Experimental study on the performance of hybrid ventilation system combining forced longitudinal flow and shaft natural ventilation in tunnels

The hybrid ventilation system combining forced longitudinal flow and shaft natural ventilation has the advantages of both ventilation strategies; for example, it can discharge buoyant smoke through the nearest downstream shaft and can actively constrain backlayering flow. A series of brine-water experiments were conducted to investigate the effects of the longitudinal velocity and shaft dimensions on the performance of the hybrid ventilation system in urban traffic tunnels. A light attenuation technique was used to obtain the width-averaged density distribution in the entire tunnel. Subsequently, ventilation efficiency and the backlayering length were easily determined. The results indicated that the extraction efficiency of the buoyant fluid decreases with increasing longitudinal velocity, particularly when the ventilation shaft is short and the longitudinal velocity is large. Moreover, the existence of a stable backlayering flow is useful for improving the extraction efficiency. For ordinary urban traffic tunnels that allow only passenger cars to pass through, the optimal height and width of the ventilation shaft were determined to be 9 and 7 m, respectively. From the results of the backlayering length and extraction efficiency, we observed that the acceptable longitudinal velocity corresponding to the optimal shaft dimensions approximately varies from 1.8 to 2.3 m/s. The study can provide useful information for the design of such hybrid ventilation systems in urban traffic tunnels.