Flow measurements around guide vanes of Francis turbine: A PIV approach Biraj Singh Thapa a, * , Ole Gunnar Dahlhaug b , Bhola Thapa a a Dept. Mechanical Engineering, Kathmandu University, P.O. Box 6250, Dhulikhel, Nepal b Dept. Energy & Process Engineering, Norwegian University of Science and Technology, Trondheim, 7491, Norway article info Article history: Available online 20 March 2018 Keywords: Francis turbine Guide vane Cascade PIV Uncertainty Velocity triangle abstract A guide vane cascade is developed for the study of flow in the distributor of a low specific speed Francis turbine. Velocity and pressure measurements are done with Reynold's number 1.15 Eþ07, at 80% of BEP flow as in a reference prototype turbine. This work illustrates the development of test setup and focuses on investigation of PIV methods applied for the velocity measurements. Techniques developed for ‘in- situ’ calibration of PIV setup and methods applied for image processing are discussed in details. Approach to estimate total uncertainty in PIV measurements and minimum no of image pairs required for sta- tistical convergence of velocity field is presented. Reference measurements are done along the plane of chord, from guide vane wall to its mid-span. Flow velocity exceeding 35 m/s, at the runner inlet of Francis turbine, is reported for the first time from such experimental studies. Flow phenomenon inside Francis turbine distributor are characterized and comparison are done with the cases for prototype turbines. The cascade setup is found to reproduce the flow conditions inside a Francis turbine distributor, except the rotor-stator-interaction. PIV methods are generalized for the cases of similar measurements and the results will be applicable to validate numerical studies. © 2018 Elsevier Ltd. All rights reserved. 1. Introduction Since the first installation of hydroelectric power plant in 1827, this green energy technology has undergone several innovations and capacity enhancements. At present hydropower sector pro- duces more than 1211GW electricity worldwide, which is about 20% of total electricity supply [2]. As thermal energy is not associ- ated with hydropower system, the efficiency of this technology may reach 100% in theory [3]. In practice, up to 93% efficiency is possible, which makes hydropower one of the most efficient and economic energy resources. Two third of technically feasible hy- dropower resources are still undeveloped. More than 80% of the undeveloped hydropower resources lie in Asia, South America and Africa [4]. Hence, future of hydropower developments will be more localized in these regions. Energy conversion process in a hydropower system occurs in several intermittent stages. Major stages include conversion of potential energy stored in reservoir to kinetic and pressure energy in the penstock, then to mechanical energy in the turbine, and finally to electrical energy in the generators. The most complicated energy conversion occurs inside the turbines, where water trans- fers its hydraulic energy and exits out from the system. The turbines are mechanical components, which include runners that are driven by hydraulic energy of water. Depending upon nature of flow pa- rameters and mechanism of energy conversion, the hydro turbines are classified into several types. Francis type of turbines, which harness both kinetic and potential energy in water, are most commonly used in hydropower systems due to its high efficiency and flexibility [5]. Fig. 1 shows cross section of a Francis turbine with flow veloc- ities at respective components. Spiral casing (SC), stay vanes (SV) and guide vanes (GV), together, are often called as the distributor part of turbine. Flow accelerates inside the distributor and normally 50% potential energy is converted to kinetic energy before water enters into runner. GV regulates the flow into runner and gives it correct tangential velocity required for the energy conversion process. Water leaves runner with a high relative velocity and exits through draft tube. Francis turbines are normally designed with the aim to maxi- mize hydraulic efficiency, minimize size and avoid cavitation [6]. These conditions demands high flow velocity and high blade * Corresponding author. E-mail address: bst@ku.edu.np (B.S. Thapa). Contents lists available at ScienceDirect Renewable Energy journal homepage: www.elsevier.com/locate/renene https://doi.org/10.1016/j.renene.2018.03.042 0960-1481/© 2018 Elsevier Ltd. All rights reserved. Renewable Energy 126 (2018) 177e188