Water supply networks represent key infrastructures to provide safe, reliable, drinking water with adequate pressure to communities, thus ensuring people’s health and well-being. These networks can be operated continuously or intermittently. Continuous water supply (CWS) is characterised by delivering permanently pressurised piped water to consumers with adequate pressure, meeting water quality standards and preventing potential contaminant intrusion. Intermittent water supply (IWS) also provides piped water, though only ensures delivery during limited periods of the day or the week, with interruptions from hours to days. This service is common in areas with limited water resources and with financial constraints. Despite the technological and management advancements in the water sector, most utilities with IWS have limited knowledge of the network performance due to unavailable or unreliable data or the lack of numerical models to better understand the systems’ operation. The development of numerical models to describe the phenomena in each IWS stage (filling, supplying and emptying) is important for design, diagnosis and management purposes. Most developed research focuses on the supply stage, using models with the assumption that the pipes are continuously pressurised. Since that is not the case, a model that allows simulating free-surface and pressurised flows is necessary to describe the other two IWS stages.

The thesis aims to develop and validate a new 1D model, based on the widely used SWMM solver, capable of describing the air-water interaction during pipe-filling events in IWS systems. The specific objectives of this research are: i) to identify, understand and characterise the most relevant air pocket related phenomena during pipe-filling events in single pipes and looped networks; ii) to learn how to incorporate the air pressurisation in SWMM solver as well as iii) the different mechanisms associated with the air pocket creation; iv) to understand the model's uncertainties related to these phenomena; and v) to test the developed model in a real-life network.

To accomplish objective i) and to contribute to objectives ii) - iv), an extensive experimental data collection program is developed to understand the phenomena related to the air pocket creation during the pipe-filling event. Collected data include time series of pressure and flow rate and video recordings of entrapped air pockets, for different pipe configurations and aeration conditions. Three pipe configurations are tested: a straight horizontal pipe, a single pipe with a high point and a single-loop pipe network. Three aeration conditions end are tested: no air release, restricted and unrestricted air release.

Several novel numerical developments are gradually implemented to fulfil key objectives ii) - iv). The first is the modification introduced in the existing SWMM hydraulic solver to incorporate the air phase. A conventional air accumulator model is implemented and coupled with SWMM flow calculations. Experimental data collected during the rapid filling of a single horizontal pipe for the three referred aeration conditions are used for model calibration and validation (fulfilling objective ii). Results show that the improved SWMM, AirSWMM(v1.0), describes better the effect of air behaviour during pipe-filling events than the original SWMM when using the EXTRAN surcharge method.

The AirSWMM(v1.0) model is improved to locate and quantify entrapped air pockets created during the pipe-filling events in single undulating pipe systems. Measurements are collected and video recordings are carried out to assess air pocket volumes for the three referred air release conditions. The stochastic nature of air pocket creation results in a range of air volumes predicted for the same aeration conditions. The new version of the model developed, AirSWMM(v2.0), is capable of simulating the air pocket creation, transport and entrainment (air and water mixing process). The stochastic nature of air pocket formation can be numerically simulated by conducting multiple runs of the new solver with different air entrainment ratios. The obtained numerical results show that AirSWMM(v2.0) can accurately locate and approximately quantify the entrapped air pocket volumes. These developments contribute to objective iii).

The AirSWMM(v2.0) model is further tested and validated using experimental data from a single-loop network laboratory setup. Experimental data consisting of pressure-head at multiple locations and video recordings of air entrapment for two high point locations and different nodal elevations, under three aeration conditions, are used. Experimental tests show that air entrapment occurs not only at the high point but along the pipe network, creating air pockets with elongated shapes and larger volumes than for single pipe systems. AirSWWM(v2.0) model results for the looped pipe network demonstrate that this model can correctly locate large air pockets with a tendency to underestimate their volumes. These developments contribute to objective iv).

The AirSWWM(v2.0) model is also tested using a case study of a real-life network published in the literature to assess the accuracy of predicted locations and volumes of the air pockets created during a pipe-filling event. For this purpose, pressure-driven analysis is implemented to better simulate the nodal demands, leading to AirSWMM(v2.1), since this feature was not originally included in SWMM. Results show that pressure-heads predicted by AirSWMM(v2.1) compare well with field data when constant spatial discretisation is used, provided the Courant number is close to 0.15. The recommendations from international guidelines for the location of air release devices (from the American Water Works Association and Deltares) are compared to the predicted air pocket locations. The locations of the estimated air pockets agree with those from the international guidelines for air valve installation. However, these locations only represent part of the air valves needed, those that are necessary for releasing entrapped air during the pipe-filling events, not accounting for other air valves important for pipe failure or conservative design purposes. These developments contribute to objective v).

Further research on AirSWMM should focus on assessing the spatial discretisation that corresponds to the best compromise between accuracy and computational effort to describe the air pocket dynamics in real-life networks. Additional numerical analyses should assess if the developed methodologies can be incorporated into the Preissmann slot pressurisation scheme. More experimental tests are needed to better quantify the air entrainment in piped flows and to analyse the effect of two-phase flows on leakage rate. Further field tests, collecting high-frequency pressure head data, should be carried out during pipe-filling events to validate the developed models.@en

Intermittent water supply systems are prone to air entrapments during the pipe filling phase. This work aims to analyse and discuss the numerical results obtained by applying the recently developed AirSWMM model, an extension of SWMM incorporating air phase, to a laboratory network. Experimental data consisting of pressure-head at multiple locations and video recordings of air entrapments are collected in a single loop network with a high point, for different pipe-filling conditions, system layouts and node elevations. Experimental tests have shown that the air entrapment occurred not only at the high point but also throughout the pipe network, creating air pockets with elongated shapes and larger volumes than for single pipes. AirSWWM model with air-entrapment formation, growth and transport is tested in the pipe network, and results are compared with measurements. AirSWWM model can correctly locate large air pockets but underestimates their volume.

