Fast Response Aerodynamic Pressure Probe Research Articles

Unsteady Investigation of Intermediate Turbine Ducts Under the Influence of Purge Flows

Abstract The aerodynamic flow field at the outlet of a high-pressure turbine (HPT) stage in a modern turbofan engine is highly unsteady and strongly influenced by periodic fluctuations from the HPT rotor, also interacting with the HPT stationary vanes. Downstream of the last HPT stage, the flow enters the intermediate turbine duct (ITD), where new unsteady components are generated, due to the interaction between the incoming flow and the struts supporting the ITD. This paper illustrates the differences between the unsteady flow behavior in two state-of-the-art ITD configurations for turbofan engines: the turbine center frame and the turbine vane frame. The experimental results discussed in this work are obtained in the Transonic Test Turbine Facility, situated at Graz University of Technology, Austria. To achieve engine-representative operating conditions at the ITD inlet and outlet sections, the tested setups include not only the duct, but also the last HPT stage and the first low-pressure turbine stage row. Additionally, the test rig features a secondary air supply system, capable of feeding purge air to every stator–rotor cavity in the test vehicle. Unsteady measurement data at the inlet and outlet of the two duct configurations are acquired with fast-response aerodynamic pressure probes, for two purge flow conditions. Azimuthal mode decomposition is applied to phase-averaged data at selected radial positions, to isolate and quantify the contribution of different interaction modes to the overall unsteady flow field.

Influence of the Rotor-Driven Perturbation on the Stator-Exit Flow within a High-Pressure Gas Turbine Stage

In stator–rotor interaction studies on axial turbines, the attention is commonly focused on the unsteady rotor aerodynamics resulting from the periodic perturbations induced by the stator flow structures. Conversely, less interest has been historically attracted regarding the influence of the rotor on the flow released by the stator, correlated to propagation of the blade potential field upstream of the rotor leading edge. In this paper, experiments in the research high-pressure turbine of the Laboratory of Fluid-Machines of the Politecnico di Milano, performed by applying a fast-response aerodynamic pressure probe, alongside fully-3D time-accurate CFD simulations of the flow, are combined with the aim of discussing the rotor-to-stator interaction. While rotating, the rotor induces periodic perturbations on the pressure and velocity field in the stator–rotor gap, altering the evolution of the total quantities and the flow rate discharged by each stator channel and eventually triggering energy-separation effects which result in total pressure and total temperature oscillations in the stator-exit flow. Such oscillations were found to rise up to almost ±10% of the stage total temperature drop.

Uncertainty Evaluation on Multi-Hole Aerodynamic Pressure Probes

Abstract In the frame of a continuous improvement of the performance and accuracy in the experimental testing of turbomachines, the uncertainty analysis on measurements instrumentation and techniques is of paramount importance. For this reason, since the beginning of the experimental activities at the Laboratory of Fluid Machines (LFM) located at Politecnico di Milano (Italy), this issue has been addressed and different methodologies have been applied. This paper proposes a comparison of the results collected applying two methods for the measurement uncertainty quantification to two different aerodynamic pressure probes: sensor calibration, aerodynamic calibration and probe application are considered. The first uncertainty evaluation method is the so-called “uncertainty propagation” method (UPM); the second is based on the “Monte Carlo” method (MCM). Two miniaturized pressure probes have been selected for this investigation: a pneumatic five-hole probe and a spherical fast-response aerodynamic pressure probe (sFRAPP), the latter applied as a virtual four-hole probe. Since the sFRAPP is equipped with two miniaturized pressure transducers installed inside the probe head, a specific calibration procedure and a dedicated uncertainty analysis are required.

