Marine propulsion
“Rim-Driven” electrical thruster
CHALLENGE
In traditional marine propulsion systems, an engine is connected by a shaft line to a propeller immersed in water. At present, the increasing use of electric motors in the nautical sector has led to new configurations, called “rim-driven”, in which the rotor of the electric motor is mounted on a ring attached to the vane; the stator windings, on the other hand, are positioned on a coaxial nacelle.
The company had a well-tested methodology for the traditional propeller design: by means of an analytical method they performed a preliminary sizing of the propellers and then, afterwards, some modifications were made to the geometry following indications provided by CFD simulations.
The rim-driven thrusters designed with this method, however, were not characterised by very high performance, since the differences in construction compared to traditional thrusters required a change in the design methodology.
SOLUTION
A rim-driven thruster has much more affinity with an axial pump than with a propeller. We have therefore been able to put our expertise in axial pump modelling and simulation into play.
Having previously developed the MPROP software for axial pump simulation, we proposed a new design approach, the through-flow method, which is widely used in aeronautical turbine design. These are the advantages:
Since with MPROP each pump configuration can be tested in a few seconds, the optimizer has been able to explore different geometries until the optimal one is reached, in a total calculation time of less than 5 minutes. Note that testing a single configuration with the CFD typically takes several hours.
The MPROP software turned out to be the best trade-off between design speed and accuracy of results.
RESULTS
The optimised geometry of the engine calculated with MPROP produced significant improvements over the configuration initially developed by the company:
Before the prototype was made, these values were verified by a single CFD analysis, which confirmed the accuracy of the design calculation (differences in the order of a few percentage points).
The improvement in performance was further confirmed by the experimental tests carried out in collaboration with the University of Southampton.
The obtained results are a concrete example of technology transfer typical of our R&D philosophy: the experience gained in the field of industrial pumps, combined with the use of a method borrowed from the aeronautical sector, has allowed technological progress in the marine propulsion sector.
