Hybrid LES/RANS Simulations of Compressible Flow in a Linear Cascade of Flat Blade Profiles

The paper reports on 3D numerical simulations of unsteady compressible airflow in a blade cascade consisting of flat profiles using a hybrid LES/RANS approach including a transition model.As a first step towards simulation of blade flutter in turbomachinery,various incidence angle offsets of the middle blade were modeled.All simulations were run for the flow regime characterized by outlet isentropic Mach number M_(is)=0.5and zero incidence.The results of the LES/RANS simulations(pressure and Mach number distributions)were compared to a baseline RANS model,and to experimental data measured in a high-speed wind tunnel.The numerical results show that both methods overpredict flow separation taking place at the leading edge.In this regard,the hybrid LES/RANS method does not provide superior results compared to the traditional RANS simulations.Nevertheless,the LES/RANS results also capture vortex shedding from the blunt trailing edge.The frequency of the trailing edge vortex shedding in CFD simulations matches perfectly the spectral peak recorded during wind tunnel measurements.

Near-Wall Flow in the Blade Cascades Representing Last Rotor Root Sections of Large Output Steam Turbines

This paper investigates the flow past two variants of root section profile cascades for a last stage rotor considering three-dimensional flow structures in the near-wall region.Analyses were drawn based on RANS numerical simulations of both variants and on the experimental data obtained by the 3 D traversing in the exit flow field of one of the variants.Extent of 3 D structures at two different regimes and its influence on aerodynamic characteristics of the blade cascades was assessed.The distributions of Mach number along the profiles were compared with 2 D optical measurements and its distortion due to the presence of the sidewall was explored.The interaction between main vortical structures was described and its influence on the loading of the blades,mechanical energy losses and exit flow angle was discussed.The results showed that for a front loaded blade the vortical structures appeared earlier and at a larger extent than for an aft loaded variant.However,due to different Mach number distribution,contribution of end wall flow to the energy losses was lower in the case of the aft loaded variant.The influence of the near wall flow on the loading was found to be rather weak while the deviation of the exit flow angle appeared to be comparable for both of the variants.

Study on the Numerical Model of Transonic Wind Tunnel Test Section

The authors consider numerical simulations of transonic flows through various turbine cascades in a confined channel which approximates boundaries of real wind tunnel.The boundaries of the wind tunnel are impermeable or there can be permeable tailboards to diminish shock wave reflections.The mathematical model is based on Favre-averaged Navier-Stokes equations closed by a turbulence model and model of transition to turbulence.The mathematical model is solved by an implicit finite volume method with multi-block grids.Several types of turbine blade cascades with subsonic or supersonic inlet are presented.The results are compared with optical measurements and simulations of periodic cascades.The validity of experimental reference flow parameters in relation to computed flow patterns is discussed.

Optimization of Root Section for Ultra-long Steam Turbine Rotor Blade

This study presents the comparison of aerodynamic performances of two successive designs of the root profiles for the ultra-long rotor blade equipped with a straight fir-tree dovetail. Since aerodynamic and strength requirements laid upon the root section design are contradictory, it is necessary to aerodynamically optimize the design within the limits given by the foremost strength requirements. The most limiting criterion of the static strength is the size of the blade cross-section, which is determined by the number of blades in a rotor and also by the shape and size of a blade dovetail. The aerodynamic design requires mainly the zero incidence angle at the inlet of a profile and in the ideal case ensures that the load does not exceed a limit load condition. Moreover, the typical root profile cascades are transonic with supersonic exit Math number, therefore, the shape of a suction side and a trailing edge has to respect transonic expansion of a working gas. In this paper, the two variants of root section profile cascades are compared and the aerodynamic qualities of both variants are verified using CFD simulation and two mutually independent experimental methods of measurements （optical and pneumatic）.