Diffusion principles of SF₆ tracer gas: constraints on SF₆ mixing distance in mine tunnels

Mine wind measurement work is an important task to ensure the normal operation of mine production and environmental safety. The traditional wind measurement method uses the average wind speed and average cross-sectional area of the tunnel to calculate the air volume. Due to the fact that the tracer gas wind measurement method does not require the measurement of average wind speed and tunnel cross-sectional area, the error is smaller than traditional wind measurement methods. The wind measurement method monitors SF₆ diffusion by stably releasing SF₆ in the tunnel, and then measuring the SF₆ concentration until achieving even distribution. The accuracy of SF₆ concentration measurement directly determines the accuracy of wind measurement. The premise of accurately measuring the uniform concentration of SF₆ distribution is to determine the SF₆ mixing distance. This study analyzes the diffusion principles of SF₆ in the tunnel, and applies the computational fluid dynamics method for three intersecting tunnels to study the SF₆-air mixing distance in the intersection tunnel under a range of conditions to assess the effects of various parameters on the SF₆ mixing distance. The SF₆ concentration distribution was investigated to determine the optimal location for concentration sensors. The results indicate that the SF₆ mixing distance increases with gravity, while the SF₆ concentration distribution rectangular and semi-circular arch tunnels follows a relationship of left and right interchangeability. For a given cross-sectional shape, shorter mixing distances are associated with larger tunnel cross-sectional areas, lower SF₆ concentrations, higher tunnel wind speeds, and greater angles between the two air inlet tunnels. The results of this study provide a reference for selecting appropriate numerical simulation models for more accurate mine wind measurement work and future intelligent ventilation wind measurement.