Experimental Study on Effects of Internal Rib and Rear Bump on Film Effectiveness
This paper reports detailed measurement of film cooling effectiveness for a scaled up film-cooling hole with an expanded exit fed by a smooth and ribbed secondary flow channel, an arrangement typical of turbine blades. The experiments are carried out at blowing ratios ranging from 0.4 to 1.25, and ten different rib patterns including forward oriented ribs and inverse oriented ribs are evaluated. Further, to develop an efficient film-cooling technique, several kinds of bumps are installed downstream of the hole exits, and the effects of the bumps on film effectiveness are investigated. The bump structures tested here are semicircular, hemispherical, and cylindrical bumps. The results show that the rib orientation strongly affects film effectiveness. When the blowing ratio is comparatively low, the forward oriented ribs afford higher film effectiveness. On the other hand, when the blowing ratio is comparatively high, the inverse oriented ribs afford higher film effectiveness. The cylindrical bump provides a better spreading of the ejected secondary flow than the other bumps, leading to higher film effectiveness. To clarify how the bumps improve the film effectiveness, computational simulations are performed. The simulations indicate that a longitudinal vortex, formed at the trailing edge of the cylindrical bump improves the film effectiveness by generating downward velocity vectors.