C. De Maesschalck

Blade Tip Carving Effects on the Aerothermal Performance of a Transonic Turbine

Tip leakage flows in unshrouded high speed turbines cause large aerodynamic penalties, induce significant thermal loads and give rise to intense thermal stresses onto the blade tip and casing endwalls. In the pursuit of superior engine reliability and efficiency, the turbine blade tip design is of paramount importance and still poses an exceptional challenge to turbine designers. The ever-increasing rotational speeds and pressure loadings tend to accelerate the tip flow velocities beyond the transonic regime. Overtip supersonic flows are characterized by complex flow patterns, which determine the heat transfer signature. Hence, the physics of the overtip flow structures and the influence of the geometrical parameters require further understanding to develop innovative tip designs. Conventional blade tip shapes are not adequate for such high speed flows and hence, potential for enhanced performances lays in appropriate tip shaping. The present research aims to quantify the prospective gain offered by a fully contoured blade tip shape against conventional geometries such as a flat and squealer tip. A detailed numerical study was conducted on a modern rotor blade (Reynolds number of 5.5 × 105 and a relative exit Mach number of 0.9) by means of three-dimensional (3D) Reynolds-averaged Navier–Stokes (RANS) calculations. Two novel contoured tip geometries were designed based on a two-dimensional (2D) tip shape optimization in which only the upper 2% of the blade span was modified. This study yields a deeper insight into the application of blade tip carving in high speed turbines and provides guidelines for future tip designs with enhanced aerothermal performances.

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

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.Copyright

Performance Robustness of Turbine Squealer Tip Designs due to Manufacturing and Engine Operation

This paper quantifies the changes in turbine performance due to manufacturing tolerances and profile degradation of the blade-tip region during engine operation. An extensive numerical study was co...

Analysis of the Heat Transfer Driving Parameters in Tight Rotor Blade Tip Clearances

Turbine rotor tips and casings are vulnerable to mechanical failures due to the extreme thermal loads they undergo during engine operation. In addition to the heat flux variations during the transient phase, high-frequency unsteadiness occurs at every rotor passage, with amplitude dependent on the tip gap. The development of appropriate predictive tools and cooling schemes requires the precise understanding of the heat transfer mechanisms. The present paper analyzes the nature of the overtip flow in transonic turbine rotors running at tight clearances, and explores a methodology to determine the relevant flow parameters that model the heat transfer.Steady-state three-dimensional Reynolds-Averaged Navier-Stokes calculations were performed to simulate engine-like conditions considering two rotor tip gaps, 0.1% and 1% of the blade span. At tight tip clearance, the adiabatic wall temperature is not anymore independent of the solid thermal boundary conditions. The adiabatic wall temperature predicted with the linear Newton’s cooling law was observed to rise to non-physical levels in certain regions within the rotor tip gap, resulting in unreliable convective heat transfer coefficients. This paper investigates different approaches to estimate the relevant flow parameters that drive the heat transfer. The present study allows experimentalists to retrieve information on the gap flow temperature and convective heat transfer coefficient based on the use of wall heat flux measurements. Such approach is required to improve the accuracy in the evaluation of the heat transfer data while enhancing the understanding of tight-clearance overtip flows.Copyright

Blade Tip Carving Effects on the Aero-Thermal Performance of a Transonic Turbine

Tip leakage flows in unshrouded high speed turbines cause large aerodynamic penalties, induce significant thermal loads and give rise to intense thermal stresses onto the blade tip and casing endwalls. In the pursuit of superior engine reliability and efficiency, the turbine blade tip design is of paramount importance and still poses an exceptional challenge to turbine designers. The ever-increasing rotational speeds and pressure loadings tend to accelerate the tip flow velocities beyond the transonic regime. Overtip supersonic flows are characterized by complex flow patterns, which determine the heat transfer signature. Hence, the physics of the overtip flow structures and the influence of the geometrical parameters on the overtip flow require further understanding to develop innovative tip designs. Conventional blade tip shapes are not adequate for such high speed flows and hence, potential for enhanced performances lays in appropriate tip shaping.The present research aims to quantify the prospective gain offered by a fully contoured blade tip shape against conventional geometries such as a flat and squealer tip. A detailed numerical study was conducted on a modern transonic turbine rotor blade (Reynolds number is 5.5 × 105, relative exit Mach number is 0.9) by means of three-dimensional Reynolds-Averaged Navier-Stokes calculations. The novel contoured tip geometry was designed based on a 2D tip shape optimization in which only the upper 2% of the blade span was modified. This study yields a deeper insight into the application of blade tip carving in high speed turbines and provides guidelines for future tip designs with enhanced aerothermal performances.Copyright