Combined effects of obstacle and fine water mist on gas explosion characteristics

Xiaoping Wen ,Mengming Wang ,Fahui Wang ,Minggao Yu *,Haoxin Deng

1 School of Mechanical and Power Engineering,Henan Polytechnic University,Jiaozuo 454003,China

2 State Key Laboratory of Coal Mine Disaster Dynamics and Control,Chongqing University,Chongqing 400044,China

Keywords:Fine water mist Gas explosion suppression Obstacle Couple effects

ABSTRACT Combined effects of obstacles and fine water mist on a methane-air explosion of a semi-closed pipe were investigated experimentally.In this study,the diameter of the water mist,the location,and the number of obstacles was considered.The results demonstrated that 5 μm water mist present a significant suppression affected while 45 μm shows a slight promotion effected on a gas explosion of the condition without obstacles.In the presence of an obstacle,however,the inhibitory effect of 5 μm water veils of mist dropped significantly during flame propagation,and the effect of 45 μm water veils of mist changed from the enhancement of inhibition,and its inhibitory effect was significant.The inhibitory effect of 45 μm water veils of mist on gas explosion weakened firstly and then enhanced with the increasing distance between obstacle location from the ignition location as well as in several obstacles.

1.Introduction

As an essential industrial energy source,coal has made great outstanding to social and economic development[1].As the depth of coal mining deepens,the frequency of gas explosion in underground coal mine increases and the risk of mining continues to increase [2].There is a hidden danger of explosion in the longdistance extraction pipeline network,which has become an urgent problem to be solved.Gas explosion suppression is an important means to effectively prevent gas explosion disasters.Experts and scholars at home and abroad have begun to study the characteristics of gas explosions in pipelines to effectively control such explosion accidents [3].The research on the explosion suppression media for gas explosion mainly focuses on inert gas,fine water mist,aerosol,porous media and solid powder[4-6].The explosion suppression mechanisms of different types of explosion suppression media are also different.Among them,fine water mist [7]has the advantages of wide source,environmental friendliness,obvious fire extinguishing and explosion suppression effects,and is widely used in many engineering fields [8,9].

Pure water and fine water mist are mainly used to suppress explosion through the effects of evaporation,isolates oxygen,and heat absorption but due to the limitation of environmental conditions,the effect is often not ideal[10].The feasibility of using fine water mist to weaken or mitigate the explosion of hydrogendoped gas is proposed by Ingramet al.[11] and Battersbyet al.[12],and the results show that water mist significantly reduces the burning rate and the rate of initial pressure rise.A large number of research results show that the diameter of water mist droplets has a significant effect on the effect of gas explosion suppression.In the gas explosion,water mist with a diameter of less than 50 μm plays an important role in gas explosion suppression.Small droplets are more effective flame suppressants than large droplets,but when the droplets are reduced to less than 10 μm,there is no further obvious suppression effect.If the droplets are too large,the evaporation rate is too slow to cause a significant cooling flame and must be split into many small droplets to achieve fast enough mitigation affect relative flow[13].The suppression effect of fine water mist is reflected in the flame spread through evaporation,isolates oxygen,and heat absorption,the dilution of unburned gas,the formation of a barrier and the attenuation of heat radiation to reduce the hazard of explosion [14,15].At the same time,scholars pointed out that it is necessary to reasonably control the density of the fine water mist [16,17].Fine water mist can reduce the sufficient speed and overpressure of the flame;on the contrary,when the concentration of fine water mist is low,the explosion can enhance the local combustion of the flame[13].The flame front is affected by the water mist,which increases the flame surface tension,promotes combustion and strengthens the degree of the explosion.Therefore,how to improve the suppression of efficiency explosion of water mist has become the focus of many scholars’ research.At the same time,the study also found that the interaction between the flame and the obstacle will inevitably increase the flame disturbance and increase the combustion reaction rate,resulting in a greatly accelerated flame acceleration rate [18,19].Similarly,obstacles will also affect the coupling effect of water mist and explosion flame [20].In general,the current research mainly focuses on the effect of different water mist on explosion suppression,about the impact of obstacles on the explosion suppression effect of different water droplet sizes and the suppression of water mist under different obstacle conditions [21,22].There are few research reports on explosive effects.Therefore,it is reasonable to assume that adding obstacles can prolong the survival time of water droplets,and the overall coupling suppression effect will be more efficacious [23,24].

