Turbine blade tip leakage loss investigation

Résumé

Although current tip-clearance calculation methods utilize experimental-based correlations to calculate turbine tip leakage losses, these values have been found to be inconsistent and thus require improvement. For example, shrouded turbine blades with straight seals have better tip clearance loss than shrouded blades with stepped seals. One of the aims of this current study was to investigate the manner in which blade tip geometry contributes to tip leakage loss in a turbine by comparing shrouded blades that had several configurations, each of which has a combination of straight outer diameter (OD), stepped OD, vertical fins, and angled fins. The second aim was to develop an improved tip-clearance-loss correlation for straight seals, by comparing existing experimental-based correlations to Computational Fluid Dynamics (CFD) results retrieved following simulation analysis. More specifically, CFD simulations were performed on several configurations of a one-stage power turbine (PT). These configurations differed from each other in terms of blade tip geometries (e.g., shroud with one-angled fin, shrouded with two-vertical fins, etc.) and casing wall or outer diameter (i.e., straight OD, stepped OD) at the blade tip area and were compared based on these characteristics. CFD analysis of all models consisted of the following process: CAD model creation, grid study, mesh models preparation, and CFD simulation applying same boundary conditions. A grid independence study was performed on one model to check for grid convergence. Due to time constraints, a compromise needed to be reached and therefore, a mesh size of around 10 million nodes was chosen. All configurations used the same grid size parameters to obtain the same approximate grid count. Each configuration had three different tip-clearance-tospan ratios. CFD results from this study revealed that stepped OD configurations had less tip loss when compared to configurations with straight OD. In addition, angled and vertical fins did not reveal a significant difference (for stepped OD and straight OD configurations) in terms of tip mass flow. The constant value used in the tip loss experimental correlation was modified for stepped and straight OD configurations in order to obtain new correlations that matched efficiencies from CFD analysis. Three updated tip loss correlations were acquired for straight OD configurations, which corresponded to one, two, and three fins respectively. On the other hand, for stepped OD configurations, only one updated correlation was obtained for any number of fins. These updated correlations were implemented in the Pre-Detailed Design System (PDDS) tool, which is a multi-preliminary design interface. Furthermore, these improved correlations were validated for one blade design of a power turbine. Further research would need to verify these improved correlations on other blade designs.

Date	20 sept. 2016
langue originale	Anglais américain
Établissement diplômant	École de technologie supérieure
Superviseur	Saïd Hany Moustapha (Directeur(-trice)) & François Garnier (Codirecteur(-trice))