Analysis of entrainment phenomenon near mechanical exhaust vent and a prediction model for smoke temperature in tunnel fire

To investigate the entrainment phenomenon near a mechanical exhaust vent in the tunnel fire, the computational fluid dynamics method employing the software Fire Dynamics Simulator (FDS) is utilized to analyse the effects of heat release rate (HRR) and exhaust velocity on the entrainment near the vent. The simulation results indicate that the total amount of fresh air entrained in the tunnel increases continuously with increasing HRR and exhaust velocity. However, when the smoke temperature is low, the mass flow rate of fresh air exhausted through the shaft is insensitive to the HRR, while is affected more by the exhaust rate. In addition, a simplified prediction model for downstream smoke temperature is developed to account for the entrainment effect on smoke temperature variation. The predicted values accord well with the simulated ones. Results in this work can provide a reference to the optimization design and management of smoke control system in tunnels.