Closed circuit steam cooling of blades and vanes in modern gas turbines is an promising alternative instead of film-cooling using compressor air. The temperature drop across the first-stage nozzle, which is convectively steam-cooled, is reduced significantly in comparison to an intensive film-cooled vane using compressor air. Thus, the firing temperature (temperature in front of the first-stage blade row) can be increased while the combustion temperature can remain as low as necessary for low-Nox purpose.
In this paper, a steam-cooled test configuration consisting of a 3-vane cascade is numerically analysed. A computer code using a Conjugate Calculation Technique is applied. The CHTflow code has been developed at the Institute of Steam and Gas Turbines in Aachen. Due to the direct coupling of fluid flow and solid body, heat transfer boundary conditions at the external and internal surfaces become unnecessary. Validation of the code for a similar convection-cooled configuration is also given here.
The presented investigations focus on the thermal load analysis and the cooling efficiency analysis of the test configuration. It consists of a planar cascade with a convection-cooled central vane where cooling fluid can be supplied to 22 radial passages. One main aspect of the paper is to show the influence of cooling steam conditions (low-, medium & high-pressure steam supply) on the local and global cooling efficiencies. The results show that, for reaching a defined cooling efficiency level, medium steam pressure supply might be advantageous in comparison to a high-pressure level in supply. Although a lower pressure level demands an increase in steam mass flow, the overall effect on the thermal efficiency of the whole process is acceptable if one keeps in mind the advantages of handling steam at lower pressure levels. For further comparison, convective air-cooling with reasonable cooling conditions and comparable flow and heat transfer characteristics is analysed. For the given geometry of the configuration, sufficient cooling of the trailing edge becomes problematic for steam- and air-cooling application.