Impact of Turbine-Strut Clocking on the Performance of a Turbine Center Frame

Abstract Previous studies have indicated a potential for improving the performance of a turbine center frame (TCF) duct by optimizing the clocking position between the high-pressure turbine (HPT) vanes and TCF struts. To assess the impact of clocking on the performance, a new test vehicle with a clockable ratio of HPT vanes to TCF struts, consisting of an HPT stage (aerodynamically representative of the second-stage HPT engine), a TCF duct with nonturning struts, and a first-stage low-pressure turbine vane, was designed and tested in the transonic test turbine facility (TTTF) at Graz University of Technology. This article quantifies the performance impact of clocking and describes the mechanisms causing TCF flow field changes, leveraging both experimental and numerical data. Other areas in the TCF duct impacted by the choice of the HPT vane circumferential position including the strength of unsteady HPT-TCF interaction modes, TCF strut incidence changes, and carryover effects to the first low-pressure turbine (LPT) vane are additionally highlighted. Five-hole-probe (5HP) area traverses and kielhead-rake traverses were used to assess the flow field at the TCF exit and to calculate the pressure loss. The flow field at the TCF exit shows significant differences depending on the circumferential position of the HPT vane. A relative performance benefit of 5% was achieved. A series of unsteady RANS simulations were performed to support the measured results, understand, and characterize the relevant loss mechanisms. The observed performance improvement was related to interaction between the HPT secondary-flow structures and the TCF struts. The impact of the HPT vane clocking on the unsteady flow field downstream of the TCF was investigated using fast-response aerodynamic pressure probe (FRAPP) area traverses and analyzed by means of modal decomposition. In this way, the individual azimuthal modes were ranked by their amplitude, and a dependency of the clocking position was observed and quantified.

Spatial and temporal resolution of a fast-response aerodynamic pressure probe in grid-generated turbulence

The spatial and temporal resolution of a fast-response aerodynamic pressure probe (FRAP) is investigated in a benchmark flow of grid-generated turbulence. A grid with a mesh size of M=6.4 mm is tested for two different free-stream velocities, hence, resulting in Reynolds numbers of Re_M= {4300,12800}. A thorough analysis of the applicability of the underlying assumptions with regard to turbulence isotropy and homogeneity is carried out. Taylor’s frozen turbulence hypothesis is assumed for the calculation of deducible flow quantities, like the turbulent kinetic energy or the dissipation rate. Furthermore, besides the examination of statistical quantities, velocity spectra of measurements downstream of the grid are quantified. Results of a small fast-response five-hole pressure probe equipped with piezo-resistive differential pressure sensors are compared to single-wire hot-wire constant temperature anemometry data for two different wire lengths. Estimates of temporal and spatial turbulent scales (e.g., Taylor micro scale and Kolmogorov length scale) show good agreement to data in the literature but are affected by filtering effects. Especially in the energy spectra, very high bandwidth content cannot be resolved by the FRAP, which is mainly due to bandwidth limits in the temporal calibration of the FRAP and the minimal resolution of the integrated sensors.Graphic abstract

Unsteady Flow Interactions Between a High- and Low-Pressure Turbine

Abstract This paper presents the unsteady flow interactions between an engine-representative high-pressure turbine (HPT) and low-pressure turbine (LPT) stage, connected by a turbine center frame (TCF) duct with nonturning struts. The setup was tested at the high-speed two-spool test turbine facility at the Institute for Thermal Turbomachinery and Machine Dynamics at Graz University of Technology and includes relevant purge and turbine rotor tip leakage flows. Due to the complexity of such a test, the unsteady component interactions in an HPT–TCF–LPT module have not received much attention in the past and require additional analysis to determine new approaches for further performance improvements on the system level. The flow downstream of an HPT is highly unsteady and dominated by stator–rotor interactions, which affect the flow behavior through the downstream TCF and LPT. To capture the unsteady flow structures, time-resolved aerodynamic measurements were carried out with a fast-response aerodynamic pressure probe (FRAPP) at three different measurement planes. In this paper, the time-resolved and phase-averaged flow fields with respect to the HPT and LPT trigger are studied. Since the two rotors are uncorrelated, the applied method allows the identification of the flow structures induced by either of them. Upstream of the LPT stage, the HPT flow structures evolving through the TCF duct dominate the flow fields. Downstream of the LPT stage, the flow is affected by both the HPT and the LPT secondary flow structures. The interactions between the various stator rows and the two rotors are detected by means of time-space plots and modal decomposition. To describe the fluctuations induced by both rotors, particularly the rotor–rotor interaction, the rotor synchronic averaging (RSA) is used to analyze the flow field downstream of the LPT. This paper highlights the need to account for the HPT-induced unsteady mechanisms in addition to the LPT flow structures and the interaction of both to arrive at improved LPT designs.

