Experimental Study on Effects of Internal Ribs and Rear Bumps on Film Cooling Effectiveness

This paper reports the detailed measurement of the film cooling effectiveness for a scaled up film-cooling hole with an expanded exit fed by a smooth and ribbed secondary flow channel, which is 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. Furthermore, in order 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 the film cooling effectiveness are investigated. The bump structures tested here are semicircular, hemispherical, and cylindrical bumps. The results show that the rib orientation strongly affects the film cooling effectiveness. When the blowing ratio is comparatively low, the forward oriented ribs afford a higher film cooling effectiveness. On the contrary, when the blowing ratio is comparatively high, the inverse oriented ribs afford a higher film cooling effectiveness. The cylindrical bump provides a better spreading of the ejected secondary flow than the other bumps, leading to a higher film cooling effectiveness. To clarify how the bumps improve the film cooling effectiveness, computational simulations are performed. The simulations indicate that a longitudinal vortex, formed at the trailing edge of the cylindrical bump, improves the film cooling effectiveness by generating downward velocity vectors.