George Gyarmathy

On fast-response probes. Part 1: Technology, calibration, and application to turbomachinery

A system for fast-response probe measurements in turbomachine flows has been developed and tested. The system has been designed for 40 kHz bandwidth and used with various in-house built probes accommodating up to four piezoresistive pressure transducers. The present generation of probes works accurately up to several bar pressure and 120°C temperature. The probes were found to be quite robust. The use of a miniature pressure transducer placed in the head of a probe showed that a precise packaging technique and a careful compensation of errors can considerably improve the accuracy of the pressure measurement. Methods for aerodynamic probe calibration and off-line data evaluation are briefly presented. These aimed, e.g., in the case of a four-hole probe, at measuring the velocity fluctuations as characterized by yaw, pitch, total pressure, and static pressure and at deriving mean values and spectral or turbulence parameters. Applications of the measuring system to turbomachinery flow in a radial compressor and to a turbulent pipe flow demonstrate the performance of the measuring system.

On Fast-Response Probes: Part 2—Aerodynamic Probe Design Studies

The influence of the probe size and geometry on the quality offast-response measurements in turbomachines has been experimentally investigated. For investigations in the static domain (time-independent flows) probes were calibrated in two continuously operating wind tunnels in the range 0.2 < M < 1.2. For dynamic calibrations in time-variant flows model experiments in water (0.025 < k < 0.4, reduced frequency) were performed. Aerodynamic characteristics were determined for a great number of probe geometries, such as circular cylinders and wedge-type probes with varied apex angles, locations of the sensing holes, and leading edge shapes. The experiments comprised investigations in tolerance ranges for prismatic total pressure probes, yaw angle sensitivity, yaw angle, and Mach number effects on calibration and influence of dynamic yaw angle fluctuation on probe characteristics. As a result of the experiments errors due to static and dynamic aerodynamic effects could be quantified. The majority of the errors arising during measurements in turbomachines can be directly related to the probe size. An important number of these errors are systematic and can be analytically modeled and hence their influence corrected. In fluctuating flows the most severe measurement errors, which often may exceed the quantity of interest, are due to dynamic stall effects. This phenomenon, which is of transient nature and cannot be corrected, is typical for sharp wedge probes, but is not present with circular cylinders, and the effects are much smaller with very blunt wedges.

Comparison of measurement data at the impeller exit of a centrifugal compressor measured with both pneumatic and fast-response probes

The main goal of these investigations was the refined measurement of unsteady high speed flow in a centrifugal compressor using the advanced FRAP{reg_sign} fast-response aerodynamic probe system. The present contribution focuses on the impeller exit region and shows critical comparisons between fast-response (time-resolving) and conventional pneumatic probe measurement results. Three probes of identical geometry (one fast and two pneumatic) were used to perform wall-to-wall traverses close to impeller exit. The data shown refer to a single running condition near the best point of the stage. The mass flow obtained from different probe measurements and from the standard orifice measurement were compared. Stage work obtained from temperature rise measured with a FRAP{reg_sign} probe and from impeller outlet velocity vectors fields by using Euler`s turbine equation are presented. The comparison in terms of velocity magnitude and angle distribution is quite satisfactory, indicating the superior DC measurement capabilities of the fast-response probe system.

Comparison of Measurement Data at the Impeller Exit of a Centrifugal Compressor Measured With Both Pneumatic and Fast-Response Probes

The main goal of these investigations was the refined measurement of unsteady high-speed flow in a centrifugal compressor by using the advanced FRAP® fast-response aerodynamic probe system. The present contribution focusses on the impeller exit region and shows critical comparisons between fast-response (time-resolving) and conventional pneumatic probe measurement results.Three probes of identical external geometry (1 fast and 2 pneumatic) were used to perform wall-to-wall traverses close to the impeller exit. The data shown refer to a single running condition near the best point of the stage.The mass flow obtained from different probe measurements and from the standard orifice measurement were compared. Stage work obtained from temperature rise measured with a FRAP® probe and from impeller outlet velocity vectors fields by using Euler’s turbine equation are presented.The comparison in terms of velocity magnitude and angle distribution is quite satifactory, indicating the superior DC measurement capabilities of the fast-response probe system.Copyright

