A hot gas stream driving a turbine is conducted from the stationary turbine guide vanes to the turbine moving blades which are fastened on disks rotating about a central turbine axis. A circular arrangement of turbine guide vanes, which are fastened with their radially outer foot regions on a stationary turbine casing wall, in this case alternates with an arrangement of turbine moving blades on a rotating disk. The radially inner head regions of the turbine guide vanes are contiguous to a U-shaped inner ring which on its outside has a labyrinth seal which seals off against a flow of hot gas around the U-ring.
Cooling air is used, as a rule, for cooling the turbine blades heated by the hot gas flowing past. Where turbine guide vanes are concerned, the cooling air flows, for example through a radial cooling-air duct, formed in the turbine guide vane, from the radially outer foot region of the turbine guide vane as far as the radially inner head region. The cooling air is introduced from the head region into the contiguous U-shaped ring. The latter is cooled by the cooling air flowing past. Moreover, an excess pressure of the cooling air is intended to prevent hot gas from penetrating into the cavity formed by the head region of the turbine guide vanes and by the U-shaped ring lying below them.
One problem, in this case, is that, for manufacturing and cost reasons, the U-shaped ring usually consists of a material of relatively low temperature resistance. When flowing through the turbine guide vane, the cooling air, as a rule, heats up to the maximum permissible temperature of the turbine guide vane. Thus, when it flows into the U-ring, the cooling air which is already at a very high temperature may not provide sufficient cooling of the U-ring in the case of small cooling-air quantities which would suffice for cooling the turbine guide vane of a rear stage which is not very hot, as compared with the other turbine guide vane stages. This presents a problem also because the cooling air introduced into the cavity formed by the U-ring and the turbine vane head region, after flowing through the cavity, is discharged and flows in the direction of the rearmost, largely uncooled heat-sensitive turbine moving blade disk.
The solution adopted hitherto for solving the problem has been to conduct a large amount of cooling air through a central bore of the turbine guide vane or through a cooling-air duct of a largely hollow-cast turbine guide vane.