Based on our previous research,the fine water mist concentration and particle size have an excellent suppression effect on the overpressure and flame of methane combustion [7,25].However,there is no in-depth analysis of the synergistic effect of water mist particle size and obstacles.The goal of the current work is to study the inhibitory effect of fine water mist particle size and several obstacles on methane/air explosion and provide guidance for the design of fine water mist inert explosion-proof engineering systems in the future [26].

2.Experimental Apparatus and Procedures

The experimental apparatus used in this work is schematized in Fig.1.The experimental apparatus contains a venting explosion vessel,a gas distribution system,an ignition device,a data acquisition system,and a water mist generation system.The explosion vessel was placed horizontally and had inner dimensions of 150 mm×150 mm×1700 mm.Same as we did before,the explosion chamber is made of 20 mm thick perspex to make the flame propagation process can be captured by optical access and the right side is sealed by a 5 mm thick steel plate while the left side is sealed by a 0.1 mm polyethylene film [7].There are three pairs of removable steel obstacles placed on the upper and lower pipe walls,and each pair of the distance of the obstacle closed end is 300 mm,600 mm,900 mm,which are named 1# obstacle,2#obstacle,3# obstacle,respectively.The size of the obstacle is 150 mm × 37.5 mm × 10 mm and the blocking rate is 0.5.The methane/air mixtures were produced by two mass flow controllers,and the gas was entirely mixed before entering the chamber.In the experiment,the volume fraction of methane in the mixed gas is 8.5%.The methane/air mixture was fed into the chamber through an office located at the sealed end and could be vented through a valve that was positioned on the chamber’s wall near the vented end.The volumetric flow rates of the mixtures were controlled by a mass flow controller which supplies 4 times the volume of the chamber to ensure that the air in the pipeline is fully discharged and the gas-filled with uniform distribution [7].

The self-made ultrasonic atomizer uses a dual spray system,which can produce 5 μm and 45 μm fine water mist,providing water mist with different particle sizes.The atomizing pipe is connected with pressure gauge,needle valve and vortex flowmeter to accurately adjust the atomized water amount.The 5 μm and 45 μm water mist was provided by ultrasonic atomization,and the gas mixtures passing through the device of ultrasonic spray by homemade will carry the mist in the pipe.The water flux of ultrasonic mist was measured by an electronic balance and the spray time was set to make the two size water mist have the same density in the chamber.The particle size of the two kinds of water mist was measured by a PDPA[7].The atomization parameters are shown in Table 1.

In this paper,we use a photoelectric sensor,a piezoelectric pressure sensor and a DAQ device to collect Flame structure diagram and pressure dynamic.The pressure sensor with a natural frequency of 15 kHz can measure the range of -0.1-1.5 bar (1 bar=105Pa),while response time 2 ms,the margin of error 0.25%.The dynamic flame propagation process of the gas explosion was recorded by a Phantom series high-speed camera vision acquisition system (2000 fps).The ignition system is fed by a 6 V regulated DC power.

Fig.1. Schematic diagram of the experiment system.

Table 1 Atomization parameters

In the experiment,we first charged the pipeline of gas mixtures for a computational time,then started spraying water mist and gas charging process continued in the process of spraying water mist.When a computational time for spraying was up,all external spray valve was turned off,and then the gas was ignited immediately.The photoelectric sensor automatically triggered the DAQ and high-speed camera software when the gas ignited,then saved the test data after completion of data acquisition.The combustion reactor was injected by a mixture component (purity above 99.99%).The initial temperature and pressure for the experiments were set to 300 K and 1.0 atm (1 atm=105Pa).Each experiment was tested at least 4-5 times in the same condition,to ensure the accuracy and reproducibility of the results.

