Experimental and Numerical Investigation of Pumps As Turbines in Water Distribution Networks

Abstract

The present Ph.D. thesis is devoted to the experimental and numerical analysis of the Pumps As Turbines (PATs) application in hydraulic systems and, specifically, in Water Distribution Networks (WDNs) to generate Energy Recovery (ER) in both urban and rural areas. The use of PATs in WDNs represents a relevant topic in the field of the innovative Best Management Practices (BMPs) of WDNs because addressed to both the pressure regulation and the hydropower generation. Thus, it could represent an alternative approach to the use of Pressure Reducing Valves (PRVs), benefitting of the exceed pressure to produce sustainable energy, otherwise dispersed. The PATs application consists in the use of pump models commercially available running in reverse mode, by inverting the flow direction and making use of the motor as an electrical generator. Their employment can be intended as an effective alternative to the traditional micro-turbines in WDNs, having lower investment and maintenance costs and allowing the model selection among a wide set of pump models available in the market. Conversely, PATs usually assure lower operative ranges, however expandable by arranging proper hydraulic and/or electrical regulations. In addition to the evaluation of the effective opportunities for pressure reduction in water systems, one of the main issue, concerning the PATs use for hydropower generation, regards the limited knowledge about their performance curves, rarely made available by manufactures. In the literature, several experimental, theoretical and numerical models have been proposed, aiming at both predicting the PAT performances and reproducing the internal fluid dynamics. In this regard, Computational Fluid Dynamics (CFD) models are widely applied, because able to both simulate the fluid dynamics of actual configurations and analyse the performance improvement obtainable by modifying the geometric properties of the considered turbo-machine. However, the current knowledge about PAT performances, available in the literature, does not allow to characterize the wide set of pump models, which are on the market, useful for running as turbines. With the aim of making this limitation smaller, in the present study an experimental and numerical investigation of centrifugal PAT models was performed, presenting an operative procedure for the optimal selection of centrifugal PATs in WDNs. A laboratory set-up was specfically installed at the Department of Civil, Architectural and Environmental Engineering (DICEA) of the University of Naples (IT) Federico II where several centrifugal PATs, having different geometric configurations and motor equipment, were tested. The experiments were performed in wide flow rate ranges, typical of WDNs supplying small-medium sized urban and rural centers, by varying the PAT rotational speed from 300 to 3000 rpm. A total of about 7400 operative configurations were investigated, in compliance with the laboratory set-up potentialities, achieving generated head drops up to about 70 m and produced electrical powers up to 16.3 kW. From the experimental analysis, with the aim of characterizing the investigated PATs, comparison with models from the literature was performed and, in order to provide effective tools for predicting the performances of centrifugal PATs, analytic relationships were derived, as a function of the geometric configuration, the number of stages and the motor efficiency class equipment. The attention was specifically drawn to the experimental characterization of: a) a classical Horizontal Axis Single-Stage PAT running at wider operative ranges than those discussed in the literature; b) PAT models, not adequately taken into account in the literature, such as centrifugal Single-Stage and Multi-Stage Vertical Axis PATs. Taking advantage of the experimental results, an operative procedure was developed for the optimal selection of PATs in WDNs, able to maximize the potential ER in WDNs. It could be intended as a basic tool for implementing a Decision Support System (DSS) for hydropower generation in WDNs. Finally, the development of a numerical CFD model, by applying the ANSYS® Fluent™ code, allowed to test the reliability of numerical simulations to both predict the PAT performances and investigate the fluid behaviour across centrifugal PATs.