Impact of Blockage Ratio on Smoke Control in High Heat Release Rate Tunnel Fires

Tunnel fires involving vehicle carriers transporting new-energy vehicles, heavy goods vehicles (HGVs), and tanker trucks are characterized by high heat release rate (HRR) and large blockage ratio (φ), posing significant challenges to tunnel smoke control. Previous studies have mainly focused on conventional tunnel fires (HRR < 40 MW), while the coupled effects of φ and HRR under large-fire conditions remain insufficiently understood. To address this gap, full-scale numerical simulations were conducted using Fire Dynamics Simulator (FDS) to investigate the critical ventilation velocity (v _c) under HRR ranging from 10 to 300 MW and φ ranging from 0 to 0.6. The numerical model was validated against the Memorial Tunnel full-scale fire tests, with a prediction error of 5.26%. A quantitative coupling relationship between φ, HRR, and v _c was established, and a quadratic predictive model was developed to characterize their combined effects. The results show that v _c decreases almost linearly with increasing φ, with a reduction of approximately 0.10-0.20 m/s for every 10% increase in φ. For large-fire scenarios (100-300 MW), the coupling degree exceeds 0.90, exhibiting a distinct weak-strong-weak coupling pattern with increasing φ. Furthermore, the influence of φ gradually weakens as HRR increases, indicating that HRR becomes the dominant factor governing v _c in large tunnel fires. These findings provide a theoretical basis for the design of longitudinal ventilation systems and emergency smoke control strategies in tunnels carrying new-energy vehicles and heavy goods vehicles.