Experimental Study On Mitigation Of Methane-coal Dust Hybrid Explosion With Ultra-fine Water Mist

China is one of the largest coal production countries in the world, and the coal has made significant contribution to China’s economic development. However, frequent safety accidents in the coal mine make the coal mine safety becoming the focus of current security issues in China and the gas explosion accidents has greatly influence the coal mine safety and coal production in China. As is known to all, the coal mine is a coexistence system of methane and coal dust, then the gas explosion shock wave tends to induce coal dust explosion, forming mixed gas and coal dust explosion. As a clean and high efficient fire extinguishing agent, water mist is widely used in construction, industrial and many other fields as fire prevention measure. Besides that researchers generally considered that water mist with particle size of less than20μm can extinguish obstacles fires and the hidden fire. Therefore, taking into account the particularity of coal mine and explosion, water mist has good applicability in coal mine. However, there is hardly any previous research in this area. Therefore, carrying out the water mist suppression on gas and coal dust explosion has important scientific significance.In this paper, self-developed small-scale experiment platform is applied to carry out experiments on hybrid explosion with different coal dust concentration, coal dust particle size, methane concentration, and initial pressure of methane-air mixture. And the characteristic parameters of explosion such as the maximum explosion pressure, the rate of explosion pressure rise, the explosion flame temperature and so on are obtained. The results show that:the maximum explosion pressure and explosion pressure rise rate of the hybrid explosion increase with the increase of the coal dust concentration and then decrease, there exists in an optimal concentration of coal dust where the maximum explosion pressure and the rate of explosion pressure rise reach the summit. When the coal dust concentration deviates from the optimal value, the maximum explosion pressure and the rate of explosion pressure rise will reduce. Simultaneously, the explosion flame temperature, explosion pressure and rate of explosion pressure rise decrease with the increase of particle size of coal dust, this because the larger the coal dust particle size is, the smaller the specific surface area is and then the strength of the explosion is smaller. At the same time, when methane concentration gets close to the optimum reaction concentration, the reaction and combustion become fierce, and more heat is released. causing the explosion flame temperature, the maximum explosion pressure and the rate of explosion pressure ris become higher. In addition, the turbulence effect caused by initial pressure of the mixed gas could accelerate the reaction and strengthen the explosion.In order to reveal the regularity and mechanism of inhibition on hybrid explosion with water mist, the inhibition effect on hybrid explosion with water mist is carried out on small-scale experimental platform and different coal dust concentration, coal dust particle size and water mist volume flux are considered. The results show that: the explosion pressure and rate of explosion pressure rise decrease with the increas of the volume flux of ultra-fine water mist, this because the endothermic effect. pxygen dilution effect and soot adsorption effect of ultra-fine mist suppress the hybrid explosion. The critical volume flux of ultra fine water mist reduces with the increas of the coal dust concentration and coal dust particle size. In addition, in order to minimize the initial airflow turbulence influence on explosion, ignition delays5s after the coal dust is dispersed into explosion vessel, founding that delaying for a period of time to ignition can be more effective to inhibit the hybrid explosion.According to the coal mine roadway with obstacles, four types of obstacles are designed in this paper and the experiment on hybrid explosion with obstacles and ultra fine water mist is carried out by changing the type of obstacles, coal dust particle size and volume flux of ultra-fine water mist. The maximum explosion pressure, rate of explosion pressure rise, deflagration index and the critical volume flux of water mist are obtained, the results show that the shape, the quantity and location of the obstacles have great influence on the strength of the hybrid explosion. Meanwhile, under the same blocking rate, the obstacles with sharp edges and corners can cause stronger turbulence effect than the obstacles with smooth surface, leading to the rise of explosion pressure and the rate of pressure rise. The turbulence reinforcement to gas flow caused by water mist and obstacles superpose together. When the volume flux of water mist is less than a certain range, the hybrid explosion is intensified, until the volume flux of water mist increases to a certain extent, and then the inhibition effect of water mist is stronger than the reinforcement effect, the explosion pressure and the rate of pressure rise gradually decrease.For the sake of intuitive observation of explosion flame propagation process, a high-speed camera is adopted to shoot the explosion dynamic process. It finds that before adding ultra-fine water mist, the pulsation of the flame front is obvious, and the flame structure curls irregularly with bright white or orange color; while after ultra-fine water mist is applied, the flame front is smooth curved as semicircle, pulsation almost disappears and the flame color becomes orange. In addition, the explosion flame turbulence intensity is significantly enhanced by obstacles, the flame is compressed and pulsates obvious around the obstacle, and lots of turbulence vortexes appear around the obstacle, indicating that the existence of obstacle make the explosion more reinforcement. In addition, the explosion flame turbulence effects caused by type Ⅲ obstacles extends from the center of the lower obstacles to the upper obstacle, forming a V-shaped structure of turbulence between two obstacles. Meanwhile, the explosion flame turbulence effect caused by type Ⅳ obstacle extends as disorder turbulence structure between the two obstacles. This result verifies that the explosion reinforcement mechanism of type Ⅲ and type IV obstacle is different.The explosion flow field under the condition of obstacles and ultra fine water mist is measured through La Vision laser diagnostic system, further validating the accuracy of the results, and revealing the influence mechanism of obstacles and ultra fine water mist on mixture explosion.