EXPERIMENTAL INVESTIGATION ON THE FLUID-DYNAMIC LOSSES IN POWER GEARBOXES FOR AEROENGINE APPLICATIONS

Enhancing the efficiency of gearing systems is an important topic for the development of future aero-engines with low specific fuel consumption. The transmission system has indeed a direct impact on the engine overall efficiency by means of its weight contribution, internal power losses and lubrication requirements. Thus, an evaluation of its structure and performance is mandatory in order to optimize the design as well as maximize its efficiency. Gears are among the most efficient power transmission systems, whose efficiencies can exceed 99 %, nevertheless in high speed applications power losses are anything but negligible. All power dissipated through losses is converted into heat that must be dissipated by the lubrication system. More heat leads a larger cooling capacity, which results in more oil, larger heat exchangers which finally means more weight. Mechanical power losses are usually distinguished in two main categories: load-dependent and load-independent losses. The former are all those associated with the transmission of torque, while the latter are tied to the fluid-dynamics of the environment which surrounds the gears, namely windage, fluid trapping and squeezing between meshing gear teeth and inertial losses resulting by the impinging oil jets, usually adopted in high speed transmission for cooling and lubrication purposes. The relative magnitude of these phenomena is strongly dependent on the operative conditions of the transmission. While load-dependent losses are predominant at slow speeds and high torque conditions, load-independent mechanisms become prevailing in high speed applications, like in turbomachinery. Among fluid-dynamic losses, windage is extremely important and can dominate the other mechanisms. In this context, a new test rig was designed for investigating windage power losses resulting by a single spur gear rotating in a free oil environment. The test rig allows the gear to rotate at high speed within a box where pressure and temperature conditions can be set and monitored. An electric spindle, which drives the system, is connected to the gear through a high accuracy torque meter, equipped with a speedometer providing the rotating velocity. The test box is fitted with optical accesses in order to perform particle image velocimetry measurements for investigating the flow-field surrounding the rotating gear. The experiment has been computationally replicated, performing RANS simulations in the context of conventional eddy viscosity models. The numerical results were compared with experimental data in terms of resistant torque as well as PIV measurements, achieving a good agreement for all of the speed of rotations. Time resolved PIV revealed strong instabilities in the flow field generated by the gear, highlighting the importance of performing unsteady simulations for a better modelling of this component. Results have been post-processed in terms of Fast Furier Transform (FFT) and Proper Orthogonal Decomposition (POD) in order to provide a reliable data base for future unsteady simulations. In design phase it is important to predict the losses increase due to the lubricating oil jet impact on the spur gear varying the different geometrical and working parameters such as the jet inclination, distance and the oil mass flow rate and temperature. For this reason the test rig was equipped with an oil control unit able to provide a controlled oil mass flow rate to a spray-bar placed within the test chamber. The oil jet can be regulated in terms of pressure and temperature, in such a way the mass flow rate can be imposed and measured by means of flow-meters. The spray-bar is equipped with a circular hole, its position can be varied as well as the inclination angle. High speed visualizations were performed for every tested condition in order to deepen the physical understanding of the phenomena and to obtain more information on the lubrication and cooling capability. The high speed camera was placed in front of the gear exploiting an optical access while a halogen lamp was used to provide the proper lightening necessary due to the very low exposure time of the acquisitions. In every test the power losses were also measured using the torque-meter, results were post-processed in order to insulate the torque increase due only to jet injection. The collected data were used for the validation of a simple 0D model able to well predict power losses due to jet injection under certain conditions.