Heterogeneous Optimization Strategies for Carved and Squealer-Like Turbine Blade Tips

Abstract

Superior rotor tip geometries possess the potential to simultaneously mitigate aerodynamic losses and severe thermal loads onto the rotor overtip region. However, classical design strategies are usually constrained to a specific type of geometry, narrowing the spread of shape topologies considered during the design phase. The current paper presents two novel multi-objective optimization methodologies that enable the exploration of a broad range of distinct tip configurations for unshrouded rotor blades. The first methodology is a shape optimization process that creates a fully carved blade tip shape defined through a Bezier surface controlled by 40 parameters. Combined with a differential evolution optimization strategy, this approach is applied to a rotor blade for two tip gap sizes: 0.85% (tight) and 1.38% (design) of the blade span. The second methodology is based on a topology optimization process that targets the creation of arbitrary tip shapes comprising one or multiple rims with a fixed height. The tip section of the blade has been divided into more than 200 separate zones, where each zone can be either part of an upstanding rim or part of the cavity floor. This methodology was tested with a level-set approach in combination with a differential evolution optimizer, and coupled to an optimization routine based on genetic algorithms. The current study was carried out on a modern high-pressure turbine operating at engine-like Reynolds and high subsonic outlet Mach numbers. A fully hexahedral unstructured mesh was used to discretize the fluid domain. The aerothermal performance of each tip profile was evaluated accurately through RANS simulations adopting the SST turbulence model. Multi-objective optimizations were set for both design strategies that target higher aerodynamic rotor efficiencies and simultaneous minimization of the heat load. This paper illustrates a wide variety of profiles obtained throughout the optimization and compares the performance of the different strategies. The research shows the potential of such novel methodologies to reach new unexplored types of blade tip designs with enhanced aerothermal performances.

Experimental and Numerical Investigation of Optimized Blade Tip Shapes: Part I — Turbine Rainbow Rotor Testing and CFD Methods

“…A perfect gas hypothesis was considered for the working fluid. The CFD solver and the numerical settings had been previously validated on a highly loaded turbine cascade featuring cooled squealer tip profiles [12]. For each geometry, a single blade passage was modeled.…”

“…An optimization procedure was employed for the design of the high-performance tip profiles, extensively described in [12]. The driving objectives were the maximization of the rotor aerodynamic efficiency and the minimization of the tip heat transfer.…”

“…Squealer tips amplify the vena contracta effect to effectively decrease the spilled mass flow, delaying its discharge into the main stream from the blade suction side [10]. While several research works emphasized the superior thermal performance of rimmed tips [11], such seal configurations are more exposed to the occurrence of hot spots [12]. As an alternative to squealer tips, a decoupling of tip aerodynamic performance and work extraction could be achieved by employing carved profiles [13].…”

Investigations into heat transfer and film cooling effect on winglet-squealer tips in a turbine stage

“…Zhong and Zhou 13 found that the leakage loss in winglet-squealer tip is reduced significantly as compared with the conventional squealer tip. By using the optimization strategies, Maesschalck et al 14 Zhang et al 15 Caloni et al 16 and Zhou et al 17 presented the optimized geometries of winglet and novel designs of squealer-winglet tip, which have shown promising benefits in aerodynamic performance improvement in the tip region. For the heat transfer and film cooling effect, Papa et al, 18 Silva and Tomita, 19 Zhou et al, 20 and Yan et al 21 reported that the winglet extension is beneficial for the heat transfer reduction as compared with the flat tip and conventional squealer tip.…”

Experimental and numerical study of novel tip configurations in a turbine cascade

“…Hu 23 studied numerically the influence of the contoured endwall on the aerodynamic performance. Maesschalck [24][25][26] apllied the carved and squealer-like tip to promote the aerodynamic thermal performance and proposed the optimization strategy. Cernat 27,28 researched the aerothermal performance of a multi-cavity squealer-like tip and a contoured shape tip by the detailed experimental and the numerical studies.…”