Study On The Inhibition Effect Of The Acceleration Of Composite Flame In The Initial Stage Of Gas/coal Dust Explosion By CO₂ And Ultra-fine Water Mist

Gas/coal dust explosion is one of the most serious thermal disasters in coal mine.It is of great significance to carry out the research on CO₂ultrafine water mist to suppress the compound flame acceleration at the initial stage of gas/coal dust explosion for the prevention and control of underground gas/coal dust explosion.In this paper,the acceleration characteristics of gas/coal dust composite flame at the initial stage of gas/coal dust explosion and the inhibition effect of CO₂ultrafine water mist on it were studied experimentally in a 20L spherical explosive device.From the aspects of flame microstructure,initial flame propagation velocity,markstein length,explosion pressure,distribution of explosion flow field and change of functional groups,the law and reason of CO₂ultrafine water mist inhibiting the acceleration of composite flame at the initial stage of gas/coal dust explosion were analyzed.The results show that:1)From the micro structure of the composite flame,the surface of the composite flame is convex at the initial stage of explosion at 6%gas concentration,and the flame surface tends to be smooth with the increase of gas concentration and coal dust mass concentration,and the"light spots"formed by coal dust combustion on the flame surface increase.2)The smaller the coal dust particle size is,the darker the flame brightness is due to the energy consumption of particle heating,and the flame propagation speed is relatively slow at the initial stage of explosion.On the contrary,the flame brightness is relatively high,but with the development of the explosion process,the specific surface area of small-size coal dust is larger,and it is easier to be heated to release volatile,more coal dust participate in the explosion,and the explosion intensity is higher.3)Under the experimental conditions,the markstein length of the flame at the initial stage of gas/coal dust explosion changes monotonously and is positive,which indicates that the composite flame tends to be stable at the initial stage of gas/coal dust explosion,prolongs the residence time of coal dust particles in the flame area,and is conducive to coal dust pyrolysis.Therefore,when the gas concentration is close to the optimal equivalent concentration,the smaller the coal dust particle size or the coal dust mass concentration is close to the optimal concentration,the composite flame acceleration can be induced,resulting in higher explosion intensity.4)CO₂water mist has a good inhibition effect on the flame acceleration in the initial stage of gas/coal dust explosion,The combination of 12%CO₂and ultra-fine water mist can prevent and reduce explosion casualties.With the increase of CO₂and ultra-fine water mist concentration,the markstein length of gas/coal dust explosion flame increases,and the number of"light spots"formed by coal dust combustion on the flame surface decreases.The flame propagation velocity,maximum explosion pressure,maximum pressure rise rate and deflagration index decrease significantly at the initial stage of explosion.5)PIV analysis shows that there is a coupling effect between coal dust particle flow direction and flame propagation.When the initial explosion intensity is high,the negative pressure in the center of the composite flame makes most coal dust particles move centripetally,and a large number of large vorticity vortices with opposite directions are formed around the flame front,which promotes the full contact between coal dust and oxygen;When CO₂and ultrafine water mist interact,the vorticity distribution on the flame surface is reduced,and the area of large vorticity vortices is reduced and mostly distributed in the interior of the flame,which indicates that the number of coal dust particles participating in combustion is reduced under the synergistic effect of CO₂and ultrafine water mist,which reduces the energy exchange rate and combustion propagation speed on the flame surface,and then the explosion intensity is weakened.6)Infrared analysis shows that the-OH、-CH₃、-CH₂,C=C and oxygen-containing functional groups of coal dust in the process of gas/coal dust explosion are effectively inhibited under the joint action of CO₂and ultrafine water mist,which reduces the reaction rate of gas/coal dust explosion and weakens the destructive power of explosion.7)CO₂can dilute the oxygen concentration and inhibit the explosion chain reaction,reduce the initial explosion energy and flame speed,enhance the flame stability,and then extend the contact time of the explosion inhibitor with flame and coal dust;The ultrafine water mist can adsorb on the surface of coal dust,promote coal dust deposition,reduce the number of coal dust particles involved in the explosion,delay and inhibit the pyrolysis of coal dust;Other suspended droplets mainly play the role of endothermic cooling,dilution and reducing thermal radiation.