The calculation formula for flame propagation speed in the pipeline is given:

whereLf(mm) is the distance between the actual flame front peak and the ignition end of the pipe,ais the ratio between the pixel numberIf(PPI) between the flame front and the ignition end of the pipeline and the total pixel numberIg(PPI) of the entire test pipeline in the picture;and the actual length of the pipe isLg(mm),the flame propagation velocity isV(m·s-1);Lfi+1(mm) andLfi(mm) are the flame front location of the flame evolution structure in the two moments,which the corresponding time points areti+1(ms),ti(ms),respectively.

3.Results

3.1.Effect of water mist characteristic

Fig.2 provides the obstacle-free explosion flame structures of under the conditions no water mist,5 μm water mist and 45 μm water mist.The flame colour and flame temperature follow the rule:the flame temperature shows warm colors,such as red,orange,and yellow,which indicates that the flame temperature is relatively low and the flame wavelength is longer.As the temperature rises,the wavelength of light becomes shorter,and the colour of the flame changes from warm to cool,such as blue and purple [27].From the structures,the flame colour under the three conditions are light purple,dark red and orange.The flame temperature with no water mist is the highest.Due to the effect of water mist,the flame temperature is reduced by 5 μm mist and 45 μm mist,the smaller the water mist particle size,the better the suppression effect.5 μm water mist has intense evaporation and endothermic properties due to smaller and more dispersed water droplets.In contrast,while 45 μm water mist is not easy to evaporate due to larger water droplets [26],and the heat absorption effect is weak.After ignition,a spherical flame appeared in the centre of the pipe,and the flame spread quickly to form a hemisphere.Then the flame continues to spread,the speed of the front end of the flame becomes more massive,causing the flame shape to stretch,the flame front gradually forms a finger flame,and continues until the pipe exit [28,29].

In the case of three different additions of water mist from Fig.2,the flame propagation time is 109 ms,214 ms,and 106 ms,separately.The 5 μm water mist extended the flame propagation time by 105 ms,and the 45 μm water mist slightly reduced the flame propagation speed,indicating that the 45 μm water mist increased the overall flame propagation speed [30,31].The reason is that water has a high heat capacity of 2450 kJ·(kg·°C)-1.The pipes are filled with fine water mist,which serves as the third mechanism(H+OH+M →H2O+M),by absorbing heat,exchanging energy with free radicals,it plays the role of auxiliary cooling effect and reducing the concentration of free radicals [32].Based on the mechanism of chemical reaction kinetics,the methane explosion of three body exist in the process of gas phase reaction collision response,namely under certain temperature and pressure condition,these three kinds collision response makes a lot of free radicals or atomic energy is transferred to the third body stable molecules,a large number of free radicals or free atoms together into the stability of low activity molecules,such as:H+OH -H2O+M+M (M of the third body stability of these molecules).When water vapor is added to the methane/air mixture,H2O participates in the tri-body collision reaction as a third stable molecule.When the amount of water vapor added is larger,the concentration of the third stable molecule (H2O) is also larger,and the three-body collision reaction is strengthened,thus reducing the concentration of highly active radicals such as H,O and OH [33].

According to the chain reaction theory of methane,in the process of methane chain reaction,highly active radicals such as H,O and OH are the chain reaction transporters.According to the chain reaction theory,H,O and OH are considered as the transporters of the chain reaction,and are the key free radicals that induce the ignition,combustion and explosion of methane.The addition of water vapor significantly reduces the concentration of highly reactive radicals such as H,O,and OH,which means that the chain reaction of methane combustion and explosion is hindered and disrupted.The blocking and destruction of the chain reaction inevitably leads to the deceleration or termination of the methane oxidation reaction (combustion and explosion).In other words,the water vapor achieves the effect of fire suppression and explosion suppression in chemical inhibition,which is the third explosion suppression mechanism.

