Improvements in CFD-based throughflow methods for analysis and design of axial turbines

The research activity presented in this work has been devoted to the extension and improvement of a CFD-based throughflow code aimed at the development of a meridional analysis tool for modern industrial design systems for turbomachinery. The throughflow code inherits its numerical scheme from a state-of-the-art CFD solver (TRAF code) and incorporates real gas capabilities, three-dimensional flow features, and spanwise mixing models. Secondary flow effects are introduced via a concentrated vortex model. Tip gap and shroud leakage effects are modelled in terms of source vectors in the system of governing equations. Also, film cooling and purge flow injections are taken into account as source terms vectors that are applied in selected regions of the meridional flowpath. The impact of part-span shrouds and snubbers are considered, on a local basis, through suitable body force fields. The advection upstream splitting method (AUSM+ -up) upwind strategy has been adopted as a basis to construct a numerical flux scheme explicitly suited for throughflow applications. The original formulation has been adapted to handle real gas flows and to embed the treatment of body force fields in a fully consistent framework. In order to relieve the time-step limitations associated with source terms, an implicit treatment of the axisymmetric and force vectors has been considered. During the research activity a methodology for gas turbines off-design analyses, based on the application of the throughflow method, has also been developed. The effectiveness of the proposed methodology will be discussed by reporting the results obtained for three of the test cases used for the validation activity, for which detailed experimental data are available (the T106 high-lift, low-pressure turbine blade, the KTH 4b subsonic high-pressure steam turbine stage, and the CT3 stage high-pressure transonic gas turbine). Each one of these allows us the assessment of the various physical models included in the framework. Finally, the capabilities of the throughflow procedure is assessed by applying it to the study of some industrial axial turbine configurations designed and manufactured by Ansaldo Energia. The first one is a four-stage, air-cooled gas turbine. A detailed analysis at design point will be presented together with an extensive off-design study over a wide range of operating conditions with varying expansion ratios and operating speed. The last two test cases are the low-pressure modules of two large steam turbines. The assessment of the throughflow predictions will be discussed by scrutinizing them against 3D CFD analyses carried out with the TRAF code and the available experimental data.