Flame Propagation Speed Research Articles

Effect of airflow velocity on flame propagation and pressure of starch dust explosion in a pneumatic conveying environment

To investigate the flame propagation and pressure characteristics of dust explosions during the airflow transportation of dust particles, a 2 m transparent pipeline, supplied with a constant airflow from a fan, was constructed as a dust explosion testing apparatus. The distribution patterns of airflow velocity were measured using an anemometer and subjected to statistical analysis. Dust explosion experiments were conducted with airflow velocities of 5, 10 and 15 m/s, and an ignition energy of 1 kJ was applied to the transported dust particles. The results revealed that the flame propagation of starch explosions undergoes distinct stages: ignition, slow propagation, accelerated propagation. The flame propagation speed and explosion pressure initially increased and subsequently decreased as the airflow velocity was raised. As the airflow velocity increased from 5 m/s to 10 m/s, the maximum flame propagation speed rose from 53.2 m/s to 67.7 m/s, and the maximum explosion pressure increased from 25.62 kPa to 31.96 kPa. At an airflow velocity of 15 m/s, both the maximum flame propagation speed and explosion pressure decreased, reaching 32.6 m/s and 19.67 kPa, respectively. In contrast to conventional closed container powder injections, chemical igniters in the pneumatic conveying environment exhibit elongated flames, with the airflow influencing the timing of dust deflagration flame appearance and accelerating flame propagation. Due to the connectivity of the airflow transport pipeline, the explosion pressure tends to be lower. Influenced by non-uniform airflow and temperature differentials, dust deflagration flames within the airflow transport pipeline manifest a spiral propagation structure.

Study on Mechanisms of NOx Formation and Inhibition during the Combustion of NH3/CH4 and NH3/CO Mixtures

Ammonia is an ideal renewable, carbon-free fuel and hydrogen carrier, which produces nitrogen and water after complete combustion in the presence of oxygen. However, ammonia has low reactivity, slow flame-propagation speed, and carries risks of high nitrogen oxide (NOx) emissions. Co-firing ammonia with an industrial by-product gas (with CH4 and CO being the main combustible materials) is a cost-effective and convenient method of improving the combustion characteristics of ammonia, but attention still needs to be paid to the NOx generation. Currently, the research on NOx formation during co-firing of ammonia with other fuel gases is still insufficient. In this study, a high-temperature furnace reaction system was used to investigate the NOx formation and inhibition mechanisms during the combustion of NH3/CH4 and NH3/CO mixtures. By varying the ammonia blending ratio, excess air coefficient (α), temperature, residence time, and fuel concentration, the key factors influencing NOx generation and inhibition were further analyzed. The results showed that when α was no less than 1, the production of NOx initially increased and then decreased with an increasing proportion of ammonia in the fuel gas. Within the temperature range of 900 °C to 1500 °C, the amount of NOx generated during the combustion of the mixed gas gradually decreased with the increase in temperature. Under the conditions of NH3/CH4 and NH3/CO, the emissions of NOx were higher than those during pure ammonia combustion.

Effect of Turbulence and Diffusional–Thermal Instability on Hydrogen‐Rich Syngas Turbulent Premixed Flame

As a kind of fuel with low‐carbon emissions, hydrogen‐rich syngas has a large resource potential and is a promising alternative energy resource. A series of experiments in a constant volume combustion bomb are carried out to investigate the effects of turbulence and diffusional–thermal (DT) instability on a hydrogen‐rich syngas turbulent premixed flame. The flow field in the constant volume combustion bomb is calibrated, and the turbulent flow field's homogeneity and isotropy were investigated. The effects of different speeds of fan (1366–4273 rpm), hydrogen fractions (55%–95%), pressures (1.0–3.0 bar), and equivalence ratios (0.6–1.0) on the hydrogen‐rich syngas turbulent premixed flame are studied. According to the analyses, the flow field in the experimental device exhibits good homogeneous and isotropic characteristics. With the increase of turbulence intensity, equivalence ratio, hydrogen fraction, or pressure, the turbulent flame propagation speed increases gradually. As the hydrogen fraction increases or the equivalence ratio decreases, the effective Lewis number decreases, and the effect of DT instability on the flame increases. The research in this article is crucial for the clean and efficient utilization of hydrogen‐rich syngas and the design of related burners.

