Influence of coal powder particle sizes on dynamic characteristics of coal and gas outburst

In order to further study the mechanism of coal and gas outburst and the mechanism of the influence of coal powder particle sizes on the dynamic characteristics of outburst gas and coal,a coal and gas outburst two-phase flow simulation test system is developed. The pressure sensors installed in the test roadways have the ability to measure the impact of the protruding gas and the impact of the moving coal powder on the sensors. In addition,considering the principle why schlieren effect fails in current schlieren system when observing the flow fields in the circular pipe,an improved schlieren system to observe the flow field in a circular pipeline is designed which greatly helps the research on the propagation of the shock wave front. The test system is used to carry out the outburst experiments of four kinds of pulverized coal particle sizes,and the parameters such as the airflow impact force,the shock wave front propagation, and the pulverized coal impact are mainly observed. The results show that the velocity of the airflow shock wave is much larger than that of the coal powders. In the test roadways,the airflow shock wave arrives at any position of the test roadways ahead of the coal powders;the pressure will reach the maximum in an extremely short time after the air- flow shock wave reaches the sensors. The peak pressure could be maintained for about 0. 01 s. The airflow impact force increases with the increase of the particle size of the pulverized coal. The gas impact force of the four selected particle sizes increases by 10. 9% ,11. 4% and 7. 6% . The increase of void ratio is not the main reason for the increase of air- flow impact. The increase of gas analysis speed caused by the increase of specific surface area is the main reason for the increase of airflow impact. The airflow impact is enhanced at first and then attenuated in the roadways,and for the coal powders size of 80-200 mesh,in the position of 2. 27,4. 27,6. 27 and 8. 27 m,the shock wave force on the sensor is increased by 13. 6% ,attenuated by 13. 4% ,and attenuated by 20. 6% . The reflection and re-reflection of the shock wave are normal scenes of the shock waves in the non-uniform propagation paths,which is the case of both coal mine airways and the test experiment. As the distance of the sensors increases,the impacts of the coal powders on the sen- sors are significantly weakened. The speed of the coal powders increases with the increase of coal powders meshes. The average coal powder velocity in the four test roadways are 34. 4,37. 3,39. 1 and 41. 7 m / s,respectively. The coal pow- der speed is accelerated and then decelerated in the roadways,and for the coal powder size of 80-200 mesh,the coal the average coal powder speed increased by 31. 4% ,12. 2% and 13. 1% in the four roadways. Both the airflow impact and the coal powder movement speed have a law of increasing at first and then decreasing,but there is no direct causal relationship between them. The schlieren system observes the prominent shock wave front,which is perpendicular to the roadway axis and moves at high speed. The propagation velocity of the wavefront calculated by schlieren system is consistent with theoretical calculation. The airflow impact denotes the stagnation pressure by the shock wave forefront to the sensor facing instantaneously,while the average speed of coal powders in the roadways indicates the velocity of the particle determined by air flow and resistance within a certain period of time. The gas impact arrival sensor instan- taneously produces a stagnation pressure that represents only the strength of the shock wave front at the forefront of the impinging airflow. The average velocity of pulverized coal in the pipeline indicates the velocity of pulverized coal deter- mined by gas power and other resistance for a certain period of time.