Experimental study on acoustic wave propagation characteristics and main paths in loose coal

Accurate detection of high temperature points in loose coals has always been a major problem in the field of coal spontaneous combustion fire prevention and control in coal piles and coal bunkers. It is of practical significance to study the detection of temperature anomalies in loose coal bodies by acoustic thermometry. The determination of the propagation path of sound waves in loose coals is the premise and basis for the application of acoustic temperature measurement technology in loose coals. In order to explore the propagation characteristics and main paths of sound waves in loose coals, this paper takes long-flame coal, lean coal, and anthracite as the research objects, considering the gradation characteristics of loose coals, the tests were conducted using the sound wave flying time test system, respectively. The flying time of sound waves in a pure gas environment and in the saturated gas environment of loose coals were calculated. The results show that the cross-correlation results of pseudo-random acoustic signals are better, and the difference between the pseudo-peaks and the main peaks of the cross-correlation results is large, and the interference to the determination of the main peaks is small. The propagation speed of the sound wave is inversely proportional to the molar mass of the gas. When the sound waves propagate in nitrogen, air and carbon dioxide, the propagation speed is the slowest in carbon dioxide, and the speed in nitrogen and air is similar. Compared to the propagation in a gas environment and the sound waves under different atmosphere conditions, the flying time in loose coals is longer, and the change law is the same as in the gas environment(the flying time in descending order is carbon dioxide, air, nitrogen),indicating that the acoustic wave flying time has a small relationship with the degree of coal metamorphism. After reloading the anthracite coal sample, the acoustic wave flying time is shortened, but the acoustic wave flying time with saturated carbon dioxide and saturated air in loose coals is still small. With different coal samples in three atmospheres, the change rate of acoustic wave flying time in nitrogen-air atmosphere is the smallest. The rate of change is the largest in a nitrogen-carbon dioxide atmosphere. The change trend of the change rate of the acoustic wave flying time in different environments is the same, indicating that the acoustic wave propagates mainly along the gaps of the loose coals when it propagates in the loose coals. The research results can provide a certain theoretical basis for the application of acoustic temperature measurement in loose coals.