Methane-air Mixtures Research Articles

Multi-objective optimization and analysis of chemical kinetics properties: Exploring the impact of different hydrogen blending ratios on LPG and methane-air mixtures

The use of hydrogen with hydrocarbon fuels like methane and LPG is crucial for carbon neutrality and environmental goals. United Nations Environment Programme (UNEP) and the International Energy Agency (IEA) emphasise reducing LPG and methane emissions to prevent temperature increases. Utilising H2-LPG and H2-CH4 blends enhances free radical formation, mitigates emissions, and combats global warming. Using the CHEMKIN tool, this research compares the combustion characteristics of H2-CH4-air and H2-LPG-air mixtures under atmospheric conditions and different operating conditions. Blending 50 % hydrogen with LPG and methane yields notable variations, including a 20.97 % higher flame speed in H2-CH4, 42.89 % more H free radical formation in H2-LPG, 50.24 % reduction in O free radical formation in H2-LPG, and a 24.01 % increase in OH free radical formation in H2-LPG. The study investigates the energy and environmental impacts of incorporating hydrogen into CH4-air and LPG-air combustion, focusing on 50 % H2 blending with LPG. The benefits of hydrogen blending encompass heightened flame speed and augmented free radical formation, ultimately contributing to emission reduction in hydrogen-blended fuels. These findings highlight H2-LPG and H2-CH4 blends as environmentally friendly alternatives with improved combustion.

Experimental study of lube oil pre-ignition in an optical gas engine test rig

Lean-premixed gas combustion is an attractive option for meeting future emission standards toward considerably lower particulate matter and emissions while obtaining an efficiency comparable to diesel combustion. However, ignition processes still pose considerable challenges, with pre-ignition being a major issue. The pre-ignition phenomenon might result from the self-ignition of lube oil in hot zones, making it challenging to observe in real engines. To study single or sparsely separated lube oil droplets under engine-like conditions, the “Flex-OeCoS” experimental test facility was used. To be able to induce such pre-ignitions artificially a novel piezo droplet injector was developed and manufactured, which enabled the metering of minor amounts of lubricating oil or even the injection of single droplets. High-speed cameras and specially developed and partially automatable and adaptive evaluation algorithms were used to record and track the behavior of the injected droplet using overlaid schlieren and OH* chemiluminescence. During the measurement campaigns, relevant operating parameters were varied, and optical investigations were carried out with an ignitable mixture on the Flex-OeCoS test facility under engine-relevant operating conditions. Spontaneous ignition of the vaporized lubricating oil was observed, which subsequently led to ignition of the whole air-fuel mixture in the optical combustion chamber. This was especially evident for lean methane-air mixtures up to an air-fuel ratio of 2.0. With this approach, information could be gathered about the necessary internal engine conditions that lead to pre-ignition in lean mixtures and where their limits lie.

Experimental investigation and numerical analysis on the confined deflagration behavior of methane-air mixtures within the suppression of typical haloalkanes

To investigate the mechanisms of haloalkanes in promoting and inhibiting explosions in methane-air mixtures, the halon alternatives of CHF3, C3HF7, and C3H2F6 were selected as inhibitors. Experimental research was conducted in a 20-liter spherical explosion vessel to examine the promotional and inhibitory effects of these haloalkanes on methane-air mixtures. The adiabatic flame temperature, heat release rate, and concentrations of the key radicals were calculated. The results showed that C3H2F6, C3HF7, and CHF3 all exhibited a dual effect, initially promoting and subsequently inhibiting explosions in fuel-lean methane-air mixtures. The critical explosion suppression volume fractions of C3HF7 and CHF3 were 1.5 times and 2.25 times that of C3H2F6, respectively, for the suppression of methane explosion at Φ = 1.2. Furthermore, C3HF7 demonstrated a dual impact on the explosion of stoichiometric methane-air mixtures. From the perspective of reaction kinetics, for fuel-lean methane-air mixture (Φ = 0.8), a small volume of halogenated hydrocarbons led to an increase in the adiabatic flame temperature due to the heat release from critical fluorine-containing elementary reactions, thereby promoting the explosion. Under fuel-lean conditions, the introduction of a small volume of halogenated hydrocarbons led to an increase in the adiabatic flame temperature due to the heat release from critical fluorine-containing elementary reactions, thereby promoting the explosion. Simultaneously, these fluorine-containing reactions scavenged the key free radicals of H, O, and OH inhibiting the explosion process. These two effects competed with each other, ultimately determining the direction of the explosion process.

