Session: 05-12 Instrumentation II: Pressure Probes

Paper Number: 121034

121034 - Accuracy of Steady Pneumatic Probes in Unsteady Turbomachinery Flows 

In this paper we show that steady pneumatic probes, as they have been used for decades, are well suited for measurements in unsteady, periodic turbomachinery flows. We did this by means of a simplified setup of a turbomachinery rotor and comparisons between various pneumatic and fast-response unsteady probes. The experiments demonstrated the absence of a frequency-dependent effect on the measurements.

We designed an experimental setup consisting of abstracted rotors placed in the jet of a free-stream wind tunnel to investigate the probes. The rotor model is conceived as a disk with slotted holes which are uniformly distributed along the circumference. The bars between the slotted holes generate wakes which are similar to those of real turbomachine rotors. The operating points of this rotors are characterised by blade passing frequency (0 kHz to 5 kHz) and Mach number (Mach 0.2 to 0.8) upstream of the rotors. We systematically investigated the impact of unsteadiness, probe head size and shape as well as axial distance between probe and rotor on the measured pressure value for six different probes at up to 90 operating points each. The probes examined included steady and unsteady five-hole and kielhead probes as well as a five-hole probe of different size and an unsteady pitot probe. We compared identically shaped pneumatic and fast-response unsteady probes for all of the operating points.

We found two primary results: First, there is no frequency-dependent effect on the measurements. The pneumatic probes reproduced the time-averaged pressure values of the unsteady probes correctly at all operating points, taking measurement uncertainties into account. Especially the pressure increase due to higher rotational speed of the rotor was measured identically with both types of probes. Therefore, steady probes can be used to determine time-averaged quantities in unsteady turbomachinery flows. Secondly, we examined the influence of probe head shape and size. A larger probe head influences the flow around it and the local enthalpy increase of the rotor directly upstream stronger due to its potential field than a smaller one. Therefore, the measured pressure is impacted as well. We quantified these effects by extensive variations of the axial distance between probe and rotor. The experiments showed significant differences at the same location, which were considerably larger than the measurement uncertainty. This explains discrepancies in measurements between steady and unsteady probe, which have been credited to the unsteady nature of the flow in the past. Numerical simulations confirmed this behaviour. Therefore, probe head shape and size as well as distance to the rotor should be evaluated on a case-by-case basis for accurate measurements, especially when measurements of probes of different sizes are compared.

In conclusion, the tests showed that a reasonable measurement of the mean flow downstream of a rotor is possible with pneumatic probes. The time-averaged value of the measured quantity is correctly reproduced. For each application, probes should be evaluated individually with regard to the interaction of their potential field with the rotor.

Presenting Author: Tim Sebastian Widera Institute of Jet Propulsion and Turbomachinery, RWTH Aachen University

Presenting Author Biography: Tim Widera is a PhD student at the Institute of Jet Propulsion and Turbomachinery of the RWTH Aachen University since June 2019. He finished his studies of aeronautical engineering at the RWTH Aachen University with a Master’s thesis on Forced Response simulations for centrifugal compressors in early 2019. Since then he has been working as a research assistant on various experimental topics including a 2.5 stage axial compressor. He is especially interested in measurement techniques and designed and build a test rig to evaluate the influence of unsteady flows on the measurements with conventional, steady, pneumatic probes.

Authors:

Tim Sebastian Widera Institute of Jet Propulsion and Turbomachinery, RWTH Aachen University
Bastian Patzer MTU Aero Engines AG
Stephan Behre MTU Aero Engines AG
Peter Jeschke Institute of Jet Propulsion and Turbomachinery, RWTH Aachen University