Experimental studies on the smoke extraction performance by different types of ventilation shafts in extra-long road tunnel fires

摘要

Ventilation shafts are widely applied in extra-long road tunnels. Experiments were conducted in a 1/20 scale model tunnel to investigate the smoke extraction performance with a ventilation shaft. Four types of shafts were compared under the normal ventilation mode and the smoke extraction mode. The influence of shaft types, the operation frequency of the jet fan and board locations were investigated. Results indicate that the smoke extraction capacity of the mechanical ventilation shaft (MTS) is almost the same as the operation frequency increases, but the smoke extraction efficiency declines as the operation frequency increases. Besides, it can be found that the smoke extraction capacity and the smoke extraction efficiency of the mechanical board-coupled shaft (MBCS) decrease as the operation frequency increases. Moreover, it was found that the smoke extraction performance of the MBCS is highly affected by the distance between the board and the shaft. And the optimal board location for the MBCS changes with the operation frequency. Furthermore, the coupling-effect of the momentum induced by the mechanical ventilation and the buoyancy induced by the hot smoke in the ventilation shaft during a fire was explored. And the dimensionless smoke extraction Froude number (Frs), was developed for characterizing the buoyancy-momentum transition during the smoke extraction process with the mechanical ventilation shaft. These studies may contribute positive significance in the smoke control and extraction design with extra-long road tunnels.