Choking conditions inside plunging flow dropshafts

Abstract

Severe rain events can subject our existing drainage systems to capacity. While the conveyance capacity of sewers can be increased by temporarily tolerating a full pipe flow condition, the complex dynamics of water–air mixture occurring in vertical dropshafts can largely restrict the overall conveyance capacity of drainage systems. In this study, the hydraulic performance of plunging flow dropshafts is investigated experimentally with respect to their conveyance capacity. The model dropshaft consisted of an upstream horizontal inflow pipe, a vertical circular shaft, and an outflow pipe discharging to the atmosphere. Five setups were built with Ds/Di from 1.0 to 3.0 for the range of drop heights H/Ds = 3.0 to 21.0, where Di is the inlet pipe diameter, Ds the shaft diameter, and H the drop height. A wide range of discharges up to Q* = 25 was tested, where Q* = Q/(gDi5)(1/2). Flow patterns recognized within the shaft (free flow, surface roller, plug flow, slug flow, and full pipe flow) help to assess the two stages of choking: the incipient and fully choked state. Criteria to predict choking occurrences were developed experimentally. The flow is considered to reach its conveyance capacity when an incipient choking state is visible. For a typical dropshaft with Ds/Di = 2, Q* was found to be approximately 5.