The Behaviour of Air Pockets in Hydraulic Structures With Particular Reference to Dropshaft/Tunnel Bends

Himmo, Sawsan Khalid Mohamad (1986) The Behaviour of Air Pockets in Hydraulic Structures With Particular Reference to Dropshaft/Tunnel Bends. PhD thesis, University of Glasgow.

Abstract

This thesis investigates the behaviour of large air pockets forming at the junction bend of a vertical dropshaft and a horizontal or slightly inclined outlet tunnel. A secondary thrust of the work concerns the behaviour of air pockets in straight pipe sections inclined at a shallow slope both upwards and downwards from the horizontal. These aims were achieved by constructing a physical model of the situation and also by deriving theoretical models of each physical situation arising. The experimental data is compared with the derived theoretical models, and both are compared with past research in this field. The physical model was tested over a range of nine different geometries. The radius of the junction bend (R'/D) was tested at values 0.5, 1.0 and 1.5, the tunnel slope (0) was varied between +1.5, 0 and -1.5, the water flow rate was varied from 0 to 0.04 m3/s, and the air flow rate, in the form of independent injection of air bubbles down the dropshaft, was varied from 0 to 0.02 m3/s. The ultimate aims of the work were as follows: (a) to obtain a deeper understanding of air pocket behaviour at vertical bends and in straight pipes. (b) to ascertain if simplified theoretical models can be made realistic enough to describe air pocket behaviour, especially the chaotic behaviour of a two-phase flow at a dropshaft/tunnel bend. (c) to provide insight and information for the designers of such hydraulic structures. In the first chapter an introduction to air presence in Civil Engineering Hydraulic Structures is given, with reference to the benefits and problems associated with air presence and the structures which have experienced problems. A state-of-the-art review of past research work on air pockets in closed conduit hydraulic structures is given in Chapter (2). Chapter (3) is concerned with producing theoretical models for air pockets at the dropshaft/tunnel bend. This includes air pockets blowing back, air pockets with a drowned jump, air pockets with a hydraulic jump and air pockets clearing downstream from the bend. Also, theoretical models are produced for air pockets in straight pipes inclined above and below the horizontal. Chapter (4) contains a description of the design of the experimental apparatus, the instrumentation used, as well as details of the experimental procedure. Chapter (5) includes the experimental results for the behaviour of air pockets at the nine dropshaft/tunnel bend geometries tests. The experimental results for each geometry tested are presented graphically, covering the various flow regimes found to exist at the bend, the depth of air pocket forming at the bend, the upper limits of air pocket blowback up the dropshaft, the lower limits of air pocket clearing downstream from the bend, as well as information on the velocity and Froude number of flow under the air pocket forming at the bend. A comparison is carried out between the three bend radii used as well as the three angles of outlet tunnel. Chapter (6) includes the results of air pockets in straight pipes. Most of the data was obtained for air pockets rising in an upward sloping straight pipe. The design of the experimental apparatus permitted only a few data points to be taken for the case of the downward sloping straight pipe, for reasons outlined in Chapter (6). Chapter (7) contains the comparison between the theoretical models and experimental data for both air pockets at the bend and in straight pipes. Empirical equations are derived for the case of air pockets clearing downstream from the bend, where no theoretical model was attempted due to the flow complexities involved. A comparison is also carried out between the author's data and theoretical models and most of the available previous research data. Chapter (8) includes a discussion on the findings of this work, conclusions and suggestions for the future work. In the broadest terms, this work shows that four stable regimes of flow are identifiable at a dropshaft/tunnel bend and these regimes can be represented with reasonable accuracy by theoretical models of the flow, based on force-momentum and energy principles. The work also shows that the bend radius (R'/D), the outlet tunnel slope (9), the Froude number of the flow (FrO), and the ratio of air to water (B) are all important parameters affecting whether or not an air pocket will blow back, remain trapped at the bend or clear downstream. Air pocket behaviour can now be accurately predicted for the case of upward sloping straight pipes, although further work is required for the case of downward sloping straight pipes.

• Item Type: Thesis (PhD)
• Qualification Level: Doctoral
• Keywords: Civil engineering, Hydraulic engineering
• Date of Award: 1986
• Depositing User: Enlighten Team
• Unique ID: glathesis:1986-77464
• Copyright: Copyright of this thesis is held by the author.
• Date Deposited: 14 Jan 2020 09:07
• Last Modified: 14 Jan 2020 09:07
• URI: https://theses.gla.ac.uk/id/eprint/77464