Technology Development of Fast-Response Aerodynamic Pressure Probes

This paper presents and discusses the recent developments on the Fast-Response Aerodynamic Pressure Probe (FRAPP) technology at the Laboratorio di Fluidodinamica delle Macchine (LFM) of the Politecnico di Milano. First, the different geometries developed and tested at LFM are presented and critically discussed: the paper refers to single-sensor or two-sensor probes applied as virtual 2D or 3D probes for phase-resolved measurements. The static calibration of the sensors inserted inside the head of the probes is discussed, also taking into account for the temperature field of application: in this context, a novel calibration procedure is discussed and the new manufacturing process is presented. The dynamic calibration is reconsidered in view of the 15-years’ experience, including the extension to probes operating at different temperature and pressure levels with respect to calibration. As for the probe aerodynamics, the calibration coefficients are discussed and the most reliable set here is evidenced. A novel procedure for the quantification of the measurement uncertainty, recently developed and based on the Montecarlo methodology, is introduced and discussed in the paper.

On the Influence of an Acoustically Optimized Turbine Exit Casing Onto the Unsteady Flow Field Downstream of a Low Pressure Turbine Rotor

Modern low pressure turbines (LPT) are designed in order to fulfil a various number of requirements such as high endurance, low noise, high efficiency, low weight, and low fuel consumption. Regarding the reduction of the emitted noise, different designs of LPT exit guide vanes (aerodynamically and/or acoustically optimized) of the turbine exit casing (TEC) were tested, and their noise reduction capabilities and aerodynamic performance were evaluated. In particular, measurements of TEC-losses were performed, and differences in the losses were reported. Measurements were carried out in a one and a half stage subsonic turbine test facility at the engine relevant operating point approach. This work focuses on the study of the unsteady flow field downstream of an unshrouded LPT rotor. The influence on the upstream flow field of a TEC design including acoustically optimized vanes (inverse cut-off TEC) is investigated and compared with a second TEC configuration without vanes (Vaneless TEC), by means of fast response aerodynamic pressure probe (FRAPP) measurements. The second configuration served as a reference concerning the influence of turbine exit guide vanes (TEGVs) onto the upstream located LPT rotor. The interactions between the stator and rotor wakes, secondary flows, and the TEGVs potential effect are identified via modal decomposition according to the theory of Tyler and Sofrin. The main structures constituting the unsteady flow field are detected, and the role of the major interaction effects in the loss generation mechanism and in the acoustic emission is analyzed. This study based on the modal analysis of the unsteady flow field offers new insight into the main interaction mechanisms and their importance in the assessment of the aerodynamic and aeroelastic performance of modern LPT exit casings.

Transport of Entropy Waves Within a High Pressure Turbine Stage

This paper presents the results of an experimental study on the transport of entropy waves within a research turbine stage, representative of the key aero-thermal phenomenon featuring the combustor-turbine interaction in aero-engines. The entropy waves are injected upstream of the turbine by a dedicated entropy wave generator (EWG) and are released in axial direction; they feature circular shape with peak amplitude in the center and exhibit sinusoidal-like temporal evolution over the whole wave area. The maximum over-temperature amounts to 7% of the undisturbed flow, while the frequency is 30 Hz. The entropy waves are released in four azimuthal positions upstream of the stage, so to simulate four different burner-to-stator blade clocking. Time-resolved temperature measurements were performed with fast microthermocouples (FTC); the flow and the pressure field upstream and downstream of the stator and the rotor was measured with five-hole pneumatic probes and fast-response aerodynamic pressure probes. The entropy waves are observed to undergo a relevant attenuation throughout their transport within the stator blade row, but they remain clearly visible at the stator exit and retain their dynamic characteristics. In particular, the total temperature distribution appears severely altered by burner-stator clocking position. At the stage exit, the entropy waves loose their coherence, appearing spread in the azimuthal direction to almost cover the entire pitch in the outer part of the channel, while being more localized below midspan. Despite the severe and unsteady interaction of the entropy waves within the rotor, they retain their original dynamic character. A comparison with measurements performed by injecting steady hot streaks is finally reported, remarking both differences and affinities. As a relevant conclusion, it is experimentally shown that entropy waves can be proficiently simulated by considering a succession of hot streaks of different amplitude.

