T. Yasa

Unsteady Strong Shock Interactions in a Transonic Turbine: Experimental and Numerical Analysis

The aerothermal performance of highly loaded high-pressure turbines is abated by the unsteady impact of the vane shocks on the rotor. This paper presents a detailed physical analysis of the stator-rotor interaction in a state-of-the-art transonic turbine stage at three pressure ratios. The experimental characterization of the steady and unsteady flowfield was performed in a compression tube test rig. The calculations were performed using ONERAs code elsA. This original comparison leads to an improved understanding of the complex unsteady flow physics of a high-pressure turbine stage. The vane shock impingement on the rotor originates a separation bubble on the rotor crown that is responsible for the generation of high losses. A model based on rothalpy conservation has been used to assess the pressure loss. The analysis of the unsteady forcing relates the shock patterns with the force fluctuations.

Performance analysis of a transonic high-pressure turbine

Abstract Efficiency is a crucial design parameter in any turbine development. This research presents a detailed investigation on the efficiency of a modern transonic high-pressure turbine. The work focuses on the study of the efficiency loss at different stage loading factors (ranging from ΔH/U2 = 1.3 to 2.7) and various flow factors (Vax/U = 0.41 to 0.90). In particular, the present research is of utmost importance to understand the effect of the vane trailing edge shocks in the turbine stage efficiency. The experimental work was carried out in a compression tube facility that allows testing of the turbine at temperature ratios, Re and Mach numbers, encountered in real engines. The efficiency is measured by the mechanical method. Experiments were performed with two different vanes (cooled and uncooled) at two stagger angles, four rotational speeds (4570–6500 r/min) and three pressure ratios (p01/Ps3 ranging from 2.42 to 5.12). The effect of the change of reaction and rotor incidence is correlated with the performance. Three-dimensional Navier-Stokes calculations aid the interpretation of the results.

Determination of the Efficiency of a Cooled HP Turbine in a Compression Tube Facility

The efficiency of a cooled transonic turbine stage was measured in a compression tube facility. The formulation takes into account mechanical losses, coolant flows and leakage flows. The proposed methodology allows computing the efficiency independently from the test rig. Owing to the short testing time (∼0.5 s), specific measurement and data reduction techniques are used. The paper details how the power, the overall mass flow, the mass-averaged inlet quantities and the stage pressure ratio are determined. The measurements of mechanical losses and of the thermodynamic properties of coolant and leakage flows are also described. Finally, results are presented and supported by an uncertainty analysis that identifies the random and systematic contributions to the final error.Copyright

Application of Hot-Wire Anemometry in a Blow-Down Turbine Facility

In order to test HP turbine stages under engine representative conditions on a heat transfer point of view, blow-down test rigs are often used. In these rigs the evolution of gas temperature, pressure, and density is similar to a step function. Hence, the use of hot-wires, which are sensitive to flow velocity, density, and temperature, is more difficult than in an incompressible flow at constant temperature. This investigation describes how the data reduction can be performed in such an environment in order to extract the velocity. The gas temperature is measured with a thermocouple and the gas density is derived from the measurement of the total pressure thanks to an iterative procedure. Once the velocity is derived, the turbulence can be computed. The effectiveness of the method is first demonstrated in a heated jet where both pressure and temperature are varied. Tests in the turbine facility are performed at turbine inlet temperatures of 480 K. Thus, overheat ratios up to 1.9 had to be used, leading to a very high temperature of the tungsten platinum coated wire. The aging of the probe was very fast, causing a drift in the voltage output between the successive tests. A technique is proposed to minimize the aging effect. It consists in adapting the calibration based on the resistance of the wire measured before each test. Measurements were carried out at the turbine inlet and rotor outlet. At the turbine inlet, velocity radial profiles are obtained together with measurements of the turbulence intensity. The time-averaged data is compared with pneumatic probe measurements. At the rotor exit, the time-resolved periodic velocity fluctuations are analyzed using a phase-locked average technique.

Impact of a multi-splitter vane configuration on the losses in a 1.5 turbine stage

This study presents the influence of a multi-body architecture on the aerodynamic performance of a low-pressure stator. The novel design has been studied numerically in a one-and-a-half stage turbine by means of a three-dimensional Reynolds-averaged Navier–Stokes simulation. This numerical research compares the behaviour of low-pressure stator composed of two different vanes against a conventional axisymmetric single airfoil row. The computational fluid dynamics predictions were calibrated using experimental aerodynamic measurements. Loss generation mechanisms were evaluated for the conventional and multi-splitter cascades at nominal and off-design conditions. At design conditions, the novel stator and conventional designs show comparable performances. However, the performance is drastically reduced at off-design conditions due to the sensitivity of the structural vanes to flow incidence. This article addresses the performance limitation for the multi-splitter vane configuration and presents a new tool to analyse the non-uniform flow conditions associated with such novel design. This procedure should help researchers in addressing any non-axisymmetric design.

Performance of a Nozzle Guide Vane in Subsonic and Transonic Regimes Tested in an Annular Sector

The understanding of shock interactions and mixing phenomena is crucial to design and analysis of advanced turbines. A nozzle guide vane (NGV) is experimentally investigated at subsonic and transonic off-design conditions (M2is of 0.6 and 0.95) in an annular sector at the Royal Institute of Technology (KTH). The effect of cooling ejection (3% of main stream mass flow rate) on the downstream flow field is also studied. The airfoil loading is monitored with pneumatic taps. The downstream pressure field is characterized at four different axial locations using a 5-hole probe and a total pressure probe that contains a single piezo-resistive transducer. The probe with a piezo resistive transducer is also used as a virtual 3-hole probe to measure the flow angle. The time-averaged yaw angle measured with the virtual 3-hole probe is in agreement with the 5-hole probe data. At subsonic conditions the wake causes a pressure loss of 7% of the upstream total pressure and covers 25% of the pitch whereas the pressure deficit is doubled in transonic operation. The coolant ejection results in an additional loss of 2% of the upstream total pressure. The flow speed does not have a significant effect on the wake width at 7% Cax . However, the low pressure region has different width at far downstream depending on the flow velocity. The fillet at the hub region has a significant effect on the secondary flow development. The frequency spectrums at the different conditions clearly reveal the shear layers. The results aim to help the characterization of mixing phenomena downstream of the NGV.Copyright

