High temperature fast response aerodynamic probe

In the last decade, significant improvements have been achieved in the development of new materials and in cooling techniques in the field of turbomachinery. The main driver for the focus on such topics is due to the trend of higher turbine inlet temperatures. This leads to a larger amount of work per unit mass flow and an improved weight-to-power ratio of engines. However, under these severe flow conditions, it is of even more importance to be able to measure unsteady flow phenomena, in order to improve our understanding of loss generation mechanisms.

The objective of this work is therefore to design, build and evaluate a new Fast Response Aerodynamic Probe with a higher upper temperature limit (533K) compared to traditional unsteady pressure probe techniques, often limited to temperatures around 390K. Such a new probe, with a higher upper temperature limit therefore opens up a wide field of applications, such as measurements in centrifugal compressors at design point, or in the first compressor stages of axial machines. The development of the probe is therefore motivated by the need for unsteady measurement techniques that are capable of withstanding the harsh environment of turbomachines.

The new probe is comprised of a pair of piezoresistive pressure sensors that measure the unsteady pressure and the steady temperature, respectively. Special care is taken to account for effects related to the higher temperature levels. Furthermore, possible probe shaft vibrations when operating the probe at high temperatures are taken into account in order to achieve a robust probe design.

The thesis details the operating principle, the design and fabrication of the high temperature fast response aerodynamic probe (FRAP-HT), as well as the integration of additional sensors in order to thermally manage the probe and to monitor possible probe shaft vibrations.

Major challenges related to topics such as sensor technology, signal conditioning, material science, packaging technology as well as electrical connections are addressed in the thesis.

The calibration procedure, the dynamic response, the measurement system and the measurement uncertainty analysis for the new probe are described in detail. Furthermore, in order to be able to apply the probe in harsh flows of any kind, several subcomponent systems, such as a sensor calibration oven and a sensor characteristic test device working in the high temperature range are VI required. Their design and evaluation is as well detailed in this thesis.

In order to demonstrate the measurement principle and to detail the unsteady flow in severe flow conditions, the new probe is applied in an axial research turbine equipped with a hot-streak generator. Furthermore measurements in a centrifugal compressor facility, in order to first time ever detail unsteady measurements at the design point of the facility are performed and discussed.

A comparison between measurements using the newly developed high temperature probe against various well-established steady and unsteady measurement techniques is performed. There is good agreement between all the techniques and therefore the new probe concept was proven to be applicable. The minor variations between the measurements of the different probe techniques might be related to combinations of different effects such as, wall proximity effects, blockage effects due to the difference in the probe diameter and relative sensor position, high total pressure gradients near the endwalls and a lower signal-to-noise ratio of the standard FRAP for elevated flow temperatures compared to the new FRAP-HT.

Based on the validation results of the new FRAP-HT probe and due to its higher degree of robustness it is concluded that the newly designed probe allows one to successfully conduct measurements in the harsh environment of real turbomachinery, such as at the exit of axial compressors as well as in a wide spectrum of centrifugal compressors. The probe therefore is a major contribution both to the turbomachinery community as well as for future developments of unsteady probe technology.