Elsevier

Combustion and Flame

Volume 234, December 2021, 111649
Combustion and Flame

The free radical mechanism of electromagnetic field affecting explosion of premixed methane

https://doi.org/10.1016/j.combustflame.2021.111649Get rights and content

Abstract

To study the mechanism of the electromagnetic field influences flammable gas explosions, 9.5% methane-air explosion tests were carried out in a closed transparent pipe and the effect of the electromagnetic field on the free radicals was analyzed through COMSOL Multi-physics. The relationship between explosion pressure, explosion product composition and its concentration and movement of key free radicals was analyzed from the results of experiment and numerical simulation. In the experiment, the reactions between the reactants and the free radicals were intensified such that the mean overpressure increases by 49.76%, the time of forming in similar flame shape was advanced by 15 ms, 39 ms and 99 ms respectively at the same location and the explosion reaction proceeded violently, and residual methane decreased by 24.57% and carbon monoxide increased by 55.67% in the product components and the generation of other hydrocarbons also decrease. From simulation, the electromagnetic field accelerated the movement of paramagnetic free radicals, which increased the effective collisions of the reaction, and promoted the chain reaction velocity. Accordingly, in places where there is a danger of flammable gas explosions, electromagnetic fields should be avoided.

Introduction

With in-depth study of flammable gas explosions, the impact of the electromagnetic field on the explosion mechanism has come into focus. Many studies about the effect of electromagnetic field on gas explosion characteristics have been carried out. Ye et al. [1]. studied the influence of the electromagnetic field on the gas explosion by applying the electromagnetic field generated by a 4000-turn coil. It was found that the electromagnetic field increased the overpressure and flame propagation velocity of the gas explosion, and the greater the electromagnetic field strength, the more obvious the promotion of gas explosions. Wu Wenfei [2] proved that a gradient electromagnetic field decreased the concentration of NOX by at least 60% on average owing to the oxygen paramagnetic force. Li Jianqiao [3] verified a two-dimensional simulation of the explosion's influence on the electromagnetic field and found that only detonation gases with a certain conductivity can freeze the electromagnetic field disturbance. Mohd. Junaid Siddiqui [4] deduced the formula for the electromagnetic field effect on the explosion. At the same time, the point explosion problem highlighted that the presence of an electromagnetic field enhanced the velocity. It may be concluded that the presence of an electromagnetic field decreased the instabilities. Chauhan Astha [5] obtained a first-order approximate analytical solution of the planar and cylindrical symmetric flow under the presence of a transverse electromagnetic field in a non-ideal medium and found that gas particles will collide more frequently as the non-ideal state increases.

A series of studies on the effect of uniform and non-uniform magnetic field on the torch burner flame have been carried out by using holo-shear lens-based interferometer, digital holographic interferometry, digital speckle pattern interferometry, circular grating Talbot interferometer and other experimental equipment. The research show that under the influence of upward decreasing and uniform magnetic field temperature inside the micro flame increases in comparison to temperature inside micro flame without magnetic field. The reason why the magnetic field affects the temperature of the micro flame is due to the paramagnetic and diamagnetic behavior of gases in air and combustion products. In combustion field, the magnitude of magnetic susceptibility of paramagnetic O2 is much more than the diamagnetic constituents (i.e. N2, CO2, CO, H2O), so the influence of gradient magnetic field on the paramagnetic O2 is more prominent than that of other species controlling the combustion. The temperature of the flame is increased under the influence of the upward decreasing magnetic field and flame temperature is decreased under the influence of upward-increasing magnetic field. Uniform magnetic field has a negligible effect on temperature of the flame [6], [7], [8], [9], [10], [11]. The effect of a permanent magnet was investigated on the radial distribution of refractive index difference and temperature distribution inside the diffused flame by using the LLFTDHI is presented and the results show that the temperature, as well as the width of the flame, is increased under the influence of a uniform magnetic field [12].S. Kinoshita [13] verified that in the microgravity without magnetic field, it is revealed that combustion products remain around the diffusion flame because of the lack of convection, and the amount of O2 diffusion to the flame region becomes retarded. When a gradient magnetic field is added, convection is induced around the diffusion flame by the magnetic field which induces magnetic buoyancy force due to the inhomogeneity of magnetic susceptibility.

