 %. As the φ increases, the LBV exhibits an inverted U-shaped relationship, reaching its maximum value near “φ = 1″. The flame stability first weakens and then strengthens. The explosion hazard is strongest in the range of ”φ = 1–1.2″. With a decrease in pressure, the total reaction rate of fuel consumption is reduced by more than 50 %. The continuous reaction distance is extended by 12 %. Changes in pressure have a relatively weak impact on the content of CO and CO2. The increase in the φ results in an increase in CO content of over 7 %. The content of CO2 reaches its maximum value around “φ = 1″. It is an essential reference for the safe delivery of hydrogen energy and the reduction of environmental pollution.

Numerical Investigation of flow and combustion process in a hydrogen direction injection rotary engine

The high power-to-weight ratio and zero carbon emission hydrogen rotary engine (HRE) is an attractive development direction of hydrogen-powered systems, which is of great significance in expanding the hydrogen energy diversified utilization and promoting greenhouse gas (GHG) emission reduction. In this paper, a direct injection HRE simulation model coupling chemical reaction kinetics is constructed. The flow and combustion processes are visually presented using computational fluid dynamics (CFD) means, and the effect of the air intake method and hydrogen injection timing on the mixture formation and combustion performance are analyzed. Research finds that the combustion pressure, peak pressure, indicated work and indicated power of the HRE are larger when using methods of the compound or side intake or delaying hydrogen injection strategy. Due to the forward flame propagation characteristics, the combustion process mainly occurs in the front part of the combustion chamber, and the mixture distribution in this area and aggregation near the ignition chamber are more favorable to improving the combustion performance. More gas fuel gathered in the leading or end regions of the chamber will affect the burn rate in the later combustion stage. The preferred scheme is compound AIM with HIT at the early compression stage (CAIM-190), and its peak pressure, indicated work and indicated power improve by 30.9%, 25.5% and 26.6% respectively while combustion duration shortens by 15.3% when comparing to the scheme of minimum peak pressure (PAIM-286). This study can offers basic data to promote the HRE achieving efficient combustion and zero carbon emissions.

Study on the explosion mechanism of two-phase organic dust — Based on polypropylene/propylene hybrid explosions as an example

A two-phase organic dust explosion is more complex than a single dust or gas explosion and poses a more significant threat to industrial safety production. To elucidate the explosion mechanism of two-phase organic dust, this study investigated the characteristics of flame propagation in PP/propylene hybrid explosions and revealed the flame propagation mechanism. The research findings indicate that adding propylene makes the flame brighter and more continuous, and velocity and temperature increase significantly. The introduction of propylene shifts the control of flame propagation from a process dominated by pyrolysis explosion kinetics to one governed by premixed gas explosion kinetics. Dimensionless characteristic numbers Bi and Da were introduced further to analyze the control mechanism of the dust explosion process. It was found that the explosion process was mainly diffusion flame combustion and premixed gas phase combustion.

A new experimental method to study the flame inhibition performance of low concentration halogenated gas

The minimum fire extinguishing concentration is a key index for the development of next generation chemical clean fire extinguishing gases, which is usually got by the cup-burner. The fire suppression performance of low concentration of chemical gas is hard to be understood by the available methods. In this work, the fundamental acknowledgement of steady flame surface attacking mechanism by halon functional group is newly discussed by conducting a series of bench scale tests with a cylindrical channel under a varied fluorinated gases. The characteristics of flame propagation under differentiated air atmosphere environmental conditions is obtained. The influence of fluorine-containing gas flow rate on flame propagation speed is clarified. A dimensionless relationship including atomic radius, bond length, polarization degree, molar mass and molar volume are proposed to describe the influence of low concentration of fluorine-containing gas on flame propagation speed. It provides an insight for the understanding of the flame inhibition performance of low concentration halogenated gas.