Numerical investigation and optimisation of a new Pelton turbine distributor system

Abstract

Pelton turbines rely on the distributor system and the injectors to feed high-qualitywater jets to the runner to operate at high efficiencies. The jet formation is highly influenced by the upstream flow history, where secondary flows are mainly created in the distributor system. A good distributor system allows for low energy losses and limited secondary flow. Previously, it was proven that for conventional distributor systems of six-jet Pelton turbines, the secondary flows upstream of the injectors, and thus, the jet quality differs significantly, leading to notable variations in turbine efficiencies. Following these observations, this thesis asks how Pelton turbine distributor systems must be designed to allow for similar flow conditions in all branches and injectors and maintain a high-quality water jet at all relevant operating conditions. A yet unexplored solution could be distributor systems with axial inflow. Hence, this thesis describes the flow phenomena in such distributor systems with axial inflow (= AxFeeder) and derives designs favourable for high-quality flow. Four core designs of the AxFeeder, the diffuser manifold (basic model), the diffuser manifold with conical frustum, the spherical manifold and the cylindrical manifold, differing mainly in the shape of the manifold and the positioning of the branch lines, were drafted. Operating parameters representative of future potential applications of the AxFeeder in small hydropower plants were derived, the operating regime was simulated, and a realistic design point for the parametric investigation was selected. In the parametric investigation, the effects of changes in the manifold and branch line geometries on the flow quality criteria, namely the power losses, the dissipation power coefficient and the secondary velocity ratio, were analysed.The parametric investigation detected those areas of the AxFeeder, where the flow reacted most sensitively to a change. These were the size of the manifold, the transition zone from the manifold head to the branch line and the injector bend. A conical frustum connecting the manifold and the branch line, combined with a steep deviation angle of the branch line, results in low losses in the distributor system and limited secondary flow upstream of the injector position. The secondary flow at this station showed a distinct reverse S-shape pattern, which appeared to be characteristic of many AxFeeder configurations. All concepts were compared by a compound quality coefficient that equally weighs losses and secondary velocities. Only a few combinations of geometrical parameters allowed for minimal losses and secondary flows simultaneously. With a reduction of the compound quality coefficient of 44 %, the best results were achieved by the diffuser-shaped manifold with a conical frustum and a converging branch line of a deviation angle of 90°.