Daniele Simoni

Off-Design Performance of a Highly Loaded Low Pressure Turbine Cascade Under Steady and Unsteady Incoming Flow Conditions

The off-design performance of a highly loaded low pressure (LP) turbine cascade has been experimentally investigated, at the Aerodynamics and Turbomachinery Laboratory of Genova University, under steady and unsteady incoming flow conditions. Tests have been performed for different Reynolds numbers (Re = 70,000 and Re = 300,000), in order to cover the typical LP turbine working range. The incidence angle has been varied between i = −9 deg and +9 deg, in order to test off-design conditions characterizing the engine. For the unsteady case, upstream wake periodic perturbations have been generated by means of a tangential wheel of radial rods. The cascade and the moving bars system have been located over a common bearing in order to make them rigidly rotating. This solution allows a proper comparison of the cascade robustness at the incidence angle variation under steady and unsteady incoming flows, since all the other operating parameters have been kept the same. In order to survey the variation of the unsteady boundary conditions characterizing the off-design operation of the downstream cascade, time-mean and time-resolved wake structures have been analyzed in detail. For what concerns the cascade performance, profile aerodynamic loadings and total pressure loss coefficients at the cascade exit have been surveyed for the different incidence angles under both steady and unsteady inflows. Different total pressure loss sensitivity at the incidence angle variation has been observed for the steady and the unsteady inflow conditions. Hot-wire anemometer has been employed to obtain the time-mean pressure and suction side boundary layer velocity profiles at the blade trailing edge for the different conditions. The integral parameters at the cascade exit plane help to justify the different loss trend versus incidence angle found for the steady and the unsteady cases, explaining the different sensibility of the blade profile when this operates under realistic unsteady inflow condition.

Loading Distribution Effects on Separated Flow Transition of Ultra-High-Lift Turbine Blades

The suction side boundary-layer evolution in two ultra-high-lift low-pressure turbine blade cascades, characterized by the same Zweifel number but two different aerodynamic loading distributions, has been experimentally analyzed under steady and unsteady incoming flows. For the steady inflow case, a suction side boundary-layer separation has been detected for both cascades. Time-mean velocity and unresolved unsteadiness distributions have been exploited to survey the dynamics of the separated flow transition mode. The spectral analysis reveals that only the midloaded cascade is affected by a Kelvin–Helmholtz instability that induces the separated shear layer rollup, which provokes high losses. Results obtained for the unsteady case reveal that linear stability mechanisms drive the amplification of velocity fluctuations carried by wakes with dynamics similar to that characterizing the steady inflow condition. A rollup vortex has been found to be generated for both cascades as a consequence of the wake–shea...

Velocity and turbulence measurements in a separating boundary layer with and without passive flow control

Abstract The present paper reports the results of a detailed experimental study on low profile vortex generators (VGs) used to control boundary layer separation on a large-scale flat plate with prescribed adverse pressure gradients. This activity is part of a joint European research program on Aggressive Intermediate Duct Aerodynamics. The inlet turbulent boundary layer and the pressure gradient over the flat plate are representative of aggressive turbine intermediate ducts. By regulating the inclination of the wall opposite to the flat plate, different pressure gradients, typical of turbine intermediate ducts, can be obtained. To avoid separation on the movable wall, boundary layer suction is applied. Previous measurements showed the effectiveness of VGs in delaying separation and revealed their optimum configuration for the different prescribed pressure gradients. In the present work, laser Doppler velocimetry (LDV) is applied to the most significant pressure gradient case, in order to obtain a more thorough knowledge of the near-wall flow field. Velocity and turbulence profiles are determined up to the near-wall region in order to provide an in-depth analysis of turbulent boundary layer at separation conditions, with and without application of control devices. LDV allowed high spatial resolution and accurate statistical analysis of the boundary layer velocities. The results show velocity and turbulence profiles typical of separated turbulent boundary layers for the baseline case, and non-conventional unseparated boundary layer profiles when VGs are installed on the flat plate.

