Experiment And Large Eddy Simulation On Methane And Deposited Coal Dust Explosion

Methane and coal dust explosions are the major accidents which seriously threat the safety in the coal mining industry. The casualty caused by methane and coal dust explosions in Chinese coal mining accidents is three times more than the casualty in other main coal countries. One important reason is that the mechanisms of methane and coal dust explosions in mines are not mastered adequately, which causes the explosion-proof and explosion mitigation appliances could not be triggered timely. Therefore, a lot of experimental and numerical simulation work aimed at flammable gas and dust explosion were conducted by many scholars in the world. However, the methane and coal dust hybrid explosion, especially the interaction between gaseous phase and coal dust, the dust dispersion effect by the blast wave and coal-dust rapid response dynamics et.al during the explosion process have not been solved ideally yet. And these mechanism-based questions are still the hot spot and difficulty in the research of this field.In view of the unresolved aspect in explosion mentioned above, methane explosion and methane-coal dust hybrid explosion in a closed pipe were studied in this thesis by the means of experiment and numerical simulation. The major work and conclusions of this thesis are as follows.(1) A methane-deposited coal dust explosion experimental system was set up, which consists of an explosion pipe with a length to diameter (L/D) ratio of23, a gas mixing tank, coal dust dispersion slot, an obstacle distributor, computer, a data-acquisition card, a high frequency pressure sensor and an ignition system. The dynamic response time of control and data acquisition unit is less than lms, and the pressure measurement accuracy is0.25%. The methane-air explosion and methane-deposited coal dust hybrid explosion experiments under different obstacle layout schemes could be conducted in this experimental system and the explosion pressure-rising under various experimental conditions were obtained.(2) The methane-air explosion experiments were conducted. There are four stage in the pressure time history of methane-air explosion in the pipe. In the first stage, the pressure rise can be negligible and it was approximately10%-15%of the total explosion time. In the second stage, the pressure rose rapidly and the duration of this stage was less than5%of the total explosion time. After that, the explosion pressure increased slowly and the duration was relatively long. In the last stage, the explosion pressure increased with pressure fluctuation. (3) A semi-circular asbestos plate as obstacle was arranged in the closed explosion pipe to study the influence of obstacle on the explosion pressure. The experimental results showed that the obstacle could result in the increasing of pressure rising rate locally and the shortening of explosion duration. When multi-obstacles were amounted in the pipe, the explosion pressure increased rapidly, and the maximum explosion pressure increased significantly. Considered the influence of the distance of the single obstacle from the ignition end on the explosion process, it was found that when the distance was more than a certain value, there are multiple pressure peaks in the explosion pressure time history.(4) A three-dimensional numerical model was developed to simulate the methane-air explosion process in closed pipe. The turbulent flow was calculated by the large eddy simulation model. The chemical reaction rate was computed by the premixed combustion model based on the gradient method. The radiation heat transfer was calculated by P1model. The coupled solution of dynamic heat transfer between the flame front and the solid wall, and the effect of the temperature and pressure on burning rate were included in the model. The simulated pressure time histories were in good agreement with experimental date from both published literature and our experiments. And the error between experimental and computed maximum pressure is less than15.6%.(5) The methane-air explosion process in closed pipe was simulated numerically. The simulation results showed that the flame front experienced such a change process of hemisphere shape, stretched in axial direction, plane shape, Tulip shape, and irregular turbulent flame in sequence. Correspondingly, the flame velocity experienced a process of accelerating, decelerating, and re-accelerating. The maximum flame propagation velocity appeared in the flame stretching stage. After analyzing the flow field of flame propagation process, it was realized that there was a backflow existing in the center of the flame front and a large-scale vortex structure appeared near the wall behind the flame front when the tulip flame formed. On that basis, the influences of L/D ratio and ignition location on the maximum flame propagation velocity were discussed. The conclusions were that the flame propagation velocity increases linearly with the increasing of L/D ratio and the maximum flame velocity is30%higher with the ignition point locates at the end than at the center of the pipe.(6) A numerical model of methane-air explosion occurred in the tube which has obstacles inside was established based on the methane-air explosion model. By simulating the methane-air explosion process under different obstacles layout schemes, the propagation rules of flame passing through the obstacles were obtained, and the relationship between the flame propagation and the explosion pressure rising was revealed. When an obstacle was located at the second stage of the pressure rising, there was only one flame speed peak due to the two superposed mechanisms of flame font elongation and cross-sectional area of the pipe reduction. When the obstacle was located at the position behind the flame decelerated, there were two flame speed peaks. When the obstacle was located at the position where the tulip flame formed, the peak of the flame speed reached to the maximum value. When the obstacle was located at the position where the tulip flame disappeared, the multi-peak phenomenon of explosion pressure were observed. When multiple obstacles were arranged in the tube from the ignition end with a spacing of300mm, the flame speed peak value and the explosion pressure significantly increased with the increasing of the number of obstacles.(7) Based on the methane-air explosion experiment in closed pipe, the hybrid explosion experiment of deposited coal dust was conducted. Compared with the methane-air explosion, when no obstacle was arranged in the pipe, the rising trendy and pressure peak value in different stages of the explosion pressure curve have no significant dissimilarity and the pressure fluctuation of the fourth stage is reduced. When multiple obstacles were arranged in the pipe, the deposited coal dust could disperse in the gas mixture much easier and react with the oxygen in the mixture more efficiently, which leaded to the explosion pressure of methane-coal dust hybrid explosion became larger than the pressure of methane-air explosion.(8) Combing the premixed combustion model of gas and the non-premixed combustion model of coal dust, the three-dimensional numerical model to describe the deposited coal dust dispersion and chemical reaction induced by the methane-air explosion was established. The simulation images of coal dust dispersion process in an unsteady state were obtained, and the dispersion states of coal dust in the pipe with and without obstacles were revealed. The heights of coal dust dispersion caused by gas explosion in the pipe without obstacles were limited and there were rarely coal dust at the top of the pipe. When obstacles were arranged in the pipe, coal dust would deposit in the front of the obstacles and pass through the obstacles with the help of the fast-moving explosion wave and then disperse around the obstacles. The coal dust could disperse richly in the whole pipe with a continuous layout of obstacles.