New Approach for Multi-Rotor Mixed-Flow Pump Design and Optimization

Counter-rotating blade rows in single stage machines have been widely investigated in several applications like naval propulsion [1], axial fan [2] and axial pump design [3]. Previous publications have presented this design philosophy as a promising way to improve efficiency and cavitation performance [4]. Differently the goal of the presented design is to achieve lower required rotational speeds and at the same time a higher power density ratio, indirectly improving also the cavitation’s performance, aspiring actually to a compact design. However, a literature survey did not produce any evidence of significant advantage in axial-flow machines from this design philosophy in terms of machine power density. This paper presents a preliminary analysis of the applicability of the counter-rotating impeller concept on mixed-flow and radial-flow pumps. The design example presented in this paper constitutes a mixed-flow first rotor with a radial second rotor that rotates in opposite direction. A 1-D design model demonstrates that power density coefficient has a maximum for optimum speeds of the rotors range. The design example parameters were selected based on the highest power density coefficient and efficiency. Unlike most published literature where the rotor speed ratios are not fixed, instead the speeds can be independently varied through two separate electric drives. Both the front and rear rotor were developed using multi-streamline curvature analysis which combines fluid dynamic loss model [5] with a slip model at the rotor exit [6]. Several hypotheses were considered to determine the most significant and independent parameters that run through a response surface optimizer tool, scripted in MATLAB^®. It detects optimum specific speed for the rotors which maximizes the benefits of counter-rotating impeller design compared to rotor-stator machine with same design point. Assuming steady numerical approximation error, the fitness function of the optimizer tool is based on steady state CFD results. The fitness function depends on total head and hydraulic efficiency, which show a maximum as a result. Optimum geometries were identified and one was chosen for manufacturing and testing in a test rig. Transient CFD analysis was also done to determine volute losses and incidence losses between first and second rotor and pressure pulsations generated. At this stage, manufacturing of hydraulic components of test model is in progress, while the mechanical design of the machine is close to completion.