Ennio Spano

Numerical Investigation of Airfoil Clocking in a Three-Stage Low-Pressure Turbine

A quasi-three-dimensional, blade-to-blade, time-accurate, viscous solver was used for a three-stage LP turbine study. Due to the low Reynolds number, transitional computations were performed. Unsteady analyses were then carried out by varying the circumferential relative position of consecutive vanes and blade rows to study the effects of clocking on the turbines performance. A clocking strategy developed in order to limit the number of configurations to be analyzed is discussed. The optimum analytically-determined clocking position is illustrated for two different operating conditions, referred to as cruise and takeoff. The effects of clocking on wake interaction mechanisms and unsteady blade loadings is presented and discussed. For low Reynolds number turbine flows, the importance of taking transition into account in clocking analysis is demonstrated by a comparison with a fully turbulent approach.

Parametric Optimization of a High-Lift Turbine Vane

Numerical optimization techniques are increasingly used in the aerodynamic design of turbomachine blades. In the present paper, an existing three-dimensional high-lift turbine cascade was redesigned by means of CFD analyses and optimization techniques, based on the blade geometrical parameterization. A new parametric design tool was developed for this purpose. Blade geometry was handled in a fully three dimensional way, using Bezier curves and surfaces for both camber-surface and thickness distribution. In the optimization procedure different techniques were adopted: a Genetic Algorithm (GA) strategy made it possible to considerably reduce two-dimensional profile losses, while the optimal stacking line was found based on a successive Design of Experiments (DOE) analysis. As a result, a new high-lift blade with higher performance was obtained; in addition, the effect of hub/tip leaning was presented and discussed.Copyright

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.

A Comparison between Radial Rakes of Sensors and Axial Arrays of Microphones for the Experimental Investigation of Tone Noise in LPTs

The experimental investigation of turbomachinery noise requires to measure the acoustic field within annular test rigs. For this purpose, different in-duct measurement techniques are available. The most widely used are the arrays of flush-mounted microphones installed at the duct walls and the rakes of dynamic pressure sensors placed along the radial direction of the duct. Both methods have been implemented and used for several test campaigns performed at the Avio cold flow rig in Turin (Italy), in order to assess the acoustic performances, from the tonal noise point of view, of a model low-pressure-turbine representative of modern turbofan applications. This paper aims at performing a back-toback comparison between the results obtained by using these different experimental techniques. The results allowed to make assessments about the effectiveness of each experimental technique at analyzing tonal noise, highlighting the peculiar features of the two methods and their relative advantages and drawbacks.

Validation of a 3D Linearized Method for Turbomachinery Tone Noise Analysis

A fully three-dimensional noise propagation analysis method accounting for 3D radial modes was developed in order to predict the tone noise emissions of a multistage low-pressure turbine. This propagation procedure employs a time-linearized aeroacoustic solver with 3D non-reflecting boundary conditions to compute the acoustic response of each single row and allows the acoustic waves to propagate across adjacent rows up to the turbine exit, where such perturbations radiate to the far field. This method follows the evolution of the acoustic modes within the machine, taking into account both circumferential and radial mode orders. The overall turbine exit sound power level, due to specific rotor-stator interactions, may thus be evaluated. Some numerical results are compared with experimental data measured in an acoustic two-stage test rig which is representative of the last stages of a low-pressure turbine in terms of row geometry, blade loading and flow deflection.Copyright

Boundary Layer Separation Control on a Flat Plate With Adverse Pressure Gradients Using Vortex Generators

The continuous tendency in modern aeroengine gas turbines towards reduction of blade count and ducts length may lead to aerodynamic loading increase beyond the limit of boundary layer separation. For this reason boundary layer separation control methods, up to now mostly employed in external aerodynamics, begin to be experimented in internal flows applications. The present paper reports the results of a detailed experimental study on low profile vortex generators used to control boundary layer separation on a large-scale flat plate with prescribed adverse pressure gradients. Inlet turbulent boundary layer conditions and pressure gradients are representative of aggressive turbine intermediate ducts. This activity is part of a joint European research program on Aggressive Intermediate Duct Aerodynamics (AIDA). The pressure gradients on the flat plate are generated by increasing the aperture angle of a movable wall opposite to the flat plate. To avoid separation on the movable wall, boundary layer suction is applied on it. Complementary measurements (surface static pressure distributions, surface flow visualizations by means of wall mounted tufts, instantaneous and time-averaged velocity fields in the meridional and cross-stream planes by means of Particle Image Velocimetry) have been used to survey the flow with and without vortex generators. Three different pressure gradients, which induce turbulent separation in absence of boundary layer control, were tested. Vortex generators height and location effects on separation reduction and pressure recovery increase were investigated. For the most effective VGs configurations detailed analyses of the flow field were performed, that demonstrate the effectiveness of this passive control device to control separation in diffusing ducts. Particle Image Velocimetry vector and vorticity plots illustrate the mechanisms by which the vortex generators transfer momentum towards the surface, re-energizing the near-wall flow and preserving the boundary layer from separation.Copyright

Numerical Investigation of Three-Dimensional Clocking Effects in a Low Pressure Turbine

One and a half stages of a low pressure turbine were investigated using a three-dimensional, time-accurate, viscous solver. Unsteady analyses were carried out by varying the circumferential relative position of consecutive vanes to study the effects of clocking on performance. Assuming that efficiency improvements by clocking are linked to the wake tangential position with respect to the successive blade, a certain circumferential shift in this position can be observed along the blade height due to blade twist and non-radial stacking, giving different contributions. In order to assess this phenomenon, results from three-dimensional computations were compared with a quasi three-dimensional analysis at mid-span. The effects of clocking on wake interaction mechanisms and unsteady blade loadings are presented and discussed.Copyright

An Experimental Investigation of Passive Control Device: Blade Wake Interaction on a Ultra High Lift Turbine Profile

The transition of the boundary layer subjected to unsteady wake-passing in a linear cascade of ultra high lift profiles has been investigated at the Avio Aerodynamics Laboratory. The blade profiles are representative of the turbine nozzle mid section of a long range aeroengine. Measurements were performed at the cruise Reynolds number. A surface hot-film array was adopted to survey the boundary layer nature and the periodic variations related to the passing wakes. A phase-locked ensemble averaging technique was employed in order to separate the random fluctuations from the periodic ones. Results have been represented in space-time plots in order to provide an overall view of the time-dependent phenomena in terms of the quasi wall shear stress statistical moments, that are important parameters for the analysis of the boundary layer transition and separation. Passive control devices may be adopted to suppress boundary layer laminar separation at critical conditions (low Reynolds numbers, ultra high lift profiles). In the present experimental investigation a wavy step device has been mounted on the suction side of the blade. The effects of this boundary layer control device on the transition process and profile losses have been investigated at cruise Reynolds number, with and without incoming wakes.Copyright

Numerical Investigation of Airfoil Clocking in a Three-Stage Low Pressure Turbine

A quasi–three–dimensional, blade–to–blade, time–accurate, viscous solver w as used for a three–stage LP turbine study Due to the low Reynolds number, transitional computations were performed. Unsteady analyses were then carried out by varying the circumferential relative position of consecutive vanes and blade rows to study the effects of clocking on the turbine’s performance. A clocking strategy developed in order to limit the number of configurations to be analyzed is discussed. The optimum analytically–determined clocking position is illustrated for two different operating conditions, referred to as cruise and takeoff.The effects of clocking on wake interaction mechanisms and unsteady blade loadings is presented and discussed. For low Reynolds number turbine flows, the importance of taking transition into account in clocking analysis is demonstrated by a comparison with a fully turbulent approach.Copyright