Study on Influence of Cavity and Water Mist on Flame Propagation of Gas Explosion in a Pipeline

For studying the in ﬂ uence of the cavity and water mist on the ﬂ ame propagation of gas explosion, a rectangular steel cavity of size of length 80cm × width 50cm × height20 cm was designed. The in ﬂ uence of the cavity and it with water mist on explosion ﬂ ame propagation in a large circular gas explosion system with a length of 34m was studied. The change of gas explosion ﬂ ame in the pipeline was analyzed. The results showed that the intensity and ﬂ ame propagation velocity increase after the explosion ﬂ ame passes through the straight pipeline, and the attenuation rates are 4.93% and -2.48%, respectively. After the explosion ﬂ ame passes through a rectangular cavity of length 80cm × width 50 cm × height20cm , its intensity and propagation speed are inhibited, and the attenuation rates are 66.58% and 45.26%, respectively. After the explosion ﬂ ame passes through the rectangular cavity of the size of length 80 cm × width 50cm × height20 cm with water mist, the intensity and propagation speed are inhibited much more, and the attenuation rates are 85.09% and 65.85%, respectively. The in ﬂ uence of the cavity with water mist on ﬂ ame attenuation of gas explosion is better than that of the cavity alone. Based on theoretical analysis, it is concluded that the inhibition in ﬂ uence of the cavity on explosion ﬂ ame propagation is mainly due to repeated re ﬂ ection of ﬂ ame in the cavity, which results in the attenuation of its energy. The inhibition in ﬂ uence of water mist is mainly due to its heat absorption by vaporization.


Introduction
The gas explosion accident is one of the most destructive accidents in coal mine production in China. Although the safety level of China's coal mining production has been greatly improved in recent years, gas explosion accidents still happen from time to time [1,2].
The mechanism of gas explosion, its suppression, and mitigation have been studied by many scholars. Yu et al. [3] implemented a comparative experimental research on the explosion flame propagation characteristics of CH 4 -air mixture with different volume fractions, by using the selfbuilt small-scale experimental platform. The results indicated that when the methane volume fraction is 9.5%, the wave pressure and explosion flame propagation velocity are the highest. Yu et al., Wen et al., and Yu et al. [4][5][6][7] studied the effect of obstacles on the propagation characteristics of gas explosion. Cao et al., Song and Zhang, and Yu et al. [8][9][10][11][12][13][14] researched the influence of water mist particle size, spray volume, water mist zone length, and additives on the inhibition influence of water mist in gas explosion suppression by experiments. The results showed that when the particle size of ultrafine water mist is within 10 μm to 15 μm, the inhibition effect of explosion intensity and the methane-air mixture explosion flame propagation velocity is the best. When the concentration of superfine water mist is below 1.5 kg/m 3 , its inhibition effect on gas explosion overpressure is not obvious. The water mist reduces the flame temperature largely by absorbing the heat of combustion and rapidly evaporating. Shao et al. [15] found in the experiments that the inhibition effect of a vacuum cavity on gas explosion is related to the volume of the cavity. When the actual volume of the vacuum cavity is larger than the critical volume, it has the inhibition effect on the explosion; otherwise, it enhances explosion propagation to some extent. Wang et al. and Su et al. [16,17] have concluded through experimental research that ethylene and hydrogen can increase the maximum explosion pressure, laminar combustion rate, and maximum pressure rise rate of methane-air mixture, while it shortens the combustion time. Li et al. and Yan et al. [18,19] designed rectangular steel cavities with different aspect ratios and installed them in a 36 m long large-scale round pipeline of gas explosion test system. The experiment results showed that the cavity has an inhibition effect on the gas explosion propagation, and the effect of explosion inhibition is related to the volume of the cavities, their aspect ratios, etc. The relationship between the methane explosion peak overpressure attenuation factor y and the aspect ratio x of the cavity is as the following: y = −1:149 exp ðx/10:089Þ + 2:405. When the attenuation factor of peak overpressure is 1, the value of the aspect ratio is the critical. When the aspect ratio of the cavity is not more than 1, the cavity has an inhibition effect on the explosion wave overpressure, and the best aspect ratio for inhibition effect is 1/10. When the aspect ratio of the cavity is greater than 1, it enhances the explosion wave overpressure, and the cavity with an aspect ratio of 5/2 has the most enhancing effect on the explosion wave overpressure. Li et al. [20] studied the effects of hydraulic pressure on mechanical behavior, pore size distribution, and permeability.
For the study of gas explosion suppression and its disaster reduction, the small-scale test platforms have been mostly used, but the large-scale test platforms are not much applied. In order to further research the influence of a cavity with water mist on the flame propagation of gas explosion in a large-scale experimental pipeline system, in the paper, theoretical analysis and experimental research were used.

