Marina Ubaldi

An experimental investigation of stator induced unsteadiness on centrifugal impeller outflow

Detailed flow measurements were taken in a centrifugal turbomachine model to investigate the aerodynamic influence of the vaned diffuser on the impeller flow. The model consists of an unshrouded centrifugal impeller with backswept blades and a rotatable vaned diffuser, which enables a continuous variation of the vaned diffuser location with respect to the measuring points. Phase-locked ensemble-averaged velocity components have been measured with hot-wire probes at the impeller outlet for 30 different relative positions of the probe with respect to the diffuser vanes. The data also include the distribution of the ensemble-averaged static pressure at the impeller front end, taken by means of miniature fast response pressure transducers flush-mounted at the impeller stationary casing. By circumferentially averaging the results obtained for the different circumferential probe locations, the periodically perturbed impeller flow has been split into a relative steady flow and a stator-generated unsteadiness. The results for the different probe positions have also been correlated in time to obtain instantaneous flow field images in the relative frame, which provide information on the various aspects of the diffuser vane upstream influence on the relative flow leaving the impeller.

Detailed Velocity and Turbulence Measurements of the Profile Boundary Layer in a Large Scale Turbine Cascade

Extensive measurements of velocity and turbulence have been performed by means of a two-component fibre-optic laser Doppler velocimeter, to investigate the profile boundary layer development on a large scale turbine cascade.Flow field investigation has been integrated with data obtained by surface-mounted hot-film gauges in order to get direct information on the boundary layer nature and on its time varying characteristics.Measurements were detailed enough to allow constructing mean velocity and Reynolds stress boundary layer profiles giving an in-depth description of the boundary layer development along both suction and pressure surfaces through laminar, transitional and turbulent regimes.Copyright

Relative Flow and Turbulence Measurements Downstream of a Backward Centrifugal Impeller

The paper presents the results of an experimental investigation on the three-dimensional relative flow at the exit of the backward bladed centrifugal impeller of the high-pressure stage of a two-stage biregulating pump-turbine model, operating at the pump nominal point. Mean velocity, Reynolds stress tensor, and total pressure of the relative flow have been measured with stationary hot-wire probes and fast response miniature pressure transducers, by means of a phase-locked ensemble-average technique. The results, shown in terms of secondary vector plots and contours of mean flow characteristics and Reynolds stress components, give a detailed picture of the flow kinematic structure and of the complex relative total pressure loss and turbulence distributions

An experimental study of the unsteady characteristics of the turbulent near wake of a turbine blade

Abstract The paper reports the results of an experimental investigation of the time-varying characteristics of the flow in the turbulent near wake behind the central blade of a large-scale turbine cascade. The blade wake has been surveyed by means of a two-component laser Doppler velocimeter (LDV) over a close experimental grid extending from the blade trailing edge to six trailing edge diameters downstream. A phase-locked ensemble averaging technique has been applied to the LDV instantaneous data in order to separate coherent contributions due to large-scale organised structures from random contributions due to turbulence. A reference signal generated by a hot-wire probe, located at a fixed position within the flow, is utilised to associate each LDV velocity realisation with the phase of the vortex shedding. The hot-wire signal provides also an univocal time reference suitable for correlating all single-point measurements, allowing to construct sequences of images which show the time evolution of the organised structures embedded in the flow. In the discussion, the mean flow properties, the periodic structure of the unsteady flow and the vortex shedding mechanism are analysed. Similarities and differences between the present results for a turbine blade wake and those for wakes of circular cylinders are highlighted. Normal stresses due to the periodic motion and those due to turbulence fluctuations are resolved separately. Both contributions have the same order of magnitude in the region close to the trailing edge, where the vortices are forming, but the normal stresses associated with the periodic fluctuations decay more rapidly in streamwise direction.

Detailed flow measurements within the impeller and the vaneless diffuser of a centrifugal turbomachine

Abstract The flow within the impeller and the vaneless diffuser of a centrifugal turbomachine model operating at the nominal point has been experimentally investigated. The radial and circumferential velocities and the associated turbulent fluctuations have been measured on seven cross-sectional circumferential surfaces, four within the impeller and three in the vaneless diffuser by means of a four-beam two-color fibre optic laser-Doppler velocimeter. To complete the flow investigation, LDV measurements have been performed on the blade-to-blade plane at mid-span. Data acquired have been processed by means of a phase locked ensemble-average technique in order to separate periodic from turbulent fluctuations. The results show the impeller flow structure, the development and decay of the blade and through-flow wakes and the main features of the diffusing flow. The present experimental work provides a well documented data base suitable for the assessment of turbomachinery flow prediction codes.

