Session: 34-03 Endwall, Seal & Leakage Flows

Paper Number: 78443

78443 - Numerical Analysis of Blade Platform Leakage in Axial Compressors 

Modern design of turbomachinery relies widely on numerical methods, such as 3D-RANS or LES simulations and usually requires many optimization iterations. The reduction of complexity of the numerical model saves time and costs when dealing with complex machines, such as multistage compressors. However, the simplification of the reality neglects subtle geometrical characteristics, such as radii, edges, steps, gaps, misalignments, or cavities. A range of investigations, numerical as well as experimental, has shown, that the impact of these so-called “real geometrical effects” (RGE) can have a non-negligible negative impact on the performance of a turbo-component, which may not be reproduced by a simplified model. One realistic, yet not well-investigated feature of turbo-components are gaps between differentially manufactured blades of an axial compressor. The impact of blade platform gaps particularly grows in off-design conditions at part-load, where the operational temperature level in the machine and, thus, the thermal expansion of its components decrease. Growing tolerances between single parts of the machine lead to increased leakage flow, potentially taking a greater effect on the machine’s aerodynamic behaviour. Due to the volatile renewable energy supply, the necessity of running stationary gas turbines at part-load poses one current cause for this scenario.

Numerous numerical and experimental studies have been made in the past on the “classic” mechanisms for aerodynamic loss in axial compressors. Aside from the well-investigated tip-leakage of un-shrouded blades, leakage paths, albeit less pronounced, may occur at many locations inside of a compressor. With growing computational power in the past two decades, a new stream of numerical research focused on a more detailed analysis regarding leakage and its interaction with the main flow. Most attention was paid to leakage flows in turbine aerodynamics, possibly affecting coolant efficacy.

For the numerical investigation of the platform spill, leakage flow paths and the associated influence on its aerodynamic performance, a full-detail CAD model of a modern axial compressor’s rear stage was prepared accordingly. The RANS solver “HYDRA” was applied to solve the steady Navier-Stokes equations using a MUSCL-based flux-differencing scheme and a semi-implicit Runge-Kutta approach. A grid convergence assessment obeyed the suggestions by Roache (P. J. Roache, “Perspective: A Method for Uniform Reporting of Grid Refinement Studies,” Journal of Fluids Engineering, vol. 116, pp. 405-413, 1994).

The main investigation focused on the aerodynamic impact of fully connected axial and inter-platform gaps of both rotor and stator blade rows, each enclosed by two idealized row models up- and downstream to enhance accuracy and convergence while saving computational resources. The preliminary investigation showed a decrease of rotor performance due to the introduction of the more realistic platform geometry with respect to simplified, smooth endwalls, especially when midpassage leakage was considered. Compared to a clean hub surface, leakage from both front and rear gaps (1), as well as in combination with inter-platform gaps of 1.3% c_ax (2)  and 2.6% c_ax (3) circumferential pitch reduced polytropic rotor row efficiency between 0.64% and 3.2%. Rotor row total pressure ratio decreased slightly by 0.24% with the inclusion of front and rear gaps only, while showing a stronger reduction between 1.86% to 2.27% when inter-platform gaps were included. A distinct local effect altered several flow variables at the hub in a two-dimensional analysis, yet, a radial shift of momentum was observed as well, impacting the entire channel flow. The inclusion of partial gaps without any interconnection of the leakage paths partially captured some negative impact of the influence of the full real geometry in contrast to an ideal hub surface while keeping additional pre-processing effort comparatively small. However, without inter-platform gaps, those features rather tended to increase total pressure and flow turning in the bladerow. Further investigations regarding a more detailed flow analysis including streamlines, contours, and leakage flow paths will follow in the full paper, as well as the investigation of inter-platform gaps in a cantilevered stator row.

Presenting Author: Jannik Petermann TU Munich

Presenting Author Biography: Research associate at the Institute of Turbomachinery and Flight Propulsion at Technical University of Munich

Authors:

Jannik Petermann TU Munich
Marvin Pommerening TU Munich
Volker Gümmer TU Munich
Bernd Becker Rolls-Royce Deutschland Ltd & Co KG