Highly Resolved Large-Eddy Simulations of a Transonic Compressor Stage Midspan Section - Part II: Effect of Rotor-Stator Gap

Abstract The need for increased power density and reduced weight in modern gas turbines often leads to shorter inter-row distances. This translates into stronger blade-to-blade and row-to-row interactions. These interactions are particularly difficult to predict for low-order methods such as URANS due to the overlap between deterministic unsteadiness frequencies and stochastic unsteadiness frequencies. This lack of a spectral gap results in complex broadband energy spectra that necessitate the use of high-fidelity scale resolving simulations to predict such flows and understand how they affect the flow behavior of subsequent bladerows. In this study, wall-resolved large eddy simulations have been conducted of a transonic compressor stage at a realistic rotor Reynolds number of 1 × 106, varying the axial gap between rotor and stator. The analysis of the data focuses on the impact of the rotor-stator gap on the downstream stator boundary layer behavior and employs phase averaging to describe the flow in a detailed manner. We then report on how the rotor-stator gap affects loss generation and loss mechanisms. Overall, a 2.5% overall stage loss decrease was found when the gap size was reduced by 25%. This reduction nearly entirely stems from the stator bladerow, despite the stator contributing only 15% to the total stage loss. The high-fidelity simulations are complemented by unsteady RANS calculations of the same configuration, to assess how accurately current design tools capture the complex row-to-row interactions. Although the overall loss magnitudes were captured reasonably well, no significant variation with gap size were found with URANS.