Fig.2. Flame evolution structure of mixture explosion:(a) without water mist;(b) with 5 μm water mist;(c) with 45 μm water mist.

Fig.3. Flame propagation speed (a) and flame overpressure dynamic (b) under three kinds of inhibitors with no obstacle (1 bar=105 Pa).

Fig.3(a)compared the flame propagation speed of gas explosion without obstacles under three spray conditions.It can be seen that the suppression effect of the water mist on the flame velocity is inversely related to the particle size,and the spray suppression effect is better when the particle size is 5 μm.As the curve slope represents the flame propagation acceleration,it can be seen that the flame accelerated firstly after ignition and kept accelerating.However,the flame propagation acceleration under the condition with 5 μm water mist was much smaller than that of the other two mist conditions,and the flame acceleration trend turned undoubtedly different as the flame continued to spread.Under the condition with 5 μm water mist,the flame front speed increased to a lesser extent,remained stable from 700 mm away from the ignition end,and the maximum flame propagation speed was 15.2 m·s-1with a downward trend appeared near the exit.In the condition with no water mist,the flame spread from the ignition position to about 1000 mm position of the chamber with a maximum speed of 31.5 m·s-1.After a slight decline in the maximum velocity tends to be stable.The flame propagation speed under the condition with 45 μm water mist maintained an increasing trend in the whole propagation process.It reached the maximum value of 40.8 m·s-1near the outlet of the chamber.In summary,under the condition without obstacle,the maximum flame propagation speed in the condition with 5 μm water mist decreased by 51.7%,which presented a pronounced inhibitory effect on the gas explosion.But the maximum flame propagation speed in the case with 45 μm water mist increased by about 29.5%,which indicates a promoting effect on flame propagation.

The overpressure history of gas explosion without obstacles in the case with no water mist,with 5 μm water mist and with 45 μm water mist are put in Fig.3(b).After ignition,a large amount of heat generated by the reaction quickly accumulates in the container,causing the gas to expand and the pressure to rise rapidly.According to the flame propagation photo,at about 30-50 ms,when the shock wave passes through the PVC (polyvinyl chloride)[34] membrane,the first pressure peak is formed due to the pressure increase of the sealing membrane and the destruction of the release pressure.When the speed of pressurization is greater than the speed of pressure relief,the explosion overpressure will continue to increase.The energy released by the continuous combustion of unburned gas causes the gas in the pipeline to continue to expand so that the explosion pressure continues to increase.At this time,the pressure increase rate is greater than the pressure relief rate,and the pressure rises.With the continuous dissipation of energy,the speed of pressure increase and pressure relief reached a balance,forming the second peak pressure.After that,the pressure relief rate keeps increasing,that is,the pressure continues to decay until the end of the reaction.

The overpressure rising trends under the conditions without water mist and with 45 μm water mist is different from that with 5 μm mist.The overpressure rises to a peak and then falls quickly due to the decompression when the explosion flame exits the chamber vent under the condition without water mist and with 45 μm mist.However,the overpressure of gas explosion kept relatively stable before the pressure drops instead of rapid decline after pressure rising under the condition with 5 μm water mist,which attributed to the low steady speed of flame propagation suppressed by water mist.The maximum explosion overpressure under the conditions without water mist,with 5 μm water mist and with 45 μm water mist are 73.98 mbar,34.76 mbar and 79.68 mbar,respectively.That means the explosion overpressure decreased by 53.0% under the condition with 5 μm water mist,but the overpressure under 45 μm water mist increased.Comparing Fig.3 with the previous analysis of speed rate of overpressure rise in Fig.2,it can be summarized that the 5 μm water mist has an apparent inhibitory effect on the gas explosion under the same condition without obstacle.In contrast,the 45 μm water mist has a particular promoting effect on the gas explosion.

