In highly loaded axial flow pumps considerable changes of the flow structures are known when lowering the flow rate from design point operation to part load operation. After the stall inception the backward tip flow increases in axial and radial direction upstream of the impeller forming the part load “vortex B”.
The backflow has a strong co-swirl, which is transmitted to the inlet flow. This leads to less Euler work and often to a drop of pump head. To overcome these negative effects of the stall, a so called double inlet nozzle can be used. At the University of Braunschweig this device is investigated experimentally. In this contribution the time dependent three dimensional flow fields have been numerically investigated by unsteady Reynolds averaged Navier-Stokes simulations.
The geometry of the test stand, the geometry of the numerical model and the used meshes are described. The experimental results and the numerical obtained results are compared to each other. First the measured overall performance characteristic is compared to the numerical results obtained by averaging the head and the efficiency over the last computed impeller revolution. In addition the numerical obtained flow field upstream of the inlet nozzle is compared to flow fields measured in Braunschweig by PIV method. Also the flow field is shown in regions of the pump which are not accessible by experimental methods.
Based on these results the shape of the double inlet nozzle is varied in different ways. The influence of the modifications on the flow field and the part load behavior of the pump is displayed and discussed.