Simulation of three-demensional unsteady flow in hydraulic pumps

The design of hydraulic turbomachines has reached the stage were improvements can only be achieved through a detailed understanding of the internal flow. The prediction of the flow in such equipment is very complicated due to the rotation and the curved three-dimensional shape of the impellers. Furthermore, the flow in turbomachines shows unsteady behaviour, especially at off-design conditions, as a result of interaction between impeller and pump casing. Considering these complexities, computer simulations will become increasingly important. In this thesis it is shown that the flow in hydraulic pumps of the radial and mixedflow type, operating at conditions not too far from design point, can be considered as an incompressible potential flow, where the influence of viscosity is restricted to thin boundary layers, wakes and mixing areas. A three-dimensional method for unsteady flow based on this model yields good results. In order to predict the efficiency of pumps, additional models to quantify the viscous losses can be employed successfully. Thus reads the overall conclusion which can be drawn from the investigation presented in this thesis. The numerical method developed for solving unsteady potential flow is based on a fully three-dimensional finite-element method. The computational mesh is divided into two parts, one for the rotor and one for the pump casing, and connected by a sliding interface. In this way the impeller rotating motion with respect to the pump casing can be simulated efficiently. Some special numerical techniques are employed in order to reduce computing time. These are based on the substructuring method combined with the implicit imposition of the Kutta conditions at the trailing edges of the impeller and diffuser blades. The losses which occur in pumps are quantified using additional models for energy dissipation in boundary layers, in mixing areas and at sudden expansions and contractions in through-flow area, as well as models for disc friction and leakage flow. Based on the velocity distribution along the rotating and stationary surfaces, as obtained from a three-dimensional potential flow computation, the state of boundary layers is determined using a one-dimensional boundary layer method. The capability of the method is demonstrated by analyses of the flow in several laboratory and industrial pumps. For these pumps, information on the velocity and pressure distribution as well as the performance characteristics are available from experiments. In general, computational results show a good agreement with experimentally obtained values. This, and the short computing times required, make the proposed method very well suited as an analysis-tool within a pump development process