Abstract
Air expulsion from an end-of-pipe orifice in a rapidly filling horizontal pipe is investigated experimentally and analytically in order to more completely characterize the system's transient response. In particular, images of air-water patterns, air-volume variations, orifice flow regimes, and measured pressure histories are synchronized to elucidate the process of air expulsion. Air expulsion typically undergoes an early stage involving pressurization, expansion, and release of a portion of the initial air, events that generally occur even before the advancing water column reaches the pipe end. The next stage depends strongly on the orifice size. For a small discharge orifice, an oscillation of the residual air occurs with the discharge orifice being intermittently choked by water; by contrast, larger discharge orifices rapidly and completely expelled the air, often leading to high water-hammer pressures. Three distinct patterns of pressure oscillation are typically observed. With small orifices, the cushioning effect of the initial air tends to dominate, whereas slightly larger orifices lead to a more complex process of expulsion and more persistent and larger pressure oscillations. Even larger orifices often lead to severe water-hammer pressures. Thus, smaller orifices tend to result in smaller pressure fluctuations. As expected, both the initial-air volume and the inlet pressure significantly influence the transient response. A derived analytical model accurately captures the measured pressure oscillations during the intermittent release of residual air, including the water hammer that can arise due to suddenly arresting the liquid water column.
Original language | English |
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Article number | 04020047 |
Number of pages | 16 |
Journal | Journal of Hydraulic Engineering |
Volume | 146 |
Issue number | 7 |
DOIs | |
Publication status | Published - 1 Jul 2020 |
Funding
The authors gratefully acknowledge the financial support for this research from the National Natural Science Foundation of China (Grant Nos. 51679066 and 51839008), the Fundamental Research Funds for the Central Universities (Grant No. 2018B43114), Fok Ying Tong Education Foundation (Grant No. 161068), and the China Scholar Council (File No. 201806715024). A. Bergant acknowledges the support of Slovenian Research Agency (Project L2-1825 and Programme P2-0162).
Keywords
- Air expulsion
- Orifice
- Pipeline
- Transient flow
- Trapped air