No abstract

Study of aerodynamic flow and aeroelastic stability in vibrating blades of cascade is the main objective of this study. Standard test configuration (STC-5) was chosen for this study as it involves transonic flow regime in compressor blade cascades. CFD analysis were carried out for 11 test cases of STC-5 configuration and pressure coefficient values were compared with test data. The range of incidence angles vary from 2° to 10° and reduced frequency varies from 0.14 to 1.02. Inflow Mach number was fixed at 0.5 and Reynolds number was fixed at 1.4 × 106. Analysis of vibrating blades and comparison of test data results of axial compressor with linear cascade stator blades of fifth standard configuration at high subsonic speed is compared with CFD results. While doing this vibration of only the center blade is concerned when all the other blades in the cascade are fixed. Fluid structure interaction approach is used here to evaluate the unsteady aerodynamic force and work done for a vibrating blade in CFD domain. Energy method and work per cycle approach is adapted for aerodynamic damping prediction. A framework has been developed to estimate the work per cycle and aerodynamic damping ratio. Final sensitivity study was carried out to evaluate the influence of blade incidence and frequency on blade damping values.

This study discusses in detail the aeroelastic flutter investigation of a transonic axial compressor rotor using computational methods. Fluid structure interaction approach is used in this method to evaluate the unsteady aerodynamic force and work done of a vibrating blade in CFD domain. Energy method and work per cycle approach is adapted for this flutter prediction. A framework has been developed to estimate the work per cycle and aerodynamic damping ratio. Based on the aerodynamic damping ratio, occurrence of flutter is estimated for different inter blade phase angles. Initially, the baseline rotor blade design was having negative aerodynamic damping at part speed conditions. The main cause for this flutter occurrence was identified as large flow separation near blade tip region due to high incidence angles. The unsteadiness in the flow was leading to aerodynamic force fluctuation matching with natural frequency of blade, resulting in excitation of the blades. Hence axially skewed slot casing treatment was implemented to reduce the flow separation at blade tip region to alleviate the onset of flutter. By this method, the stall margin and aerodynamic damping of the test compressor was improved and flutter was avoided.

This paper describes a methodology for obtaining correct blade geometry of high aspect ratio axial compressor blades during running condition taking into account of blade untwist and bending. It discusses the detailed approach for generating cold blade geometry for axial compressor rotor blades from the design blade geometry using fluid structure interaction technique. Cold blade geometry represents the rotor blade shape at rest, which under running condition deflects and takes a new operating blade shape under centrifugal and aerodynamic loads. Aerodynamic performance of compressor primarily depends on this operating rotor blade shape. At design point it is expected to have the operating blade shape same as the intended design blade geometry and a slight mismatch will result in severe performance deterioration. Starting from design blade profile, an appropriate cold blade profile is generated by applying proper lean and pre-twist calculated using this methodology. Further improvements were carried out to arrive at the cold blade profile to match the stagger of design profile at design operating conditions with lower deflection and stress for first stage rotor blade. In rear stages, thermal effects will contribute more towards blade deflection values. But due to short blade span, deflection and untwist values will be of lower values. Hence difference between cold blade and design blade profile would be small. This methodology can especially be used for front stage compressor rotor blades for which aspect ratio is higher and deflections are large.