Measurement and Computation of Energy Separation in the Vortical Wake Flow of a Turbine Nozzle Cascade

This paper describes the observation, measurement and computation of vortex shedding behind a cascade of turbine nozzle guide vanes which have a blunt trailing edge. At subsonic discharge speeds periodic wake vortex shedding was observed at all times at a shedding frequency in the range 7–11 kHz. At high subsonic speeds the wake was susceptible to strong energy redistribution. The effect was greatest around an exit Mach number of 0.95 and results are presented for that condition. An unusually cold flow on the wake center line and hot spots at the edges of the wake were measured. These were found to be a manifestation of Eckert-Weise effect energy separation in the shed vortex street. Experimental identification of these phenomena was achieved using a new stagnation temperature probe of bandwidth approaching 100 kHz. Using phase averaging techniques it was possible to plot contours of time-resolved entropy increase at the downstream traverse plane. Computational work has been undertaken which gives qualitative confirmation of the experimental results and provides a more detailed explanation of the fine scale structure of the vortex wake. The topology of the wake vortical structures behind blunt trailing-edged turbine blades is becoming clearer. These measurements are the first instantaneous observations of the energy separation process occurring in turbine blade wake flows. This was also the first demonstration of the use of the probe in the frequency, Mach number and temperature ranges typical of operation behind the rotors of high performance turbomachines such as transonic fans.