Thermal and fluidic characteristics of CO2 diffusion in tunnel fires: Role of transverse air curtains

• Study of transverse air curtains was conducted under a wide range of environmental pressures. • Effects of jet velocity and fire heat release rate on air curtain smoke control were investigated. • Dimensionless analyses of temperature and gas concentration fields show unified scaling laws. • Environmental pressure’s effect on air curtain efficiency is a key novel finding. • Analyzing temperature and CO 2 under jet velocity, fire, and pressure clarified mechanisms. Tunnel fire smoke control is a critical issue for ensuring safe evacuation of occupants. Thermal and fluidic characterization of CO 2 concentration by transverse air curtains under varying environmental pressures remain insufficient. In this study, we employ FDS numerical simulation and analysis to investigate the efficacy of a transverse suction–blowing air curtain in controlling CO 2 concentration in tunnel fires. Four key parameters are examined: air curtain supply velocity (6–16 m/s), heat release rate (HRR, 1–15 MW), fire source spacing (5–50 m), and environmental pressure (40–140 kPa). A non‑dimensional fitting model is developed to quantify the curtain’s blocking performance. The results indicate that increasing supply velocity enhances the jet momentum of air curtain and thus its CO 2 blocking efficiency. Conversely, higher HRR strengthens smoke inertial forces, thereby reducing the curtain’s effectiveness. Fire source spacing also affects smoke control performance, while elevated environmental pressures markedly improve both blocking and dilution efficiencies of the air curtain. Through fitting analysis, non‑dimensional CO 2 concentration models are established as functions of supply velocity, HRR, and environmental pressure, revealing the governing relationships for CO 2 control. By quantifying the CO 2 blocking effectiveness of transverse air curtains, our study confirms that supply velocity is the primary control parameter; at high HRR, additional measures are required; and high-pressure environments favor smoke segregation and dilution. These findings deepen our understanding of the smoke-control mechanisms of transverse air curtains and provide essential guidance for optimizing curtain parameters and formulating emergency evacuation strategies in tunnel fires under varying environmental pressures.