Air Voids at Downshaft‐tunnel Bends

Abstract

In closed conduit water flows containing a small amount of free air, an air void can form at a bend which connects a vertical shaft to a horizontal tunnel. A sudden reduction in the water flow rate can cause this void to vent back up the shaft. Tests were conducted on eight perspex pipe bends whose center‐line radius varied from 0.5 times–1.5 times the pipe diam, and over a range of pipe diameters from 2.8 in.–7.5 in. (0.072 m–0.190 m). The air volumetric flow rate was up to 2% of the water volumetric flow rate. A combined theoretical and analytical approach yielded equations which can be used to predict void formation and venting at a downshaft‐tunnel bend. Extrapolation of the results shows that these voids will not occur in large diameter bends with moderate air flows.