EXPERIMENTAL AND NUMERICAL INVESTIGATION OF A COUNTER-ROTATING MIXED-FLOW SINGLE STAGE PUMP

摘要

A prototype of a new generation of centrifugal pumps has been developed, with aim to improve typical weaknesses like narrow operable range due to slip and incidence losses in off-design conditions. The peculiarity of this pump is 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 rotating diffuser produces a flexible adaptability against aforementioned limits. Keep on adding energy to the fluid instead of just diffusing the flow, permits to reach a downsized hydraulics and an increased entire machine power density. The characteristic of such a pump needs to be analyzed as 3D-surface, both speeds are actually independent and for a fixed discharge the head rise and efficiency become surfaces. A new definition of optimized characteristic curve, with variable speed ratio, could be identified based on those performance maps. A special test rig has been built to measure the machine performance, distinguishing the mechanical losses of both shafts separately with no-load measurements. The sensitivity of the system on speed ratio variation has been explored. The experiments show the presence of different best speed ratios which maximize alternately the head or the efficiency. Additionally, the results confirm the possibility to expand the working range acting on the rotational speeds. Head rise and efficiency curves with varying speed ratio, as functions of the flow rate are finally shown using several diagrams, highlighting the advantages of this new design.