Migration and Evolution of Metallic Particles in an Intermediate Reducer under Splash Lubrication Conditions

To systematically clarify the migration and evolution mechanisms of metallic particles in the solid-liquid-gas three-phase flow field of a helicopter intermediate reducer under splash lubrication, this study innovatively develops a three-phase numerical model based on the CFD-DPM framework, and establishes a high-precision oil-churning visualization test rig for experimental validation via high-speed imaging technology. Under the baseline operating condition (gear rotational speed of 1500 r/min, dimensionless oil immersion depth λₕ=6.78), the effects of multiple hydrodynamic forces (gravity, pressure gradient force, virtual mass force, Magnus lift, and Saffman lift) on particle trajectories and dynamic responses are comprehensively evaluated. Meanwhile, comparative analyses are conducted to quantify the regulatory effects of particle density (intrinsic property variation) and initial release location (meshing region, spalling region, sidewall/bottom) on migration behavior. Numerical results reveal that the pressure gradient force and virtual mass force dominate macroscopic particle transport: the former prolongs the quasi-steady stage by 40%–50%, while the latter accelerates particle velocity decay to near-zero within 0.3–0.5 s. In contrast, the Saffman lift and Magnus lift play secondary roles, mainly inducing high-frequency velocity fluctuations (velocity perturbation amplitude ≤15% for Saffman lift) and exhibiting strong medium dependence (Magnus lift doubles particle migration distance in the gas phase but is negligible in the oil phase). Increasing particle inertia significantly weakens flow-following capability, and the particle velocity response intensity follows the order: meshing region > spalling region > sidewall/bottom. Visualization experiments verify a complete particle transport cycle inside the gearbox (extrusion from the meshing zone-airborne flight-wall collision-gravity-driven return), which is in good agreement with numerical predictions. Further experimental results show that increasing the rotational speed from 100 to 400 r/min significantly enhances oil splash height and particle adhesion on the top and side walls. At a high immersion depth (λₕ=11.86), intense turbulent splashing leads to global dispersion of the oil-particle mixture; at a low immersion depth (λₕ=1.70), particle motion is dominated by local entrainment with weak wall retention. Additionally, as the particle volume fraction increases from 0.1% to 1.0%, the oil-particle mixture exhibits a pronounced thickening effect, suppressing oil splashing and enhancing particle deposition. These findings provide robust theoretical support and technical guidance for the optimization of splash lubrication design and reliability assessment of helicopter transmission systems.