Combustion Of Methane With Microdroplets Of Water In A Weak Electric Field

The effect of a weak electric field on the flame of a Bunsen burner, including the case of an aerosol consisting of water microdroplets in an air-methane mixture has been studied in detail. The propagation velocities of the laminar flame have been determined. If the stabilization area (burner edge) and the flame tip are excluded from consideration, the average propagation velocity of a laminar flame in the absence of an electric field is constant. For a stoichiometric mixture of methane with air and aerosol combustion, it is equal to Su≈0.35 and 0.3 m/s, respectively. When an electric field is applied, there is a common trend for cases of combustion of methane and a gas-droplet mixture, i.e., maintaining the flame front propagation velocity from the anode side and increasing by about 8-10% from the cathode side.

ЦИФРОВЕ МОДЕЛЮВАННЯ РУЙНУВАННЯ ДЕГАЗАЦІЙНИХ ТРУБОПРОВОДІВ У ПІДЗЕМНИХ ГІРНИЧИХ ВИРОБКАХ

Purpose. The purpose of the work is to simulate the process of corrosion formation on a degassing pipeline using Solid Works and COMSOL Multiphysics 5.6 software products. Methodology. Development of a mathematical model for establishing corrosion zones of a steel degassing gas pipeline and modeling the corrosion process in a mine environment using the SolidWorks and COMSOL Multiphysics 5.6 software package. Results. It has been established that the degassing network of mines is constantly under the influence of mechanical-electrochemical interaction, which manifests itself during longitudinal deformations of the rocks at the bottom of the mine workings. The high level of corrosion processes in underground pipelines is the result of the interaction of the metal, which acts as an electrode, with groundwater, which acts as an electrolyte; at the same time, the determining factors in the corrosion process are the electrical conductivity of the soil and deformation processes in pipelines. The primary corrosion protection of underground degassing pipelines can be considered as the improvement of methods for monitoring and maintaining the technical condition of mine degassing gas pipelines to eliminate mechanical deformations and the effects of aggressive groundwater. Scientific novelty. It has been established that the high level of corrosion processes in underground pipelines is the result of the interaction of the metal, which acts as an electrode, with groundwater, which acts as an electrolyte. At the same time, the determining factors of the corrosion process are the electrical conductivity of the rocks at the bottom of the mine workings and deformation processes in pipelines. Practical implication. The reasons for the low service life of underground degassing pipelines under the constant influence of mechanical and electrochemical corrosion have been determined. Keywords: degassing, vacuum underground gas pipeline, methane-air mixture, monitoring, corrosion, throughput.

LES of pilot n-heptane ignition and its interaction with the lean premixed methane–air mixture in a dual-fuel combustion engine

Large eddy simulations of pilot fuel ignited, lean premixed, natural gas engines are performed to study the pilot-ignition process and its subsequent interaction with the premixed charge. The injection pressure (Pinj) and injection duration (Δinj) are varied (i.e. 800 bar/760μs, 800 bar/500μs, and 400bar/760μs) to study the impact of the injection process on the subsequent combustion evolution. Open-cycle simulations considering the full engine geometry are used to predict the in-cylinder flows, while combustion is modeled using a finite-rate chemistry model. In-cylinder methane (CH4) is shown to delay the low-temperature ignition of the pilot fuel, regardless of the pilot injection setting, which subsequently prolongs the overall pilot fuel ignition delay. Moreover, all simulated cases show the occurrence of back-supported combustion (BSC), where the burning of CH4-air mixture is “back-supported” by pilot fuel radicals. Despite both the 800 bar/500μs and 400 bar/760μs cases having the same injected pilot fuel mass, the peak in-cylinder pressure and burning rate of the premixed CH4-air mixture in the former case are higher. Higher Pinj and shorter Δinj lead to better mixing between the pilot fuel and the premixed CH4-air charge. Subsequently, this forms a larger volume of regions with elevated equivalence ratio due to the presence of pilot fuel (ϕ>ϕCH4) which, consequently promotes the formation of BSC. The impact of in-cylinder flow fields on the dual-fuel combustion process is investigated by performing two closed-cycle 800 bar/500μs cases with one assuming solid-body rotation and another without solid-body rotation (i.e. zero velocity field). In-cylinder flow field is shown to have a visible impact on the transition stage between the pilot ignition stage and the premixed flame propagation stage, but have an insignificant effect on the pilot fuel ignition process. In the transition stage, slower flame propagation is observed in the zero-velocity case. The results show that this is not only due to the turbulence effect on premixed flame but also due to differences in the volume and distribution of pilot fuels that impacts BSC.

