Experimental Investigation of a Counter-Rotating Mixed-Flow Single Stage Pump Under Cavitation Conditions

The increase in power per unit volume in modern pumps has been driven by manufacturing cost reduction. The first prototype of a new generation of centrifugal pumps has been experimentally and numerically investigated. It presents a particular and novel design characterized by the absence of any stator blade, which has been substituted with a counter-rotating radial impeller. According to an exhaustive literature survey, the usage of a mixed-flow impeller as a front rotor, followed by a radial-flow impeller seems to be a novel approach in pump design. The combination of a high specific speed impeller with a low specific speed counter-rotating diffuser produces flexible adaptability against varying working conditions. It also gives a rise to an increase of pressure coefficient values beyond limits of similar volute envelope. Applying the counter-rotating design principle to a radial pump increases power density, however cavitation-related issues remains a limiting factor. Counter-rotating design also features an additional degree of freedom to the system due to the independency of motor speeds of one another. This aspect improved cavitation inception characteristics especially at overload capacities. Moreover, such an arrangement required building a special test rig in order to accommodate for the double motor configuration. In this study, the NPSH3% -curve and the NPSHic cavitation inception characteristics have been measured. The system dependency on speed ratio variation has been also investigated along with the influence of the speed ration on the cavitation. Results of the cavitation inception visualizations were obtained using an endoscope at front rotor in order to analyze the behaviour of the pump under cavitation conditions. Test results showed two distinct speed ratios where maximum head and best cavitiaiton behavior were achieved. Additionally, results also confirmed that the cavitaion-free range can be optimized by using different speed ratios. A head drop-efficiency curve with variable speed ratios, which have been progressively adjusted for several flow capacities, is developed. This curve highlights the advantage of this new design compared to a conventional pump particularly under off-design conditions. It is clearly evident that delaying head deterioration, due to low inlet available suction energy, is solely attributed to the variable speed ratio of the runners.