Self-aerated Flow in Steep, Partially Filled Pipes

The self-aerated flow in steep and partially filled conduits of circular shape is investigated. Prototype measurements are carried out on pipelines with slopes up to 1.0 (45°) and diameters from 110–900 mm. Instruments for measuring the air concentration, the velocity of the mixture and the mixture level are described. For the mixture flow, the isotachs, the contour lines of air concentration, and the shape of the free surface are analyzed. Formulae are presented for the mean air concentration, the mean mixture velocity as well as for the critical slope that initiates a water-air flow (air entrainment). The results are compared with known formulae for open rectangular cross-sections. The calculation of the pipe diameter that guarantees partially filled flow (also the width for not-too-wide rectangular channels) is illustrated by examples.