Influence of Aerodynamic Loading on Rotor-Stator Aerodynamic Interaction in a Two-Stage Low Pressure Research Turbine

The unsteady flow within a two-stage low-pressure research turbine equipped with high lift profiles has been investigated in detail for three different aerodynamic loading conditions. Experiments have been carried out at low speed. Velocity and turbulence intensity in the blade-to-blade plane at midspan have been measured by means of a crossed hot-wire probe, upstream and downstream of each blade row. The probe has been traversed circumferentially over 1.5 bladings pitch and the phase-locked data acquisition and ensemble average technique have been used to reconstruct the flow in space and time. The effects of multistage configuration have been identified and analyzed by considering the velocity components and turbulence intensity. Potential interaction from the downstream blading in relative motion, periodic wake perturbations from the upstream blading and preceding stage perturbations make the flow in the second stage extremely complex. Overall the flow downstream of rotors is perturbed in space by upstream and downstream stators, while flow downstream of stators is mostly perturbed in time by rotor effects. As expected, high lift profiles are significantly sensitive to incidence variation, with this effect further enhanced by the multistage cumulative interactions.

Endwall Effusion Cooling System Behaviour Within a High-Pressure Turbine Cascade: Part 1—Aerodynamic Measurements

The secondary flow field in a large-scale high-pressure turbine cascade with micro-holed endwall cooling has been investigated at the Genova Laboratory of Aerodynamics and Turbomachinery in cooperation with Avio S.p.A in the framework of the European Project AITEB-2. The experimental investigation has been performed for the baseline configuration, with a smooth solid endwall installed, and for the cooled configuration with a micro-holed endwall providing micro-jets ejection from the wall. Two different cooling flow rates were investigated and the experimental results are reported in the paper. Different measurement techniques have been employed to analyze the secondary flow field along the channel and in a downstream tangential plane. Particle Image Velocimetry has been utilized to quantify the blade-to-blade velocity components in a plane located close to the endwall and in the midspan plane. Hot-wire measurements have been performed in a tangential plane downstream of the blade trailing edges in order to survey the micro-jets effects on the secondary flows behavior. The total pressure distributions, for the different blowing conditions, have been measured in the downstream tangential plane by means of a Kiel pneumatic probe. The results, represented in color plots of velocity, pressure loss coefficient and turbulent kinetic energy distributions, allow the identification of the endwall effusion cooling effects on location and strength of the secondary vortical structures. The thermal investigation of the effusion system is discussed in Part 2 of the paper.Copyright

Radial Inflow Turbine Design Through Multi-Disciplinary Optimisation Technique

Multidisciplinary design optimisation (MDO) is nowadays widely employed to obtain advanced turbomachines design. The aim of this work is to provide a complete tool for the aeromechanical design of a radial inflow gas turbine. The high rotational speed of such machines, especially if used for micro cogenerative power plants, coupled with high exhaust gas temperature, exposes blades to really high centrifugal and thermal stresses; thus the aerodynamics optimisation has to be necessarily coupled with the mechanical one. Such an approach involves two different computational tools: a fully 3D Reynolds Averaged Navier-Stokes (RANS) solver is used for the aerodynamic optimisation, while an open source Finite Element Analysis (FEA) solver is employed for the mechanical integrity assessment. The geometry parameterization is handled with a commercial tool that employs b-spline advanced curve for blades and vanes definition. The aerodynamic mesh generation is managed via dedicated tools provided by the CFD software and it is a fully structured hexahedral multi-block grid. The FEA mesh is built by means of a harmonic map approach, which is able to provide high quality second order unstructured grid preserving geometrical features starting from boundary surfaces of the fluid domain. The finite element calculation provides stresses, displacements and eigenmodes that are used for mechanical integrity assessments while the CFD solver provides performance parameters and local thermodynamic quantities. Due to the high computational cost of both these two solvers, a metamodel, such as an artificial neural network, is employed to speed up the process. The interaction between two codes, the mesh generation and the post processing of the results is obtained via in-house developed scripting modules. Results obtained are presented and discussed.© 2015 ASME

Boundary Layer Development on a High-Lift LP Turbine Profile Under Passing-Wakes Conditions

