Three-dimensional simulations of airfoil clocking in a 1-1/2 stage turbine

Abstract

Axial turbines have inherently unsteady flow fields because of the relative motion between rotor and stator airfoils. This relative motion leads to viscous and inviscid interactions between blade rows. As the number of stages increases in a turbomachine, the buildup of convected wakes can lead to progressively more complex wake/wake and wake/airfoil interactions. Variations in the relative circumferential positions of airfoils in adjacent rotating or non-rotating blade rows can change these interactions, leading to different unsteady forcing functions on airfoils and different turbine efficiencies. In addition, as the Mach number increases the interaction between blade rows can be intensified due to potential effects. It has been shown, both experimentally and computationally, that airfoil clocking can be used to improve the efficiency and reduce the unsteadiness in multiple-stage axial turbomachines with equal blade counts. While previous investigations have provided an improved understanding of the physics associated with airfoil clocking, most of the work focused on two-dimensional effects. More research is needed to determine how airfoil clocking can be incorporated in the design of three-dimensional airfoils. In the current study three-dimensional simulations of airfoil clocking have been performed for a 1-1/2 stage high-pressure turbine operating at subsonic flow conditions. Comparisons have been made with the available time-averaged experimental data at a single clocking position. The predicted results indicate approximately a 1% efficiency variation as the secondstage stator airfoils are clocked with respect to the first-stage stator airfoils. / cv Cp P P ps Pt f S ss t U a /? 7 TJ in min max ms out st tt oo 1/2 1 2 3 4 NOMENCLATURE Frequency Specific heat at constant volume (P-Pt)/(l/2pUms) Static pressure Pressure surface Total pressure Entropy Suction surface Time Rotor velocity Absolute frame flow angle (from axial) Relative frame flow angle Ratio of specific heats Efficiency Flow coefficient Density