The suppression effect of fine water mist is reflected in the physical and chemical coupling effect of water mist,including evaporative heat absorption,dilution of premixed gas,and absorption of free radicals.Firstly,after ignition,the water mist around the combustion area will quickly evaporate and absorb a lot of heat.After the combustion heat decreases,the combustion reaction temperature and combustion reaction rate also decrease,and the smaller the particle size,the more obvious the suppression effect.Secondly,the steam generated from the water mist can dilute the free radicals of the explosion,which can interrupt the chain reaction and enhance the explosion suppression effect.The O,H active molecules and OH groups generated by the methane explosion chain reaction will be absorbed by the droplets or react with the droplets,and the active molecules are reduced,thereby interrupting the branched-chain reaction.These significantly weaken the strength of the chain reaction during the explosion.Finally,the fine water mist not only absorbs heat in the process of gasification but also expands rapidly in volume,which can be expanded by 1700 times.According to the law of Dalton’s partial pressure theorem,a 5 μm droplet vaporizes rapidly with the partial pressure of water vapor increases,and the partial pressure of water vapor becomes larger,and the volume of the explosion pipeline also becomes more extensive.This large particle size leads to a decrease in oxygen content,thereby isolating oxygen to inhibit flame propagation.The particle size of the 5 μm water mist is small,which is enough to evaporate and form the water vapor in the reaction mixture,enhancing the endothermic and dilution effect [35,36].In the case of stronger adsorption of free radicals during the combustion reaction,compared with the same number of droplets,the larger the particle size of the water mist particles,the larger the surface area and the stronger the adsorption force.And,the 45 μm water mist with large particle size and cannot be evaporated quickly at the explosion temperature,causing blockage and disturbing the propagation of the explosion flame,causing local turbulence and promoting the propagation of the explosion flame.Leading to the promotion effect of 45 μm water mist on flame propagation is more significant than its inhibitory effect,so ultimately promotes the flame as a whole.Therefore,the more giant particle size water mist will enhance the flame propagation speed and the flame explosion overpressure.

3.2.Effect of coupling action of obstacle and water mist

The images of gas explosion flame with 3# obstacle under the conditions with no water mist,with 5 μm water mist and with 45 μm water mist are compared (Fig.4).The same hemispherical flame can also be observed in the early stage of ignition.As the flame continues to spread,the speed of the flame at the front end of the flame increases,and the flame surface is stretched and deformed,gradually forming a finger-shaped flame.Before the flame front reaches the 3# obstacle,the flame slowly contacts the wall surface,and the propagation speed becomes slower due to heat loss.Form a flat flame.When approaching the 3#obstacle,the flame front narrows due to the blocking effect of the obstacles on both sides,increasing the compression effect of the blocking rate,and the flame propagation speed increases rapidly.The presence of the obstacle promotes the turbulence of the airflow and forms turbulence locally [37].After the flame passes through 3#obstacle,the front edge of the flame begins to stretch and curl on both sides behind 3#obstacle,which increases the flame propagation speed and dramatically enhances the turbulence of flame.As the flame front curls further,the contact area between the flame front and the unburned gas increases,the combustion reaction increases,the turbulence intensity increases,and the flame propagation speed accelerates [38].

It can be seen from Fig.5 that the flame propagation speed has a small increase due to the presence of 3# obstacle.But there is no obvious change in the flame propagating velocity of 5 μm,so it can be inferred that the suppression effect of 5 μm fine water mist is better.After the 45 μm fine water mist passes 3# obstacle,the flame propagation speed decreases.Due to the existence of the obstacle,the water droplets are broken into smaller particle sizes.Thus the inhibition effects are much better.The particle size of water mist has a significant influence on the disturbance degree and evaporation rate of the flame [39],thus affecting the combustion rate of the flame.The larger the particle size of the fine water mist,the higher the turbulence level of the flame is,and the higher the methane combustion rates will be,as shown in Fig.5.The degree of action between fine water mist and flame is directly related to the particle size of fine water mist,so the research on the influence of particle size on the gas-liquid two-phase action process can ensure better heat absorption of water mist.

