NONAXISYMMETRIC ENDWALL CONTOURING AND LEADING EDGE MODIFICATIONS ON TURBINE NOZZLE GUIDE VANES
Open Access
- Author:
- Turgut, Ozhan Hulusi
- Graduate Program:
- Aerospace Engineering
- Degree:
- Doctor of Philosophy
- Document Type:
- Dissertation
- Date of Defense:
- October 20, 2011
- Committee Members:
- Cengiz Camci, Dissertation Advisor/Co-Advisor
Cengiz Camci, Committee Chair/Co-Chair
Savas Yavuzkurt, Committee Member
Deborah A Levin, Committee Member
Anil Kamalakant Kulkarni, Committee Member - Keywords:
- nonaxisymmetric endwall contouring
secondary flows
gas turbine
leading edge fillets - Abstract:
- The three main sources of the total pressure deficit in a turbine stage are profile loss, leakage loss, and the endwall boundary layer loss. The total pressure loss related to endwall boundary layer in a turbine passage may represent about one third of the total loss measured. The endwall loss definition involves the secondary flows such as the horseshoe vortex, passage vortex, and the cross-passage flow from the pressure surface to the suction surface. Researchers have been trying various methods to reduce the effect of these flow structures. The successful approaches for meaningful aerodynamic improvements can be counted as the axisymmetric/nonaxisymmetric endwall contouring, blade leading edge modifications, using shape optimized blades, and the insertion of endwall fences. One of the promising methods for reducing the secondary flows in a turbine stage is the nonaxisymmetric endwall contouring. Besides the improvement with axial endwall contouring having cylindrical endwalls, nonaxisymmetric contouring could significantly contribute to the energy efficiency in turbomachinery systems. The modern turbine stages with reduced blade count could undergo additional secondary flow losses which can be counteracted by nonaxisymmetric endwall contouring. Additionally, the aerodynamic losses related to the horseshoe vortex can be minimized using a leading edge (LE) fillet, which fills the intersection of the nozzle guide vane (NGV) and the hub endwall. It forms a smooth transition from the NGV leading edge to the endwall surface. These LE fillets improve the aerodynamics of the flow and the heat transfer effectiveness, as well. The main goal of this study was to minimize the secondary flows by developing methods such as designing nonaxisymmetric endwall contouring and LE fillets. The study is unique in itself because it was performed in the well simulated turbine stage, the Axial Flow Turbine Research Facility (AFTRF), that is including an actual rotor. Many current studies of this sort were performed either in linear cascade facilities with no rotor influence or isolated annular NGV cascades. The procedure was to evaluate the new conceptual designs computationally, perform experimental investigations, and compare the simulation results with the experimental data for validation and further analysis. The AFTRF installed at the Turbomachinery Aero-Heat Transfer Laboratory of the Pennsylvania State University is a low speed, single-stage, cold flow turbine having a diameter of 91.66cm. It has a stationary NGV assembly with 23 vanes and a High Pressure (HP) turbine rotor row with 29 blades. The NGV inlet and exit Reynolds numbers based on midspan axial chord are around 300000 and 900000, respectively. The current research presented the total pressure measurements at a plane perpendicular to the axial direction, between the NGV and the rotor blade. The measurements were taken by a Kiel probe which was highly insensitive to yaw and pitch angles of the flow. The computational fluid dynamics (CFD) evaluations were accomplished by a commercial finite-volume viscous flow solver which was based on the Reynolds-Averaged Navier Stokes (RANS) equation model. The computational validation of the baseline case was achieved by comparing the CFD results with the already available experimental data of a previous researcher. This validation showed the difference of the grid structure type and the body fitted, multi-block structured mesh was found to be superior to the unstructured grid. The flow transition effect and the influence of corner fillets at the vane-endwall junction were also studied. The CFD validation also reported that it was necessary to include the rotating domain in the simulations. Another important criterion to be included was the rim seal leakage flow. The consideration of these two parameters in the simulation improved the agreement of the CFD results with the experimental measurements. The contoured endwall and the LE fillet designs were manufactured using stereolithography (SLA) technique. The SLA models of the designs were installed at the AFTRF and tested experimentally. The contoured endwall successfully reduced the crossflows from the pressure side to the suction side of the vane. Also, LE fillets achieved to minimize the horseshoe vortex formation. The experimental results showed 15.06% total pressure loss reduction, while CFD estimated a 7.11% reduction.