Design of an experimental stand for hydrogen explosions

Hydrogen, with its remarkable potential as a clean and abundant energy carrier, has gained significant attention as a promising solution for a sustainable future. However, the handling, storage, and utilization of hydrogen come with inherent risks, particularly the potential for explosions. The present work deals with the design of an experimental stand for hydrogen explosions in interconnected spaces, based on previous experience in the field of air-methane mixtures explosions. Considering the explosive properties of hydrogen, much more aggressive than methane gas, a comparative analysis is carried out between the results of a physical experiment of a methane explosion carried out on a spiral stand and the results obtained from a computational simulation of a hydrogen explosion on a similar geometry. The purpose of the comparative analysis is to highlight the vulnerable points of the future construction, bringing improvements to the new experimental model in the sense of increasing operational safety, while preserving, at the same time, the possibilities of recording the explosion parameters (pressures, velocities, flame front behavior) at a higher level of accuracy.

Improving the efficiency of the maintenance and monitoring system for mine degassing pipelines

Degassing pipelines in mine workings frequently change position, causing leaks and joint failures due to geomechanical processes and floor rock heaving, which lead to fatigue and corrosion. This paper reviews maintenance and renovation methods for these pipelines, aiming to develop innovative solutions to enhance operational performance. Research focuses on assessing current maintenance techniques, improving reliability, and implementing new technical solutions under real mine conditions. The proposed technological complex aims to enhance the safety and quality of pipeline maintenance and renovation. Typical production scenarios and monitoring methods for effective control of the technical state of mine degassing systems are considered. Continuous control of degassing wells and pipelines using structural modules of the Automated Mine Degassing Control System (AMDCS) is emphasized. Implementing these modules allows for automatic monitoring of the methane-air mixture and measurement of pressure drops, absolute pressure, temperature, and gas flow volume, thereby providing critical data to operational personnel.

Fundamentals of forecasting explosive gas accumulation formation in working areas of coal mines

This article presents a methodological approach to solving the problem of predicting local methane accumulations in order to prevent occupational injuries resulting from explosions of methane-air mixtures. On the basis of the analysis of accidents that occurred in coal mines and industrial injuries of miners (mine workers) it is established that the leading role belongs to explosions of methane-air mixtures. The method/methodology considered in the article is based on a comprehensive analysis, first of all, of both mining and geological and other influencing factors and conditions of operating mines of the Pechersk coal basin, as well as aerogas-dynamic processes of mass transfer of air flows in mine workings. The assumption about influence of mining equipment on the process of formation of methane accumulation sites is made. The algorithm of consecutive actions of managerial engineering and technical personnel to reduce industrial injuries is proposed. The developed algorithm implies the use of applied software for modelling aerogas-dynamic processes in the mining excavation operating region. FlowVision and ANSYS Fluent application software packages were selected to determine the methane accumulation locations based on the analysis of existing application programmes.

Investigation into the Effect of H2-Enriched Conditions on the Structure and Stability of Flames in a Low-Swirl Combustor Derived from Aero-Engine Design

This study introduces an innovative approach involving the injection of hydrogen into a low-swirl, non-premixed flame, which operates with gaseous fuels derived from an air-blast atomizer designed for aero-engine applications. The aim is to characterize how hydrogen enrichment influences flame structures while maintaining a constant thermal output of 4.6 kW. Using high-speed chemiluminescence imaging, three fueling conditions were compared: the first involved pure methane/air, while the second and third conditions introduced varying levels of hydrogen to an air–methane mixture. The results reveal significant effects of hydrogen enrichment on flame characteristics, including a slightly shorter length and a wider angle attributed to heightened expansion within the Combustion Recirculation Zone. Moreover, the emission of UV light underwent considerable changes, resulting in a shifted luminosity zone and reduced variance. To delve deeper into the underlying mechanisms, the researchers employed Proper Orthogonal Decomposition (POD) and Spectral Proper Orthogonal Decomposition (SPOD) analyses, showing coherent structures and energetic modes within the flames. Hydrogen enrichment led to the development of smaller structures near the nozzle exit, accompanied by longitudinal oscillations and vortex shedding phenomena. These findings contribute to an advanced understanding of hydrogen’s impact on flame characteristics, thereby propelling efforts toward improved flame stability. Additionally, these insights hold significance in the exploration of hydrogen as an alternative energy source with potential environmental benefits.