Theoretical Analysis of Influence on Gas
Explosion Propagation Process 2.1. The Propagation Mechanism of Gas Explosion. The propagation mechanism of gas explosion is the feedback mechanism of the precursor shock wave created by the explosion flame to the heating and compression of the unburned premixed gas. In the process of premixed combustion of a substance, the reaction zone separates the glowing combustion products from the unburned premixed combustibles, as shown in Figure 1 [21]. From the results of combustion, it can be seen that T 0 and C A0 of the premixed combustible gas are transformed into T f and C Af of the reaction products after combustion, and they are separated by combustion wave in space. According to the combustion theory of premixed flame, the turbulent premixed flame velocity S T is expressed as the ratio of the volume flow q v of the combustible premixed gas flowing through the flame to the apparent area A f of the turbulent flame, as shown in: The main reaction of premixed gas/air explosion is shown in the following equation: The above chemical reaction formula only expresses the final result of gas explosion. Many studies show that gas explosion is a very intricate chain reaction. When premixed CH 4 /air absorbs a certain amount of heat, the molecular chain breaks and turns into free radicals. Then, the free radicals become the reaction activation center. Under the right conditions, the free radicals will continue to decompose, and as the number of free radicals increases, the reaction will become faster and faster, resulting in an explosion.

Theoretical Analysis of the Effect of Cavity on Flame
Propagation of Gas Explosion. It is assumed that the mixture of CH 4 and air is uniform, and relatively static after, the mixture was prepared in the experimental pipeline system. The flame will spread to the two ends, and the periphery of the round pipeline with the ignition source as the detonation center after the mixture is ignited. At this point, the wall of the pipeline will interfere with the flame propagation, and the laminar flame will become turbulent propagation, which will lead to the distortion of the flame front and increase the flame burning speed. After the flame enters the cavity, part of the flame comes out from the cavity to form a primary flame, and the other part of the flame is stirred and mixed in the cavity to form a secondary flame. When the flame passes through the cavity, because of the influence of the cavity disturbance, the primary flame intensity attenuates and the secondary flame intensity increases. However, with the increase of the cavity length, the magnitude of the secondary flame increase decreases, and the time interval of the secondary flame also increases, and the overall attenuation of the flame front is positively correlated with the length of the cavity. Yan et al. [22] studied the mechanism of gas explosion suppression by the cavity by simulating the propagation process of gas explosion shock wave and flame in 2 Geofluids cavity. According to the method described in reference [22], the mode of the gas explosion flame premixing in the cavity is shown in Figure 2.

Theoretical Analysis of the Influence of Water Mist on
Gas Explosion Propagation. Lentati and Chelliah [23] found through research that the water mist mainly inhibits explosion through physical effect, and its influence through chemical effect is less than 10%. Therefore, in the paper, only its related physical effects were analyzed. The main physical effects of the water mist include heat absorption by vaporization and energy absorption, which mainly effect on the explosion flame. According to the calculation, when the size of the droplet d is less than 200 microns, the spray speed V is less than 30 m/s, the mass concentration Q of water mist is less than 899 g/m 3 , and the rate of absorption of flame energy by the droplets is far less than the order of magnitude of the latent heat of vaporization and the sensible heat absorption rate. So the suppression effect of water mist on the flame is mainly based on its heat absorption.

Experimental Study
The influence of the cavity and it with water mist on the flame propagation of gas explosion was studied by monitoring the parameters of the explosion flame in the gas explosion experiment system.

Experimental System.
A large-scale gas explosion experiment system with a 34 m-long pipeline is shown in Figure 3.
The experiment system consists of five parts: explosion experiment pipeline subsystem, ignition subsystem, gas distribution subsystem, data acquisition and storage subsystem, and explosion suppression subsystem. In the study, the large-scale experimental pipeline system consists of 34 m long circular pipeline and separately installed in it a straight pipe with the length of 50 cm and the diameter of 20 cm, a rectangular steel cavity of length 80 cm × width 50 cm × height 20 cm, or a rectangular steel cavity with length 80 cm × width 50 cm × height 20 cm. The experimental conditions are shown in Table 1. The purpose is to find the influence of the cavity alone or the cavity with water mist on the propagation of gas explosion to provide a reference for studying the suppression of the methane-air mixture explosion.
(1) Explosion experiment pipeline subsystem is made of steel round pipes with a thickness of 0.01 m, diameter of 0.2 m, and compressive strength of 20 MPa, which are connected by flanges and bolts and nuts. The air tightness is guaranteed by rubber gasket between the flange plates. (2) The ignition subsystem is composed of power supply, wire, ignition electrode, and electric fuse. The ignition electrode is installed on the flange plate of the end side of the experiment pipeline system, and the electric fuse is used for ignition.  (1) Check the Air Tightness of the Experiment System.
The ignition electrode was sealed, the experiment system was pumped to -20PV by vacuum pump, and then waited 5-10 min to observe the negative pressure of the experiment system, if there is no change, it shows that the air-tightness of the experiment system is good and the experiment begins