The overpressure of gas explosion with obstacle under the conditions without water mist,with 5 μm water mist and with 45 μm water mist is shown in Fig.6.As can be seen from the figure,the explosion overpressure under three mist conditions had a similar trend with time.The overpressure began to rise slowly from the ignition time and then declined slightly after 60-70 ms,which due to the pressure relief caused by sealing membrane rupture.With the flame spreading,the pressure increases slowly before reaching the barrier.After the flame passed through the obstacle,flame propagation speed increased rapidly due to the turbulence effect on the explosion flame by obstacles [40].When the flame approached the tube open port,the combustion gas inside the pipe can promptly leak to make the explosion pressure drop quickly.The maximum overpressures of the pressure peak under three water mist conditions are 1151 mbar,957 mbar and 976 mbar,respectively.In the condition with obstacles,the gas explosion overpressure descended by about 16.9% and 15.2% with 5 μm water mist and 45 μm water mist,respectively.The 5 μm and 45 μm water mist presented the effective suppression effect of gas explosion in the presence of obstacles,by combining with the previous analysis of the flame propagation speed [41].

Fig.4. Three flame evolution structures of mixture explosion with 3# obstacle:(a) without water mist;(b) with 5 μm water mist;(c) with 45 μm water mist.

Fig.5. The flame propagating velocity with 3# obstacle under three kinds of spraying conditions.

Fig.6. Explosion overpressure in the conditions of 3#obstacle under three kinds of spraying conditions (1 bar=105 Pa).

Fig.6 shows the gas explosion overpressure with obstacles in the absence of water mist,5 μm water mist and 45 μm water mist.It can be seen from the figure that the explosion overpressure under the three water mist conditions has the same trend with time.After ignition,the flame quickly spreads forward,releasing a lot of energy,producing a pressure peak.However,relative to the situation without obstacles,the flame peak delay is about 100 ms.When the flame approaches the opening of the tube,the combustion gas in the tube will leak quickly,the combustion energy will decrease,and the explosion pressure will drop quickly.After that,the unburned gas will continue to burn and cause small pressure oscillations.The maximum overpressures of the pressure peaks in the three cases are 1151 mbar,957 mbar and 976 mbar,respectively.Compared with the barrier-free working conditions,under the dual action of 3# obstacle and 5 μm water mist and 45 μm water mist,the gas explosion overpressure dropped by about 16.9%and 15.2%,respectively.Combined with the analysis of flame propagation speed in Fig.5,the dual coupling effect of water mist and obstacles can greatly reduce the degree of explosion[42].

Under the condition with 3#obstacle,the 45 μm water mist has a particular promotion effect on the gas explosion.The presence of obstacles increases the probability of water droplets breaking.When obstacles exist,the maximum flame propagation speed is far greater than the critical speed.The blast will break up spray droplets into smaller droplets,the droplet gasification endothermic effects were enhanced by more dispersed break-up water mist droplets at the same time.And the droplet is exposed in the explosion pipeline the hydrodynamic force generated is much greater than the surface tension of the droplet.Water droplets will be more easily broken down into smaller water droplets to evaporate to achieve the effect of explosion suppression quickly.The effect of 45 μm water mist on gas explosion decreases from explosion suppression due to the existence of obstacles.Due to the promoting effect of obstacles on the blend of explosion flame and water mist,the adsorption and consumption of the free radical of the combustion zone in the gas by fog particles were enhanced,thus strengthened the explosion suppression effect.

Fig.7. Flame front speed and overpressure without or with 45 μm mist with different location of obstacles:(a)with 1#obstacle,(b)with 2#obstacle,(c)with 3#obstacle(1 bar=105 Pa).

