Towards desensitization of gas turbine performance to tip Clearance: Design optimization and engine analysis

Abstract Increased scrutiny on aviation’s environmental footprint has precipitated a dramatic increase in gas turbine technology development, with a focus on engine performance improvements and the reduction of noise and emissions. In a somewhat limited approach, these studies are often performed at the component level and at a single engine operating condition. In order to achieve future fuel consumption, noise, and emissions targets, it will be essential for designers to optimize the overall propulsive system across the entire trajectory. Accurate engine models will be needed to quantify the overall system performance and select the optimal components and the aircraft mission, highlighting the interconnection between optimization and engine modeling. In this paper, an engine modeling approach is described that provides a framework for analysis of the impact of component changes on the overall system for an entire trajectory. The engine model is built using a modeling environment developed by NASA called Toolbox for the Modeling and Analysis of Thermodynamic Systems (T-MATS). The modeling environment allows for transient updates to the boundary conditions, similar to what an engine would experience during operation. The emphasis on trajectory-driven design led to the development of a new blade tip optimization approach that improved turbine efficiency across a range of tip clearances rather than at a single clearance. The robust blade tip design approach used a multi-objective optimization with a differential evolution strategy and Computational Fluid Dynamics software to solve Reynolds-Averaged Navier-Stokes equations at three tip clearances. The optimization objectives were to increase the average and decrease the standard deviations of the stage efficiency for the three clearances. This approach demonstrated efficiency improvements as large as 0.50% with variations of standard deviations near zero. The robust blade tip optimization approach is discussed in detail, as well as a brief description of the physical mechanisms responsible for the performance improvement. Finally, in a demonstration of capability and applicability, turbine-specific updates are made to the baseline engine model. The updates include heat transfer and structural modules that are used to predict the turbine blade running tip clearance, which changes during the trajectory from 1.1% to 1.6% of span. Brief discussions of each module are included as well as a description of the turbine performance assessment. The impact of the blade tip clearance on the turbine efficiency across the trajectory is reported for three blade tip geometries: a baseline squealer design, a single point optimization design, and the robust blade tip optimization design. The results demonstrate the importance of trajectory-influenced design at both the component and engine level.