Geofluids
(3) Gas Distribution. Dalton partial pressure method was used for gas distribution. First, a vacuum pump was used to vacuum the experimental system to make the system pressure reach −20 PV (the maximum negative pressure of the polyethylene membrane used in the experiment was measured as -25 PV). Then, the experimental system is filled with methane gas with a concentration greater than 99.9%, and stop filling methane gas when the system pressure rises to -10 PV. Open the valve to allow air to enter the experiment system and close the valve when the pressure in the system rises to 0 PV, and a mixture of gas with a methane concentration of 10% was prepared (according to theoretical analysis, the gas concentration of 9.5% is the concentration of the maximum explosion intensity under the experimental conditions) [6]. Because the experiment precision is accurate to 1%, so the concentration of CH 4 in the gas mixture was prepared as 10%        Table 2. 3.3.1. The Variation of Flame Intensity of Gas Explosion under the Three Experiment Conditions. After the mixture gas exploded in the experimental device, the evolution process of the flame at measuring point F2 and F3 with time is shown in Figure 5. The influence of experiment condition 1 on flame propagation of gas explosion was presented in Figure 5(a). According to Figure 5(a), the flame intensity at F2 is 0.07813, and the flame intensity at F3 is 0.07428. The attenuation rate of flame intensity from F2 to F3 is 4.93%. Therefore, the methane explosion flame intensity is enhanced after it passes through the straight pipeline. The influence of experiment condition 2 on flame propagation of gas explosion was shown in Figure 5(b). According to Figure 5(b), the flame intensity at F2 and F3 is 0.05134 and 0.01716, respectively. The attenuation rate of explosion flame intensity from F2 to F3 is 66.58%, which indicates that the cavity has a suppression effect on the flame intensity. The influence of experimental condition 3 on flame propagation of gas explosion was presented in Figure 5(c).

Geofluids
According to Figure 5(c), the flame intensity at F2 and F3 is 0.04535 and 0.00676, respectively. The attenuation rate of flame intensity from F2 to F3 is 85.09%. The experiment condition 3 has a suppression influence on flame propagation. The cavity with water mist has better effect on inhibition of explosion flame propagation than the cavity alone.

The Variation of Flame Propagation Velocity of Gas
Explosion under Three Working Conditions. After the mixture gas exploded in the experimental device, the evolution of the flame at each measuring point F1, F2, F3, and F4 is shown in Figure 6. It can be seen from Figure 6 Compared with experimental condition 1, experimental condition of cavity combined with water mist has a stronger suppression effect on the methane explosion flame velocity, and it is better than the restraining effect of only attaching a cavity.

Analysis of Explosion Suppression by Coeffect of Cavity and Water
Mist. After the explosion flame enters the cavity, it expands and dissipates. When it propagates at the outlet, part of the methane explosion flame passes out of the steel cavity, and the other part is blocked by the walls of the cavity and reflected, forming a reverse explosion flame and propagating in the opposite direction. Due to the different reflection angle, part of the reverse explosion flame enters the steel cavity inlet and passes out of the cavity after being superposed. The reverse explosion flame that cannot enter the inlet of the cavity is blocked by the walls of the cavity. The flame is reflected again and propagates toward the outlet. This process is repeated so that the flame disappears as the premixed gas is exhausted. Therefore, the flame of explo-sion attenuates obviously after passing through the cavity, and the functions of flameout and wave elimination are realized. The coeffect of cavity and water mist increases the effect of flame suppression because of the reasons such as (1) when the flame enters the cavity, the temperature of the water mist is lower than that of the explosion flame, and heat transfer occurs between water mist and methane explosion flame, resulting in the temperature of the flame decrease. (2) The water mist with high density in the confined space of the cavity can cool down the temperature and isolate the oxygen, so that the enhancement of the secondary flame in the cavity is weakened and the explosion flame is suppressed.
(3) As an inert droplet, water can directly interfere with the chemical reaction in the explosion reaction zone, and thus, has the effect of chemical inhibition. The suppression effect of the flame propagation velocity is better because the water mist forms a "water wall" in the cavity, which hinders the explosion flame propagation, thus, resulting in greater inhibition effect on the methane explosion flame propagation speed.

Conclusions
(1) The attenuation rate of the explosion flame intensity by using the cavity with the aspect ratio of 5/8 is 66.58%, and the attenuation rate of the flame propagation velocity is 45.26%. The attenuation rates by using the cavity increased by 61.65% and 47.74%, respectively, compared with those in the straight pipelines (2) The attenuation rate of the methane explosion flame intensity under the coeffect of the cavity with the aspect ratio of 5/8 and the water mist is 85.09%, and the attenuation rate of the flame propagation velocity is 65.85%. The attenuation rates have increased by 80.16% and 68.33%, respectively, compared with those in the straight pipes. The attenuation rates increased by 18.51% and 20.59%, respectively, compared with those by using cavity alone. The suppression effect on the intensity and speed of gas explosion flame by coeffect of the cavity with the aspect ratio of 5/8 and water mist is better than by using the cavity with the aspect ratio of 5/8 alone (3) The repression effect of the steel cavity on the explosion flame propagation is mainly due to the repeated reflection of the flame in the steel cavity, causing its energy to be attenuated. The repression effect of the water mist is mainly due to its vaporization and heat absorption