A Parametric Study of Actuator Requirements for Active Turbine Tip Clearance Control of a Modern High Bypass Turbofan Engine

The efficiency of aircraft gas turbine engines is sensitive to the distance between the tips of its turbine blades and its shroud, which serves as its containment structure. Maintaining tighter clearance between these components has been shown to increase turbine efficiency, increase fuel efficiency, and reduce the turbine inlet temperature, and this correlates to a longer time-on-wing for the engine. Therefore, there is a desire to maintain a tight clearance in the turbine, which requires fast response active clearance control. Fast response active tip clearance control will require an actuator to modify the physical or effective tip clearance in the turbine. This paper evaluates the requirements of a generic active turbine tip clearance actuator for a modern commercial aircraft engine using the Commercial Modular Aero-Propulsion System Simulation 40k (C-MAPSS40k) software that has previously been integrated with a dynamic tip clearance model. A parametric study was performed in an attempt to evaluate requirements for control actuators in terms of bandwidth, rate limits, saturation limits, and deadband. Constraints on the weight of the actuation system and some considerations as to the force which the actuator must be capable of exerting and maintaining are also investigated. From the results, the relevant range of the evaluated actuator parameters can be extracted. Some additional discussion is provided on the challenges posed by the tip clearance control problem and the implications for future small core aircraft engines. INTRODUCTION Turbine tip clearance refers to the distance between the turbine blades and their containment structure. The tip clearance changes over the course of a flight due to thermal expansion, centrifugal forces of the spinning components, and the mechanical loads applied to the structures by aerodynamic forces and internal stresses. Axisymmetric tip clearance variations are the most significant and include the contributions of thermal expansion and the elongation of moving components due to axisymmetric thermal and mechanical loads. Capturing these components of the tip clearance variation is the focus of the tip clearance model used in this study. A physical explanation of the variation of the tip clearance gap begins with any change in engine operating condition. Consider an increase in power. As the rotor and blade increase in speed, the centrifugal force exerted on these components increases causing them to expand. Additionally, as the temperature in the gas path increases the turbine components heat up and expand. Due to differences in size, geometry, materials, and heat transfer rates, the components of the turbine expand at different rates and reach different steady-state deformations. Note that throughout this paper deformation will be used to characterize an elongation or contraction of a turbine component. This is not to be confused with twisting or bending. Deformation of the blade and rotor occurs relatively quickly due to acceleration of the high pressure spool (HPS). The blade deformation is accelerated further by its relatively fast thermal expansion because of its relatively low mass and large surface area, and its direct exposure to the hot gas path. The rotor and the containment structure around the turbine are larger and experience weaker heat transfer leading to much slower thermal transients and therefore slower expansion. These differences in magnitude and rate of expansion, particularly between the internal engine components and containment structure, create ‘pinch points’ where the tip clearance is significantly reduced during fast accelerations of the engine that are accompanied by rapid changes in the gas path temperature. These pinch points lead to conservative and less efficient design decisions. Modern commercial gas turbine engines employ slow acting thermal management techniques for controlling the tip clearance in the high pressure turbine (HPT) and low pressure turbine (LPT) [1]. Due to the lack of tip clearance sensors https://ntrs.nasa.gov/search.jsp?R=20170008736 2018-05-22T18:03:08+00:00Z