Unsteady Probe Research Articles

Experimental validation of a three-dimensional through-flow model for high-speed compressor surge

This paper describes the validation of surge simulations produced using the low-order code ACRoSS with experimental data from a 4. 5-stage high-speed rig, representative of the front stages of a modern high-pressure compressor (HPC). ACRoSS is an unsteady, 3D through-flow code developed at Cranfield University to predict compressor performance during stall events. The experimental data was derived during the Rig250 build 6B test campaign, carried out in 2016 by the DLR Institute of Propulsion Technology. To correctly represent the surge events, the ducting of the testbed upstream and downstream of the compressor is included in the simulation.The results from the low-order model are compared with measurements from unsteady probes for two surge events at design speed with different downstream plenum sizes. The surge frequency and pressure profiles in time are closely reproduced by the low-order model. Analysis of the unsteady pressure measurement and the acoustic waves modelled by ACRoSS indicates that the surge period is likely to be influenced by the reflection of the initial surge wave in the inlet duct.Using the ACRoSS model, surge can be accurately reproduced both in 3D and 2D. Two-dimensional, axisymmetric simulations are shown to be sufficient for the cases investigated, and surge can be simulated with a computational cost of less than one hour per event using just 40 CPUs. This represents over an order of magnitude improvement in computational power and time required to simulate surge, compared to traditional URANS 3D CFD.

Response of a Tandem-Staged Compressor to Circumferential Inflow Distortion

Abstract Tandem rotor, as a concept, promises a higher diffusion capacity than a single rotor. However, it is reported to have a lower operating range than the conventional rotor. Inflow distortion can lead to significant changes in the flow dynamics of the axial-flow compressor. Further, a highly loaded tandem blade is likely to be severely affected due to the inflow distortions. In this study, circumferential distortion is imposed at the inlet of a tandem stage and its response is studied, using steady and unsteady probes. The performance characteristics and other aerodynamics parameters under the circumferential distortion are compared with the clean inflow case. As expected, the tandem stage experiences a significant drop in the total pressure rise and operating range under circumferential distortion. A large deficit in the total pressure, efficiency, and axial velocity is observed in the distorted region. The casing pressure traces, Morlet wavelet analysis, and Fast Fourier Transform techniques are used to analyze the unsteady data. The stall is incepted in the form of a low-intensity spike, which later evolved into a fully grown stall within 2.5 rotor revolutions. Further, the amplitude of the stall cell under the circumferential distortion is found to be higher than the clean flow.

Development of unsteady multi-hole pressure probes based on fiber-optic pressure sensors

For measurements of unsteady flow phenomena with multi-hole pressure probes, pressure transducers are integrated in the probe near the probe tip. The application of additive manufacturing enables a wide variation in probe geometries for complex use cases. The spatial characteristics of the unsteady probe are determined by the steady state calibration in a known free-jet wind tunnel. Furthermore, the acoustic/pneumatic line-cavity system, that emerges inside the channels of the probe, is investigated in detail in the temporal calibration. In order to realize multi-hole probes with higher temporal resolution, which can be operated in harsh environments, a fiber-optic pressure sensor is developed. The measurement principle of the fiber-optic sensor is based on the Fabry-Pérot interferometer effect. The sensor is operated differentially with a pressure capillary by either pressurizing the sensor or using the surrounding static pressure as the reference pressure. Besides calibration of the sensor, comparisons with a state-of-the-art piezo-resistive pressure transducer have been performed. The focus of this work is on the reproducibility of both frequency response and amplitude.

