Experimental study of air demand in high head gated tunnels

Abstract

In the present work air demand in a gated tunnel with a circular cross-section was studied experimentally. Effects of gate opening, relative depth (i.e. ratio of flow depth to tunnel diameter) and head losses in the air vent, as well as along the air passage from the tunnel outlet, on air demand were studied. It was concluded that when losses in the air vent increase, flow of air begins from the tunnel outlet. A relationship is suggested for the minimum sub-atmospheric pressure after the gate at which air flow starts from the tunnel outlet. Measurements showed that with a relative depth of less than 40%, even when the vent was fully closed, air was supplied from the tunnel outlet. In this condition higher negative pressures were recorded after the gate. The present work revealed the importance of head losses along the air passage in the tunnel as well as the head losses in the air vent on vent air discharge. These factors are not considered in empirical equations for estimation of vent air discharge.