Effect of ammonia addition on combustion characteristics of hydrogen/air using passive turbulent jet ignition

Hydrogen (H2) and ammonia (NH3) are ideal carbon-free fuels, and the application of NH3/H2 in internal combustion engines offers the potential to reduce carbon emissions. And turbulent jet ignition (TJI) is an advanced ignition strategy that can effectively enhance the combustion. This work aims to experimentally investigate the effect of NH3 addition on the combustion characteristics of H2/air mixtures under passive TJI conditions. Combustion images and instantaneous pressure were collected for processing and analysis. The effect of NH3 substitution and equivalence ratio were investigated. The results indicate that the peak combustion pressure decreases with the addition of NH3, accompanied by longer combustion duration and ignition delay, as well as lower jet velocity and flame area growth rate. The hot jet will be quenched by the strong heat transfer, causing the change in the ignition mechanism as the NH3 substitution ratio increases. In addition, compared to lean conditions, the combustion appears to be less sensitive to the equivalence ratio on the rich side and always shows high jet velocity and flame propagation speed. However, a relatively small amount of NH3 addition changes the ignition mechanism on the rich combustion side compared with lean conditions, resulting in a longer ignition delay.

Comparison of the flame stabilities during oxy-methane and air-methane combustion in a two-layer porous burner

The development of the oxy-fuel porous media combustion technique not only takes advantage of the characteristics of porous media combustion, such as a relatively low lean-burn limit and small volume, but also dramatically facilitates carbon capture and storage. In this paper, a two-layer porous burner model is established, in which a two-temperature equation model is adopted for calculation purposes. Differences in combustion behaviour between oxy-fuel and air-fuel conditions are compared. The results show that the difference in physical properties between CO2 and air is the main reason for the significant variation of combustion behaviours under the conditions of oxy-fuel compared to air-fuel. The specific performance is lower combustion temperature and flame propagation speed. The temperature under oxy-fuel combustion conditions is 300 K lower than that under air-fuel combustion conditions at an equivalence ratio of 0.6, and the stable range of oxy-fuel combustion is only approximately 50% of air-fuel combustion. The impact of porous media material parameters on combustion behavior has been investigated. These variables include the equivalence ratio, material thermal conductivity, volume heat transfer coefficient, and extinction coefficient, while the ratio of O2/CO2 remains fixed at 0.21/0.79. The influence of these variables on the stable velocity range and temperature field is consistent, but a large difference in values occurs. Reducing the thermal resistance of the burner by adjusting the properties of the porous matrix can increase the velocity limit of stable combustion. However, even if the thermal conductivity and extinction coefficient are increased by 5 times, there will be no significant impact on the stable range. Reducing the pore size of the combustion zone by 50% will increase the stable range by over 100%.

Study on Gasoline–Air Mixture Explosion Overpressure Characteristics and Flame Propagation Behaviors in an Annular Cylindrical Confined Space with a Circular Arch

Gasoline–air mixture explosions mostly occur in buried tank rooms, which are annular cylindrical confined spaces with circular arches. In this paper, explosion experiments at different gasoline–air mixture volume fractions are carried out in an annular cylindrical steel bench with a circular arch curvature radius of 900 mm and an annular half-perimeter to radial width ratio of 12π. The results show that the development process of explosion overpressure is clearly divided into four stages after first-order differentiation treatment. Compared with other types of confined spaces, 1.70% is still the most dangerous gasoline–air mixture volume fraction. However, this type of confined space has a larger inner surface area in the same volume condition, which will inevitably increase the heat absorption rate, reduce the chemical reaction rate, and slow down the flame propagation speed. Meanwhile, this spatial structure will inevitably make the explosion flames collide, which will promote positive feedback coupling between explosion flames and pressure waves, making the explosion more violent and dangerous. These results can provide theoretical and technical support for the explosion prevention design of buried tank rooms.

