PASSAGE SECONDARY FLOW EFFECTS ON TURBINE ENDWALL DISCRETE HOLE FILM COOLING - A REVIEW WITH UNIQUE NEW EVIDENCE

Film cooling effectiveness measurement results for gas turbine endwalls are discussed with emphasis on the effects that complex near-endwall flows in the passage have on them. The complexity comes from the secondary flows naturally occurring in a turbine passage. Important is their influence on coolant migration and film cooling effectiveness distributions downstream of coolant injection. Film cooling holes are typically distributed over the upstream portion of the endwall surface or over the entire surface in an attempt for full coverage. Endwall surface wall shear fields documented in the literature and the hole pattern design establish the angle between the near-wall flow approaching a selected coolant hole and the axis of the corresponding hole. Often, shear field data are not available; also, downstream injection would affect the shear field. Thus, approach flow directions can accurately be applied to only the most upstream rows of holes (particularly, upstream of any coolant injection). Measurements of coolant coverage with discrete hole injection show migration of coolant on the endwall as it is affected by the momentum of injection, the passage main flow direction, and the effects of vortices in the vicinity of each hole. The latter provide some evidence regarding the effect of vortices near the endwall on coolant migration, giving a valuable description of vortex effects. Results from recent studies in the literature that show interactions of emerging coolant flows and nearby vortices are applied to develop a description of the effects of passage vortices and other secondary flows on endwall coolant migration. One example is the discrete hole located under a vortex 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 pressure and suction surfaces difficult regions to provide coolant coverage by discrete hole injection. Cases discussed herein have no or low injection upstream of the passage inlet. Studies with high levels of injection upstream of the inlet (high ratios of passage inlet momentum flow near the endwall to passage average momentum flow) are not discussed herein. Such cases would have strong injection along the endwall immediately upstream of the passage inlet and, thus, would have a different passage secondary flow pattern, not discussed herein. Such a pattern is discussed in [21] as the “impingement vortex.”