On the Development and Application of the FRAP® (Fast-Response Aerodynamic Probe) System for Turbomachines: Part 1 — The Measurement System

This contribution gives an overview of the current state, performance and limitations of the fast-response aerodynamic probe measurement system (FRAP® System) developed at the Turbomachinery Lab of the ETH Zurich. In particular, the following topics are addressed:• Probe technology: Miniature probes with tip diameter ranging from 0.84 to 1.80 mm (1-sensor and 3-sensor probes respectively) have been developed. New technologies derived from microelectronics and micromechanics have been used to achieve an adequate packaging of the microsensor chips used. Both the sensor packaging and the sensor calibration (time-independent and time-dependent) are crucial issues for the DC accuracy of any measurement.• Aerodynamic probe calibration: The methods used for the sensor calibration and the aerodynamic probe calibration, the pertinent automated test facilities and the processing of the output data are briefly presented. Since these miniature probes are also capable of measuring the mean flow temperature, aspects related to the recovery factor and the self-heating of the probe tip are treated and some recommendations related to sensor selection are given.• Measurement system and data evaluation: The early measurement chain described in Gossweiler, Kupferschmied and Gyarmathy 1995 has evolved into the FRAP® System. This automatic system incorporates dedicated measurement concepts for a higher accuracy and a more efficient operation in terms of time and failures. An overview of the data evaluation process is given.The FRAP® System has been tested in real-sized turbomachines under industrial conditions within the temperature limits of 140°C imposed by the sensor technology (axial-flow turbofan compressor, axial-flow turbine, centrifugal compressor). These applications confirmed the potential of the system and encouraged its further development. Now, the system is routinely used in the facilities of the Turbomachinery Lab and in occasional measurement campaigns in other laboratories.Part 2 of this contribution (Roduner et al.) will focus on the application of the FRAP® System in a transonic centri fugal compressor of the ETH Turbomachinery Laboratory, while Part 3 (Koppel et al.) treats more sophisticated data analysis methods.© 1999 ASME

On Fast-Response Probes: Part 2 — Aerodynamic Probe Design Studies

The influence of the probe size and geometry on the quality of fast-response measurements in turbomachines has been experimentally investigated.For investigations in the static domain (time independent flows) probes were calibrated in two continuously operating wind tunnels in the range 0.2< Ma < 1.2. For dynamic calibrations in time variant flows model experiments in water (0.025 < k < 0.4, reduced frequency) were performed.Aerodynamic characteristics were determined for a great number of probe geometries such as circular cylinders and wedge-type probes with varied apex angles, locations of the sensing holes and leading edge shapes. The experiments comprised investigations in tolerance ranges for prismatic total pressure probes, yaw angle sensitivity, yaw angle and Mach number effects on calibration and influence of dynamic yaw angle fluctuation on probe characteristics. As a result of the experiments errors due to static and dynamic aerodynamic effects could be quantified.The majority of the errors arising during measurements in turbomachines can be directly related to the probe size. An important number of these errors are systematic and can be analytically modelled and hence their influence corrected.In fluctuating flows the most severe measurement errors, which often may exceed the quantity of interest, are due to dynamic stall effects. This phenomenon, which is of transient nature and cannot be corrected, is typical for sharp wedge probes but is not present with circular cylinders or the effects being much smaller with very blunt wedges.© 1994 ASME

On the Development and Application of the FRAP® (Fast-Response Aerodynamic Probe) System in Turbomachines: Part 2 — Flow, Surge and Stall in a Centrifugal Compressor

