Low-head high specific speed Kaplan turbine for small hydropower – design, CFD loss analysis and basic, cavitation and runaway investigations: A case study

Abstract

In an era of growing energy demand, it is necessary to reach for lower water potentials to be used by a hydro unit. Due to low damming, a hydro unit must be designed to allow the flow of as much water as possible with maximum efficiency. This ensures optimal use of hydropower and minimization of turbine size. Therefore, this paper presents the construction of a model Kaplan turbine (runner diameter of 300 mm) with a high specific speed, designed to work under ultra-low and low heads up to 3 m.Adjustable guide vanes and runner blades were designed by means of an inverse problem method using a two-dimensional axisymmetric model. In the case of the adjustable guide vane, two shapes have been designed. The first has a classical shape and the second has a non-classical shape. The influence of both guide vanes on the turbine operation was analysed using the CFD technique. As a result, the turbine with a non-classical shape blade has a higher hydraulic efficiency and the efficiency curve is flatter. As part of these calculations, many variants of turbine settings were analysed (192 cases), namely 4 angles of classical guide vanes, 4 angles of non-classical guide vanes, and 4 angles of runner as well as 2 angles of a conical draft tube and 3 rotational speeds. The energy losses occurring in the flow elements of the turbine and their mutual contribution are presented in this paper.Model experimental tests of the turbine were carried out on the test stand with a wide range of settings of the guide vanes (the tests used a guide vane with a non-classical shape), the runner and its rotational speed. The research is aimed at determining the basic parameters of efficiency, the effect of cavitation and the runaway speed. Cavitation and runaway characteristics as well as the results of the resistance torque (friction losses) and its share in the total moment are presented.As a result of design, analytical, and experimental works the low-head high specific speed model Kaplan turbine was obtained and diagnosed. The realization of large flows, combined with the high efficiency of the machine is always a difficult task. The paper briefly describes the shapes of the designed guide vanes and runner blade obtained by the inverse problem method. However, more emphasis has been focused on the presentation of model tests that are rare for this type of machine. Therefore, the measurement uncertainty, measuring devices and the course of the conducted basic performance, cavitation and runaway tests are described in detail. Additionally, for the selected cases, a visualization of the flow with forced cavitation in the runner during cavitation tests is also presented.