Abstract
Liquid nitrogen is a vital medium during the extinguishing and chilldown process of coal spontaneous combustion in coal mine goafs. In this paper, the heat and mass transfer of liquid nitrogen in a loose medium was investigated. A laboratory system including a temperature sensing system was designed and built to explore the effects of different nitrogen injection positions and angles on the chilldown effect. The results indicate that after liquid nitrogen injection, the temperatures in the liquid nitrogen flow area and the bottom of the model can be quickly reduced to −196 °C, which was the best chilldown effect zone. With the vaporization of the liquid nitrogen, the cryogenic nitrogen gradually diffused. At 20,000 s, the bottom temperature was about –63 °C, the middle was –30 °C, and the upper was 0 °C. When the model angle was 0°, the effective chilldown zone was the largest. As the angle increased, the effective chilldown volume and holding time decreased significantly. The model angle had a greater impact on the chilldown. For the position of nitrogen injection, the inlet was set slightly farther from the gas outlet’s position, leading to a larger coverage for the liquid nitrogen and cryogenic nitrogen chilldown. This study can provide a theoretical basis for the flow and chilldown of nitrogen injection in goafs.
Highlights
The goaf in a coal mine is a confined space formed by the gradual breakdown and collapse of remaining coal and rock after mining, which can be regarded as a loose medium [1]
The vaporized cryogenic nitrogen gathered near position 6# for a short time, so the temperature was kept at −196 ◦ C for 90 s longer than the injection position 6# for a short time, so the temperature was kept at −196 °C for 90 s longer than the injection time
Temperature sensing system was applied to monitor theThe temperature changes over a processes of liquid nitrogen and cryogenic nitrogen were investigated in the loose medium
Summary
The goaf in a coal mine is a confined space formed by the gradual breakdown and collapse of remaining coal and rock after mining, which can be regarded as a loose medium [1]. The process of fire extinguishing and cooling with liquid nitrogen in goafs involves a two-phase flow, heat transfer, and phase transformation in the loose medium, which is extremely complicated. To explore the heat and mass transfer in goafs, Lu et al [21] built a tunnel model with temperature sensors in porous coal media, and obtained the optimum position for the liquid nitrogen injection. A laboratory experimental platform was adopted to investigate the chilldown effect of liquid nitrogen in a loose medium, reflecting the influence of inclination angle and injection position on the heat and mass transfer of liquid nitrogen in a loose medium; the effective chilldown zone is proposed for the first time.
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