On Turbulence Measurements and Analyses in a Two-Stage Two-Spool Turbine Rig

Since the experiment in turbulence research is of very high importance for evaluating turbulence hypothesis, turbulence measurements were carried out in a two-stage two-spool transonic turbine test rig at the Institute for Thermal Turbomachinery and Machine Dynamics in Graz in which the two rotors are counter-rotating with two different rotational speeds. For the current measurement campaign, triple hot-wire probes, which represent a very new measurement technique in this test rig, were used and their results validated with a fast response aerodynamic pressure probe (FRAPP). Based on the data measured with this device, turbulence intensities may be determined using a method called Fourier filtering. If the classical ensemble averaging procedure with only one trigger is applied, the periodic fluctuations of the other rotor will artificially increase the stochastic fluctuations. Therefore, the two trigger signals of the two rotors require a special analysis method, which was established at Graz University of Technology. The results from this method will be compared to the classical triple decomposition, which uses only one trigger signal. With this analysis tool, it is not only possible to evaluate unsteady signals triggered by one of the two rotors, but also the unsteady interactions of the rotors can be determined and investigated.

Non-Ideal Compressible-Fluid Dynamics of Fast-Response Pressure Probes for Unsteady Flow Measurements in Turbomachinery

The dynamic response of pressure probes for unsteady flow measurements in turbomachinery is investigated numerically for fluids operating in non-ideal thermodynamic conditions, which are relevant for e.g. Organic Rankine Cycles (ORC) and super-critical CO2 applications. The step response of a fast-response pressure probe is investigated numerically in order to assess the expected time response when operating in the non-ideal fluid regime. Numerical simulations are carried out exploiting the Non-Ideal Compressible Fluid-Dynamics (NICFD) solver embedded in the open-source fluid dynamics code SU2. The computational framework is assessed against available experimental data for air in dilute conditions. Then, polytropic ideal gas (PIG), i.e. constant specific heats, and Peng-Robinson Stryjek-Vera (PRSV) models are applied to simulate the flow field within the probe operating with siloxane fluid octamethyltrisiloxane (MDM). The step responses are found to depend mainly on the speed of sound of the working fluid, indicating that molecular complexity plays a major role in determining the promptness of the measurement devices. According to the PRSV model, non-ideal effects can increase the step response time with respect to the acoustic theory predictions. The fundamental derivative of gas-dynamic is confirmed to be the driving parameter for evaluating non-ideal thermodynamic effects related to the dynamic calibration of fast-response aerodynamic pressure probes.

Development of a Fast-Response Aerodynamic Pressure Probe Based on a Waveguide Approach

Currently, fast-response aerodynamic probes are widely used for advanced experimental investigations in turbomachinery applications. The most common configuration is a virtual three-hole probe. This solution is a good compromise between probe dimension and accuracy. Several authors have attempted to extend the capabilities of these probes in terms of bandwidth and operating conditions. Even though differences exist between the solutions in the literature, all of the designs involve the positioning of a dynamic pressure sensor close to the measurement point. In general terms, the higher the frequency response, the more the sensor is exposed to the flow. This physical constraint puts a limit on the probe applicability since the measurement conditions have to comply with the maximum allowed operating conditions of the sensor. In other applications, when the conditions are particularly harsh and a direct measurement is not possible, a waveguide probe is commonly used to estimate the local pressure. In this device, the sensor is connected to the measurement point through a transmitting duct which guarantees that the sensor is operating in a less critical condition. Generally, the measurement is performed through a pressure tap and particular attention must be paid to the probe design in order to have an acceptable frequency response function. In this study, the authors conceived, developed, and tested a probe which combines the concept of a fast-response aerodynamic pressure probe with that of a waveguide probe. Such a device exploits the benefits of having the sensor far from the harsh conditions while maintaining the capability to perform an accurate flow measurement.