The Effect of Methane–Ammonia and Methane–Hydrogen Blends on Ignition and Light-Around in an Annular Combustor

Abstract The use of hydrogen and ammonia in gas turbines, either alone or blended with natural gas, poses various technical challenges for combustion systems, including ignition. Depending on the fuel composition, the laminar flame speed and the ratio of unburned to burned gas density (dilatation ratio) of hydrogen and ammonia flames can be well outside the range seen in natural gas flames. Previous studies in annular combustion chambers have provided evidence of the importance of these properties in determining the ignition dynamics including light-around times. So far, these studies have mostly considered hydrocarbon fuels, have been limited to only a few runs, and have not yet systematically investigated variations in the dilatation ratio and the flame speed but rather have considered them as a lumped parameter. To investigate these effects in more detail, experiments characterizing the light-around times were carried out on an atmospheric annular combustor in which the dilatation ratio and the laminar flame speed was independently varied. This was achieved by varying the equivalence ratio and employing a variety of different hydrocarbon fuels (ethylene, propane, and methane) and fuel blends of methane–ammonia and methane–hydrogen. Light-around times were evaluated from global chemiluminescence measurements obtained using an azimuthal array of photomultipliers placed round the combustor chamber as well as high speed imaging. To improve statistical certainty, more than 3000 ignition and light-around times were measured with 30 repetitions obtained for each operating condition. To provide some insight into the light-around dynamics in specific cases, 900 of the 3000 sets included high-speed OH* chemiluminescence images. Light-around times for premixed pure hydrocarbon flames showed a similar dependence on the laminar flame speed as reported in previous studies. For the range of ammonia fuel blends investigated, an increase in laminar flame speed leads to a predictable increase in the flame propagation speed, as in the case of hydrocarbon fuel. Furthermore, collapse of this dependence for all blends could be achieved when corrected for an effective Lewis number, noting that all Lewis numbers for these blends were above unity. However, for hydrogen fuel blends, a decrease in dilatation ratio was found to decrease the light-around time counter to existing experimental results on the ignition of hydrocarbon fuels for which we currently do not have an explanation.

Mechanism of influence of water mist in a closed vessel on the explosion overpressure and flame propagation characteristics of magnesium dust

The primary purpose of this paper is to deeply investigate the impact of water mist on the explosive characteristics of magnesium dust and to broaden the inerting and suppression strategies for magnesium dust explosions. The explosion characteristics and flame propagation behaviors of magnesium dust/water mist mixtures were examined in a 16.2-L visualized closed vessel. The explosion products were further analyzed using SEM and XRD. The results indicate that with the addition of water mist, the explosivity of low-concentration magnesium dust (≤600 g/m3) increases. The explosive strength of three higher concentrations of magnesium dust (800 g/m3, 1200 g/m3, 1600 g/m3) notably decreases, with the Pmax dropping by 69.86%, 54.08%, and 46.22% respectively compared to pure magnesium dust. The leading edge of the preheat zone exhibits red radiation, the fluctuation range of the instantaneous flame propagation speed decreases, and cubic particles appear in the explosion products. These changes are mainly attributed to the hydrogen produced in the explosion of the magnesium dust/water mist mixture. Under the combined effects of hydrogen and steam, the preheat zone significantly expands. Moreover, as the concentration of hydrogen increases, its presence weakens the suppressing effect of the water mist, manifested by the evident slowing of the average flame speed, the maximum flame speed, and the rate of decrease in explosive strength. The suppression of magnesium dust explosions by water mist mainly relies on physical actions such as heat absorption, blocking thermal radiation, and diluting oxidizer concentration.

