Theoretical And Experimental Study On The Flow And Thermal Effects Of Oil Air Two-phase In A Bearing Chamber

Bearing chamber is the compartment to contain support bearings of main shaft in an aero engine. The important role of bearing chamber is collecting lubrication oil, separating the bearing from the hot zones and dirt of engine. As the interface of secondary air and oil systems, a complex air/oil two-phase flow and heat transfer pattern, which involve oil droplets, sealing air and oil film, appear inside the bearing chamber. That gives a lot of difficulty and challenge to lubrication and heat transfer designs of bearing chamber. The lubrication and heat transfer designs of bearing chamber largely depend on the correct understanding and accurate prediction on the complex air/oil two-phase flow and heat transfer pattern. The design trends of modern aero engine, high shaft speed, operating temperature and reduction of weight and dimension, have more strict demands on the design of transmission and oil system, which consists of bearing chamber. However, due to various factors, the research level about the air/oil two-phase flow in bearing chamber has lagged behind the engineering application. Therefore, in present paper, special efforts are pointed toward the fundamental research on air/oil two-phase flow in bearing chamber, specially the characteristics of oil droplet generation, motion and deposition, as well as oil film flow. The work is summarized in detail.(1) According to the remarkable similarities of droplets generation and motion process between bearing chamber and rotary atomizer, the overall structure of rotational disk chamber test facility is designed. The partial system, components and parts are built and selected subsequently. The measurement techniques about oil ligament number, oil droplet trajectory and velocity, as well as width of oil film are determined. The relative experiments are carried out, and that provides foundation for the following analysis.(2) To overcome the weakness of experimental approach, the theoretical analysis about oil ligament and oil droplet shed from rotational disk are also carried out. Firstly, the centrifugal force on the oil film flowing on rotational disk is determined. The mechanical equilibrium equation is proposed based on the behavior of Newtonian fluid. The oil film thickness and velocity are calculated. Secondly, the instability and disintegration of oil film are discussed. By means of force analysis, the equation to predict critical state of disintegration is deduced. The ligament number generation by disintegration of film and Sauter mean diameter of oil droplet break by ligament are obtained. Lastly, the correctness of theoretical analysis is validated by the experimental results collected on the rotational disk chamber test facility.(3) Providing input parameters for oil droplet motion and thermal analysis, the numerical simulation and similarity analysis of air flow in bearing chamber are performed. Firstly, the continuity, momentum, energy and turbulence equations of air flow and heat transfer in bearing chamber are listed. The computational domain is built and discretized with the structured grid comprising hexahedral cells, as well as the boundary conditions are determined. The air flow pressure, velocity and temperature are solved by above numerical model. Secondly, the similarity criteria of air flow in bearing chamber are derived based on dimensional analysis. The airflow distribution general formulas are proposed based on the numerical results of airflow velocity and temperature. The general formulas include various similarity criteria. The errors of general formulas are shown by the others’ experimental results.(4) Based on the above work, the motion and thermal states of oil droplet in bearing chamber are analyzed, as well as the energy transfer between droplets and air. Firstly, with the heat transfer of droplets and air, the temperature equation of oil droplet is inserted to its velocity equation. The differential form of above equations are described by a difference method. The velocity and temperature of droplet are obtained using a step-by-step method. Secondly, the probability density function of oil droplet diameter is determined. The volume and mass fraction of oil droplets, and their energy transfer with the air flow in bearing chamber are calculated. The reliability of calculation about oil droplet trajectory, velocity and volume fraction is validated by some experimental results.(5) The characteristics of oil droplets impact on chamber housing are also investigated. Firstly, the transition criteria for various outcomes of droplet/wall interaction is summarized. The mass, momentum and heat energy fractions of oil droplet deposits on housing are determined under different outcomes. Secondly, the calculation model of whole oil droplets impact on housing is built considering diameter distribution of droplets. The total deposition mass, momentum and heat energy rates are obtained. Thirdly, the thickness, average velocity and temperature of oil film on bearing chamber housing are calculated based on the condition of force balance and mass conservation.(6) The treatment that oil droplets/air mixture in bearing chamber is equivalent to homogeneous phase is proposed. To describe the oil/air two-phase flow in bearing chamber, the numerical model of homogeneous phase/oil film stratified flow is built. Therefore, the velocity, pressure and temperature of oil/air two-phase flow could be determined. A test facility to investigate into the oil/air two-phase flow in bearing chamber is designed. As following, the temperature and pressure of homogeneous phase and oil film are measured. Lastly, the rationality of numerical model is supported by the experimental results.The work in present paper makes up some limitations of previous study. For example, the theoretical approach to investigate into oil droplets generating from bearing is strengthened. Dimensional analysis is applied to airflow analysis of bearing chamber. That makes the analysis model is suitable for a wide range of geometry and operating conditions. In addition, it is convenient to link the analysis model with the other analysis model. Moreover, the temperature solution is added to the oil droplet motion analysis. That could promote the calculation accuracy of droplet trajectory, velocity and temperature. The work in present paper may have certain significance for perfecting research system and improving research level of air/oil two-phase flow and heat transfer in an aero engine bearing chamber.