At the micro-level, a strong electromagnetic field affects the movement of electron in the atoms of a substance. The electromagnetic fields involved in chemical reactions can change the chemical reaction process and the reaction rate of the molecules or atoms [14]. Abramov Vv [15] pointed out that the influence of the electromagnetic field depends on the type of incident particles. Qiu Jihua [16,17] first pointed out that chemical reactions between paramagnetic substances, diamagnetic substances, or ferromagnetic substances will also be significantly affected by an electromagnetic field owing to the different magnetic properties of the reactants and products participating in the chemical reaction, and found that an electromagnetic field of 0.25 T can reduce the relative ignition temperature of anthracite by approximately 100 °C. The electromagnetic field affects the following mechanisms of combustion [18]: the electromagnetic field directly affects the chemical kinetics, the Lorenz force of the electromagnetic field affects combustion; and the gradient electromagnetic field force affects the flow. Zhang Hui et al. [19] systematically studied the influence of the electromagnetic field on the formation of a laminar flame and nitrogen oxides in gas explosions. The study found that an applied electromagnetic field can enhance the combustion of laminar flame, and in the gradient of the electromagnetic field, the higher flame temperature is found where the strength of the electromagnetic field is high. Owing to the influence of the Lorentz force, the original random movement of nitrogen-containing ions or ion groups in the direction of the force and their collision probability with oxygen molecules decrease, thereby reducing the formation of nitrogen oxides. Aoki Takashi [20] tested and showed the infinitesimal effect of the Lorentz force on the combustion characteristics and found that in upward-decreasing electromagnetic fields, the emission intensities of ·OH, ·CH, and ·C2 radical transitions increased, the flame temperature increased, the flame dimension decreased, and a bluing tendency of the flame occurred. E. Nchesnokov [21] investigated the ultrafast dynamics of OH radicals investigated by their free induction decay (FID) signal. ZHANG Wen [22] pointed out that the formation rate of surface hydroxyl radicals was increased by 11.7% because of presence of an electromagnetic field through fluorescence technology. LIU Zhen-tang [23] proposed analyzing the explosion mechanism by testing the products. J. Herzler [24] analyzed the product components through experiments and matched them with a simulation. Luo Zhenmin [25] inferred the methane explosion process by analyzing the spectra of ·H and CH2O and found that the ·H from the reaction of ·CH3 and H2 formation triggered the explosion reaction of other flammable gases, agreeing with the results of the numerical simulation. Lei Baiwei [26] used Chemkin to study the reaction and influencing factors on hydrogen formation during methane explosion and found that the free radical H is the key factor to influence the formation of H2 generated from gas explosion. The results from the numerical simulation calculation showed that the radical reaction H2O + ·H ⇔ ·OH + H2 was the main cause of hydrogen formation. ·H + O2⇔ ·O + ·OH is a key reaction in the explosion [27,28], and it has a promoting effect on the overall reaction, including shortening the ignition delay time, accelerating the flame propagation, and lowering the concentration of O2 [29]. The competition between the chain-branching reaction and chain termination reaction has a far-reaching effect on the flammability limit [30]. The inhibitory effect is mainly to reduce the concentration of key free radicals [31]. Xiaozhe Yu et al. obtained the reaction mechanism of explosion through SEM and XPS analysis of the explosion residues [32]. Haitao Li et al. analyzed the severity of the explosion by qualitative and quantitative methods, including gas chromatography, SEM-EDS, etc. [33].

The development of microscopic mechanisms and the study of magnetic field effects are more conducive to the analysis of the influence of magnetic fields on explosions. Explosion reaction is a reaction involving a large number of free radical particles, and free radical particles are paramagnetic or diamagnetic. The magnetic field generated by the electromagnetic field will directly act on the paramagnetic or diamagnetic particles, resulting in changes in their motion state, and ultimately affect the whole explosion reaction process. This paper hopes to analyze the action mode of magnetic field from the perspective of microscopic free radicals or particles, so as to help the future research on the influence of magnetic field on explosions.

Section snippets

Experimental system

The experimental system primarily consisted of a closed transparent pipeline for flammable gas explosion, a transient pressure acquisition device, a high-speed camera, an ignition device, a sampling system of gas explosion products, and other parts. Compared with actual homologous systems, this experimental system included a gas sampling system to effectively collect the gas products after explosion, used synthetic air (79% N2, 21% O2) to reduce errors in explosion products and added nitrogen

Pressure of premixed methane-air explosion

The explosion pressure curve of the premixed 9.5% CH4/air gas with and without electromagnetic field shows the three transient pressure sensors monitor the pressure changes at three different positions in the pipeline. The 1#, 2# and 3# transient pressure sensors were evenly distributed along the axis of the explosion test pipeline. The abnormal signal of the pressure curve at time zero in the figure was the ignition signal.

The representative explosion pressure curve of the premixed 9.5% CH4

Discussion of results

After the 9.5% CH4/air premix was ignited, in the presence or absence of the applied electromagnetic field, the explosion reactions showed similar pressure curve, the same flame propagation shape and similar product compositions. But the values of pressure, flame propagation velocity and product component concentration were different. The explosion pressure peak measured by each sensor gradually increased along the length of the experiment pipeline. Compared with gas explosion without magnetic

Conclusion

  • 1)

    In view of the results of the explosion test, it can be concluded that the electromagnetic field promoted the explosion reaction, as confirmed by the following facts: the mean peak pressure increases by 49.76%; the time of forming the same flame shape was advanced by 15 ms, 37 ms and 99 ms respectively; and methane residue decreased by 24.57%, carbon monoxide increased by 55.67% and the generation of other hydrocarbons decreases.

  • 2)

    Through COMSOL, it can be concluded that the paramagnetic

Declaration of Competing Interest

None.

Acknowledgments

The authors would like to thank Elsevier for the refinement of this article. This research work is supported by Beijing Municipal Education Commission (KM201910017001), Capital Collaborative Innovation Center for Clean Energy Supply, and Use of Safeguard Technology (No. PXM2017_014222_000041).

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