The deflagration suppression effect of ammonium salt-modified dry water on methane-air mixtures: An experimental investigation

To evaluate the efficacy of ammonium salt-modified dry water in suppressing the deflagration of methane, in this study, four kinds of ammonium salts were chosen to modify the properties of dry water powders. The physical features of the modified dry water were tested and analyzed. Their suppression characteristics on methane explosion were experimentally explored, and the corresponding deflagration parameters were obtained. Results show that the Pmax and the (dP/dt)max first diminish and then increase with increasing amounts of modified dry water. The effect of ammonium dihydrogen phosphate-modified dry water (MAP-DW) on the explosion pressure is superior to that of other kinds. The delay effect of MAP-DW and ammonium polyphosphate-modified dry water (APP-DW) on explosion time is better for methane at equivalence ratios of 0.8 and 1.2. Moreover, the explosion suppression mechanism of the modified dry waters was proposed. The findings of this work are helpful for optimizing gaseous deflagration suppressants.

Investigation of Sub-Grid Scale Turbulence-Radiation Interaction Effects on Turbulence Energy Transport and Varying Thermophysical Properties Using Large Eddy Simulation

Abstract The main objective of this article is to investigate sub-grid scale turbulence–radiation interaction (SGS TRI) effects on SGS turbulence kinetic energy (TKE) fluctuations and varying thermophysical properties in a partially premixed combustion system for a laboratory-piloted methane/air flame. The large eddy simulation approach is employed to simulate the turbulence of the compressible reactive flow. SGS quantities, including turbulent stress and fluxes of enthalpy and species in the sub-grid scale, are computed using the standard Smagorinsky–Lilly model. The radiative transfer equation is modeled by applying the spherical harmonic P1 approximation by considering the radiative heat source related to the SGS TRI contribution. Optically thin fluctuation approximation is utilized to simplify the radiative absorption term. A chemical reaction mechanism comprising 41 steps and 16 species is applied to model methane–air mixture combustion. Diffusion flamelet-generated manifolds are employed to govern the species transport equation. About 87% of TKE is resolved by applying the finest grid consisting of 1,822,580 cells. Impacts of SGS TRI on the spatially filtered density, eddy viscosity, SGS velocity and TKE, overall radiative emission, RMS temperature fluctuations, and nitrogen monoxide (NO) formation are studied. The results reveal that considering SGS TRI in the simulation leads to remarkable discrepancies, particularly in SGS velocity and TKE by 6.70% and 7.40%, respectively. Meanwhile, SGS density and eddy viscosity deviate negligibly in the presence of SGS TRI. Also, the filtered mass fraction of NO reduces up to 17.54% on average by considering TRI.

Results of the testing of an automated control system and the application of natural gas burners

Purpose. Testing an automated control system to improve the accuracy of calculating natural gas costs in coalfields and provide reliable information to suppliers and consumers regarding mutual settlements. Research methods. The conditions of operation of the mine degassing system and the methods of monitoring its operational indicators for effective management of the technical condition of the object in the real conditions of the mine environment are considered. Approaches to continuous monitoring of well parameters using structural modules of the ACS of the DSH have been determined. Findings. The DSH ACS module was implemented in the real conditions of the mine environment, which made it possible to carry out automatic monitoring of the methane-air mixture, measurement and preparation for issuing to the operational personnel the current values of pressure drops on the diaphragms, absolute pressure, temperature and volume flow of gas. Originality. A new method of increasing the accuracy of commercial accounting of natural gas of coal deposits has been developed, which consists in constant monitoring of the technical condition of the degassing system and automated accounting of natural gas based on the «DIYA» measuring complex. The average hourly values of gas mass flow were obtained, as well as the difference between the data of the channel of the operating ASUTP KS and the channel of the "Ergomera"-126.MU controller in the complex with the "DIYA" operator station. Practical implications. A mine experiment was conducted on monitoring and control of mine degassing gas pipelines using innovative technical solutions for the rapid and systematic delivery of information about the parameters of the mine degassing system to the dispatch service. The structure, principle of operation, methodology for calculating the consumption of natural gases and coal field gases were developed, and industrial tests of an experimental sample of the ACS of the DSH were carried out with automatic entry of results into electronic reporting journals.

