A novel pneumatic structure for reducing train-induced mass and heat transfers in a permafrost tunnel

As a train passes quickly through a confined space such as a tunnel, it induces dramatic airflow movement in the tunnel. The substantial mass transfer results in a noteworthy energy transfer in the air-tunnel system, forming complex aerodynamic and thermodynamic processes (ATPs). The aerodynamic process has been studied well enough to solve many practical engineering issues. However, little attention has been given to the thermodynamic process in tunnels, especially heat transfer in permafrost tunnels. To study the train-induced mass and heat transfers in the Fenghuoshan permafrost tunnel and design a novel pneumatic structure (PS) to mitigate thaw damage to the tunnel, we built a train-induced mass-heat transfer (MHT) model for permafrost tunnels based on mass, momentum and energy conservation and simulated the ATPs in tunnels without and with the PS. By comparing the mass and heat transfers in the two tunnel structures, we found that the air temperature maximally increases by 1.7 °C in the tunnel without the PS, whereas it increases by only 0.5 °C in the tunnel with the PS. In addition, the PS also reduces the convective heat transfer coefficient (CHTC) by 38.6 % compared with that of the tunnel without the PS. Therefore, the PS can significantly decrease the temperature rise in the tunnel system and prevent excess heat energy from penetrating into the surrounding permafrost. Consequently, it is an efficient method for mitigating thaw damage to permafrost tunnels. Moreover, the study is also helpful for comprehending train-induced mass and heat interactions in permafrost tunnels.