3.3.Effect of obstacle conditions on the inhibitory with 45 μm water mist

By changing the influence of the position and number of obstacles on the development stage of adding fine water mist flames,the mechanism of suppressing explosion was studied to obtain the best explosion suppression effect.

Fig.8. Flame front speed and overpressure without or with 45 μm mist with different number of obstacles:(a) without obstacle,(b) with 1# obstacle,(c) With 1# and 2#obstacles.

The flame propagation speed and overpressure of gas explosion with 1#obstacle,2#obstacle and 3#obstacle conditions under conditions without water mist and with 45 μm mist are contrasted in Fig.7.As can be seen that under the three obstacle conditions,the flame propagation speed occurred a peak when the explosion flame arrived at the left side of the obstacles due to the blocking effect of the obstacle[43].Besides,the peak velocity occurs later with the distance between obstacle and ignition ends farther away.After the explosion flame passed the obstacle,the propagation speed began to accelerate due to the disturbance of the obstacles in the process of flame propagation,and the maximum flame propagation speed formed when the flame propagates to the outlet of the pipeline.The explosion overpressure reached the maximum value simultaneously.Then the overpressure decreased rapidly due to rapid relief of the high-temperature combustion gas when the flame spread to the outlet of the pipeline[44].Under the three kinds conditions of obstacles,the maximum propagation velocities of the explosion flames under the conditions with no water mist are 186 m·s-1,247.7 m·s-1and 220.0 m·s-1,and the maximum explosion overpressures are 455.82 mbar,1141.56 mbar and 1023.72 mbar.The maximum flame propagation speed under the condition with 45 μm water mist are 179.8 m·s-1,231 m·s-1and 208.6 m·s-1[45],and the maximum explosion overpressure are 409.44 mbar,872.47 mbar and 907.44 mbar.Under the different position conditions of the obstacle,the flame propagation speed with 45 μm water mist decreased by 3.3%,6.7% and 5.5% and the maximum explosion overpressure decreased by 10.2%,23.6%,15.2%,respectively.

In summary,under the conditions with 1#obstacle,2#obstacle,3#obstacle,the 45μm water mist showed obvious inhibitory effects on the gas explosion.It can be seen from the flame propagation speed in Fig.7 that the detonation suppression effect is the best under the condition of 2# obstacle [46,47].The more incredible flame propagation speed is accompanied by earlier crushing critical value[48],the better degree fragmentation of water mist obtained.Therefore the explosion suppression performance is better.In the three position conditions of obstacle[49],the maximum flame propagation speed is larger and reaches the minimum critical fragmentation velocity of water mist in the earlier time under the condition of 2#obstacle,so 45μm water mist presented the best inhibitory effect under the condition of 2#obstacle with the maximum overpressure decreased by 23.6%[50].However,due to the blocking effect of a part of the water mist without breaking,the water mist did not show an obvious inhibition effect on the flame propagation speed[51].

The flame propagation speed and overpressure of gas explosion without obstacle,with 1 obstacle(1#obstacle)and with 2 obstacles(both 1#obstacle and 2#obstacle)in the conditions without water mist and with 45 μm water mist is shown in Fig.8,respectively.As is shown that flame propagation speed and overpressure are at a low level under the condition without obstacle,and the maximum flame propagation speed and overpressure under the condition without water mist are 31.5 m·s-1and 73.9 mbar while 40.8 m·s-1and 79.6 mbar under the condition with 45 μm water mist.

The 45μm water mist has a particular promoting effect on the gas explosion under the condition without obstacle,which can attribute to that the blocking effect of larger size droplets of mist is more significant than its inhibition effects[52].In the condition with 1 obstacle,the maximum flame propagation speed and the maximum explosion pressure are 186 m·s-1and 455.8 mbar under the condition without water mist and which are 179.8 m·s-1and 409.4 mbar under the condition with 45 μm water mist.The maximum flame propagation speed and maximum explosion pressure decreased by 3.3%and 10.2%,respectively.In the condition with 2 obstacles,the maximum flame propagation speed and overpressure declined from 290.2 m·s-1and 1023.72 mbar to 266.3 m·s-1and 729.92 mbar by 45 μm water mist,which decreased by 8.3%and 28.7%,respectively.