Experimental investigation on initiation mechanism, overpressure, and flame propagation characteristics of methane-air mixtures explosion induced by hexogen in a closed pipeline

Gas explosion accidents under shock wave stimulation cause more severe hazards than electric spark ignition. To explore the initiation mechanism and explosion characteristics of methane induced by shock waves. A series of experimental investigations of methane-air mixtures initiated by hexogen (RDX) was performed based on a self-developed closed pipeline. The effects of incident shock wave intensity and methane concentration on the characteristics of methane explosion were studied. The results indicate that methane participating in the reaction can significantly increase the positive pressure duration compared with RDX explosion. With the increase of incident shock Mach number (Ma), methane deflagration pressure increases and counteracts the attenuation of RDX detonation pressure. When the ignition distance is 0 cm, the overpressure inside the pipeline is directly proportional to the methane concentration from 5 vol% to 13 vol%. When the concentration approaches the upper explosion limit, the flame velocity exceeds the theoretical detonation velocity, resulting in the unstable detonation of methane in the pipeline.

Experimental investigations of the influence of radial gas mixing in an inert ceramic foam bed on thermal combustion of lean methane-air mixtures

The paper discusses the application possibilities of ceramic foam in a thermal combustion process of a lean methane-air mixture. The experiments were done in a ceramic foam bed. The foam (Vukopor ® A) was made mainly of Al 2O 3. The foam samples were packed in a tubular reactor symmetrically placed in a laboratory furnace. It was assumed that the tested foam should have a surface close to the monolith surface area which was tested in a previous work (Pawlaczyk and Gosiewski, 2015). Pore density of the tested foam was 10 PPI. The tested air mixture contained 0.51 - 0.76 vol. % of methane. The results show that thermal methane oxidation in foam is possible in the acceptable range of temperatures. The combustion process in foam is characterized by similar ignition temperature to tests carried out in monolith, a more intense course, and better methane conversion at lower temperatures.

Ultra-lean premixed combustion of a holder-stabilized 80 %hydrogen–20 %methane–air mixture in a preheated mesoscale combustor

To achieve cleaner emissions, the ultra-lean premixed flames of 80 %H2–20 %CH4–air with the coupling effect of flow and heat recirculation were experimentally investigated. The flame behaviors for various Re values are observed and discussed in detail. The conventional stable flame, stable wedge-shaped residual flame, and wedge-shaped residual flame with upward and downward oscillations appeared in sequence as the equivalence ratio decreased. For a stable wedge-shaped residual flame with a decreasing Re value, the operating range broadens first and then narrows. By contrast, the operating range of the wedge-shaped residual flame with oscillations narrowed monotonically and slightly. With a decreasing Re value, the resident location of the conventional stable flame base remained nearly the same. When the equivalence ratio was reduced further, a stable wedge-shaped residual flame appeared. Subsequently, a wedge-shaped residual flame with oscillation occurred when the equivalence ratio was further reduced. Finally, the blow-off mechanisms were revealed.

Effect of Co3O4 powder on the explosion characteristics and CO products of methane-air mixture

Rapid removing toxic CO for methane-air mixture explosions is essential in reducing casualties, but relevant research is rare. This paper proposed a novel approach for efficient CO removal and explosion suppression for methane-air mixture explosions using Co3O4 powder. The effect of Co3O4 powder on the explosion characteristics and basic parameters to quantify the CO removal performance have been studied, mainly including maximum explosion pressure (Pm), combustion time (tc), flame propagation velocity, and average CO removal rate (Rave.). Results indicated that explosion intensity and CO concentration decreased significantly with Co3O4 powder, due to increased heat absorption capacity and the active site of CO catalytic oxidation. However, the average CO removal rate decreased as the Co3O4 powder usage and CH4 partial pressure increased, which could reduce the dispersion degree of the Co3O4 dust cloud. The CO removal was achieved through the CH4 catalytic combustion and CO catalytic oxidation, which could reduce the CO generation and oxidize the generated CO to harmless CO2, respectively. This work provides a novel routine to lessen the explosion hazards and has promising applications in emergency rescue.

