Investigation of three-dimensional flowfield at the exit of a turbine nozzle

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The objective of the research reported in this article is to gain a detailed understanding of the flowfield at the exit of turbine nozzles. The flowfield was measured at two axial locations downstream of the nozzle of a single-stage turbine with a miniature five-hole probe. An area traverse was carried out to resolve the flowfield accurately, including the nozzle wake, secondary flow region, horseshoe vortex, and losses. All three components of the velocity, stagnation pressure, static pressure, and pitch and yaw angles have been resolved accurately. A distinct vortex core has been observed near the tip and at the hub. The indications are that the horseshoe vortex and the passage vortex have merged to produce a single-loss core region. Roughly, a third of the blade height passage near the tip and a third of the blade height near the hub is dominated by secondary flow phenomena. Only the middle third of the nozzle behaves according to design. The nozzle wake decay is compared to the wake decay of other blades. The nozzle wake decays much faster than a compressor cascade wake, an annular turbine nozzle cascade wake, or a turbine nozzle wake with a large rotor-stator spacing. This faster decay is attributed to effect of the downstream rotor at very small rotor-stator spacing (20% of nozzle axial chord). These and other data are presented, interpreted, and synthesized to understand the nozzle flowfield.