In summary,suppression characteristics of 45 μm fine mist under the condition with 2 obstacles is vastly improved than that of 45 μm fine mist with 1 obstacle.However,the disturbance of two obstacles is more intense compared with one obstacle in the flame propagation process,which caused higher flame propagation speed and greater flame acceleration.Since the fragmentation of water mist is greater,and the water mist is more dispersed,the explosion suppression effect of 45μm water mist is more significant.

When I finally took my lunch break, I was surprised to find the cafeteria elaborately decorated for the season. I sat down next to one of the staff nurses from the unit. During lunch with Andrea, I had the chance to ask a burning question. “Who is this George fellow? And why is he here on Christmas Day?”

4.Conclusions

The gas explosion is one of the fatal incidents in underground coal mines,which brings many problems for both human beings and the surrounding environment.In this paper,an experimental study is conducted to investigate the combined effects of obstacle and fine water mist on gas explosion characteristics inside a narrow semi-closed pipeline by employing high-speed camera and pressure transducer.Safe and effective explosion suppression measures can reduce the potential risk of methane explosion.

The influence of fine water mist particle size on the propagation characteristics of explosion flame is analyzed.Only when the particle size of fine water mist is the best,the inhibition effect is the best.The results show that in the pipeline without obstacle,the inhibition of 5 m water mist was very obvious;the maximum flame propagation velocity and explosion overpressure decreased by 51.7% and 53%,respectively.But the experiment result of 45 m water mist showed the promotion effect,the maximum flame propagation velocity increased by 29.5%,and the maximum explosion overpressure increased by 7.7%.The main reason for the difference in explosion suppression effect is the difference between water mist particle size.The water mist particle size can change the heat exchange rate of gas-liquid phase by affecting the degree of interaction with the flame reaction zone and the evaporation rate,and then affect the flame propagation rate and explosion strength,so as to realize the effective inhibition of explosion strength.The conditions for the water mist particle size selection should ensure the complete vaporization of water mist in the reaction zone.

The presence of obstacles has an exciting effect on the methane explosion.By comparing the changes of flame and explosion overpressure before and after the obstacle,it can be seen that the flame will generate shock reflection process when passing through the obstacle,which can induce the formation of turbulent flame and increase the explosion pressure.The number and location of obstacles are the key factors affecting the intensity of the gas explosion.With the addition of 45 μm water mist,the maximum explosion overpressure of obstacles 1#,2# and 3# at different locations decreased by about 10.2%,23.6%and 15.2%,respectively.With the increase in the distance between the obstacle position and the ignition position,the explosion suppression effect of 45 μm water mist was firstly enhanced and then weakened.It can be seen that in the early stage of flame combustion,taking effective measures has guided the significance of reducing explosion accidents and accident hazards.As the number of obstacles increases from 0 to 2,the effect of 45 μm water mist on gas explosion changes from promotion to inhibition,and the inhibition effect are enhanced.In the future,a new technology of prevention and control of gas explosion accident in a coal mine will be putting forward.

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgements

This paper would like to acknowledge the financial supports of the National Key Research and Development Program of China(2018YFC0808103) and the National Natural Science Foundation of China (51774115,51604095).

Nomenclature

aratio between the pixel numberIfbetween the flame front and the ignition end of the pipeline and the total pixel numberIgof the entire test pipeline in the picture

Ifpixels between the flame front and the ignition end of the pipe,PPI

Igtotal number of pixels in the image of the entire test tube,PPI

Lfdistance between the actual flame front peak and the ignition end of the pipe,mm

Lfithe flame front location of the flame evolution structure,mm

Lgactual length of the pipe,mm

tifire forward position corresponds to time point,s

Vflame propagation velocity,m·s-1