Influence of Blowing Ratio on the Double-Jet Ejection of Cooling Air Available to Purchase

Further improvement of the thermal efficiency of modern gas turbines can be achieved by a further reduction of the cooling air amount. Therefore, it is necessary to increase the cooling effectiveness so that the available cooling air fulfils the cooling task even if the amount has been reduced. In particular, the cooling effort for the vanes and blades of the first stage in a modern gas turbine is very high. The task of the film-cooling is to protect the blade material from the hot gas attack to the surface. Unfortunately, aerodynamic mixing processes are enhanced by secondary vortices in the cooling jets and, thus, the film-cooling effectiveness is reduced shortly behind the cooling air ejection through the holes. By improvement of the hole positioning, the negative interaction effects can be reduced. One approach is the Double-jet Film-cooling (DJFC) Technology presented recently by the authors. It has been shown by numerical simulations that for a special and precise arrangement of two holes, the interaction of the secondary vortices can be used for a significant increase in film-cooling effectiveness. This is reached by establishing an anti-kidney vortex pair in a combined jet from two jets starting from two cylindrical ejection holes. The influence of the blowing ratio on the double-jet ejection is investigated numerically. The configurations of the double-hole arrangements have been investigated only for a relative high blowing ratio (M = 1.7). The present investigations focus on moderate blowing ratios (1.0 < M < 1.5) and on a higher blowing ratio of M = 2.0. It can be shown that also for moderate blowing ratios the anti-kidney vortex pair is generated in the combined cooling jet. Thus, high adiabatic film-cooling effectiveness can be reached also for the case with a moderate blowing ratio. The lateral distribution of the cooling air is reduced compared to the cases of higher blowing ratios (M = 1.7, M = 2.0).