On the Periodically Unsteady Interaction of Wakes, Secondary Flow Development, and Boundary Layer Flow in An Annular Low-Pressure Turbine Cascade: An Experimental Investigation

Abstract

The experimental results reported in this contribution address the time-dependent impact of periodically unsteady wakes on the development of profile and end wall boundary layers and consequently on the secondary flow system. Experimental investigations are conducted on an annular 1.5 stage axial turbine rig at Ruhr-Universität Bochum’s Chair of Thermal Turbomachines and Aeroengines. The object under investigation is a modified T106 profile low-pressure turbine (LPT) stator row at a representative exit flow Reynolds number of 200,000. By making use of an annular geometry instead of a linear cascade, the influence of curvilinear end walls, nonuniform, increasing pitch across the span and radial flow migration can be represented. Incoming wakes are generated by a variable-speed driven rotor equipped with cylindrical bars. Special emphasis is put on the wake-induced recurrent formation, suppression, weakening, and displacement of individual vortices and separated flow regimes. For this, based on a comprehensive set of time-resolved measurement data, the interaction of impinging bar wakes and boundary layer flow and thus separation and its periodic manipulation along the passage end walls and on the blade suction surface are studied within the frequency domain.

Near-Wall Flow in Turbomachinery Cascades—Results of a German Collaborative Project

“…Rotatable casing elements with multiple probe accesses facilitate the recording of two-dimensional flow field traverses in various planes. Following, the most important information regarding the setup are given, a more detailed description was provided in [51][52][53].…”

“…Rotatable casing elements with multiple probe accesses facilitate the recording of two-dimensional flow field traverses in various planes. Following, the most important information regarding the setup are given, a more detailed description was provided in [51][52][53]. Following a combination of flow straightener cells, a turbulence grid and a contraction for generating uniform inflow within the settling chamber, the flow passes an IGV row (NACA 8408 profiles) to ensure proper inflow angles to the T106 RUB stator whilst leaving the flow as far as possible unaffected by wakes and secondary flow.…”

“…Thus, especially in the hub region, a distinction between passage vortex (PV) and the pressure side leg of the horse shoe vortex (HSV-PL) is enabled for t/T BP = 1/3, as the HSV-PL slides beneath the PV and pushes it along the suction side trailing edge radially inward, before it blends again with the PV. As could be shown in detail in [52,53,55,56], this periodic and short-duration event is based on the upstream wake impact on the developing HSV-PL, which is massively diverted in the front part of the blade passage by the impinging wake structure. Also, the unsteadiness of the suction side corner separation, shaping the concentrated shed vortex (CSV) can be realized.…”

“…The presented investigations give a brief outline of key findings from recent publications by the authors, where the unsteady impact of periodic bar wakes on the flow field downstream of the stator row as well as on the stator profile pressures and boundary layers were discussed. For details, please see [51][52][53][54][55][56][57][58].…”

“…Monitoring of the operating point was realized with Prandtl-probes at different characteristic planes and a combined temperature/humidity sensor at the rig inlet. A detailed description of the applied transducers and devices is provided in [51][52][53][54].…”