Aerodynamic Analysis of an Innovative Low Pressure Vane Placed in a S-Shape Duct

In this paper the aerodynamics of an innovative multisplitter LP stator downstream of a high-pressure turbine stage is presented. The stator row, located inside a swan necked diffuser, is composed of 16 large structural vanes and 48 small airfoils. The experimental characterization of the steady and unsteady flow field was carried out in a compression tube rig under engine representative conditions. The one-and-a-half turbine stage was tested at three operating regimes by varying the pressure ratio and the rotational speed. Time-averaged and time-accurate surface pressure measurements are used to investigate the aerodynamic performance of the stator and the complex interaction mechanisms with the HP turbine stage. Results show that the strut blade has a strong impact on the steady and unsteady flow field of the small vanes depending on the vane circumferential position. The time-mean pressure distributions around the airfoils show that the strut influence is significant only in the leading edge region. At off-design condition (higher rotor speed) a wide separated region is present on the strut pressure side and it affects the flow field of the adjacent vanes. A complex behavior of the unsteady surface pressures was observed. Up to four pressure peaks are identified in the time-periodic signals. The frequency analysis also shows a complex structure. The spectrum distribution depends on the vane position. The contribution of the harmonics is often larger than the fundamental frequency. The forces acting on the LP stator vanes are calculated. The results show that higher forces act on the small vanes but largest fluctuations are experienced by the strut. The load on the whole stator decreases 30% as the turbine pressure ratio is reduced by approx. 35%.Copyright

Robust Post-Processing Procedure for Multi-Hole Pressure Probes

Aerodynamic probes have been extensively used in turbine performance measurements for over 60 years to provide flow direction and Mach numbers. In turbomachinery applications the absence of adequate optical access prevents the use of laser-Doppler-anemometry (LDA), laser-two-focus velocimetry, particle-image-velocimetry (PIV). Moreover, multi-hole pressure probes are more robust than hot-wire or hot-fiber probes, and less susceptible to gas contamination. The pressure readings are converted into flow direction using calibration maps. Some researchers tried to model theoretically or numerically the calibration map to speed up the process. Due to manufacturing abnormalities, experimental calibration is still essential. The calibration map is obtained in a wind tunnel varying the yaw and pitch angles, while recording the hole-pressures. With the advent of powerful computers, researchers introduced sophisticated techniques to process the calibration data. Depending on the geometry or manufacturing imperfections a conventional calibration map is distorted, with multiple crossings resulting in the inability to identify a unique flow direction. In the current paper, a new calibration and data processing procedure is introduced for multi-hole probe measurements. The new technique relies on a set of calibration data rather than a calibration map. The pressure readings from each hole are considered individually through a minimization algorithm. Hence, the new technique allows computing flow direction even when a hole is blocked during the test campaign. The new methodology is demonstrated in a five-hole probe including estimates on the uncertainty.Copyright

Development of Experimental Techniques for the Characterization of Helicoidal Fin Arrays in Transonic Flow Conditions

The current research focuses on the aerodynamic investigation of the three dimensional turbofan bypassflow and its interaction with a finned air/oil surface heat exchanger integrated at the inner wall of the secondary duct. This paper addresses the experimental methodology employed in a new transonic facility characterized by a complex 3D test section. The development of accurate and relatively short time response measurement techniques is essential for the evaluation of the flow by means of map measurements at different locations along the test section. Flow angles, total and static pressures are computed using a new methodology for the processing of the pressure readings acquired by means of hemispherical five-hole probes. The sensibility of the probes and the errors on the angle determination are analyzed. Errors lower than 0.4 deg. for the determination of yaw and flow angles are obtained. The response of the shielded thermocouples designed for the temperature measurements are studied by means of conjugate heat transfer simulations. The numerical methodology is described and the steady and transient temperature effects evaluated. An innovative procedure for shear stress measurements based on oil-dot techniques is discussed and applied on a representative test bench for validation. Results are comparable with theoretical and empirical wall shear stress correlations for the case of a flat plate in subsonic flow conditions.

Experimental Heat Transfer Investigation in a Multisplitter LP Vane Architecture

This paper reports the external convective heat transfer in an innovative low pressure vane with multisplitter configuration. Three small aerodynamic blades are positioned between each structural vane, providing a novel architecture for ultra-high by-pass ratio aero-engines, with increased LP vane radius and swan-neck diffuser to link the HP turbine. The measurements have been performed in the compression tube test rig of the von Karman Institute, using single layered thin film gauges. Time-averaged and time-resolved heat transfer distributions are presented for the three aerovanes and for the structural blade, at three pressure ratios tested at representative conditions of modern aeroengines, with M2,is ranging from 0.87 to 1.07 and a Reynolds number of about 106 . This facility is specially suited to control the gas-to-wall temperature ratio. Accurate time-averaged heat transfer distributions around the aerovanes are assessed, that allow characterizing the boundary layer status for each position and pressure ratio. The heat transfer distribution around the structural blade is also obtained, depicting clear transition to turbulence, as well as particular flow features on the pressure side, like separation bubbles. Unsteady data analysis reveals the destabilizing effect of the rotor left-running shock on the aerovanes boundary layer, as well as the shift of transition onset for different blade passing events.Copyright