Modulation and Radial Migration of Turbine Hub Cavity Modes by the Rim Seal Purge Flow

In the present paper, the results of an experimental and numerical investigation of the hub cavity modes and their migration into the main annulus flow are presented. A one-and-a-half stage, unshrouded and highly loaded axial turbine configuration with three-dimensionally shaped blades and cylindrical end walls has been tested in an axial turbine facility. Both the blade design and the rim seal purge flow path are representative to modern high-pressure gas turbines. The unsteady flow field at the hub cavity exit region has been measured with the fast-response aerodynamic probe (FRAP) for two different rim seal purge flow rates. Furthermore, fast-response wall-mounted pressure transducers have been installed inside the cavity. Unsteady full-annular computational fluid dynamics (CFD) simulations have been employed in order to complement the experimental work. The time-resolved pressure measurements inside the hub cavity reveal clear cavity modes, which show a strong dependency on the injected amount of rim seal purge flow. The numerical predictions provide information on the origin of these modes and relate them to pronounced ingestion spots around the circumference. The unsteady probe measurements at the rim seal interface show that the signature of the hub cavity induced modes migrates into the main annulus flow up to 30% blade span. Based on that, an aerodynamic loss mechanism has been found, showing that the benefit in loss reduction by decreasing the rim seal purge flow rate is weakened by the presence of turbine hub cavity modes.

Assessment of Unsteady Pressure Measurement Uncertainty—Part II: Virtual Three-Hole Probe

With the advancements in miniaturization and temperature capabilities of piezoresistive pressure sensors, pneumatic probes—which are the long established standard for flow-path pressure measurements in gas turbine environments—are being replaced with unsteady pressure probes. On the other hand, any measured quantity is by definition inherently different from the “true” value, requiring the estimation of the associated errors for determining the validity of the results and establishing respective confidence intervals. In the context of pressure measurements, the calibration uncertainty values, which differ from measurement uncertainties, are typically provided. Even then, the lack of a standard methodology is evident as uncertainties are often reported without appropriate confidence intervals. Moreover, no time-resolved measurement uncertainty analysis has come to the attention of the authors. The objective of this paper is to present a standard method for the estimation of the uncertainties related to measurements performed using single sensor unsteady pressure probes, with the help of measurements obtained in a one and a half stage low pressure high speed axial compressor test rig as an example. The methodology presented is also valid for similar applications involving the use of steady or unsteady sensors and instruments. The static calibration uncertainty, steady measurement uncertainties, and unsteady measurement uncertainties based on phase-locked average (PLA) and ensemble average are presented by the authors in Dell'Era et al. (2016, “Assessment of Unsteady Pressure Measurement Uncertainty—Part 1: Single Sensor Probe,” ASME J. Eng. Gas Turbines Power, 138(4), p. 041601). Depending on the number of points used for the averaging, different values for uncertainty have been observed, underlining the importance of having greater number of samples. For unsteady flows, higher uncertainties have been observed at regions of higher unsteadiness such as tip leakage vortices, hub-corner vortices, and blade wakes. Unfortunately, the state of the art in single sensor miniature unsteady pressure probes is comparable to multihole pneumatic probes in size, preventing the use of multihole unsteady probes in turbomachinery environments. However, the angular calibration properties of a single sensor probe obtained via an aerodynamic calibration may further be exploited as if a three-hole directional probe is employed, yielding corrected total pressure, unsteady yaw angle, static pressure and Mach number distributions based on the PLAs with the expense of losing the time-correlation between the virtual ports. The aerodynamic calibration and derivation process are presented together with the assessment of the uncertainties associated to these derived quantities in this contribution. In the virtual three-hole mode, similar to that of a single sensor probe, higher uncertainty values are observed at regions of higher unsteadiness.

Assessment of Unsteady Pressure Measurement Uncertainty—Part I: Single Sensor Probe

