Graduation Semester and Year

2022

Language

English

Document Type

Dissertation

Degree Name

Doctor of Philosophy in Mechanical Engineering

Department

Mechanical and Aerospace Engineering

First Advisor

Dereje Agonafer

Abstract

Gas Turbines Hot Gas Path components represent the major contribution to generating power in Gas Turbines They are the components that mainly extract power from hot gasses that come out of the combustion Chamber (Combustor). Due to harsh temperature and environment, these components are vulnerable to high thermal and mechanical stresses which affect its durability, life, and efficiency. Hot Gases go through these components can reach very high max temperature. Max blade alloy metal temp in Gas Path blades can reach 1800-1900 F. Because of these severe operation conditions, a very careful prediction of stresses, thermal distribution, losses and flow characteristics need to be studied in depth to focus more into variables and factors that can affect gas turbine’s hot gas path components durability, living & efficiency. In many research studies, it has been noted the impact of purge flow and platform contour near the endwall for nozzle and platform blades on the overall secondary losses and specifically on the horseshoe vortex that can be formed near the hub of the blade and nozzles and can increase turbulence and lower thermal efficiency. It was also found that one of the main reasons of horseshoe vortex formation and intensity is the ΔP value between PS and SS near the blade aerofoil hub that can control the lateral flow component that migrates from PS to SS and this cause an increase of vortex intensity which will contribute on platform temperature increase that will consequently lower the Gas Path component life and crack initiations capability that could propagate and even increase the losses further more. The focus of this study is Gas Path Turbine Blades Platform Contouring Impact on secondary flow and losses near platform surface using adjoint solver optimization tool in order to lower the pressure difference between Pressure Side and Suction Side near the hub as much as possible to control vortex and turbulence near the platform surface by lateral flow component. Used for the study, Gas Path 2nd stage blade internally cooled with a serpentine core and flow exit from blade tip and TE ejections. Gas Path components are apparently critical components in design and analysis to capture as accurate as possible factors, variables, and unknowns that might impact the component performance and durability. Currently, it has been reported during engine inspection that there are multiple gas turbines that OEMs Gas Path blades show signs of failures and shortfall of durability in addition to the failures that can cause engine shutdown. After investigating and going through inspection reports on many Gas Path blades that came from the field for damage inspection, some consistent cracks and damage have been noted in the platform section at multiple locations of the blades, and that is the area of focus during this study. After further investigation we came to a conclusion that one of the major reasons has been known for platform crack initiation is the thermal gradient through the blade, especially in the platform- aerofoil root area which can imply tremendously high thermal stresses at some specific locations that can eventually cause a crack initiation at these critical locations. The first part of this study will be focusing on building Computation Fluid Dynamics (CFD) model with adaptive mesh method that will allow to get the most accurate results as possible for the baseline blade platform with a flat surface same scale of the linear cascade that will be used on the blade test for a blade flow passage. The boundary conditions that are most represent the real model, are applied in addition the linear periodic boundary conditions that represents a one blade passage and flow conditions such as pressure, temperature, inlet mass flow rate for mainstream, inlet mass flow for purge flow and flow exit conditions. The result of this study will be a baseline pressure distribution for a blade flat platform near the hub and a very accurate focus on the pressure at the hub location of intersections edges of blade platform with aerofoil PS and SS edges. ΔP value will be then calculated to represent the average static pressure value for the PS platform edge subtracted by the average static pressure value of the SS platform edge, this will represent the ΔP for the baseline flat platform blade. The second part of the study was focusing on optimization for the baseline blade platform non-axisymmetric contour that will lower the overall ΔP value between the PS and SS edges to lower the lateral flow component for three different contours and adjust the mesh after each iteration for data validation. The outcome of this study showed that non-axisymmetric contour on blade platform can have a tremendous impact on static pressure at PS and SS edges near the hub that we have three different geometries with pressure reduction of 5%, 10%, and 20%. The mesh has been adjusted and the model has been validated with an updated CFD model run with modified contour mesh and it was in agreement with the optimizer’s initial estimate.

Keywords

Blade platform, Platform contour, Non-axis symmetric, Linear cascade, Losses near platform, Blade platform optimization, Contoured platform, Horse-shoe vortex, Platform secondary losses, Platform cracks

Disciplines

Aerospace Engineering | Engineering | Mechanical Engineering

Comments

Degree granted by The University of Texas at Arlington

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