Evaluation of flammability characteristics of mixed refrigerants R134a/R1270, R134a/R170, and R170/R1270

Hydrocarbons (HCs) are considered promising alternative refrigerants due to their excellent thermodynamics, transport properties, and environmental friendliness. However, hydrocarbons are flammable. Conversely, R134a is a widely used non-flammable refrigerant but was listed in the Montreal Protocol-Kigali Amendment due to its relatively high global warming potential (GWP). Therefore, in order to reduce the GWP of R134a and inhibit the flammability of hydrocarbon refrigerants, this study investigated the flammability characteristics of mixed refrigerants R134a/R1270, R134a/R170, and R170/R1270. The flammability limits of different concentrations of R170/R1270, R134a/R1270, and R134a/R170 were measured under standard conditions, and the inhibition mechanism of R134a was briefly analyzed. Additionally, the flammability limits of R170/R1270, R134a/R1270, and R134a/R170 were predicted using the polynomial fitting method. Subsequently, the flame propagation speed and the flame color of R1270 and R170, with and without R134a, were compared and analyzed. Furthermore, the cycle performance of R134a/HCs was calculated. The research results are expected to provide guidance for the safe and effective utilization of R1270, R170, and their mixtures in refrigeration systems.

The effect of pipeline sudden contraction structure on the deflagration characteristics of hydrogen-methane-air

Based on the visualized gas cloud explosion pipeline test platform, the influence of the sudden contraction structure on the deflagration characteristics of the hydrogen-methane-air gas cloud is investigated. The results show that the changes of flame propagation velocity, explosion overpressure, and flame temperature after deflagration of the premixed gas cloud all present three stages: rapid increase, rapid decline, and secondary acceleration caused by the sudden contraction of pipeline structure. The equivalence ratio significantly affects the hydrogen-methane-air detonation process in the sudden contraction of the pipe structure. As the hydrogen content increases, the flame propagation becomes faster and the explosion impact becomes greater. The formation of “Tulip” flame is closely related to the flame propagation speed. During the process of flame propagation, inertia dominates, causing the flame propagation speed to reach its first peak at the abrupt contraction structure.

Fuel reactivity stratification assisted jet ignition for low-speed two-stroke ammonia marine engine

Ammonia (NH3) has gained much attention as a carbon-free fuel due to its great potential to reduce carbon emissions in the shipping industry. However, ammonia is difficult to use directly in existing marine engines due to its low flame propagation speed and high auto-ignition temperature. In this work, a new combustion mode for low-pressure injection low-speed two-stroke ammonia marine engines: Reactivity Stratification Assisted Jet Ignition (RSAJI) is proposed and explored. The results of the study show that the RSAJI mode is very promising. Especially at low and medium loads, the peak pressure can be increased by 27.3%, and NH3 and N2O emissions in the exhaust gas can be reduced by 87.1% and 99.7%. Two key parameters: direct injection timing and ammonia substitution rate (ASR) determine the reactivity stratification in the main-chamber, which directly affects the jet ignition effect and turbulent flame speed. A thorough analysis of engine performance shows that the injection timing should be appropriately far from the Top Dead Center (75°CA BTDC & 86.4°CA BTDC). Further increases in ammonia substitution rates (ASR = 90%, 95%) will result in a reduction in power performance gains but will improve the indicated thermal efficiency (>50%).

Experimental study on turbulent burning velocities of premixed flames for natural gas/air mixtures

Experiments on the spherical expanding flame of turbulent premixed natural gas/air mixtures were conducted in a fan-stirred turbulent combustor for the first time. As the belief that the methane burning velocity data can be obtained for natural gas is not justified, natural gas constitutes of 90% CH4, 7% C2H6 and 3% C3H8 by volume fraction was used. The operation conditions are equivalence ratio of 0.7–1.3, initial temperature of 300–400 K, initial pressure of 0.1–0.3 MPa, turbulence intensity of 1.0–2.7 m/s, oxygen volume fraction of 15–21% and carbon dioxide volume fraction of 0–20%. A self-developed program and post-processing procedure was developed to highly restore the real information of flame front without ignoring small local information. It was concluded that methane cannot be taken as a representative of natural gas for studying turbulent burning velocities. The increase of flame propagation speed with initial pressure is not as significant as that with initial temperature and turbulence intensity under low-intermediate pressure range. Dispersions exists for previous correlations relating to turbulent flame Reynolds number, Karlovitz number, Damköhler number, respectively, and a new correlation were proposed based on the ratio of Karlovitz number and Damköhler number. The new correlation was validated by both current and previous experimental data and its advantage and applicability were summarized. It will be validated by turbulent flames of various kinds of fuels over a wide range of working conditions in the further study.

