High turn-down operating of heavy-duty gas turbines in modern Combined Cycle Plants requires a highly efficient secondary air system to ensure the proper supply of cooling and sealing air. Thus, accurate performance prediction of secondary flows in the complete range of operating conditions is crucial. The paper gives an overview of the secondary air system of Ansaldo F-class AEx4.3A gas turbines. Focus of the work is a procedure to calculate the cooling flows, which allows investigating both the interaction between cooled rows and additional secondary flows (sealing and leakage air) and the influence on gas turbine performance. The procedure is based on a fluid-network solver modelling the engine secondary air system. Parametric curves implemented into the network model give the consumption of cooling air of blades and vanes. Performances of blade cooling systems based on different cooling technology are presented. Variations of secondary air flows in function of load and/or ambient conditions are discussed and justified. The effect of secondary air reduction is investigated in details showing the relationship between the position, along the gas path, of the upgrade and the increasing of engine performance. In particular, a section of the paper describes the application of a consistent and straightforward technique, based on an exergy analysis, to estimate the effect of major modifications to the air system on overall engine performance. A set of models for the different factors of cooling loss is presented and sample calculations are used to illustrate the splitting and magnitude of losses. Field data, referred to AE64.3A gas turbine, are used to calibrate the correlation method and to enhance the structure of the lumped-parameters network models.

Different labyrinth seal configurations are used in modern heavy-duty gas turbine such as see-through stepped or honeycomb seals. The characterization of leakage flow through the seals is one of the main tasks for secondary air system designers as well as the evaluation of increase in temperature due to heat transfer and windage effects. In high temperature turbomachinery applications, knowledge of the heat transfer characteristics of flow leaking through the seals is needed in order to accurately predict seal dimensions and performance as affected by thermal expansion. This paper deals with the influence of clearance on the leakage flow and heat transfer coefficient of a contactless labyrinth seal. A scaled-up planar model of the seal mounted in the inner shrouded vane of the Ansaldo AE94.3A gas turbine has been experimentally investigated. Five clearances were tested using a stationary test rig. The experiments covered a range of Reynolds numbers between 5000 and 40000 and pressure ratios between 1 and 3.3. Local heat transfer coefficients were calculated using a transient technique. It is shown that the clearance/pitch ratio has a significant effect upon both leakage loss and heat transfer coefficient. Hodkinson’s and Vermes’ models are used to fit experimental mass flow rate and pressure drop data. This approach shows a good agreement with experimental data.

In order to achieve significant secondary air savings in heavy duty gas turbines, a remarkable item of improvement is the reduction of seal flows for turbine stator-rotor cavities. The optimization of such flows allows to avoid waste of air, obligatory with standard labyrinth seals, to ensure the minimum sealing flow rate in all operating conditions. Based on the experience gained in the design of sealing system of stator-rotor cavities with standard seals, the project of installation of inter-stage brush-seals was undertaken incorporating such devices into the vane seal rings of 2nd and 3rd turbine stages of a AE94.3A Gas Turbine (GT). The paper offers a detailed description of the installation project. The following describes in detail the design flow process and the calculation methodologies used, step by step, to define the geometry of brush-seals in order to ensure mechanical integrity and durability, needed in the commercial operation, without thereby affecting the performance. The first prototype of brush-seal devices has been installed on a AE94.3A4 unit of the Ansaldo fleet. In order to verify the behavior of stator-rotor sealing system, in particular in terms of temperature and pressure variations, vane seal rings have been equipped with special instrumentation. A series of tests to optimize the set points of bleed control valves was carried out.