Parametric instability in propagating flames and the impact of hydrogen enrichment on the onset of secondary instability

A self-excited acoustic instability of laminar premixed flames propagating in an open-ended tube with a length of 700 mm and a radius of 10 mm was simulated by solving the reacting unsteady compressible Navier–Stokes equations, to understand the way of massive acoustic generation and its onset behaviors. Four fuel–air mixtures with an equivalence ratio of 1.2 were considered, namely, methane–air and methane–hydrogen–air mixtures, to identify the role of hydrogen in rich methane–air mixture. Parametric instability, which generated huge acoustic disturbance and violent flame pulsations, was observed only for a particular methane–hydrogen–air mixture with RH = 0.2, consistent with previously reported experimental observations. For the investigation of the reinforcement mechanism of acoustic instability under parametric instability, the flame surface area modulation was examined. It was found that violent subharmonic flame front pulsations could strongly modulate the flame surface area in the fundamental mode, resulting in a fluctuating heat release rate and increased thermoacoustic coupling. When hydrogen addition was small, attaining a higher level of primary instability, which is the precursor of the parametric instability, was more dominant than increasing the threshold level for the onset of the parametric instability. With larger hydrogen addition, the increase in the threshold level was more dominant than attaining a higher level of the primary instability. In particular, as the flame propagation time decreased, the level of the primary instability was saturated in larger hydrogen addition. This study elucidates the mechanism for the acoustic generation of propagating flames under the parametric instability, and the effects of hydrogen enrichment within rich methane–air mixtures.

Дегазация сближенных угольных пластов направленной трассы скважинами

The technology of conducting degassing operations at the mines of Russia is determined by the experience of many years of using means of extracting methane from coal seams and worked-out spaces by drilling wells on coal seams under development, on converged layers, mainly under-worked, unloaded by a working face from rock pressure. The parameters of the degassing of methane release sources at the extraction areas of coal mines were determined based on the results of scientific studies of the processes of methane release into degassing wells and mine workings, methane content of coal seams, gas content of workings, extraction modes of captive methane-air mixtures, their gas composition, suitability for disposal. The dependences were established related to the methane content of coal seams on the yield of volatile substances, gas recovery indicators, and the rate of reduction of methane release during the operation of wells drilled in the plane of the coal seam at high-performance coal mines. The analysis of the results of the operation of coal mines in Russia was carried out, where the degree of methane extraction based on degassing and gas suction at mining sites was 760 m3/min or 54 % of all methane at 193 exploited sites. In 30 areas where degassing was used, this figure reached 1050 m3/min (75 % of the total). At the same time, the flow rate of captured methane was 365 m3/min, including 237 m3/min (60 %) at 6 mines. Studies of degassing parameters of the converged coal seams were carried out in mining areas where the development workings are separated from the mined-out space of the entire coal. In areas of mining operations with high methane release from undermined coal seams, a complex of underground degassing schemes was applied using long directional wells drilled from a local working with its horizontal part brought into the plane of the adjacent seam. The technology, performance and parameters of degassing are determined taking into account the mining conditions of occurrence and mining of coal seams, and the provisions of the regulatory instructions.

Effects of transverse concentration gradients on the vented explosion of methane-air mixtures

Experiments were performed in a 90-L vessel with a side vent to investigate the effects of transverse concentration gradients on the vented explosion of non-uniform methane-air mixture. Controlled by a mass flow meter, methane was injected uniformly into the vessel through 18 nozzles that are evenly distributed at the top to configure a methane-air mixture with an average methane volume fraction of 10.9%. Various transverse concentration gradients were obtained by altering the free diffusion time of the methane in the vessel. The deflagration flame was recorded through a high-speed camera at a frequency of 1000 Hz, and the internal overpressure at different locations was captured through three piezoelectric sensors. Experimental results reveal that larger concentration gradient resulted in lower flame propagation rate, longer time for the burst of the vent cover, and stronger pressure oscillations. Concentration gradient affects the typical flame structure, especially the formation and development of the tulip flame. Under the present experimental conditions, Helmholtz oscillations occurred in all tests. Concentration gradient has a negative correlation with the frequency of Helmholtz oscillations but a positive correlation with the duration of oscillations. In conclusion, the variation of concentration gradient obviously affects flame propagation and pressure evolution during vented explosions, which deserves more attention in the design of explosion protection.