Numerical Investigation of Fluid-Dynamic Losses in Power Gearboxes For Aero-Engine Applications

The Geared Turbofan (GTF) technology is one of the most promising engine configurations to sensibly reduce the specific fuel consumption (SFC) by increasing the engine bypass ratio. In this architecture, a power epicyclic gearbox is interposed between the fan and the low-pressure spool, resulting in advantages both at engine and component level. The SFC is directly affected by the transmission efficiency of the gearbox and indirectly by the weight and the size of the cooling system. Therefore the gearbox performance becomes a key technology to achieve the benefits introduced by the GTF architecture. Although the transmission efficiency is usually higher than 99 %, power losses are important in high power application such as the one under consideration. A performance enhancement can be achieved developing a physical understanding of the losses within the transmission system. These are classified into load-dependent and load-independent groups. The former are primarily related to a mechanical power loss due to friction at the gear contact, while the latter are related to fluid-dynamic effects. Whilst there has been a large body of work dealing with load-dependent power losses, and suitable models for the prediction are already available, the fluid-dynamic losses still need to be studied and properly modelled. This research study is aimed at defining CFD methodologies to be used in the comprehension of fluid-dynamic losses in gearbox systems for aero-engine applications, to develop predictive tools to be used in industrial design process. A complete simulation of the multiphase flow within the epicyclic gear train would be too expensive and not useful in the understanding of the various loss mechanisms, so that each one has to be studied individually. Therefore the power losses related to the windage phenomena and to the oil-jet lubrication method were investigated. The windage losses of a spur gear in free and enclosed configuration have been analysed by means of RANS simulations, defining a reliable numerical setup. The results have been validated by comparison with experimental data. A greater insight into the phenomenon was achieved, which allowed modifying and improving a correlative approach available in literature for calculating the windage power loss, by introducing the effect of the fluid volume enclosed in the gearbox. Concerning the losses due to the oil-jet lubrication, numerical studies were performed using the Volume of Fluid (VOF) method, within the context of transient RANS calculations. These simulations were aimed at improving the description of the complex physical phenomena characterizing the oil-jet lubrication in high speed gearing systems. However VOF approach requires a very fine mesh in the liquid region, leading to considerable computational efforts. In this respect, a strategy for automatic grid adaptation was developed, reducing heavily the numerical effort. VOF simulations were performed varying the oil injection angle and the injection velocity, to assess how these parameters affect the power losses as well as the lubrication performance. As main results, a simplified formulation for the prediction of the power losses due to the oil-jet lubrication has been developed, and then an optimum configuration for such a system was defined. The presented VOF study is one of the first in literature which provides detailed insight on the physic of the oil-jet lubrication. Finally, the methodologies developed in this work have been partially validated by comparisons with preliminary experimental measurements performed on the High-Speed Test Rig, set up at the University of Florence, which was designed with input from the present research work.