Stefano Cecchi

A Redesign Strategy to Improve the Efficiency of a 17-Stage Steam Turbine

A three-dimensional Reynolds averaged Navier–Stokes solver was applied to the aerodynamic redesigning of a 17-stage steam turbine. The redesign procedure was divided into three steps. In the first one, a single embedded stage was considered, and an optimization of stator lean and rotor twist was carried out by applying suitable repeating inlet/outlet boundary conditions. In the second step, a proper geometrical transformation between the original reference stage and the optimized one was identified and then applied to all other turbine stages, thus leading to a first approximation of the redesigned turbine. Finally, a neural-network-based refinement of the stator and rotor twist of each stage was performed to account for its actual position and operating conditions within the meridional channel. In this work, a detailed description of the redesign procedure is provided, and the aerodynamic characteristics of the optimized geometry are discussed and compared with the original ones.

The Impact of Gas Modeling in the Numerical Analysis of a Multistage Gas Turbine

In this work a numerical investigation of a four stage heavy-duty gas turbine is presented. Fully three-dimensional, multistage, Navier-Stokes analyses are carried out to predict the overall turbine performance. Coolant injections, cavity purge flows, and leakage flows are included in the turbine modeling by means of suitable wall boundary conditions. The main objective is the evaluation of the impact of gas modeling on the prediction of the stage and turbine performance parameters. To this end, four different gas models were used: three models are based on the perfect gas assumption with different values of constant c p , and the fourth is a real gas model which accounts for thermodynamic gas properties variations with temperature and mean fuel/air ratio distribution in the through-flow direction. For the real gas computations, a numerical model is used which is based on the use of gas property tables, and exploits a local fitting of gas data to compute thermodynamic properties. Experimental measurements are available for comparison purposes in terms of static pressure values at the inlet/outlet of each row and total temperature at the turbine exit.

A CFD-based throughflow method with an explicit body force model and an adaptive formulation for the S2 streamsurface

The paper describes the development and validation of a novel CFD-based throughflow model. It is based on the axisymmetric Euler equations with tangential blockage and body forces and inherits its numerical scheme from state-of-the-art CFD solver (TRAF code), including real-gas capabilities. A crucial aspect of the numerical procedure is represented by an adaptive approach for the meridional flow surface, which employs a new time-dependent equation to accommodate incidence and deviation effects, and which allows the explicit calculation of the blade body force. A realistic distribution of entropy along the streamlines is proposed in order to compute dissipative forces on the basis of a distributed loss model. The throughflow code is applied to the investigation of the NASA rotor 67 transonic fan and of a four stage low-pressure steam turbine at design conditions. The performance of the method is evaluated by comparing predicted operating characteristics and spanwise distributions of flow quantities with the results of CFD, steady, viscous calculations and experimental data.

A Method for Axial Compressor Start-Up Assessment

This paper presents a method to assess compressor mass flow, pressure line and stability, including blow-off lines behavior, during a heavy duty gas turbine (GT) start up. A Company proprietary compressor mean line analysis 1-dimensional code (C1D) has been calibrated on 3D RANS calculations and matched with a 1D Fanno model to simulate the compressor behavior during GT start-up; discharge compressor pressure and secondary air system boundary conditions are provided by filed test data. C1D is based on correlations tuned with available CFD and experimental data; within the present work the code has been validated with experimental data at low mechanical speed too, so that it can be used in such applications where conventional CFD analyses are most likely to fail. In this paper C1D is used to analyze the compressor start-up characteristic from idle to full speed no load operation. Pressure rise along the compressor and blow-off line mass flow are compared and validated throughout a field test campaign. Finally the method developed is applied to an evolutionary compressor in order to analyze how the stage-wise load distribution varies with mechanical speed and blow-off mass flow.Copyright

Sensitivity Analysis to Flutter for Front Stages Compressor Blades

Heavy duty gas turbine front stages compressor blades aero-elastic behavior is deeply analyzed and investigated by means of an uncoupled, non-linear and time-accurate CFD URANS solver. The travelling-wave approach and the energy method have been applied in order to assess the aerodynamic damping (in terms of logarithmic decrement) for each inter blade phase angle (IBPA) and thus to localize the flutter stability region. The work is mainly focused on a sensitivity analysis with respect to blade operating conditions, eigen-mode shapes and frequency in order to improve the understanding of flutter mechanism and to identify the key parameters. Transonic, supercritical and subsonic blades are investigated at different operating conditions with their corresponding eigenmode and eigen-frequency (first and second flexural mode and first torsional). The results show that non-linear effects can be neglected for subsonic blades. Besides, the modal-shape and the shock structure, if any, are identified to play a key role for flutter stability.Copyright

Axial Compressor Degradation Effects on Heavy Duty Gas Turbines Overall Performances

The operation of a gas turbine is the result of the aero-thermodynamic matching of several components which necessarily experience aging and degradation over time. An approach to treat degradation phenomena of the axial compressor is provided, with an insight into the impact they have on compressor operation and on overall GT performances. The analysis is focused on the surface fouling of compressor blades and on rotor tip clearances variation.A modular model is used to simulate the gas turbine operation in design and off-design conditions and the aerodynamic impact of fouling and rotor tip clearances increase is assessed by means of dedicated loss and deviation correlations implemented in the 1D mid-streamline code of the compressor modules.The two different degradation sources are individually considered and besides the overall GT performance parameters, the analysis includes an evaluation of the compressor degradation impact on the secondary air system.Copyright