Experimental Investigation of Periodically Unsteady Wake Impact on the Secondary Flow in a 1.5 Stage Full Annular LPT Cascade With Modified T106 Blading

Abstract

This paper transfers findings from linear cascade studies to the annular system. Experimental studies have been conducted on the newly designed 1.5 stage full annular rotating axial turbine rig at Chair of Thermal Turbomachinery, Ruhr-Universität Bochum. Therefore, an existing large scale low speed test rig was retrofitted with newly designed T106RUB low pressure turbine (LPT) blading, state-of-the-art measurement technologies and multi-dimensional traversing devices to allow for highly resolved measurements of unsteady wake stator flow interaction in both space and time. Incoming wakes are generated by a variable-speed driven rotor disk equipped with cylindrical bars. The measuring concept for an in-depth analysis of unsteady flow phenomena is presented and results from highly resolved time-averaged and time-resolved flow field traverses are discussed and compared. In detail the time-dependent interaction of periodically passing bar wakes with the boundary layers and secondary flow structures of the T106RUB stator row is investigated. Special emphasis is put on time-varying dilatation and location of individual components of the vortex system and on potential flow separation along the blade suction surface. It is evaluated how these factors can contribute to a time-dependent homogenization of stator exit flow and a consequent loss reduction in the present configuration.

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.…”

“…It was developed to match Reynolds number, blade loading distribution c p at midspan and thus an equivalent boundary layer development of the original T106 profile at the rig's low Mach number flow. The principles of the transformation procedure were described by Sinkwitz et al [51].…”

“…This way, the influences of curvilinear endwalls, non-uniform, radially increasing pitch and radial flow migration are incorporated, increasing the degree of complexity over linear setups. The modified stator blade profile, labeled as T106 RUB , was developed within this collaborative project for matching the transition and separation characteristics of the original T106 profile (also applied in sub-project C) at low flow speeds, thus facilitating measurements to be taken in an annular, large-scale test rig (see [51]). The stator flow is periodically perturbed by incoming wakes from a rotating wake generator.…”

“…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].…”