Embedded LES methods for turbomachinery flows

RWTH Publications (RWTH Aachen)

Abstract

The objective of this thesis is the development of an embedded LES method for turbomachinery flow. In the industrial design process of turbomachinery applications Reynolds-averaged Navier-Stokes (RANS) methods are commonly used, for which the accurate prediction of rim seal gap flow dynamics remains challenging. RANS models offer computational efficiency and robustness for industrial applications, however, they often struggle to capture the complex, multi-modal ingress mechanisms and strongly unsteady shear-layer dynamics characterizing rim seal flows present in the stator-rotor gap in axial turbines. Large-eddy simulations (LES) provide higher fidelity turbulence modeling with substantially improved predictive capabilities but remain computationally prohibitive, especially for full-annulus configurations, when large-scale azimuthal vortex structures critical to ingress behavior must be resolved. The method presented in this work aims to bridge the gap between current computational capabilities and the routine use of LES in the industrial design process. The embedded LES method employs a priori defined RANS and LES regions, where a preliminary RANS solution provides initial conditions and physically consistent boundary conditions for a reduced LES domain. The information is exchanged from the steady RANS computation to the embedded, time-resolved LES region, thereby limiting the LES to a predefined region of interest while reducing overall computational cost and convergence time. The requirements for the embedded LES boundary conditions depend on the specific flow case. The formulation and assessment of boundary conditions for the embedded LES for turbomachinery flows are the focus of this thesis. The approach is specifically designed to integrate into the industrial design process of turbomachinery applications, where RANS simulations remain standard practice. The embedded LES thus acts as a localized high-fidelity extension of the RANS solution, enabling detailed, time-resolved analysis of specific flow regions without requiring a full-domain LES. The method was first developed and evaluated for a generic curved-backward facing step geometry designed to replicate the characteristic flow conditions expected at the embedded LES interfaces in axial turbomachinery applications, with the LES domain restricted to the separation region. The developed boundary conditions for the embedded LES were subsequently adapted for axial turbomachinery flows and validated for the prediction of hot gas ingress into stator–rotor wheel spaces in both linear and curved axial turbine cascades. The embedded LES method was used to reduce the LES region to the rim seal gap. Finally, a coupling strategy enabling the information exchange of preliminary sectoral RANS simulations with a 360◦ full-annulus embedded LES is proposed and validated for the curved turbine configuration. The full-annulus solution is compared to the sectoral solution to analyze the impact of the assumption of azimuthal periodicity on the cooling effectiveness.

Authors 1

1. Jannik Borgelt corresponding Aachen

   RWTH Aachen University

   Affiliation as printed

   RWTH Aachen