Passage secondary flow effects on turbine endwall discrete holes film cooling

Abstract

In this thesis, film cooling effectiveness measurement results representative of gas turbine endwall cooling are discussed with emphasis on describing the effects that features in the complex turbine passage flow have on them. Film cooling is when cool air from elsewhere in the turbine is supplied through walls of the turbine passage that require cooling providing a protective layer of cool air over the surface to partially isolate it from the hot mainstream. Complexity comes from secondary flows naturally occurring in the passage. Important, is their influence on film cooling effectiveness. Holes for film cooling injection are typically distributed over the upstream portion of the passage walls in an attempt to cover and protect the full passage. The present study documents separate and combined effects present in the passage by studying a single hole of various shapes, hole orientation angles with respect to the main flow, injection-to-main-flow velocity ratios, and passage flow with and without vortices generated upstream. Representative endwall surface wall shear fields are documented in the literature as are hole pattern designs. They establish the angle between the near-wall flow approaching a selected hole and the centerline direction of the corresponding hole. Often, shear field data are not available within the passage and approach flow directions to some holes cannot accurately be applied. Then, the flow approaching only holes that are upstream in the passage (particularly, upstream of any coolant injection) can be accurately described. Nevertheless, measurements showing migration of coolant on the endwall downstream of selected holes have been valuable to document how coolant is affected by features in the surrounding flow, features such as the momentum of injection, the passage main flow direction, and the effects of vortices in the vicinity of the hole. One example is the discrete hole located under a vortex created at the airfoil leading edge and residing near the passage entrance where ejected coolant is swept by the vortex away from the endwall, resulting in low local values of surface effectiveness. This leaves the upstream endwall regions near the pressure and suction surfaces difficult to provide coolant coverage by discrete hole injection. The measurements documented in the present study provide guidance for interpreting such downstream distributions of coolant. Cases discussed herein have low injection rates upstream of the passage. Studies with high injection rates upstream of the inlet (high ratios of passage inlet momentum flow near the endwall to passage average momentum flow) are fundamentally different and are not discussed herein. Such cases have strong injection along the endwall immediately upstream of the passage inlet (strong combustor wall cooling) and, thus, would have a different passage secondary flow pattern. Such a pattern is discussed in Nawathe, et al., 2023 as the “impingement vortex.” The measurements of the present study document the endwall coolant coverage and coolant distributions in the flow at several planes downstream of injection. Such data not only show the cooling effectiveness but track the coolant in the passage flow. Flow measurements describe profiles of mean velocity, describing the boundary layer growth and momentum deficit of streamwise momentum upstream and downstream of injection. Understanding the interaction of secondary flows with film cooling in the turbine is significant to gas turbine designers. Dominant flow features of secondary flow are convected vortices in the mainstream, endwall crossflow, and suction and pressure legs of the vortex formed at the passage leading edge.