Cooling performance of the steam-cooled vane in a steam turbine cascade

With the increasing demand of the electricity, an efficiency improvement and thereby a reduced CO2 emission of the coal-fired plants are expected in order to reach the goal of the Kyoto protocol. In the medium term, increase of thermal efficiency of the steam cycle can be achieved by a rise of the process parameters. Currently, live steam pressures and temperatures up to 300 bar and 923 K are planned as a next step. This means an increase of 50-100 K in comparison to plants presently under operation. Such a large increase will only be possible, if all thermally high loaded components of the plant are made of new materials and/or new technologies for cooling and thermal insulation are applied. Closed circuit steam cooling of blades and vanes in modern steam turbines is a promising technology in order to establish elevated live steam temperatures in future steam turbine cycles. Due to the cooling of the components, materials can be used, which are common in todays application. In this paper, a steam-cooled test vane in a cascade with external hot steam flow is analysed numerically by applying a Conjugate Calculation Technique. The boundary conditions of the hot steam are derived from the planned inlet parameter of the HP steam turbine in a medium term and a long term in order to show the application potential of such a cooling technique. The supply steam parameters of the cooling are set equal to those of the outlet steam from the HP steam turbine. The results show that the steam cooling technique can reach necessary cooling effectiveness in the steam turbine by using only minor amounts of cooling steam. The internal pan of the vane is cooled homogeneously in both investigated cases. Basic features and phenomena of the steam cooling technique in steam turbines are discussed in the paper with respect to future application. It has been found that the trailing edge region of the steam-cooled vane is less critical with respect to high thermal load than in gas turbine application with the same configuration. At the leading edge of the vane, high thermal load can be detected because of the direct impingement of the hot steam. Improvement of the leading edge cooling is recommended by increasing cooling steam amount for the leading edge channel. Thus, a larger diameter of the channel can increase internal cooling efficiency for the leading edge. Due to the different heat transfer characteristics on the suction side and pressure side, the suction side shows also a higher temperature distribution in the investigated cases.