With the advancements in miniaturization and temperature capabilities of piezoresistive pressure sensors, pneumatic probes—which are the long established standard for flow-path pressure measurements in gas turbine environments—are being replaced with unsteady pressure probes. Any measured quantity is by definition inherently different from the “true” value, requiring the estimation of the associated errors for determining the validity of the results and establishing respective confidence intervals. In the context of pressure measurements, the calibration uncertainty values, which differ from measurement uncertainties, are typically provided. Even then, the lack of a standard methodology is evident as uncertainties are often reported without appropriate confidence intervals. Moreover, no time-resolved measurement uncertainty analysis has come to the attention of the authors. The objective of this paper is to present a standard method for the estimation of the uncertainties related to measurements performed using single sensor unsteady pressure probes, with the help of measurements obtained in a one and a half stage low pressure (LP) high speed axial compressor test rig as an example. The methodology presented is also valid for similar applications involving the use of steady or unsteady sensors and instruments. The static calibration uncertainty, steady measurement uncertainties, and unsteady measurement uncertainties based on phase-locked average (PLA) and ensemble average are presented in this contribution. Depending on the number of points used for the averaging, different values for uncertainty have been observed, underlining the importance of having greater number of samples. For unsteady flows, higher uncertainties have been observed at regions of higher unsteadiness such as tip leakage vortices, hub corner vortices, and blade wakes. Unfortunately, the state of the art in single sensor miniature unsteady pressure probes is comparable to multihole pneumatic probes in size, preventing the use of multihole unsteady probes in turbomachinery environments. However, the angular calibration properties of a single sensor probe obtained via an aerodynamic calibration may further be exploited as if a three-hole directional probe is employed, yielding corrected total pressure, unsteady yaw angle, static pressure, and Mach number distributions based on the PLAs with the expense of losing the time-correlation between the virtual ports. The aerodynamic calibration and derivation process are presented together with the assessment of the uncertainties associated to these derived quantities by the authors in Dell'Era et al. (2016, “Assessment of Unsteady Pressure Measurement Uncertainty—Part II: Virtual Three Hole Probe,” ASME J. Eng. Gas Turbines Power, 138(4), p. 041602). In the virtual three-hole mode, similar to that of a single sensor probe, higher uncertainty values are observed at regions of higher unsteadiness.

Unsteady Entropy Measurements in a High-Speed Radial Compressor

The time-dependent relative entropy field at the impeller exit of a centrifugal compressor is measured. This study is part of a broader effort to develop comprehensive measurement techniques that can be applied in the harsh environment of turbomachines. A miniature unsteady entropy probe (diameter of 1.8 mm) is designed and constructed in the present study. The unsteady entropy probe has two components: a one-sensor fast-response aerodynamic probe and a pair of thin-film gauges. The time-dependent total pressure and total temperature are measured from the fast-response aerodynamic probe and pair of thin-film gauges, respectively. The time-dependent relative entropy derived from these two measurements has a bandwidth of 40 kHz and an uncertainty of ±2 J/kg. The measurements show that for operating Condition A, φ=0.059 and ψ=0.478, the impeller exit flowfield is highly three dimensional. Adjacent to the shroud there are high levels of relative entropy and at the midspan there are low and moderate levels. Independent measurements made with a two-sensor aerodynamic probe show that the high velocity of the flow relative to the casing is responsible for the high relative entropy levels at the shroud. On the other hand, at the midspan, a loss free, jet flow region and a channel wake flow of moderate mixing characterize the flowfield. At both the shroud and midspan, there are strong circumferential variations in the relative entropy. These circumferential variations are much reduced when the centrifugal compressor is operated at operating Condition B, φ=0.0365 and ψ=0.54, near the onset of stall. In this condition, the impeller exit flowfield is less highly skewed; however, the time-averaged relative entropy is higher than at the operating Condition A. The relative entropy measurements with the unsteady entropy probe are thus complementary to other measurements, and more clearly document the losses in the centrifugal compressor.

Stator-clocking effects on the unsteady interaction of secondary flows in a 1.5-stage unshrouded turbine

