Effect of lateral mechanical smoke exhaust on smoke backflow behavior and temperature profile in a mountain tunnel: a numerical investigation and analysis

In this study, a series of numerical simulations were performed to investigate smoke movement and longitudinal temperature distribution under varied heat release rates (HRRs) and mechanical exhaust velocities. Significant differences in smoke migration characteristics were observed between two mechanical smoke‑extraction schemes: ceiling‑opening shafts and lateral‑opening shafts. For the lateral‑opening shaft scenario, the asymmetric flow‑field distribution was identified and physically interpreted. Quantitative analyses together with numerical observations were carried out to characterize the upstream longitudinal smoke‑temperature attenuation and smoke back‑layering length in tunnels equipped with lateral‑opening shafts. As the exhaust velocity increases at a given HRR, the upstream smoke temperature and smoke back‑layering length decrease markedly; by contrast, both quantities rise with increasing HRR under fixed exhaust velocity. Accordingly, two empirical models were proposed for predicting the longitudinal temperature decay and smoke back‑layering length, respectively. The present findings can provide valuable references for the ventilation and smoke‑extraction system design of mountain tunnels with lateral‑opening shafts.