CFD simulation of power losses and lubricant flows in gearboxes

Energy efficiency represents one of the most relevant drivers in many application fields, and power transmission and gears play a critical and fundamental role in many of them, contributing to improve the overall efficiency and to reduce energy consumption and emissions. In the design phase, the availability of tools able to reliably anticipate the losses of a gearbox can provide a relevant contribution to the aim of improving efficiency. The power losses of gearboxes are the sum of different contributions. For some of them, the available analytical models can provide acceptable information but they are not suitable for an accurate prediction of the load-independent losses that are generated by the interaction of the gears with the lubricant. Computer simulation, in particular the application of Computational Fluid Dynamics (CFD), represents an approach to overcome the problem of predicting load-independent power losses accurately. Moreover, it can provide a description of the lubricant flows inside the gearbox, describing the oil supply to the critical components of the transmission, thus assessing the effectiveness of the lubrication system with respect to reliability. Nevertheless, the application of CFD to gears is challenging due to the geometrical properties of the volume domain to be studied and its variation during the meshing cycle, with the consequent complexity of the volume mesh handling. Many approaches have been proposed to apply CFD to gearboxes, and they differ in the accuracy of the results and of the simulation time. The paper proposed by the authors, on the basis of a preliminary review of the different approaches, describes the application of CFD to gearboxes based on an original global-remeshing technique, which enables accurate predictions in relatively short simulation times, compatible with the industrial design practice. The method is validated with data obtained by means of experimental tests, both on laboratory back-toback test rigs and on industrial gearboxes, including planetary gearboxes, for which the load-independent power losses are particularly relevant. The results of the CFD simulation are in very good accordance with the tests, both for the amount of the losses and for the flow distribution, and they also provide a tool to understand the origin of the losses, including the effects of churning, windage, pocketing, and cavitation. The results of the practical application used for the validation are included and discussed in the paper.