Turbine thermomechanical modelling during excessive axial movement and overspeed

Abstract

This manuscript discusses the numerical (finite element) and analytical modelling of structural interactions between gas turbine components in case of excessive axial movement and overspeed. Excessive axial movement, which may occur after a shaft failure, results in contact between rotating and static turbine components under high forces. These forces create friction which can act as a counter torque, potentially retarding the ‘free-rotating’ components. The study is based on a shaft failure scenario of a ‘three-shaft’, high ‘bypass’ ratio, civil ‘large-fan’ engine. Coupled analytical performance and friction methods are used as stand-alone tools to investigate the effect of rubbing between rotating and stationary components. The method is supported by ‘high-fidelity’, ‘three-dimensional’, thermomechanical finite element simulations using LS-DYNA software. The novelty of the work reported herein lies in the development of a generalised modelling approach that can produce useful engine design guidelines to minimise the terminal speed of a free running turbine after an unlocated shaft failure. The study demonstrates the advantage of using a fast analytical formulation in a design space exploration, after verifying the analytical model against finite element simulation results. The radius and the area of a stationary seal platform in the turbine assembly are changed systematically and the design space is explored in terms of turbine acceleration, turbine dislocation rate and stationary component mass. The radius of the friction interface increases due to the increasing radius of the nozzle guide vane flow path and stationary seal platform. This increases the frictional torque generated at the interface. It was found that if the axial dislocation rate of the free running turbine wheel is high, the resulting friction torque becomes more effective as an overspeed prevention mechanism. Reduced contact area results in a higher axial dislocation rate and this condition leads to a design compromise between available friction capacity, during shaft failure contact and seal platform structural integrity.

Study of Starting Performance of a Series Hybrid Aero Propulsion System

“…Specifically, on one hand, the low moment of inertia of high-speed turbine/generator rotors may pose a risk of overspeed runaway, particularly during starting with small loads and light torque. The introduction of protection strategies to prevent turbine blade runaway such as increased friction [21], fuel correction [22], and other measures [23] is crucial in such scenarios. On the other hand, the redundancy assessment conducted by Loder et al [24], the power flow calculation by Lei et al [25], and the endogenous failure analysis by Balaji et al [26] mainly focus on the steady-state of the high-voltage DC microgrid in series hybrid aero propulsion systems.…”

Multi-blade shedding in turbines with different casing and blade tip architectures

“…The intention in overspeed protection is to release all the blades at the same time which minimizes the occurrence of an unbalanced motion.This section discusses the application of a simultaneous blade release for overspeed prevention by use of an overspeed trajectory of a turbine after a shaft failure. The overspeed analysis is based on the methodology described by Eryilmaz and Pachidis[27]. Turbine performance parameters, such as the total pressure evolution in front of the turbine stage, are calculated from an experimental high power surge case.…”