Mechanism and Regularity of Wet Modes in a Highly Integrated Marine Magnetic Levitation Pump Rotor Under Confined Water Conditions

Designed to mitigate the significant low-frequency vibration and noise inherent in conventional marine centrifugal pump systems, the magnetic levitation pump constitutes a novel form of centrifugal pump employing active magnetic bearing technology. While this fully levitated design effectively enhances vibration and noise performance, it results in the complete immersion of the rotor within a confined fluid domain, which contains narrow fluid clearances. This poses significant challenges for the accurate computation of rotor wet modes, which is crucial for the structural design of the rotor system to avoid the resonance induced by flow. Despite exerting a substantially greater influence on rotor wet modal characteristics than unconfined domains, the analysis of rotors under confined fluid conditions has received comparatively little research attention. This study focuses on two types of magnetic levitation pump rotors. From the perspective of analytical modeling, an improved analytical method for wet modal computation based on added mass correction is proposed. The validation of this method included examining two distinct computational approaches for the added mass, the thickening treatment for axially elongated disk components, and the methodology for implementing disk equivalent density. Based on this foundation, wet modal analysis was performed on both rotors utilizing the proposed analytical method, alongside acoustic fluid–structure interaction simulations. The results indicate that for the first bending mode, the errors between the analytical and experimental values are 1.2% and 4.1%, respectively, while the discrepancies between the simulated and experimental values are 0.1% and 3.2%. Finally, regularity analysis was conducted on the wet modal characteristics of the rotor under confined water, considering various fluid clearances. The results reveal that the first three bending modes generally exhibit an increasing trend with the enlargement of the fluid clearance, with a triple-size annulus serving as a transition point. However, increasing the annulus size does not always elevate the modal frequencies above their initial values. This study contributes to understanding the influence mechanisms of confined water on the wet modal properties of magnetic levitation pump rotors. Furthermore, the proposed analytical method improved computational efficiency for the early design stages of water-immersed rotors, alongside a model of greater accuracy essential for magnetic bearing control.