Experimental and numerical investigation on the premixed methane/air flame propagation in duct with obstacle gradients

This work experimentally and numerically investigates the effect of the obstacle gradient on the characteristics of the methane/air explosion in obstructed ducts. The obstacle gradient is expressed through various blockage ratios, and three different obstacle gradients are investigated, i.e., C357, C555, and C753. Noted that C357 means that the blockage ratios of three obstacles arranged sequentially in the duct are 0.3, 0.5 and 0.7 respectively, and so do C555 and C753. A two-dimensional (2D) model is adopted and the Scale-Adaptive Simulation method with the thickened flame model is considered. Experimental results show that the obstacle gradient significantly affects the flame evolution structure, flame propagation speed and overpressure. The obstacle gradient is accountable for the “tulip flames” appearing downstream of the obstacle. For the case with a fixed methane volume fraction, the average flame front speed is fixed. However, with the increasing obstacle gradient, the maximum flame front speed increases until it achieves the maximum at C357, and so does the maximum overpressure. The numerical simulation can predict the flame evolution behaviour precisely. It is evident that the generation of different flame shapes appearing is derived from the flow field evolution of unburned gas downstream of the flame front.

A Review of Current Advances in Ammonia Combustion from the Fundamentals to Applications in Internal Combustion Engines

The energy transition from hydrocarbon-based energy sources to renewable and carbon-free energy sources such as wind, solar and hydrogen is facing increasing demands. The decarbonization of global transportation could come true via applying carbon-free fuel such as ammonia, especially for internal combustion engines (ICEs). Although ammonia has advantages of high hydrogen content, high octane number and safety in storage, it is uninflammable with low laminar burning velocity, thus limiting its direct usage in ICEs. The purpose of this review paper is to provide previous studies and current research on the current technical advances emerging in assisted combustion of ammonia. The limitation of ammonia utilization in ICEs, such as large minimum ignition energy, lower flame speed and more NOx emission with unburned NH3, could be solved by oxygen-enriched combustion, ammonia–hydrogen mixed combustion and plasma-assisted combustion (PAC). In dual-fuel or oxygen-enriched NH3 combustion, accelerated flame propagation speeds are driven by abundant radicals such as H and OH; however, NOx emission should be paid special attention. Furthermore, dissociating NH3 in situ hydrogen by non-noble metal catalysts or plasma has the potential to replace dual-fuel systems. PAC is able to change classical ignition and extinction S-curves to monotonic stretching, which makes low-temperature ignition possible while leading moderate NOx emissions. In this review, the underlying fundamental mechanism under these technologies are introduced in detail, providing new insight into overcoming the bottleneck of applying ammonia in ICEs. Finally, the feasibility of ammonia processing as an ICE power source for transport and usage highlights it as an appealing choice for the link between carbon-free energy and power demand.

Study on influence of magnetic field on ethane explosion mechanism

To explore new gas explosion prevention and control technology, experiments on the influence of magnetic field on the explosion characteristics of ethane were carried out, and the effect of magnetic field on the explosion pressure, product component changes and other explosion characteristics were obtained. CHEMKIN-PRO software was used to simulate the explosion chain reaction process and determine the key radicals and main reaction paths. The forces of radical particles under magnetic fields were analyzed through theoretical calculations. Fukui function and population analysis of elementary reaction C2H6+·OH=·C2H5 + H2O were carried out by Material Studio. The results showed that magnetic fields decreased the ethane maximum explosion pressure, pressure rise rate, and average flame propagation speed, while increasing CO and CO2 production and decreasing residual ethane. ·H, ·O, ·OH, ·CH3, and ·C2H5 were identified as the key radicals in the ethane explosion process, with ·OH having the greatest effect on the ethane consumption rate. H5 has the strongest nucleophilic sensitivity and C2 has the strongest electrophilic sensitivity, which leads to that H5 and C2 are the most easily broken sites of chemical bonds during C2H6 explosion. C2H6+·OH has the strongest adsorption capacity and the most unstable structure, and the two are more likely to react. The magnetic field affects the explosion of ethane by weakening the bond energy of CH, and the change of bond energy directly affects the enthalpy change, resulting in the change of ethane consumption rate. At the same time, the magnetic field will also change the migration trajectory of ·OH by applying the magnetic field force, reducing the multi-chain reaction rate of ·OH participation, thereby reducing the chain reaction rate of ·HCO → CO → CO2, showing that the magnetic field has a significant inhibitory effect on the explosion of ethane.

