Analysis of noise characteristics of high-speed gear pumps in series-boost centrifugal pumps

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.