# Research on Performance Prediction Model of Impeller-Type Breather
## 1 Introduction  
When an aircraft engine lubrication system starts operating, part of the sealed air will enter the bearing chamber and mix with the lubricating oil forming an oil-gas mixture. Direct discharge of the oil-gas mixture from the bearing chamber will cause loss of lubricating oil. Since the amount of the lubricating oil is certain, the loss of the lubricating oil will affect the mechanical properties of the engine [1]. Therefore, installation of a breather in the air passage of the bearing chamber will separate the oil-gas mixture effectively. Breathers come in various forms. Typical breathers include centrifugal breathers and impeller-type breathers; both types share the same operating principle. When the engine is operating, the oil carried by the gas enters the breather. The spinning rotors create a centrifugal force to the oil-gas mixture, thus forcing the oil droplets to the inner wall of the shell and flowing back to the transmission chamberunder the dynamic pressure. The separated air is exhausted through the hole in the shaft due to the pressure difference [1]. The two-phase flow driven by the blades of the breather includes swirling and strong turbulence. The characteristics of the flow fields directly affect the performance of the breather; therefore, scholars have carried out relevant experiments and theoretical research on it. In 2006, [PERSON] et al. [2] used the method of numerical simulation to study the two-phase flow field in the oil-gas separator. The trajectory of oil droplets in the turbulent flow field was simulated by the stochastic trajectory model. The results show that oil droplet impingement has a great influence on separation characteristics. In 2008, using the method of theoretical analysis, [PERSON] [3] proposed a mathematical model for the optimal design of rotor structure parameters of a centrifugal breather, and obtained the effects of different rotor structure parameters on separation efficiency of centrifugal breathers. Also, in 2008, [PERSON] et al. [4] conducted a systematic study of the performance of breathers equipped with metal foams and metal matrix and obtained the quantitative effects of the relevant parameters affecting the separation efficiency and pressure loss, such as a variation of airflow, oil flow, shaft speed, and droplet size. In 2010, [PERSON] et al. [5] used a numerical simulation method to study the oil-gas two-phase flow field of a cyclone separator, obtained the distribution characteristics of vortices, velocity fields, and pressure fields in a cyclone separator, and provided reference suggestions for separator design and optimization. In 2010, [PERSON] et al. [6] carried out numerical research on the two-phase flow field of an impeller breather based on the Euler-Lagrange model. Disregarding the aggregation and fragmentation of oil droplets in the process of oil droplet movement, the calculated results are in good agreement with the experimental data, which shows that the Euler-Lagrange model can simulate the two-phase flow field of oil and gas in an impeller breather well. In 2011, [PERSON] et al. [7] described the thickness of liquid film in the cavity of centrifugal oil/gas separation and pumping using the Bernoulli equation and the mass conservation equation. The influencing factors and variation of the gas containing rate at the oil outlet and the oil containing rate at the gas outlet were explained indirectly. The study also pointed out that the backpressure of the oil outlet and rotor speed have great influence on the oil/gas separation. In 2011, [PERSON] et al. [8] used the method of coupling the RSM (Reynolds Stress Model) model with the DPM model to study the two-phase flow field inside the axial breather and analyzed the influence of turbulence fluctuation on the separation characteristics of each section of the axial breather. It was concluded that turbulent fluctuation can effectively improve the separation characteristics of the axial breather, and the most obvious role was in the ventilation duct.