The Influence of Clocking on Unsteady Forces of Compressor and Turbine Blades

Abstract

An important part of the unsteady effects in turbomachinery is generated by the relative motion between rotor and stator airfoils. The main sources of unsteadiness due to the rotor-stator interaction are potential flow (or inviscid) interaction and viscous interaction. Recently, the concept of indexing, or clocking, has been introduced to increase the efficiency of turbine and compressor stages. Previous research on airfoil clocking has been confined to studies of efficiency variation. The present paper addresses the clocking effects on the variation of unsteady forces on compressor and turbine blades. Numerical simulation of compressor and turbine flow shows that the pressure amplitude on the airfoil surface increases when the clocking position corresponds to higher efficiencies. Based on the numerical results, it is hypothesized that, at the maximum efficiency clocking position, the inlet guide vane is intensifying the pressure response of the stator which then increases the potential flow interaction between the rotor and stator. This coupling resonance between the inlet guide vane and the stator, which occurs at maximum efficiency, energizes boundary layer, producing a reduction of airfoil losses. ∗Assistant Professor, Member AIAA †Associate Professor, Senior Member AIAA Copyright c ©1999 by Paul Cizmas and Daniel Dorney. Published by the American Institute of Aeronautics and Astronautics, Inc. with permission. NOMENCLATURE Cf Skin friction coefficient f Frequency P Pressure P.S. Pressure surface Pt Total pressure S Arc length S.S. Suction surface t∗ Non-dimensional time Utip Rotor tip velocity α Absolute frame flow angle (from axial) β Relative frame flow angle η Efficiency θ Momentum thickness