The present paper, Part 2 of a trilogy, is primarily focussed on demonstrating the capabilities of a FRAP® system configuration based on the simplest type of fast-response probe. A single cylindrical probe equipped with a single pressure sensor is used to measure absolute pressure and both velocity components in an essentially two-dimensional flow field. The probe is used in the pseudo-3-sensor mode (see Part 1). It is demonstrated that such a 1-sensor probe is able to measure high-frequency rotor-governed systematic fluctuations (like blade-to-blade phenomena) alone or in combination with flow-governed low-frequency fluctuations as rotating stall (RS) and mild surge (MS). However 3-sensor probes would be needed to measure stochastic (turbulence-related) or other aperiodic velocity transients.The data shown refer to the impeller exit and the vaned diffuser of a single-stage high-subsonic centrifugal compressor. Wall-to-wall probe traverses were performed at the impeller exit and different positions along the vaned diffuser for different running conditions. The centrifugal compressor was operated under stable as well as under unstable (pulsating or stalled) running conditions. The turbomachinery oriented interpretation of these unsteady flow data is a second focus of the paper. A refined analysis of the time-resolved data will be done in Part 3 where different spatial/temporal averaging methods are compared.Two different averaging methods were used for the data evaluation. Impeller based ensemble averaging for blade-to-blade systematic fluctuations (with constant period length at a constant shaft speed) and flow-based class averaging for the relatively slow MS and RS with slightly variable period length.Due to the ability of fast-response probes to simultaneously measure velocity components as well as total and static pressure, interesting insights can be obtained into impeller and diffuser channel flow structures as well as into the time behavior of large-domain phenomena as RS and MS.Copyright

Energy Input of a Centrifugal Stage Into the Attached Piping System During Mild Surge

Subjected to an oscillating flow rate a compressor may feed additional (excitational) energy into the attached piping system. The relation between this additional energy input and the instantaneous behavior of a centrifugal compressor stage is dealt in a first part. Modeling the stage behavior by taking into account either inertia of the enclosed fluid mass or a first-order transient element or transient stall in any component leads to a different energy input.The energy input at a flow rate oscillation of given frequency and amplitude was calculated as a function of the slope of the characteristic and the reduced frequency applying a previously published model to describe the instantaneous behavior of the stage. In this model transient stall in the diffuser is taken into account. At reduced frequencies above unity the energy input of the diffuser was reduced by a considerable amount due to the specified instantaneous behavior of the diffuser. This indicates a potential to reduce the additional energy input of the diffuser either by increasing the time constant of the stall process or by increasing the mild surge frequency. For the investigated diffuser size the required reduced frequencies imply mild surge frequencies in a range being too high for industrial application (> 200 Hz). Still, this method turned out to give useful insight into the link between the instantaneous behavior of the compressor and its energy input.In a second part for the same centrifugal compressor the energy contribution of several stage segments during mild surge oscillations was determined from detailed instantaneous measurements. As a result the contribution of each stage segment to the conservation of the mild surge pulsation emerges. Although at the investigated mild surge frequencies the stage segments no longer behave strictly quasi-steadily their contribution to the additional energy input is found to be mainly determined by the slope of their quasi-steady characteristic.© 1997 ASME

On Fast-Response Probes: Part 1 — Technology, Calibration and Application to Turbomachinery

A system for fast-response probe measurements in turbomachine flows has been developed and tested. The system has been designed for 40 kHz bandwidth and used with various in-house built probes accommodating up to 4 piezoresistive pressure transducers. The present generation of probes works accurately up to several bar pressure and 120°C temperature. The probes were found to be quite robust.The use of miniature pressure transducers placed in the head of probes showed that a precise packaging technique and a careful compensation of errors can considerably improve the accuracy of the pressure measurement.Methods for aerodynamic probe calibration and off-line data evaluation are briefly presented. These aimed, i.e. in the case of a 4-hole probe, at measuring the velocity fluctuations as characterised by yaw, pitch, total pressure and static pressure and at deriving mean values and spectral or turbulence parameters.Applications of the measuring system to turbomachinery flow in a radial compressor and to a turbulent pipe flow demonstrate the performance of the measuring system.Copyright