On energy conversion mechanism of a marine submerged waterjet

The submerged waterjet (SWJ) is a novel marine propulsor for surface ships, which shows remarkable energy performances in a mixed-use profile. Less research has been done to reveal the mechanisms of its distinguished energy and propulsion performances. Based on energy flux balance theory, this paper proposes a method on the analysis of energy conversion process of the SWJ system, where a link is established between propulsion performance and the quantitative evolution of energy flux components occurring both in the internal and external flows through a control volume system. The detailed investigation in this paper indicates that the various interactions, among rotor, stator and duct, exhibit different impacts on enhancing the overall energy performance of the SWJ system. The duct majorly changes the working environment of the rotor from the external flow operation to the internal flow ones. After adding a duct to the rotor, the shaft power increases sharply, due to the overall increased pressure distribution in the rotor domain. Afterward, the SWJ system can experience stronger conversion of pressure work into axial kinetic energy flux, which enables the propulsor to have greater potential to generate larger thrust. As for the post-stator, it improves open water efficiency in the SWJ system dominantly through the enhancement of system hydraulic efficiency. In detail, the positive impact of the post-stator is mainly achieved by increasing the proportion of the pressure work and promoting the conversion of transversal kinetic energy flux into axial ones in the pump inner domain. For the flows around the downstream of the duct, the post-stator also plays a significant role in improving the hub vortex as well as enhancing the stability of the rotor tip leakage vortex. A more stable flow field is one of the important prerequisites for improving energy efficiency as well as reducing vibration and noise of the SWJ system. Another important impact of the post-stator is to enhance the performance through generating additional thrust by its guide vanes as well as reduce the drag of the duct and stator hub at the same time.