@en

The current research aims to analyse the effect of the number and shape of the blades and the curvature of the flume bottom on the performance curves of undershot water wheels, based on experimental tests conducted in a fully instrumented laboratory facility. Six wheels are tested: four wheels with plane blades (16, 24, 36, 48) and two with 24 curved blades for two flume bottom configurations. Torque, mechanical power and mechanical efficiency performance curves are determined for several rotational speed and flow rate values. Results demonstrate that the maximum efficiency is achieved for the 36-plane blade wheel, the curved flume bottom reduces water losses under the wheel and increases efficiency, and the blades’ shape strongly influences the wheel efficiency. Non-dimensional performance curves are provided to generalise the results. This research provides relevant contributions towards the development of a low-cost energy recovery solution to be applied in water infrastructures.

@en

The paper proposes a novel methodology to locate and quantify entrapped air pockets created during pipe-filling events often found in intermittent water supply systems. Different filling conditions were tested in an experimental pipe with a high point. Measurements were taken and video recordings were carried out to assess air pocket volumes for different air release conditions at the downstream end of the pipe. The stochastic nature of air pocket creation resulted in varying air volumes. A new numerical model capable of simulating the air pocket creation, dragging and entrainment has been proposed. The new model, AirSWMM, was implemented as an extension of the Stormwater Management Model (SWMM) with stochasticity of air pocket formation reproduced by simulations with different air entrainment rates. The obtained numerical results show that the proposed model, even though based on a single-phase one-dimensional flow, can accurately locate and approximately quantify the entrapped air pocket volumes.

@en

Stormwater management model (SWMM) software has recently become a modeling tool for the simulation of intermittent water supply systems. However, SWMM is not capable of accurately simulating the air behavior in the pipe-filling phase, missing therefore a relevant factor during pipe pressurization. This work proposes the integration of a conventional accumulator model in the existing SWMM hydraulic model to overcome this gap. SWMM source code was modified to calculate the air piezometric head inside the pipe based on the system boundary conditions, and the air piezometric head was incorporated in the SWMM flow rate pressure component. Experimental data were collected during the rapid filling of a pipe system for three possible configurations that are likely to occur in intermittent water supply pipe systems: no air release, small air release, and large air release. Results show that the improved SWMM better describes the effect of the air behavior using the extended transport (EXTRAN) surcharge method when compared to the original SWMM. Results also show that the SLOT method with predefined slot width is not suitable for this purpose; thus, further research is needed to assess if an adjusted slot width could provide better results.

@en

The current research focuses on the analysis of different dynamic effects of an air pocket located at mid-pipe length on transient pressures based on experimental data. Different flow rates and air pocket volumes are analysed. Several features are identified in the pressure-head signal associated with the air pocket: initial pressure drop, higher maximum overpressure peaks, increased pressure wave damping and delay. These features result from the superposition of pressure waves associated with the entrapped air contraction and expansion. Smaller air pockets generate minor reflections, whereas larger air-pockets tend to disperse and artificially attenuate maximum pressures. Video recording images of the air pocket during the transient event allow observation of two types of behaviour: one associated with lower transients, characterized by an air pocket volume variation with a clear air–water interface; and another one associated with more severe transients in which air mixes with water. Finally, there is a critical air pocket volume, independent of the initial flow rate, that leads to the highest overpressures.@en

Pumps running as turbines are pointed out as a cost-effective solution for energy recovery in pressurised water supply systems. However, these hydraulic machines feature low efficiency under variable discharge operation due to the lack of an inlet flow control component. Variable speed operation is an approach for controlling the discharge at the pump as turbine inlet aiming at increasing the operational efficiency. This research work presents the experimental investigation for measuring the variable speed characteristic curves of pumps running as turbines, focusing on the turbine and on the extended operation modes. Three single-stage end-suction closed-impeller centrifugal pumps with different unit specific speed values are tested. Turbine mode test results show that the discharge-specific energy operating range is broadened with increasing efficiency if the machines are operated with variable speed. Extended operation results show that these hydraulic machines do not feature the instability region near the runaway conditions, the so-called the “s-curve”. Outcomes of this experimental investigation provide the required insights for establishing the design technical specifications of micro hydropower plants with variable speed pumps running as turbines, aiming at maximizing the energy recovered in pressurised water supply systems.@en

The aim of this paper is the experimental characterization of a ball valve under steady and unsteady conditions, based on experimental tests carried out in a pressurized pipe system assembled at Instituto Superior Técnico. Data analysis showed that the response of the valve under steady-state conditions depends not only on the valve geometry and closure percentage, but also on the flow regime (i.e., Reynolds number); the comparison with values published in literature confirmed that typical valve head loss curves presented in manuals and textbooks refer to turbulent flows. The behavior under unsteady conditions was also analyzed based on transient pressure head measurements showing that the valve effective closure time varies between 4 and 10% of the total time of the maneuver. Two mathematical functions (hyperbolic and sigmoidal) are proposed to describe the discharge variation as a function of the total time of the maneuver and of the initial steady-state discharge; the most appropriate function depends on the simplifications imposed on the hydraulic transient simulator. A computational fluid dynamics (CFD) model was used to support the observed experimental behavior and to highlight the effect of the pipe length on the discharge variation.@en