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.

Unsteady Aerodynamics of an Aeroengine Double Swirler Lean Premixing Prevaporizing Burner

Lean premixing prevaporizing (LPP) burners represent a promising solution for low-emission combustion in aeroengines. Since lean premixed combustion suffers from pressure and heat release fluctuations that can be triggered by unsteady large-scale flow structures, a deep knowledge of flow structures formation mechanisms in complex swirling flows is a necessary step in suppressing combustion instabilities. The present paper describes a detailed investigation of the unsteady aerodynamics of a large-scale model of a double swirler aeroengine LPP burner at isothermal conditions. A three-dimensional (3D) laser Doppler velocimeter and an ensemble-averaging technique have been employed to obtain a detailed time-resolved description of the periodically perturbed flow field at the mixing duct exit and associated Reynolds stress and vorticity distributions. Results show a swirling annular jet with an extended region of reverse flow near to the axis. The flow is dominated by a strong periodic perturbation, which occurs in all the three components of velocity. Radial velocity fluctuations cause important periodic displacement of the jet and the inner separated region in the meridional plane. The flow, as expected, is highly turbulent. The periodic stress components have the same order of magnitude of the Reynolds stress components. As a consequence the flow-mixing process is highly enhanced. Turbulence acts on a large spectrum of fluctuation frequencies, whereas the large-scale motion influences the whole flow field in an ordered way that can be dangerous for stability in reactive conditions.

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...

Reynolds stress distribution downstream of a turbine cascade

Abstract The results are presented of an experimental investigation on the three-dimenstional turbulent flow developing downstream of a turbine cascade. The cascade has an aspect ratio of 1.05 and the flow turning is about 84°. Measurements were performed in four planes located respectively at 10, 30, 54, and 90% of an axial chord downstream of the blade trailing edge. The mean velocity field and the energy loss coefficient distributions were obtained by five-hole pressure probe measurements, while the six Reynolds stress tensor components were determined by using hot-wire probes. The tests were made at low-velocity conditions, corresponding to an isentropic outlet Mach number of 0.3. The results show that high turbulence levels, both for normal and shear stress components, are associated with regions affected by secondary vortices. The evolution of the Reynolds stress distributions follows closely the development of secondary vortices, showing that convection plays a significant role in determining Reynolds stress distributions. In the first plane turbulence was found to be highly anisotropic, but in the following planes a clear trend to isotropy can be detected. The local production of turbulent kinetic energy in the four measuring planes is also presented. The eddy viscosity distributions, calculated independently from the three shear stresses and the corresponding mean strain components, show the vectorial nature of this quantity and put in evidence some severe limitations to the applicability of the eddy viscosity hypothesis to these complex flows. The results presented can be used as reference data to validate Navier-Stokes numerical methods for three-dimensional flows and in particular to improve turbulence models for turbomachinery applications.

TURBULENCE MEASUREMENTS DOWNSTREAM OF A TURBINE CASCADE AT DIFFERENT INCIDENCE ANGLES AND PITCH-CHORD RATIOS

An experimental investigation on a linear turbine cascade has been carried out to study the effects induced by incidence angle and pitch-chord ratio variations on the three-dimensional turbulent flow downstream of the cascade.Previous mean flow measurements have shown how these parameters influence the energy losses and the secondary velocity field. Now detailed hot wire measurements have been performed on a plane located at 22 per cent of an axial chord downstream of the trailing edge, in order to determine the distribution of all the six Reynolds stress tensor components, for three incidence conditions (i = −30, 0, +30 deg) and for three pitch-chord ratios (s/c = 0.58, 0.72, 0.87).Significant changes of the turbulent flow structure, interesting magnitude and distribution of the Reynolds stress components, have been observed for all the considered test conditions.The analysis of the results shows the correlation between the mean flow features and the turbulent quantities and the relationship between the energy loss production and the blade loading variation.The presented data are also suitable for assessing the behaviour of turbulence models in complex 3D flows, on design and off-design conditions.Copyright