Effect of Upstream Step on Flat Plate Film-Cooling Effectiveness Using PSP

The effect of a step positioned upstream of a row of film-cooling holes on the film-cooling effectiveness is studied systematically using the steady state pressure sensitive paint technique. The upstream step effect is studied on four separate hole geometries: simple angled (axial angle of 30 deg) and compound angled (axial angle of 30 deg and compound angle of 45 deg) and cylindrical and fan-shaped film-cooling holes. Each plate considered has seven holes, each hole 4 mm in diameter. The plates with cylindrical holes have a spacing of 3 diameters (12 mm) between the centers of two consecutive holes while the fan-shaped holes have a spacing of 3.75 diameters (15 mm). Three different step heights (⁠$12.5%d$⁠, $25%d$⁠, and $37.5%d$⁠) are studied. The effect of the width of the step is also studied; the distance of the step upstream of the hole and the positioning of the step downstream of the film-cooling hole. Four separate blowing ratios are reported for all tests: $M=0.3$⁠, $M=0.6$⁠, $M=1.0$⁠, and $M=1.5$⁠. All studies have been conducted with a mainstream of 25 m/s velocity at an ambient temperature of $22°C$⁠. Results indicate an increase in film-cooling effectiveness in the region near the hole due to the upstream step for all the plates considered. This increase due to the step is found to be most significant in the case of compound angled cylindrical holes and least significant in the case of simple angled fan-shaped holes.