2016 International Conference and Exposition on Electric and Power Engineering (EPE 2016), 26-22 October, Iasi, Romania 978-1-5090-6129-7/16/$31.00 ©2016 European Union Numerical Investigation of the Flow in a Modified Bánki Turbine with Nozzle Foreseen with Guide Vanes Andrei Dragomirescu Department of Hydraulics, Hydraulic Machinery and Environmental Engineering University Politehnica of Bucharest Bucharest, Romania andrei.dragomirescu@upb.ro Abstract—This paper presents results of a study aimed at verifying whether the usage of a cascade of guide vanes inside the nozzle of a Bánki turbine could improve the turbine performance. Bánki turbines are small cross-flow water turbines used to harvest renewable energy, usually in a range from a few kilowatts to a few hundred kilowatts. First, an existing turbine with classical nozzle was experimentally and numerically investigated to provide a basis for comparison. A mesh sensitivity analysis was performed to find the most appropriate mesh parameters in terms of accuracy and computational resources. After choosing the mesh, the numerical results were validated against experimental data. Second, the turbine with the nozzle modified to accommodate a cascade of guide vanes was numerically investigated. Comparisons of the results obtained on the two nozzle geometries show that the turbine with nozzle foreseen with guide vanes performs clearly better than the classical turbine. The guide vanes assure absolute velocity angles that are closer to the design value. The moment delivered by the modified turbine is significantly higher than that of the classical turbine. Hence, the usage of a nozzle with guide vanes is expected to provide a higher turbine efficiency. Keywords—cross-flow turbine; Bánki; modified nozzle; guide vanes; efficiency; CFD; renewable energy I. INTRODUCTION A Bánki turbine is a partial admission cross-flow water turbine, in which a nozzle produces a water jet that passes through the runner twice, following a roughly transverse path. Cross-flow turbines are small turbines used to harvest renewable energy. They produce low powers, that span from a few kilowatts to a few hundred kilowatts. Such turbines are very well suited, among others, for providing electricity in remote areas which are not connected to the grid. They possess some clear advantages, such as simple construction and maintenance, low price, flat efficiency curve, and good behavior with partial loads, but have lower efficiency than other turbine types – Pelton, Francis or Kaplan. Donát Bánki’s theory regarding the design of cross-flow turbines and results of laboratory tests on such a turbine are extensively presented by Mockmore and Merryfield [1]. A design value of 16° is recommended for the absolute velocity angle at runner entry, 1. More recently, various studies, carried out using mostly CFD, were focused on studying the flow inside cross-flow turbines and on optimizing both nozzle and runner geometry [2–6]. Bánki’s theory showed and subsequent studies confirmed that most of the water energy – about 66% of it – is transferred to the turbine during the first pass of the water through the runner. In the classical design, the nozzle of a Bánki turbine is foreseen only with a flap that allows to adjust the discharge. When the flap is partly closed, a wall jet forms at the end of the nozzle. Such a jet acquires the nature of a boundary layer near the wall while behaving like a free jet at a larger distance from it [7]. Hence, the velocity distribution inside the jet is highly asymmetric with respect to jet axis. Moreover, when the flap is fully opened and the admission angle is large, it becomes difficult to control the value of the tangential velocity when the jet attacks the runner. Consequently, the absolute velocity angle shows significant variations around the design value, that range between 2° and 35° according to De Andrade et al. [3]. This strong deviation of the absolute velocity angle from the design value can be considered a major cause of the relatively low efficiency of the cross-flow turbines when compared with other turbine types. To alleviate this problem, Dragomirescu [8] proposed the usage of an array of stay vanes (or guide vanes) inside the nozzles of Bánki turbines and presented a method to properly design the stay vanes, so that the water jets formed at nozzle exit attack the runner at roughly constant angles of the absolute velocity. In this paper, the usage of a cascade of guide vanes inside the nozzle of a Bánki turbine is investigated by numerical simulations. First, the flow through an existing turbine with classical design is simulated. A mesh sensitivity analysis is performed and the numerical algorithms and methods used are validated by comparing the numerical results with data obtained experimentally. Second, the flow through the turbine modified so that the nozzle is foreseen with guide vanes is studied. Results obtained on the two geometries are compared in order to verify to what extent the new nozzle design could improve the turbine performance. II. TURBINE PARAMETERS AND EXPERIMENTAL INSTALLATION A sketch of the Bánki turbine investigated in this study is presented in Fig. 1. The runner has 19 blades, a breadth of 300 mm, and the outer and inner diameters of 250 mm and 165 mm respectively. The nozzle is horizontal and, in the case