Blade Interaction Forces in a Mixed-Flow Pump With Vaned Diffuser

Hydrodynamic forces in turbomachinery can cause problems ranging from excessive wear to vibrations. Of the many different causes of fluid-induced forces, their source attributed to blade interaction is perhaps best studied. Most studies, however, focus on high head pumps. This paper presents a study of blade interaction forces in a mixed-flow pump with vaned diffuser. It covers both experimental and numerical results. A mixed-flow pump is built into a closed-loop test rig. In order to measure the instantaneous forces and bending moments on the impeller, a co-rotating dynamometer is used, which is built in-between the impeller and the shaft of the pump. Extensive calibration proved necessary to determine the dynamic behavior of the shaft system. Measurements of forces at blade passing frequency show an unexpected dependency on flow rate. Another important observation is that blade excitation forces cause the impeller to whirl in a direction opposite to shaft rotation. Results of measurements are compared with unsteady CFD calculations using FLUENT®. Computed global characteristics show a good agreement with measurements. Also the magnitude of blade interaction forces shows a good qualitative agreement. Over a large range of flow rates, the dependency of magnitude on flow rate agrees well with the trend found in the measurements. Deviations are explained.