Unsteady Flow Evolution Through a Turning Midturbine Frame Part 2: Spectral Analysis

This paper analyzes the propagation of the aerodynamic deterministic stresses through a two-spool counter-rotating transonic turbine at Graz University of Technology. The test setup consists of a high-pressure stage, a diffusing midturbine frame with turning struts and a counter-rotating low-pressure rotor. The discussion of the data is carried out in this second part paper on the basis of spectral analysis. The theoretical framework for a double Fourier decomposition, in time and space, is introduced and discussed. The aim of the paper is the identification of the sources of deterministic stresses that propagate through the turbine. A fast-response aerodynamic pressure probe has been employed to provide time-resolved data downstream of the high-pressure rotor and of the turning strut. The fast-response aerodynamic pressure probe measurements were acquired together with a reference signal (a laser vibrometer) downstream of the high-pressure rotor to identify different sources of deterministic fluctuations. The discussion is completed by fast-response pressure measurements on the strut surface and computational fluid dynamics computation to detail which deterministic stress related to the high-pressure stage propagates through the duct. The double Fourier decomposition shows that structures at the periodicity of the rotor blade number decay, whereas the unsteadiness at the outlet of the duct is the result of vane–rotor–vane interaction.

Unsteady Flow Evolution Through a Turning Midturbine Frame Part 1: Time-Resolved Flow

This paper identifies and analyzes the propagation of aerodynamic deterministic stresses through a two-spool counter-rotating transonic facility representative of modern and future turbine aeroengine sections. The test setup consists of a high-pressure stage, a diffusing turning midturbine frame with turning struts, and a counter-rotating low-pressure rotor. The flowfield downstream of the high-pressure stage is strongly influenced by the stator–rotor interaction. Such a mechanism interacts again with the downstream turning midturbine frame leading to a vane–rotor–vane interaction, which affects the behavior of the low-pressure stage. The results presented were obtained using a fast-response aerodynamic pressure probe for unsteady measurements as well as three-dimensional unsteady Reynolds-averaged Navier–Stokes calculations. The work is presented in two parts. This first part focuses on the explanation of the flow physics that governs the convection of unsteady three-dimensional flow through the midturbine duct. Viscous and inviscid mechanisms are discussed as main drivers for the convection of wakes, secondary vortices, and shocks. The flowfield in the duct is characterized by three superimposed effects: 1) duct diffusion and radial pressure gradient together with turning strut potential field, 2) rotor unsteady work source, and 3) vane/blade interaction phenomena. The understanding of these mechanisms will eventually help to control the unsteadiness content in future architectures where reduced engine component length will enhance the interaction effects.

Experimental characterization of stall phenomena in a single-stage low-pressure axial compressor

The experimental characterization of the rotating stall phenomena appearing in a single-stage high-speed low-pressure axial compressor is presented in this paper. The compressor design is representative of an advanced direct drive turbofan booster. The stage is equipped with both fast response and steady instrumentation. Fast response pressure sensors are placed in the rotor casing and in the hub at the outlet of each row. The former are employed to perform a detailed survey on the tip leakage flows, and the latter allow to determine the extension of the stall cells across the stage. A fast response aerodynamic pressure probe is located at the rotor outlet to detect the size of the stall cells. The global performances of the machine are recorded by time-averaged measurements at the inlet and at the outlet of the stage. Tests recording the stall inception and the complete stall transients are performed at different speed lines in the VKI-R4 closed loop compressor test rig. The inception mechanism is determined by the comparison between the time mean and the time-resolved data. In terms of stall inception, the compressor exhibits long length scale at lower rotational speed and an abrupt stall at higher rotational speed. The characterization of the stall cells is performed in terms of number, size, and speed by using the unsteady data acquired with the fast response instrumentation. The compressor presents a single full span stall cell in all the speed lines investigated.

Measurement and decomposition of periodic flow structures downstream of a test turbine

This paper shows the experimental methodology which has been employed for the simultaneous aerodynamic and aeroacoustic analysis of a test turbine by means of a fast-response aerodynamic pressure probe. Two different phase references are used to resolve the fluctuations in time of the velocity and the pressure of phenomena occurring at different and unrelated periodicity. The phase-averaged fields are further post-processed by decomposing them into spatial-temporal Fourier coefficients. The deterministic unsteadiness is then simply represented by a limited number of Fourier coefficients, the physical meaning of which is discussed in details. In particular, this approach has been used to distinguish between propagating and non-propagating noise sources.