The boundary layer development on the suction side of a high-lift LP turbine profile has been experimentally investigated under steady and unsteady flow conditions in the range of Reynolds numbers between 70000 and 300000. Upstream wake periodic perturbations are generated by means of a tangential wheel of radial rods. The paper reports the results of the investigations performed for both steady and unsteady inflow cases (reduced frequency f+ = 0.62) for Re = 300000 and Re = 70000, representative of nominal and reduced Reynolds number operating conditions, respectively. A phase-locked ensemble-averaging technique has been employed to reconstruct the phase-averaged velocity and unresolved unsteadiness boundary layer profiles from the hotwire instantaneous velocities. Phase sequences of the boundary layer development, as well as time-space plots of velocity and unresolved unsteadiness in normal and streamwise directions highlight the complex wake/boundary layer interaction mechanism. While at the larger test Reynolds number the wake/boundary layer interaction does not substantially influence the transition process, at the lower test Reynolds number the boundary layer wake receptivity triggers the transition process, strongly attenuating the large separation bubble occurring at steady conditions.Copyright

Secondary Flows in Low-Pressure Turbines Cascades: Numerical and Experimental Investigation of the Impact of the Inner Part of the Boundary Layer

Due to the low level of profile losses reached in low-pressure turbines (LPT) for turbofan applications, a renewed interest is devoted to other sources of loss, e.g. secondary losses. At the same time, the adoption of high-lift profiles has reinforced the importance of these losses. A great attention, therefore, is dedicated to reliable prediction methods and to the understanding of the mechanisms that drive the secondary flows. In this context, a numerical and experimental campaign on a state-of-the-art LPT cascade was carried out focusing on the impact of different inlet boundary layer (BL) profiles. First of all, detailed RANS analyses were carried out in order to establish dependable guidelines for the computational setup. Such analyses also underlined the importance of the shape of the inlet BL very close to the endwall, suggesting tight requirements for the characterization of the experimental environment. The impact of the inlet BL on the secondary flow was experimentally investigated by varying the inlet profile very close to the endwall as well as on the external part of the BL. The effects on the cascade performance were evaluated by measuring the span-wise distributions of flow angle and total pressure losses. For all the inlet conditions, comparisons between CFD and experimental results are discussed. Besides providing guidelines for a proper numerical and experimental setup, the present paper underlines the importance of a detailed characterization of the inlet BL for an accurate assessment of the secondary flows.

Phase-Locked Investigation of Secondary Flows Perturbed by Passing Wakes in a High-Lift LPT Turbine Cascade

The present work describes the experimental investigations carried out at the Aerodynamics and Turbomachinery Laboratory of Genoa University aimed at characterizing the unsteady features of the secondary flows in a High-Lift Low Pressure Turbine cascade perturbed by incoming wakes. The investigations have been carried out at the nominal exit flow Reynolds number of 300000 in a 5-blade large-scale linear cascade. Hot-wire phase-locked ensemble-averaging technique has been applied to analyze in depth the time-dependent velocity and turbulence intensity distributions in a downstream tangential plane during a wake period. A multiple rotation technique has been used in order to measure the three velocity components as well as the Reynolds stress tensor terms. Acquired data are presented in terms of the phase-dependent mean velocity, turbulence and vorticity maps in order to distinguish between the contributions due to incoming wake velocity defect and those due to the turbulence carried by wakes on the phase-dependent secondary flow structures. Results clearly highlight a significant distortion and weakening of the passage vortex when the upstream wake passes through the measuring domain. Also an evident displacement of the passage vortex position has been observed in the wake period. This analysis allows understanding the difference in the three dimensional time mean structures of the exit flow field between the steady and unsteady operations.Copyright

Near Wall Measurements in a Separating Turbulent Boundary Layer with and without Passive Flow Control

The present paper reports the results of a detailed experimental study on low profile vortex generators used to control a turbulent boundary layer separation on a large-scale flat plate with prescribed adverse pressure gradient, which conditions are representative of aggressive turbine intermediate ducts. Laser Doppler Velocimetry has been employed to investigate the velocity fields in different measurement planes to characterize the turbulent boundary layer at separation conditions, without and with control devices application.