Airfoils Clashing Technology for Turbofan LPT Overspeed Prevention

The thesis was developed at Avio Aero with the aim of analyzing the airfoil clashing technology for turbofan LPT overspeed prevention. The overspeed is a condition when an engine is allowed to exceed its design limit. If out of control, it can cause a hazardous event. For this reason, EASA and FAA impose requirements to guarantee the engine safety in all working conditions. Overspeed can be related to a shaft failure. In this situation, the broken part, being no longer slowed by the fan, continues to accelerate under the action of hot gases. According to the architecture of the engine, which affects the possibility of the damaged part to translate or not, are provided different overspeed detection and control systems. A solution is clashing, in which the rotor blades airfoils are destroyed to interrupt the acceleration process. By eliminating the elements responsible for converting gas energy into force, the broken shaft is slowing down, preventing the discs from reaching the overspeed condition. During the translation some surfaces come into contact, before the rotor and stator airfoils impact; during contact the turbine continues to accelerate. These contacts are called "false bearing". The material in contact heats up by friction and is melted. The time that allows the airfoils to come into contact, also having to eliminate the false bearings, must be less than the time that can cause the burst of rotor disks (considering appropriate safety margins). The output process, based on energy balances, results in the calculation of the time between the shaft breakage and the airfoils impact and the rotational speed of the rotor at the moment of impact. The study presented is part of the preliminary design, in which there is limited information, tight development times and continuously optimized geometries. It is preferred to use hand-calc approaches to dynamic finite element analyzes which would require long computation times. The thesis can be divided into: - patent overview of overspeed detection and control systems - study and review of the approach currently used in Avio Aero - simplified model for impact analysis - sensitivity on the effect of geometric tolerances. The results obtained show that the total impact time is a minimal percentage of the entire process. The tolerances, on the other hand, have a significant effect on the percentage variation of the RPM. As future research is proposed a further investigation of the impact based on FEM 3D analysis, as well as an in-depth study on the shapes and geometries of the elements involved in clashing. Regarding tolerances is proposed the integration of process capability data for evaluations on a real engine configuration.