Experimental investigation of the effects of tip geometry on the flow and loss characteristics in a linear turbine cascade

Alican, Ozan
In gas turbines, there are a number of factors causing efficiency decrease. When internal flow in turbomachines is considered, flow vortices are one of those factors. This study aims to investigate the main mechanisms behind the efficiency losses occurring due to Tip Leakage Vortex (TLV) in gas turbine rotor blades. Additionally, according to these mechanisms, two squealer tip geometries were applied to the turbine blades and the improvements were reported. This work is the experimental branch of an optimum tip geometry investigation and an optimum solution from different squealer geometries were tested and compared with the CFD-based investigations. In the scope of this work, experiments were planned as two cases; flat tip and squealer tips. These were named as “No Treatment” and “Treated Tip Cases” respectively. No Treatment case was considered as the reference case and Treated Tip cases’ results were compared to the reference. In the Treated Tip cases; suction side squealer and full squealer blade tip geometries were manufactured as a solution for TLV. Both cases were observed in a linear turbine cascade which included seven High Pressure Turbine (HPT) blades and measurements were taken by means of Kielprobe and Five Hole Probe (FHP). Measurements were taken with traversing the probes above 50% span and at one axial chord downstream to the blades. When velocity fields and total pressure measurements were gathered and examined in detail, it was seen that a complex vortex system consisting of TLV and passage vortex (PV) existed in the observed area. In addition, when squealer geometries were applied, TLV preserved its location but pressure loss was reduced and PV became very small and migrated through TLV. Also, there was one more vortex observed which was periodic; some interrogation and predictions about its identity were also made about it. All reported consequences of tip geometries were evaluated in both cases, and test results showed that full squealer performs better for reducing the pressure loss under the circumstances. Calculations made by the means of blade-passage averaged total pressure loss coefficient indicated that, with respect to the flat tip geometry, full squealer and partial squealer tip geometry reduced the pressure loss about 19% and 3%, respectively. And, for comparing and demonstration of the repeatability of the tests, FHP and Kiel probe total pressure loss distributions were cross-checked. 
Citation Formats
O. Alican, “Experimental investigation of the effects of tip geometry on the flow and loss characteristics in a linear turbine cascade,” M.S. - Master of Science, Middle East Technical University, 2017.