Aero-thermal flow characterization downstream of an NGV cascade by five-hole probe and filtered Rayleigh scattering measurements

The characterization of pressure, temperature and velocity fields in turbomachinery flows typically relies on well-proven probe-based technology such as pneumatic probes, hot-wire sensors or thermocouples. These devices have to be introduced into narrow flow channels and by that obstruct part of the duct at the actual measuring position, which can significantly alter the aerodynamic performance of the components under investigation. In this contribution, measurement results of a commercially available five-hole probe with mounted temperature sensor and laser-optical-filtered Rayleigh scattering diagnostics are acquired downstream of a nozzle guide vane cascade with lean-burn combustion representative inlet flow distortions. Pressure results obtained by both methods are found to be on a similar absolute level. However, the unavoidable movement of optical elements following the facilities’ heat-up procedure prevents a sufficient sensitivity of pressure data obtained by filtered Rayleigh scattering and, therefore, an identification of certain flow structures related to probe interference on the static pressure field. Concerning temperature data, results of both methods, despite the probe data being shifted in lateral direction, are found to be in good agreement in topology as well as absolute level. The strongest intrusive influence of the probe body as well as a bias to probe readings in certain flow regions is identified when comparing velocity data by means of the optical Doppler frequency-shift. Whereas filtered Rayleigh scattering results follow a smooth and continuous trend, five-hole probe data are characterized by steep gradients inside the airfoil wakes as well as differing slopes in the flow passages. These findings are backed by a comparison of five-hole probe velocities with a CFD solution, revealing an erroneous reading of axial velocities inside the wake areas. The study concludes that the presented filtered Rayleigh scattering diagnostics, with estimated accuracies of 50–90 hPa, 2–3 K and 2–3 MHz in pressure, temperature and Doppler frequency-shift, is a viable alternative to conventional probe-based measuring approaches in characterizing the aero-thermal properties of turbomachinery flows.