AIR-POCKET TRANSPORT IN CONJUNCTION WITH BOTTOM-OUTLET CONDUITS FOR DAMS

Ting Liu

Division of River Engineering
Department of Land and Water Resources
Royal Institute of Technology (KTH)

Licentiate seminar: Thursday January 19th at 10.30
Venue: V3, Teknikringen 72
Main advisor: Prof. James Yang, Division of River Engineering, KTH
Opponent: Prof. Staffan Lundström, Division of Fluid Mechanics, Luleå University of Technology

ABSTRACT

Undesired air entrainment in bottom outlet conduits of dams may cause pressure transients, leading to conduit vibrations, blowback, discharge pulsation and even cavitation, and jeopardize the operational safety. It is impossible to create an air free environment in a pressurized pipe. Therefore to understand air transport in enclosed pipes is essential to provide guidance in bottom-outlet design and operation.

The classical Volume of Fluid (VOF) model combined with the k-ε turbulence model is adopted for the computation of the critical velocity, the water flow velocity for starting moving an air pocket. The governing parameters investigated in this study include pipe slope and diameter, wall shear stress and air-pocket volume. Meanwhile, the carrying capacity (air-pocket velocity/ flow velocity) at different pipe slopes are evaluated. Experimental study was conducted to measure the critical velocity for a 240-mm pipe. The results are in agreement with the experiments in a 150-mm pipe performed by HR Wallingford (HRW) in 2003 in terms of the effects of pipe slope and air-pocket volume. However, the critical Froude pipe number is slightly smaller in this study. In rough pipes, a larger critical velocity is required compared with that in the smooth pipe and the removal mechanism in the rough pipe involves the successive loss of air caused by turbulence.