The film cooling technique has been widely applied to gas turbines with operating temperatures that exceed the operational limits of the component materials. However, because the excessive injection of cooling fluid can decrease the thermal efficiency of a gas turbine, sophisticated designs are required to protect surfaces effectively with limited amounts of the cooling fluid.
Gaps between gas turbine components, including the combustor-turbine, stator-rotator, and mid-passage of the blade interfaces, are subjected to high thermal stresses due to high temperature gradients. These gaps(or slots) can be used as cooling fluid passages. Slot injection is applied to prevent the reverse flow of the combustion gas into internal components, as well as to protect components from the injection of cooling fluid though these gaps. With these reasons, many studies on slot film cooling have been conducted for last decades of years. In this study, experimental investigations were carried out to improve the film cooling effectiveness of inclined slots using the Coanda effect and the DBD(Dielectric Barrier Discharge) plasma actuator.
In order to generate the Coanda effect, a protrusive structure, named Coanda bump, was installed at the slot exit so that the cooling fluid could be able to be induced toward the wall surface. As a fundamental research, parametric study for the shape of the Coanda bump was experimentally performed in 30° and 45° inclined slot, respectively. In order to evaluate the effects of the Coanda bump, three shapes with different height and width and three bump installation locations were considered. The film cooling effectiveness was measured at the fixed mainstream velocity, 10m/s, using PSP(Pressure Sensitive Paint) technique. The mainstream to cooling fluid blowing ratio was ranged from 0.5 to 2.0, and the Nitrogen gas was used as the cooling fluid so the mainstream to cooling fluid density ratio was about 1.0. As a result, it was confirmed that the film cooling effectiveness was improved at higher blowing ratio, M=2.0, with a particular Coanda bump of wider width and higher height. With the results of the parametric study, the optimization study on the Coanda bump shape was carried out using CFD(Computational Fluid Dynamics) and DOE(Design of Experiment).
As a following study, the optimized shape of Coanda bump was applied to surfaces with various curvatures ? i.e. flat, convex, and concave wall. the mainstream velocity was fixed as 10m/s, and three blowing ratios(0.5, 1.0, and 2.0) and two density ratios(about 1.0 and 1.5) were considered. The results showed that the film cooling effectiveness was significantly improved by installing the optimal Coanda bump at the slot exit. And the improvement was more significant at higher blowing ratio.
In addition, DBD plasma actuator was simultaneously applied with the optimal Coanda bump. The exposed electrode was installed at immediately downstream of the optimal Coanda bump and the insulated electrode was asymmetrically installed at the bottom of the test surface. For input signal, 9kV of AC voltages and 1kHz of frequencies were supplied to the electrodes. The results showed that further improved film cooling effectiveness was confirmed because the DBD plasma induced the flow of cooling fluid toward the wall surface. Especially, the film cooling effectiveness in lateral direction was improved by DBD plasma actuator. In general, less film cooling effectiveness is measured near the lateral side region due to the vortex and lifted off cooling fluid.