Simplification of the combustion mechanism of Jatropha biodiesel surrogate fuel and reaction path analysis

The objectives of this study were to explore the evolution process of the chemical reaction pathways in the combustion of a biodiesel surrogate fuel and to reduce the calculation burden in the numerical simulation of this combustion process due to the complicated mechanism. A mixture of methyl decanoate, methyl 9-decanoate, and n-heptane was selected as the Jatropha biodiesel surrogate fuel. A simplified combustion mechanism of the biodiesel comprising 170 components and 737 reaction components was formed by systematically simplifying 3299 components and 10806 reaction mechanisms using a multistep directed relation graph method, path flux analysis, and sensitivity analysis. The simplified mechanism was compared with the detailed mechanism and experimental data to effectively verify the accuracy of the mechanism. The simplified mechanism could reproduce the detailed mechanism, and the results were in good agreement with the experimental data in terms of the ignition time of a methyl decanoate/air mixture in a shock tube, the concentrations of the components generated in the combustion process of the biodiesel in a jet stirred reactor, and the flame propagation speed of methyl decanoate and n-heptane/air mixture. The rate of production analysis method was used to explore the main reaction pathways and the evolution process of important intermediate components in the Jatropha biodiesel surrogate fuel generated at high and low temperatures. Methyl decanoate and methyl 9-decanoate formed hydrogen peroxide ketone and oxy groups mainly through dehydrogenation, oxygenation, and isomerization at low temperatures. The high-temperature stage mainly involved bond-breaking decomposition reactions, partial dehydrogenation, and isomerization reactions of low-temperature reaction products, resulting in the formation of small molecular products.

A numerical investigation on combustion characteristics in a thermally orthotropic narrow channel with hydrocarbon fuels

A novel approach of using thermally orthotropic wall materials to enhance stabilization of premixed methane/air and propane/air flames in a narrow channel between two parallel plates is numerically demonstrated. The streamwise and spanwise wall thermal conductivities kxx and kyy are respectively adjusted from 2 to 128 W/m⋅K by a step of eight times, to investigate the influence of directional heat transfer in solid walls on the combustor’s performance in terms of the flame stability limits and heat loss resistance. Results show that the lower velocity limit (LVL) is nearly insensitive to the wall thermal conductivities while the upper velocity limit (UVL), on the contrast, can be significantly affected. The thermally orthotropic combustor with a higher level of kxx but lower level of kyy, is observed to show a unique kind of wall temperature profile in the pre-flame region, which leads to a larger extent of preheatings to the unburnt mixture and less heat losses to the ambient, and consequently an elevated flame propagation speed. The resistance to external heat losses can be enhanced by decreasing the heat conductivity in either the streamwise or spanwise direction. However, since the latter has exhibited much more noticeable effects, and in order to avoid sacrificing the flame stability limit, the thermally orthotropic combustor thereby takes the superiority in overall performance.

