Mapping model of sound velocity and coal temperature for quasi-porous media in loose coal

The accurate determination of coal temperature in the hidden spaces such as goaf has always been a worldwide problem that needs to be solved urgently. The study of acoustic temperature measurement technology is of great practical significance for a fast and accurate detection of loose coal temperature, and the scientific and accurate construction of the mapping relationship between sound velocity and coal temperature is the key to acoustic coal temperature measurement. In this paper, considering the influence of the characteristic parameters of spontaneous combustion process of loose coal on sound wave propagation, a mapping model of sound velocity and coal temperature in the quasi-porous media of loose coal was established. An acoustic temperature measurement experimental system for loose coal was designed and built. By conducting the acoustic temperature measurement tests for the loose coal with various particle sizes (0.6～1.5, 1～3, 3～5 cm), the effect of gas composition and concentration on sound propagation during the low-temperature oxidation stage (30～50 ℃) of coal samples with different particle sizes was studied, and the equivalent path conversion factor λ was determined. Based on the relationship between the measured sound velocity and the equivalent path conversion factor, the medium correction coefficient φ of the gas component was proposed and determined, and the feasibility of the sound-temperature mapping relationship model was verified. The results show that the acoustic wave propagation path of loose coal is affected by the characteristic parameters of coal particle size, temperature and the change of gas component medium, etc. A mapping model of sound velocity and coal temperature was established. At normal temperature, the propagation path of sound waves in the quasi-porous medium of loose coal is mainly affected by the pores between the coal particles. The equivalent path conversion factor λ of sound waves is 1.426 1, 1.371 1 and 1.315 2, respectively, when the coal particle size is 0.6～1.5, 1～3, 3～5 cm, which is negatively correlated with the particle size. The influence of coal temperature on acoustic wave propagation is manifested by the change of gas component medium in the process of spontaneous combustion of coal, which verifies the feasibility of the mapping relationship between sound velocity and coal temperature. The error rate between the inversion model and the experimental coal temperature is 0～6.46%, and the average coal temperature error is 2.52 ℃, which can reflect the coal body temperature well.