Unsteady Pressure Measurement

 

For the measurement of unsteady pressures, IST uses various types of pressure transducers, which vary greatly depending on the application and operating conditions. If only the fluctuating signal components are of interest, piezoelectric pressure transducers are often used. Due to their robust design, these transducers can be used in many different applications. The charge shifts generated by the pressure forces are converted into voltage signals by means of charge amplifiers. If, in addition to the fluctuating signal components, the steady components are also of interest, piezoresistive pressure transducers are often used, which are particularly useful in applications characterized by high pressure fluctuations. Due to their compact design they offer an extremely high spatial resolution. For data acquisition, modern transient recorders from different manufacturers can be used, which allow synchronous recording of several channels in real time. The correlation between the measured voltage signal and the applied pressure is determined by calibration tests and can be strongly non-linear depending on various factors. Among other things, the applied pressure, the temperature, the frequency of the pressure signal and the installation situation play a role. The calibration of the sensors is carried out both statically and dynamically and is carried out directly at IST or by long-standing partners, depending on the application. By this means, the step response of the pressure transducer can be determined in the shock tube frequently used in this process, allowing conclusions to be drawn about the frequency and phase response.

Application and field of use

In various research projects, aeroacoustic phenomena are investigated at IST in both axial and centrifugal machines. Due to the high pressures and temperatures frequently encountered in turbomachinery, the use of microphones is often not feasible, which is why piezoelectric and piezoresistive sensors are used for these investigations. Based on the information about amplitude and phase of the signals obtained at different measuring points, the pressure fluctuations can be assigned to acoustic modes. Due to the high circumferential mode orders that occur, a very large number of measuring points is required to determine the modes. In order to reduce the number of measuring points, modern signal processing methods from the field of compressed sensing are used at IST, which allow the solution of the system of equations if the number of occurring mode orders is limited. Furthermore, unsteady pressure transducers are also used to measure unsteady total pressures. For example, probe rakes and probes are used to analyze the backflow phase during the compressor surge.