FRAP

Fast Response Aerodynamic Probes

The knowledge of the unsteady flow pattern inside turbomachines - showing large fluctuations and a high turbulence level - is the key to improve predictions of performance, dissipation rate, stall, surge, blade flutter, etc. Additionally, with the increased performance of numerical computational methods for fluid dynamics the need for their "calibration" and validation has grown larger. Thus, innovative fast-response measurement techniques are required to investigate these flows under industrial conditions. To meet this demand a high-frequency response aerodynamic probe measurement technique has been developed at the LEC, with the ability to measure the steady, the periodic and the random components of the flow vector. That means in detail, flow parameters including total and static pressure, flow angles velocity components and Mach number can be measured at frequencies up to 40 kHz. Next to the advantage that they offer a high analogue bandwidth, they show a high signal-to-noise ratio, a high reliability and that they are far more robust than hot wires.

The aerodynamic fast-response probes are equipped with miniature silicon pressure transducers placed directly in the probe head. Single and multi sensor probes are used. The mainstream flow field is typically measured using a standard two sensor FRAP probe with 1.8 mm outer diameter in virtual multi-sensor mode providing time resolved information. FRAP probe technology provides also steady (1Hz) temperature measurements. The maximal operating temperature range of the existing FRAP probes equals 130°C. The relative measurement accuracies of FRAP probes are for total pressure ±0.13%, for static pressure ±0.26% and for the flow angles yaw and pitch respectively ±1.5%.

A number of custom-made FRAP probes with a single, double or multiple sensors have been developed in recent years. In particular, close to endwalls and in regions of leakage or highly shearing flows, miniature size FRAP probes are used up to a minimum of 0.84 mm in tip diameter. This probe category is unique and the smallest in open literature.