Numerical investigations on improving the head curve stability of a radial pump by influencing the part load recirculation

Depending on the pump application, the head curve form can be critical for stable operation. In the case of systems with high static head component, the head curve must have negative slope for the whole range. Same applies for pumps installed in parallel. 1D theories cannot correctly describe 3D flow structures inside the pump at part load, where inlet and outlet recirculation are provoked. Inlet recirculation is mostly believed to be the reason for instability. This thesis deals with the assumption that the outlet recirculation is not of less significance for the head production. It is generally described in the literature sources, without being associated to any exact structure. The thesis is concentrated on investigating its structure before and after applying proposed modifications. Firstly, a model test rig was designed, consisting of two rotating disks with installed blades between them, simulating the shut-off conditions thereby. The model was investigated via LDV and CFD. Two recirculating vortices were found at the shrouds. Second step was concentrated on the full-scale pump. It was measured in a closed-loop test rig; a head curve instability was found. Transient CFD simulations had shown the best accordance to the experimental results. Two recirculating vortices were found in the area of the shrouds for the unmodified impeller. Diagonal trimming and slotting of the impeller were investigated, which brought stability to the head curve. Recirculating structures were found to obtain significant asymmetry. Some areas were found, where the angular momentum values exceeded the theoretically possible by using 1D approach. The fluid was found to gain additional circumferential velocity component by interacting with the blade surface, leading to shut-off head increase.