Experimental modeling of air-water interaction in horizontal pipe with vertical riser

Abstract

Underground stormwater drainage systems are important urban infrastructure to store and convey the stormwater for preventing urban flooding. Due to climate change, the frequency of extreme rainfall event has increased, which results in overloading of the drainage systems beyond its design capacity. Unexpected accidents like geysers can occur during heavy rainfall, causing damages to the drainage systems and threatening the safety of pedestrians. Geysers are strong splash of air-water mixture through manholes. The understanding of the mechanism of geyser occurrence is still far from complete due to the lack of detailed observation of air-water interaction. This study aims to better understand air-water interaction in drainage systems and reveal the mechanism of geysers through experimental investigation and theoretical analysis. A physical model of a simplified drainage system, which consists of a vertical riser and a horizontal pipe (diameter D) connected to a constant head tank, has been designed and constructed. Four series of experiments with different riser diameter (D_r), upstream head (H ₀), initial air pocket length/volume (L₀/V_air) and pipe-end orifice diameter (d) have been conducted. The entire process of air pocket propagation in the horizontal pipe, rise in vertical riser and vertical surge of the air-water mixture have been studied. The trajectory of air pockets are measured by videos and a high speed camera. Pressures are measured using pressure transducers near the pipe end and at the bottom of riser. The air pocket propagation in a horizontal pipe is found to be similar to a gravity current flow. The propagation speed seems to be independent on the initial air or water length and the end gate condition. The air pocket migration in a water-filled vertical riser is found to be similar to a slug flow. The rising velocity of the air pocket relative to the free surface (V_net), is found to be almost constant and close to the speed of a Taylor bubble. No geyser is observed. When an external pressure head is applied to the horizontal pipe, and with orifice at the pipe end, the initial pressure difference will generate a hydraulic transient flow. The amount of the trapped air and pressure variation is highly dependent on the orifice size. When air pocket arrives at the riser, two types of flow are observed: (i) for large riser diameter and small air volume, the air pocket resembles a Taylor bubble; the air breaks within the riser and no geyser is observed. (ii) for small riser diameter and large air volume, geyser events are observed with pressure surge, rapid acceleration of air and water, and jetting out of air-water mixture from the riser top. It is found that geyser is highly related to the compression of air, it is more likely to occur when a large volume of air (V_air/(πD>_r²/4H₀ ≥ 4.73) is trapped in a pressurized tunnel and released through a small riser (D_r/D ≤ 0.62). The initial pressure variation before the air pocket reaches the riser can be interpreted by a one-dimensional model based on rigid column assumption.

Date of Award	2016
Original language	English
Awarding Institution	The Hong Kong University of Science and Technology