Design of the hydraulics for a Kaplan residual water turbine

A mid-sized company commissioned the development of the hydraulics for a Kaplan residual water turbine. Due to the changes in the European Water Framework Directive, the demand for turbines for this kind of applications is increasing. As part of a design study, the nominal data and the main data were determined: maximum flow Q_max = 1.5m³/s, minimum flow Q_min = 0.8m³/s and specific speed n_q = 300rpm. The framework conditions were determined by the following parameters: diameter D = 0.65m and a 7-pole generator.

Based on the design for a meridian section a completely new design has been developed. As regulatory concept of the residual water turbine a semi-regulated version with an adjustment of the blade at constant vane position was designed. However, the vanes of the residual water turbine had to be designed in such a way that the guide vane in case of clogging by floating debris can be manually “turned over” the regular blade pitch.

As of the semi-regulation of the Kaplan turbine it is particularly important to develop a blade profile whose leading edge is, to a large extent, resistant to an inflow from the wrong direction. Two different concepts have been applied – the dimensioning of the profile according to Scholz-Czibere and the design of a blade from the NACA 65 series profiles which is popular in water turbine applications although it is normally a standardized profile used for airfoils.

In addition to the efficiency optimisation for every single span a variety of profile thicknesses were simulated as well as various leading edge radii. As outer contour a hemispherical as well as a spherical design has been analysed.

The sigma values reached are far below the theoretically calculated values and illustrate good cavitation behaviour of the blade. Choosing a fixed guide vane position for the hydraulics being operated as semi-regulated residual water turbine has an enormous impact on the efficiency curve shape achieved. The decision whether or not to select a constant guide vane position has to be taken based on the actual flow range. Figure 3 shows the steady flow after the runner by means of an analysis of the meridional flow rate cm at various guide vane positions.