POD analysis of passage-layout effect on unsteady internal flow in a realistic blade serpentine coolant channel with low aspect ratios

Abstract

Internal serpentine ribbed coolant channel with low aspect ratio (AR) has been widely employed to provide excellent thermal protection for advanced first-stage turbine blades. Various height differences (ΔHs) of adjacent passages along pressure side can be generated under the restriction of actual blade profile. The present work made an attempt to acquire detailed influences of ΔH on the time-averaged and transient secondary motions near sharp bends of low-AR coolant channels. An actual blade ribbed three-pass coolant channel with two ΔHs was chosen as experimental model. Planar time-resolved particle image velocimetry technique was applied to capture the transient flow fields in 21 typical cross sections. Time-averaged results were validated by standard PIV measurements and flow visualizations. Snapshot proper orthogonal decomposition was employed to extract the dominant flow structures and identify the underlying small-scale flow patterns in time-resolved velocity fields. The experimental results indicated the low-AR channel with real blade profile induces a new type of the secondary-vortex superposition downstream of bends, in comparison with the large-AR channels in open literature. The crucial reason is the ΔH along pressure side leads to an intensive impingement on the bend wall, which weakens the effects of bend, rib and curvature of suction side. Another new discovery is the effect of ΔH on unsteady internal flow. In the small ΔH case, the oscillatory frequency of the secondary vortices is nearly similar with the counter-rotating Dean-vortices; however, in the large ΔH case, the oscillatory frequency is much lower, and the rotating direction of vortices is same and unchanged in a period. Increasing inlet Reynolds number can significantly change the underlying small-scale flow patterns and improve the oscillatory frequency of large-scale structures, although the basic time-averaged flow patterns are nearly same. Effect of injection from auxiliary-hole (AH) on unsteady internal flow is obvious and the optimum mass flow ratio of AH-to-inlet is 5%.