This paper introduces the new fast response aerodynamic probe, which was recently developed at the ETH Zurich. The technique provides time-resolved, three-dimensional flow measurements using the virtual four sensor technique. The concept and the evaluation of the virtual four sensor probe is discussed in detail. The basic results consist of yaw and pitch flow angles as well as the total and static pressure. They combine to form the unsteady, three dimensional flow vector. The outer diameter of the cylindrical probe head was miniaturized to 0.84mm, hence probe blockage effects as well as dynamic lift effects are reduced. The shape of the probe head was optimized in view of the manufacturing process as well as aerodynamic considerations. The optimum geometry for pitch sensitivity was found to be a cylindrical surface with the axis perpendicular to the probe shaft. The internal design of the probes led to a sensor cavity eigenfrequency of 44kHz for the yaw sensitive and 34kHz for the pitch sensitive probe. Data acquisition is done with a fully automated traversing system, which moves the probe within the test rig and samples the signal with a PC-based A/D-board. An error analysis implemented into the data reduction routines revealed acceptable accuracy for flow angles as well as pressures for many turbomachinery flows. Depending on the dynamic head of the application the yaw angle is accurate within ±0.35° and pitch angle within ±0.7°. In the final section, a comparison of time averaged results to five hole probe measurements is discussed. The advantages of the new probe, beside its unique smallness, are the complete unsteady kinematic information and the improved recording of unsteady total pressure measurement as it is pointed out in a comparison against a 2D virtual three sensor probe.

This paper presents time-resolved flow field measurements at the exit of the first rotor blade row of a two stage shrouded axial turbine. The observed unsteady interaction mechanism between the secondary flow vortices, the rotor wake and the adjacent blading at the exit plane of the first turbine stage is of prime interest and analyzed in detail. The results indicate that the unsteady secondary flows are primarily dominated by the rotor hub passage vortex and the shed secondary flow field from the upstream stator blade row. The analysis of the results revealed a roll-up mechanism of the rotor wake layer into the rotor indigenous passage vortex close to the hub endwall. This interesting mechanism is described in a flow schematic within this paper. In a second measurement campaign the first stator blade row is clocked by half a blade pitch relative to the second stator in order to shift the relative position of both stator indigenous secondary flow fields. The comparison of the time-resolved data for both clocking cases showed a surprising result. The steady flow profiles for both cases are nearly identical. The analysis of the probe pressure signal indicates a high level of unsteadiness that is due to the periodic occurrence of the shed first stator secondary flow field.

The need to increase overall turbine efficiency is always a driving force for redesigning a turbine stage. In particular, the labyrinth leakage flows in the endwall regions contribute to an increase of the overall loss generation. In order to asses this mechanism, a detailed study of the effects of labyrinth seal geometry variation on the blade performance is presented. Two different shroud seal geometries have been experimentally investigated in a two stage low speed turbine facility. The seal geometries differ in the size and shape of the re-entry cavity. The baseline seal is designed with a large rectangular re-entry cavity volume in order to dissipate the kinetic energy of the accelerated leakage flow after the seal gap. The re-entry cavity volume of the alternative seal design is reduced in size and a spline shaped contour is added to the endwall using annular inserts. This modification alters the gas path of the leakage jet and changes the incidence angles on the downstream blade rows. The measurements are performed with state of the art pneumatic and fast response pressure probes at various planes within the turbine stage. It is found that the inserts improved the flow profile uniformity at the endwalls. The measurements within the stator passage reveal the origin of the tip passage vortex formation at the blade suction side, already at the inlet to the stator passage. This result does not conform to the classical secondary flow theory, which suggests that the passage vortex migrates from the pressure to the suction side within the stator passage. The origin and formation of the secondary flow passage vortices at rotor hub and stator tip is described in a flow schematic. The generation of streamwise and tangential vorticity at the interaction area of leakage and main flow field also is studied and discussed. The measured overall polytropic turbine efficiency for the second seal configuration, relative to the baseline case, is reduced by 0.3%. The change in the re-entry flow angle of the leakage gas path reduces the negative incidence angle on the rotor hub and increases it at the stator tip leading edge. The secondary flow and mixing loss is reduced at the hub and increased at the tip in the second test case with the smaller cavity volume. Hence, the combination of small clearances and inserts in the re-entry cavities shows no beneficial effect on the overall turbine efficiency.

This paper describes the design and construction of a new two stage axial turbine test facility, christened “Lisa”. The research objective of the rig is to study the impact (relevance) of unsteady flow phenomena upon the aerodynamic performance, this being achieved through the use of systematic studies of parametric changes in the stage geometry and operating point. Noteworthy in the design of the rig is the use of a twin shaft arrangement to decouple the stages. The inner shaft carries the load from the first stage whilst the outer is used with an integral torque-meter to measure the loading upon the second stage alone. This gives an accurate measurement of the loading upon the aerodynamically representative second stage, which possesses the correct stage inlet conditions in comparison to the full two stage machine which has an unrealistic axial inlet flow at the first stator. A calibrated Venturi nozzle measures the mass flow at an accuracy of below 1%, from which stage efficiencies can be derived. The rig is arranged in a closed loop system. The turbine has a vertical arrangement and is connected through a gear box to a generator system that works as a brake to maintain the desired operating speed. The turbine exit is open to ambient pressure. The rig runs at a low pressure ratio of 1.5. The maximum Mach number at stator exit is 0.3 at an inlet pressure of 1.5 bar. The maximum mass flow is 14 kg/sec. Nominal rotor design speed is 3000 RPM. The tip to hub blade ratio is 1.29, and the nominal axial chord is 50 mm. The rig is designed to accommodate a broad range of measurement techniques, but with a strong emphasis upon unsteady flow methods, for example fast response aerodynamic pressure probes for time-resolved flow measurements. The first section of this paper describes the overall test facility hardware. This is followed by a detailed focus on the torque measurement device including stage efficiency measurements at operating conditions in Lisa. Discussion of measurement techniques completes the paper.

This paper focuses on the flow within the inlet cavity of a turbine rotor tip labyrinth seal of a two stage axial research turbine. Highly resolved, steady and unsteady three-dimensional flow data are presented. The probes used here are a miniature five-hole probe of 0.9 mm head diameter and the novel virtual four sensor fast response aerodynamic probe (FRAP) with a head diameter of 0.84mm. The cavity flow itself is not only a loss producing area due to mixing and vortex stretching, it also adversely affects the following rotor passage through the fluid that is spilled into the main flow. The associated fluctuating mass flow has a relatively low total pressure and results in a negative incidence to the rotor tip blade profile section. The dominating kinematic flow feature in the region between cavity and main flow is a toroidal vortex, which is swirling at high circumferential velocity. It is fed by strong shear and end wall fluid from the pressure side of the stator passage. The static pressure field interaction between the moving rotor leading edges and the stator trailing edges is one driving force of the cavity flow. It forces the toroidal vortex to be stretched in space and time. A comprehensive flow model including the drivers of this toroidal vortex is proposed. This labyrinth seal configuration results in about 1.6% turbine efficiency reduction. This is the first in a series of papers focusing on turbine loss mechanisms in shrouded axial turbines. Additional measurements have been made with variations in seal clearance gap. Initial indications show that variation in the gap has a major effect on flow structures and turbine loss.