Subcritical Velocity to Maintain the Fire-Induced Smoke Stratification in Tunnels During Construction Under Press-in Ventilation

A tunnel during construction is a typical long-narrow space with one closed end. Due to the cost-effectiveness and convenience, press-in ventilation is the most widely used in tunnels during construction. In the case of fire in tunnels during construction, press-in ventilation needs to simultaneously suppress the smoke back-layering length on the upstream side and maintain the smoke stratification stability on the downstream side. Based on the two-region theoretical model of fire-induced smoke in confined spaces, the relative flow of smoke and air on the downstream side is analyzed under press-in ventilation using the hydrostatic pressure difference and mass conservation. The effects of the heat release rate and press-in ventilation velocity (\({u}_{p}\)) on the smoke stratification characteristics, longitudinal velocity, and mass flow rate are studied using numerical simulation. The results show that the smoke stratification stability on the downstream side is divided into stage I, critical moment A, and stage II. On this basis, the concept of subcritical velocity (\(\:{u}_{sub}\)) is proposed. Stage I: \({u}_{p}\) < \({u}_{sub}\), the flow directions of smoke and induced air are opposite, and the smoke stratification is stable. Critical moment A: \(\:{u}_{p}\) = \({u}_{sub}\), the lower airflow slowly flows and approaches stagnation, and the smoke stratification transits from a relatively stable state to an unstable state. Stage II: \({u}_{p}\) > \({u}_{sub}\), the flow directions of smoke and excess press-in ventilation are consistent, and the smoke stratification is destroyed. The prediction models of the subcritical and critical velocities are established. The critical velocity is twice the subcritical velocity in the same fire scenario. The results can provide a theoretical foundation for the application of press-in ventilation in tunnel fires during construction.