An Aerothermal Study of Ejector-Based, Coandă-Surface-Driven, Gas Turbine Tip Leakage Mitigation Schemes: A Computational and Experimental Approach
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- Author:
- Khokhar, Gohar
- Graduate Program:
- Aerospace Engineering
- Degree:
- Doctor of Philosophy
- Document Type:
- Dissertation
- Date of Defense:
- February 18, 2022
- Committee Members:
- Sven Schmitz, Major Field Member
Michael Krane, Outside Unit & Field Member
Jose Palacios, Major Field Member
Cengiz Camci, Chair & Dissertation Advisor
Dennis K Mc Laughlin, Special Member
Amy Pritchett, Program Head/Chair - Keywords:
- Ejector-Based
Coandă-Surface-Driven
Gas Turbine Tip Leakage Mitigation
Gas Turbine
Tip Leakage
Ejector
Coanda Surface - Abstract:
- In gas turbine systems, the mitigation of aerothermal losses due to the flow of high-energy combustor exhaust fluid within the region between the rotating turbine blades and the stationary endwall has been the focus of research efforts for the last four decades. This narrow region, known as the tip clearance gap, gives rise to the highly three-dimensional flow phenomenon known as tip leakage flow. The tip leakage flow, unlike the primary turbine flow, does not undergo a thermodynamic expansion, and thus penalizes the work extraction potential in the turbine stage. The high-pressure and high-temperature leakage fluid also results in high thermal loads on the blade tip, leading to oxidation and structural detriment over time. In addition to the adverse thermal loads and the non-participation in work extraction, the tip leakage fluid separates as it emerges onto the suction side of the turbine blade, forming a strong vortex before mixing with the main fluid flow. Collectively, the non-participation of leakage flow in work extraction, convective heat transfer to the blade, and dissipation of kinetic energy due to the formation of the tip leakage vortex, account for approximately one-third of the aerothermal losses in the turbine stage. This dissertation demonstrates the capability of several ejector-based, Coandă-surface driven, tip leakage mitigation schemes. The schemes are based on direct geometric modifications to the turbine blade, that promote the diversion and redirection of tip leakage fluid using specifically optimized channels. The proposed ejector channels operate in conjunction with strategic Coandă-surfaces, to alter the path of the leakage fluid, promoting ejection into the wake region of the blade. Multiple schemes, including single-channeled, single-channeled with “hybrid” squealer, double-channeled, and triple-channeled technologies are assessed. The novel schemes are evaluated computationally and experimentally in the large-scale Axial Flow Turbine Research Facility (AFTRF) at Penn State University. Extrapolatory data are generated for high-speed transonic environments computationally via a rotor-only linear cascade. This dissertation presents the predicted and measured changes in key performance metrics compared to an untreated blade and a conventional squealer tip geometry. The key performance metrics assessed are the stage total-to-total isentropic efficiency, tip-gap mass flow rate, and rotor-exit total pressure and entropy-based figures of merit. Exchange rates between performance metrics are presented to substantiate the aerothermal loss reductions projected by this study. The computational and experimental studies carried out in this work identify significant predicted and measured aerothermal improvements of the novel ejector-based tip mitigation implementations over untreated and conventional squealer geometries. Incompressible computational predictions project upper-bound efficiency gains of 0.49% and upper-bound mass flow reductions of 14.80% compared to an untreated flat tip. Compared to a squealer tip, upper-bound efficiency gains of 0.10% and upper-bound mass flow reductions of 8.30% are predicted. Similar trends were observed between the incompressible and compressible phases of the research.
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