Abstract: When a fire occurs in tunnels during construction, press-in ventilation is used to ensure evacuation safety by maintaining the smoke stratification stability on the downstream side. In this paper, the fire-induced smoke stratification characteristics in a tunnel during construction with press-in ventilation are studied by numerical simulation. The concept of subcritical velocity is proposed. The smoke layer height, thermal stratification intensity, and Fr number on the downstream side are analyzed. The subcritical velocity can control the smoke back-layering on the upstream side and the smoke stratification on the downstream side in an acceptable range at the same time. The subcritical velocity is also the critical value at which the smoke layer on the downstream side ranges from relatively stable to unstable. When ventilation velocity is less than subcritical velocity, the slopes of smoke layer height and thermal stratification intensity along the longitudinal direction of the tunnel decrease as ventilation velocity increases, and smoke stratification maintains relatively stable. When ventilation velocity exceeds subcritical velocity, smoke layer height and thermal stratification intensity rapidly decrease, and smoke stratification is destroyed. When the temperature ratio（ΔTcf/ΔTaver） and the Fr are used to characterize the smoke stratification, ΔTcf/ΔTaver is not sensitive to the Fr when the ventilation velocity is small.

Key words: tunnel during construction, press-in ventilation, numerical simulation, smoke stratification characteristics, subcritical velocity