Effect of swirled leakage flow on endwall film-cooling

Mass transfer measurements are performed on a naphthalene coated endwall surface in a five-blade linear cascade to study the effect of swirled leakage flow on endwall film-cooling. A 45° inclined slot equipped with turning vanes upstream of the blades is used to model the leakage flow path between a stator and rotor section. The turning vanes are used to impart swirl on the leakage flow, which simulates the relative motion between the stator and rotor walls found in a rotating turbine. For a Reynolds number of 6 × 10⁵, based on blade chord and cascade exit velocity, a parametric study was conducted for three turning vane angles and three leakage flow blowing ratios. The effects on both mass transfer coefficient and film-cooling effectiveness were determined by injecting naphthalene-free and naphthalene-saturated air through the leakage flow slot upstream of the blade row. Endwall film-cooling coverage was found to improve with increased leakage flow rates and with decreased swirl angles (zero swirl defined as leakage flow aligned with axial direction, positive swirl defined as leakage flow angled towards suction surface). Endwall mass transfer coefficients were found to increase with increased leakage flow rate. The leakage flow path was found to be significantly influenced by the endwall secondary flows for the low leakage flow rates. For realistic leakage flow rates and swirl angles, leakage flow alone was found to do a poor job at providing coolant coverage on the endwall surface. The downstream portion and the pressure side of the endwall surface were found to be especially difficult to cool, so additional cooling sources, such as discrete film-cooling holes, should be utilized to provide more complete coolant coverage.