Numerical investigations of unsteady heat transfer in ultra-long roadways with periodic fluctuation of airflow temperature

With the increasing number of high-temperature mines, there has been a growing focus on research into thermal environment management and prediction in long-distance ventilation roadways. Mastering the principles of heat transfer in roadways is crucial for enhancing ventilation performance and achieving energy savings in mine heat hazard control. A one-dimensional ventilation ultra-long roadway with high precision and low weight is presented in this study, which accurately depicts the dynamic heat transfer between the airflow and three-dimensional surrounding rock (SR) of roadway. The calculation time of the one-dimensional roadway numerical model is reduced by over 90%. The accuracy of the proposed numerical model is verified by the heat convection within roadway wall and the heat transfer in SR. In addition, the evolution of the temperature field and Nusselt (Nu) number of ultra-long roadway wall regarding ventilation time and periodic fluctuation of initial temperature of airflow (TA) are analyzed. The results indicate that the temperature of SR and Nu number of ultra-long roadway wall both periodic changes with the fluctuation of TA, and the TA and Nu number of ultra-long roadway wall fluctuate in opposite phases. When the TA temperature ranges from 15 to 25°C, the Nu number of ultra-long roadway wall changes in the range 90 in a year. The Nu number of ultra-long roadway wall decreases with the increase of ventilation distance. Additionally, the TA in ultra-long roadway exhibits a linear relationship with both the initial TA, the thermal conductivity of SR, and the initial temperature of SR, but it is subject to negative exponential decay in relation to ventilation speed. As the length of the roadway increases, fluctuations in the initial TA have less impact on the thermal environment within the long-distance ventilation roadway, while ventilation speed, thermal conductivity of SR, and initial temperature of SR become more influential factors.