Analysis tools for the unsteady interactions in a counter-rotating two-spool turbine rig

Counter-rotating turbines represent the state of the art of actual and future aero-engines, built for a considerable reduction of weight and fuel consumption. However, in the open literature there is a limited number of experimental and computational research about the unsteady interactions in such complex configurations. The paper presents an experimental investigation on the two-spool counter-rotating transonic turbine at Graz University of Technology. The rig was designed in cooperation with MTU Aero Engines and Volvo Aero and considerable efforts were put on the adjustment of all relevant model parameters. The test setup consists of a high pressure (HP) stage, a diffusing mid turbine frame with turning struts and a shrouded low pressure (LP) rotor.A two-sensor fast response aerodynamic pressure probe (2S-FRAPP) has been employed to provide time-resolved aerodynamic area traverses downstream of the LP turbine. Structures at the blade passing frequency of each rotor are resolved by the use of the shaft encoders. Furthermore, a newly developed phase averaging procedure is introduced to take into account the combined rotor–rotor interactions. Different reconstructions of the time resolved flow field are then presented in the paper, where a detailed description of the post-processing tools and their interpretation is provided.

A method for the determination of turbulence intensity by means of a fast response pressure probe and its application in a LP turbine

This paper describes the measurements and the post-processing procedure adopted for the determination of the turbulence intensity in a low pressure turbine (LPT) by means of a single sensor fast response aerodynamic pressure probe. The rig was designed in cooperation with MTU Aero Engines and considerable efforts were put into the adjustment of all relevant model parameters. Blade count ratio, airfoil aspect ratio, reduced massflow, reduced speed, inlet turbulence intensity and Reynolds numbers were chosen to reproduce the full scale LP turbine. Measurements were performed adopting a phase-locked acquisition technique in order to provide the time resolved flow field downstream of the turbine rotor. The total pressure random fluctuations are obtained by selectively filtering, in the frequency domain, the deterministic unsteadiness due to the rotor blades and coherent structures. The turbulence intensity is derived from the inverse Fourier transform and the correlations between total pressure and velocity fluctuations. The determination of the turbulence intensity allows the discussion of the interaction processes between the stator and rotor for engine-representative operating conditions of the turbine.

Three Dimensional Clocking Effects in a One and a Half Stage Transonic Turbine

This paper presents an experimental investigation of the flow field in a high-pressure transonic turbine with a downstream vane row (1.5 stage machine) concerning the airfoil indexing. The objective is a detailed analysis of the three-dimensional aerodynamics of the second vane for different clocking positions. To give an overview of the time-averaged flow field, five-hole probe measurements were performed upstream and downstream of the second stator. Furthermore in these planes additional unsteady measurements were carried out with laser Doppler velocimetry in order to record rotor phase-resolved velocity, flow angle, and turbulence distributions at two different clocking positions. In the planes upstream of the second vane, the time-resolved pressure field has been measured by means of a fast response aerodynamic pressure probe. This paper shows that the secondary flows of the second vane are significantly modified by the different clocking positions, in connection with the first vane modulation of the rotor secondary flows. An analysis of the performance of the second vane is also carried out, and a 0.6% variation in the second vane loss coefficient has been recorded among the different clocking positions.

A Parametric Study of the Blade Row Interaction in a High Pressure Turbine Stage

An extensive experimental analysis on the subject of the unsteady periodic flow in a high subsonic high pressure (HP) turbine stage has been carried out at the Laboratorio di Fluidodinamica delle Macchine of the Politecnico di Milano (Italy). In this paper the aerodynamic blade row interaction in HP turbines, enforced by increasing the stator and rotor blade loading and by reducing the stator-rotor axial gap, is studied in detail. The time-averaged three-dimensional flowfield in the stator-rotor gap was investigated by means of a conventional five-hole probe for the nominal (0 deg) and highly positive (+22 deg) stator incidences. The evolution of the viscous flow structures downstream of the stator is presented to characterize the rotor incoming flow. The blade row interaction was evaluated on the basis of unsteady aerodynamic measurements at the rotor exit, performed with a fast-response aerodynamic pressure probe. Results show a strong dependence of the time-averaged and phase-resolved flowfield and of the stage performance on the stator incidence. The structure of the vortex-blade interaction changes significantly as the magnitude of the rotor-inlet vortices increases, and very different residual traces of the stator secondary flows are found downstream of the rotor. On the contrary, the increase in rotor loading enhances the unsteadiness in the rotor secondary flows but has a little effect on the vortex-vortex interaction. For the large axial gap, a reduction of stator-related effects at the rotor exit is encountered when the stator incidence is increased as a result of the different mixing rate within the cascade gap.