Severe acoustic emissions and structural vibrations have been caused by insufficient suction oil supply in external gear pumps at elevated rotational speeds. In high-speed hydraulic applications, fluid inertia and inlet flow restrictions often lead to severe fluid starvation, massive cavitation, pressure pulsations, and consequently, intense hydrodynamic noise. Taking a centrifugal pump in series as a pre-pressurization stage, the inlet conditions of the gear pump have improved significantly to prevent unexpected acoustic behaviors. It is often required to clarify the precise flow-acoustic coupling mechanisms and the optimal matching criteria between the two pump stages. This study aims to systematically investigate the influences of a series-boost centrifugal pump on the noise characteristics of a primary gear pump. The testbed was initially designed and then constructed to measure the high-speed gear pump noise. Preliminary experimental investigations revealed that there was a highly non-linear relationship between the vibration and noise levels of the gear pump and the varying supply flow rates delivered by the auxiliary centrifugal pump. An acoustic-flow field coupling model was also established for the tandem centrifugal-gear pump. The numerical framework was developed to combine Computational Fluid Dynamics (CFD) with Lighthill’s acoustic analogy. A decoupling simulation analysis was performed on the four primary noise generation mechanisms: turbulence-induced noise, trapped oil (fluid confinement) noise, kinematic flow pulsation noise, and cavitation-induced noise. The experimental and numerical results indicate that the volumetric efficiency of the gear pump shared a two-stage evolutionary trend—initially increasing sharply and subsequently plateauing—as the supply flow rate increased from the centrifugal pump. Conversely, the overall vibration and noise levels demonstrated the parabolic trend, initially increasing before ultimately experiencing a significant reduction. The flow matching critical point was identified precisely after the dynamic process, when the supply flow rate of the centrifugal pump was equivalent to 1.2 times the theoretical displacement flow rate of the gear pump (defined as the flow ratio Q_C/Q_G = 1.2). Prior to the critical flow ratio, the volumetric efficiency ascended proportionally with the increase in supply flow. The maximum enhancement of 8.68% was achieved under high-speed conditions, compared with the baseline performance of a standalone gear pump. Interestingly, an upward trend was observed in the fundamental frequency sound pressure level at the pump's inlet and outlet during the initial stage. The acoustic emission was attributed to the initial reduction in the gas volume fraction within the fluid. As the aeration and cavitation bubbles dissolved, due to the pre-boost pressure, the effective bulk modulus of the hydraulic oil rose sharply, leading to a restoration of the fluid's inherent stiffness. Consequently, the transmission efficiency of the fluid was significantly enhanced as an acoustic propagation medium. Simultaneously, the acoustic damping and energy absorption were diminished under the two-phase bubble mixture, leading to the more intense sound waves. Once the supply flow rate surpassed the critical matching ratio (Q_C/Q_G> 1.2), the gear pump reached the complete fluid saturation state, and the volumetric efficiency remained constant. The overall sound pressure level also declined. The noise reduction was driven by the synergistic attenuation of cavitation intensity, flow pulsation rates, and trapped oil behavior. The source drastically reduced the acoustic output because the turbulent kinetic energy and the fluid stiffness no longer increased to exacerbate acoustic transmission. Quantitative acoustic source allocation revealed that the cavitation was the overwhelmingly dominant factor contributing to the overall noise reduction, accounting for 60.91% of the total decrease in the sound pressure level. In contrast, the contribution from the trapped oil pressure was fundamentally minimized to be negligible within the overall noise spectrum. In conclusion, there were complex nonlinear dynamics between pre-boost flow rates and acoustic emissions in tandem hydraulic architectures. The findings can provide a robust theoretical foundation and practical guidelines for engineering high-speed gear pumps. Simultaneously, exceptional volumetric efficiency and low-noise performance can be expected for industrial and aerospace fluid power applications.

High-speed hydraulic pump aims to comply with the inevitable electrification trend of hydraulic power units. Among them, the internal gear pump can be expected in agricultural and engineering machinery, due to its simple structure, high power density, low noise, and small flow pulsation. However, the ever-increasing wear of friction pairs has limited the high-speed operation. The microscopic degrees of freedom of the pinion gear and the ring gear cannot be constrained under the spline or flat key connection. The extremely complicated kinematic and dynamic behaviors of the gear pairs can often occur under the inertia force, oil film pressure, and medium temperature rise. This study aims to explore the wear mechanism and lubrication conditions of the friction pairs in the high-speed internal gear pump. A transient dynamic model of the internal gear pump was then established. The casing of a real internal gear pump cartridge kit was simplified as a circular ring, and the low and the high-pressure side plates were simplified as the side plates 1 and 2, respectively. Specifically, the side plates were used to limit the axial displacement, whereas, the radial displacement of the ring gear was limited by the circular ring. The inner spline surface of the pinion gear and the spline shaft, the pinion tooth surface, and the inner tooth surface of the ring gear were defined as the surface-to-surface contact. The rest parts were defined as the general contact for their unknown contact state. The clearance of each friction part was determined, according to the actual product. The implicit dynamic analysis was used to solve the final state between each component. The contact pressure was used to represent the contact state of each component. The contacts then occurred, when the contact pressure was not zero. Furthermore, an in-situ measurement was carried out, where three eddy current displacement sensors were arranged on the same axis to monitor the spokes of the ring gear, the ring gear teeth, and the pinion gear teeth. The micro-motion of the ring gear was clarified to combine the simulation and experiment, in which the axial micro-motion of the ring gear was verified by an in-situ measurement experiment. The results show that the ring gear concurrently shared the radial and axial micro-motion under the inertia force. The higher the rotational speed was, the greater the radial micro-motion of the ring gear was. There was also the random position of contact with the pump body. Moreover, the ring gear was biased to a certain side of the pump casing, and the contact frequency of the friction pair increased, although the axial micro-motion amplitude gradually decreased with the increasing speed. At the same time, the amplitude of the axial micromotion of the ring gear decreased by 27 μm, when the rotational speed increased from 1 006 to 3 002 r/min. But the frequency of contact with the mating surface increased by 1.46 times per second. There was a rebound behavior after contact and rebound again after contact between the end face of the ring gear and the mating surface of the pump casing. The different position of contact was then observed at the second time. The simulated and actual contact positions were highly compatible. The pair of gears wore significantly at the rotational speed of over 3 000 r/min. There was more significant wear on the end face of the ring gear in the flat key connection, indicating that the spline connection was more favorable at high rotational speed. No significant wear occurred on the pinion gear in either the flat key or spline connection, indicating that the axial micromotion of the pinion gear was less than that of the ring gear, with a maximum of 5 μm. Comprehensive tests and simulations show that the micro-motion behavior of the ring gear practically existed, where the rotational speed was one of the influencing factors. The friction vice clearance of the conventional rotational speed products cannot meet the high-speed operation. In addition, the influencing factors of the micro-motion of ring gear can greatly contribute to the anti-wear and friction reduction of the friction pair in the high-speed internal gear pump.