Experimental studies of air pocket movement in a pressurized spillway conduit

Abstract

Undesired air entrainment in a bottom outlet conduit causes pressure transients, leading to conduit vibrations, blowbacks and discharge pulsations and thus endangers operational safety. In this study, the propagation velocity of a solitary air pocket and the characteristics of its critical velocity were examined in experiments conducted using a 240-mm-diameter pipe. Air pocket movement depends on the pipe diameter, slope, roughness and air pocket size. The critical pipe Froude number for initiating downstream movement of an air pocket is smaller in a larger pipe, most likely due to the scale effect and/or to a smaller reduction in the effective cross-sectional area. The critical velocity in rough pipes was found to be independent of the air pocket size. A minimum Froude number was suggested for a rough pipe instead of a critical pipe Froude number because the air removal process was found to involve successive air losses from the air pocket caused by turbulence.

Keywords:

• Air pocket
• bottom outlet
• critical velocity
• large diameter pipe
• roughness effect

Acknowledgements

This study is part of a PhD programme in the area of hydraulic design, financed by the Swedish Hydropower Centre (Svenskt Vattenkraftcentrum, SVC), Stockholm.