Juri Bellucci

The Influence of Roughness on a High-Pressure Steam Turbine Stage: An Experimental and Numerical Study

This work deals with the influence of roughness on high-pressure steam turbine stages. It is divided in three parts. In the first one, an experimental campaign on a linear cascade is described, in which blade losses are measured for different values of surface roughness and in a range of Reynolds numbers of practical interest. The second part is devoted to the basic aspects of the numerical approach and consists of a detailed discussion of the roughness models used for computations. The fidelity of such models is then tested against measurements, thus allowing their fine-tuning and proving their reliability. Finally, comprehensive computational fluid dynamics (CFD) analysis is carried out on a high-pressure stage, in order to investigate the influence of roughness on the losses over the entire stage operating envelope. Unsteady effects that may affect the influence of the roughness, such as the upcoming wakes on the rotor blade, are taken into account, and the impact of transition-related aspects on the losses is discussed.

Numerical aerodynamic damping evaluation of high-pressure steam turbine blades for aeromechanical characterization

A validated non-linear uncoupled method for flutter stability analysis was employed to estimate the aerodynamic damping of an HP (High-Pressure) steam turbine blade row.Usually such blade rows are not affected to flutter instability problems, yet an estimation of the aerodynamic damping can be useful for an accurate aeromechanical characterization of these kind of blade rows. The geometry under investigation is a typical steam turbine blade row at design point. Computational aeroelastic analyses are performed on the more relevant modeshape, sampling the nodal diameters, in order to well describe the typical aeroelastic stability curve. The presence of the tip shroud implies a strong mechanical coupling between adjacent blades resulting in complex modeshapes with high frequency, significantly different from those usually analyzed in the flutter analysis.The results in term of aerodynamic damping curves are rather different from the usually sinusoidal shape. This is due to the large variation of the frequency over the analyzed nodal diameters, especially at low nodal diameters range. This results are useful to give a better insight in the aeroelastic response of this type of blades.© 2016 ASME

Support vector machine classification applied to the parametric design of centrifugal pumps

ABSTRACT In this article the parametric design of centrifugal pumps is addressed. To deal with this problem, an approach based on coupling expensive Computational Fluid Dynamics (CFD) computations with artificial neural networks as a regression meta-model was proposed in 2015 by Checcucci, Schneider, Marconcini, Rubechini, Arnone, De Franco, and Coneri, ‘A novel approach to parametric design of centrifugal pumps for a wide range of specific speeds’—Proceedings of the 12th international symposium on experimental and computational aerothermodynamics of internal flows, Lerici (SP), Italy. Paper No. 121. Here, the previously proposed approach is improved by also including the use of support vector machines as a classification tool. The classification process is aimed at identifying parameter combinations corresponding to manufacturable machines among the much larger number of unfeasible ones. A binary classification problem on an unbalanced dataset has to be faced. Numerical tests show that the addition of this classification tool helps to reduce considerably the number of CFD computations required for the design, providing large savings in computational time.

Some Experiences About the Impact of Unsteadiness in Turbine Flows

This paper describes some experiences about impact of unsteadiness in turbine flows, with a special focus on the effects of potential interaction on aerodynamic performance. The main motivation consists in trying to identify some design areas in which some further margins of improvement could be found, provided the designer chooses the proper computational framework. The underlying idea is that the approximations associated with the steady-state picture of a turbine stage might prevent the designer from unlocking the full potential of the stage, especially when the design requirements imply a challenging aerodynamics. To this end, three common design topics are presented in which the step from the classical steady-state approach to the time-accurate one unveils relevant issues, which in turn have an impact on aerodynamic performance: stator/rotor interaction in transonic stages, the choice of the axial gap between stator and rotor, and the choice of the blade count ratio. In all reported cases, significant departures are found between steady and time-averaged results, and the basic fluid mechanisms responsible for them are examined. In particular, an attempt is made to emphasize limitations deriving from of the steady-state picture of the turbine flow field, in order to warn the designer about the possible traps of the steady-state assumption.© 2015 ASME

Using CFD to Enhance the Preliminary Design of High-Pressure Steam Turbines

This paper focuses on the use of the CFD for improving a steam turbine preliminary design tool.Three-dimensional RANS analyses were carried out in order to independently investigate the effects of profile, secondary flow and tip clearance losses, on the efficiency of two high-pressure steam turbine stages. The parametric study included geometrical features such as stagger angle, aspect ratio and radius ratio, and was conducted for a wide range of flow coefficients to cover the whole operating envelope. The results are reported in terms of stage performance curves, enthalpy loss coefficients and span-wise distribution of the blade-to-blade exit angles. A detailed discussion of these results is provided in order to highlight the different aerodynamic behavior of the two geometries.Once the analysis was concluded, the tuning of a preliminary steam turbine design tool was carried out, based on a correlative approach. Due to the lack of a large set of experimental data, the information obtained from the post-processing of the CFD computations were applied to update the current correlations, in order to improve the accuracy of the efficiency evaluation for both stages. Finally, the predictions of the tuned preliminary design tool were compared with the results of the CFD computations, in terms of stage efficiency, in a broad range of flow coefficients and in different real machine layouts.Copyright

Optimization of a High-Pressure Steam Turbine Stage for a Wide Flow Coefficient Range

In this paper a multi-objective, aerodynamic optimization of a high-pressure steam turbine stage is presented. The overall optimization strategy relies on a neural-network-based approach, aimed at maximizing the stage’s efficiency, while at the same time increasing the stage loading. The stage under investigation is composed of prismatic blades, usually employed in a repeating stage environment and in a wide range of operating conditions. For this reason, two different optimizations are carried out, at high and low flow coefficients. The optimized geometries are chosen taking into account aerodynamic constraints, such as limitation of the pressure recovery in the uncovered part of the suction side, as well as mechanical constraints, such as root tensile stress and dynamic behavior. As a result, an optimum airfoil is selected and its performance are characterized over the whole range of operating conditions. Parallel to the numerical activity, both optimized and original geometries are tested in a linear cascade, and experimental results are available for comparison purposes in terms of loading distributions and loss coefficients. Comparisons between measurements and calculations are presented and discussed for a number of incidence angles and expansion ratios.Copyright