Effects of stator clocking on the unsteady interaction of stator and rotor secondary flows and performance are investigated in a 1.5-stage, high-pressure turbine. Due to the low aspect ratio design of the blade rows, vortical structures dominate the inlet flow field of the second stator row. Therefore the interaction of first stator and rotor secondary flows in relation to the stator-clocking position must be considered in order to achieve an optimized multi-stage performance. Four different stator-clocking positions are studied in this experimental investigation. The data comprise unsteady and steady probe measurements, which are acquired downstream of the rotor and the second stator blade row of a 1.5-stage, unshrouded turbine. A two-sensor fast response aerodynamic probe technique is used in the measurement campaign. It is found that multi-row interaction effects of vortical secondary flow features dominate the loss creation in turbine stages having low aspect ratio geometries. The periodic interaction of shed passage vortices of the first stator with secondary flow structures of the rotor creates regions characterized by reduced total pressure and increased turbulence, and entropy at fixed locations relative to the subsequent stator. Circumferential position of these interaction regions defines the spanwise distribution of total pressure loss in the downstream stator. Stator-clocking controls the spanwise distribution of loss in high-pressure turbines. However, the potential of reducing the loss is limited due to the three-dimensional nature of the flow, and hence, the non-uniform effect of a clocking position on the spanwise performance of a downstream stator.

Unsteady Calibration of Fast-Response Pressure Probes, Part 2: Water-Tunnel Experiments

This part of our work presents the development of a water-tunnel setup to experimentally determine the instantaneous pressure distribution over a sphere in unsteady flow, the unsteady pressure coefficient, and to provide validation of the unsteady probe calibration techniques discussed in Part 1. To generate an unsteady flowfield with significant inertial effects, a water-tunnel setup was developed where flow unsteadiness was generated by oscillating a spherical probe with a diameter of 50.8 mm at frequencies up to 4 Hz. The steady and unsteady pressure coefficients were experimentally determined

Unsteady Calibration of Fast-Response Pressure Probes, Part 3: Air Jet Experiments

A facility was developed that produces a known and repeatable oscillating air jet used to test the unsteady probe calibration techniques developed in Parts 1 and 2 for fast-response, multihole pressure probes. Its characteristics. in the steady and unsteady operation modes were studied. The facility can produce flow velocities up to Mach 0.3 with oscillation frequencies up to 1500 Hz and oscillation amplitudes of up to ±20%. A hemispherical-tip, fast-response five-hole probe was developed and tested in this facility. The unsteady flowfield was measured via hot-wire anemometry, to document the instantaneous unsteady flow velocity, and a specially designed fast-response static-pressure probe in order to measure the jet's instantaneous unsteady static pressure

The Study of Thermal Diffusivity Measurement of Composite Materials by Unsteady Probe Method (2nd Report)

The Study of Thermal Diffusivity Measurement of Composite Materials by Unsteady Probe Method (2nd Report)

The Study of Thermal Diffusivity Measurement of Composite Materials by Unsteady Probe Method

The Study of Thermal Diffusivity Measurement of Composite Materials by Unsteady Probe Method

非定常プローブ法による温度伝導率測定の研究

非定常プローブ法による温度伝導率測定の研究

A Distinction Between Different Types of Stall in a Centrifugal Compressor Stage

The flow at the stall line of a centrifugal compressor with vaneless diffuser was investigated at different speeds. A distinction between three kinds of stall phenomena could be made. One type of stall with regurgitation of fluid at the impeller inlet was of a nonperiodic character, whereas two different types of periodic stall appeared at higher speeds. The rotating nature of these two types of stall was verified from a comparison of signals of peripherally spaced pressure transducers. The low-frequency rotating stall exhibited features of diffuser generated stall and a lobe number of three was measured. From a detailed investigation of the high-frequency rotating stall, which included unsteady probe measurements upstream and downstream of the impeller, it can be shown that this type of rotating stall is generated in the impeller by a periodic breakdown of energy transfer from the rotor to the flow. This conclusion is supported by the distribution of shroud static pressures.

The dynamic response of constant resistance anemometers

The dynamic response of constant resistance anemometer systems is related to the thermal equilibrium of the probe during unsteady changes in surface cooling and also to the characteristics of the feedback system which supplies the heating current. This maintains the probe resistance and average temperature at approximately constant values. In this paper the thermal equilibrium and feedback equations are combined, taking account of the inductance of the probe cables, to derive the governing equation for the unsteady probe current. The probe inductance raises the order of this equation from first to second.