Numerical study of the effects of excess air ratio on passive pre-chamber jet performance and ignition mechanism

Pre-chamber (PC) ignition is a promising lean-burn technology to improve the efficiency of spark ignition (SI) engines while achieving ultra-low NOx emissions. However, the significant effects of the excess air ratio (λ) on the PC jet performance and ignition mechanism were still not fully unraveled. The current study numerically investigated the jet characteristics and combustion behaviors of a narrow-throat PC with two rows of orifices on a passive PC engine fueled with methane. The numerical model was validated extensively by the pressure, heat release rate, and natural flame luminosity (NFL) imaging of the PC flame jet. A detailed comparison of the NFL images of the jets and the simulated flame iso-faces presented excellent matching in the flame jet penetration and propagation process. The results reveal that the λ = 0.9 case has the best jet performance, but it doesn’t lead in every jet characteristic. For instance, the jet temperature is highest under λ = 1.0, and OH concentration is most under λ = 1.1. The transition from cold jet to flame jet produces noticeable jet non-uniformity, which is reported for the first time. This observation can be remarked as a more accurate indication of the initiation of pre-chamber flame jet discharge. The asymmetrical mixture inflow from the main chamber produces asymmetrical turbulent kinetic energy (TKE) and temperature distributions; these result in different flame propagation in the PC, jet non-uniformity, and varied stages of jet discharge. The upper jets are quenched in every case because of the much higher TKE, while the lower jets don’t present quenching when the λ ranges from 0.9 to 1.3; especially, the λ = 1.1 case has the best ignition performance for its most OH radicals. The “jet ignition regime”, in which flame quenching takes place, results in highly non-uniform flame propagation and low flame propagation speed. The ultra-rich case shows more signs of jet ignition and thus exhibits worse jet performance than the lean cases. This study uncovers the mechanisms and the optimal λ value for the pre-chamber ignition of the main chamber charge, providing valuable guidance for PC design.

The effect of flexible obstacles with different thicknesses on explosion propagation of premixed methane-air in a confined duct

The effect of flexible obstacles with varying thicknesses on the explosion characteristics of combustible gas in a simulated confined duct (cross section 80 mm × 80 mm, length 3 m) was experimentally investigated, aiming to reduce the huge losses caused by gas explosion accidents in the process industries and mining industries. In this paper, plant fiber membranes with an opening area of 0 and thicknesses of 0.105 mm, 0.210 mm, 0.315 mm, 0.420 mm, 0.525 mm, and 0.630 mm were selected as flexible obstacles. The thickness of the flexible obstacle determines the strength of its compressive resistance. The characteristics of overpressure and flame during methane explosions are analyzed and conclusions are drawn. Results indicate that several shock wave reflection processes occur before the diaphragm ruptures, resulting in turbulent flames. In addition, the explosion wave generated numerous shock reflections during the rupture process of the diaphragm, which was gradually discharged downstream of the pipe by ejection as the pressure wave accumulated in front of the diaphragm. It should be noted that the thickness of the flexible obstacle determines the pressure accumulation in front of the membrane. Generally, the thinner the flexible obstacle, the less intense the turbulent flame is induced by the flexible obstacle, decreasing the contact area between the unignited gas downstream of the pipeline and the turbulent flame area. In conclusion, with an increase in the thickness of the flexible barrier, it exhibits a mechanism of initially suppressing and subsequently enhancing the impact on methane explosions. The increase of the thickness of the flexible obstacle motivates the flame propagation speed, which leads to an increase of turbulence intensity and explosion intensity.

Study on Oxy-Methane Flame Stability in a Cylindrical Porous Medium Burner

Combustion in a porous medium can be beneficial for enhancing reaction rate and temperature uniformity. Therefore, considering the combination with oxy-fuel combustion can address some shortcomings in oxy-fuel burners, a cylindrical two-layer porous burner model is established based on OpenFOAM in this paper. A two-temperature equation model is adopted for the simulation of the heat transfer process. The CH4 skeletal kinetic mechanism is adopted for complex chemistry integration based on OpenSMOKE++. Corresponding experimental methods were used for complementary studies. The walls of the burner are wrapped with three types of thermal insulation materials to present different levels of heat loss. The results show that considering the convection and radiative heat loss of the burner wall, the temperature near the wall is reduced by more than 300 K compared to the adiabatic condition. As a result, the flame propagation speed and CO oxidation rate slowed down. The stable range will be destructively narrowed by more than 50%, and CO emissions will increase by more than 10 times. These defects will be aggravated by increasing the diameter of the burner. It is observed that when the diameter of the burner increases from the initial 5 cm to 10 cm, the effect of heat loss on the stable range is insignificant.