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Kim D.-J.,Sungkyunkwan University | Shin I.-H.,Sungkyunkwan University | Koo J.-M.,Sungkyunkwan University | Seok C.-S.,Sungkyunkwan University | Lee T.-W.,Gas Turbine Technology Service Center
Surface and Coatings Technology

The aim of this study is to investigate the failure mechanisms of coin-shaped plasma-sprayed thermal barrier coatings (TBCs) for gas turbine blades due to cyclic thermal fatigue. For this purpose, cyclic thermal fatigue tests at 1100 °C were performed for a commercial TBC system that was used for the first-stage blade of a gas turbine. The failure location was near the interface between the top coat and the thermally grown oxide (TGO). For thermally aged TBC specimens, bond tests were performed to investigate the failure modes and to observe the decrease in the bond strength. The spallation of a TBC was preceded by external crack initiation at the edge of a specimen, and the bond strength gradually decreased with increasing thermal cycles due to the expansion in the delaminated zone. Through the thermal stress analysis it was found that edge delamination originated from repeated "cooling shock", which occurs right after forced air quenching, where the normal stress the edge increases sharply within a few seconds. © 2010 Elsevier B.V. Source

Kim K.M.,Yonsei University | Park J.S.,Yonsei University | Lee D.H.,Yonsei University | Lee T.W.,Gas Turbine Technology Service Center | Cho H.H.,Yonsei University
Engineering Failure Analysis

Prediction of heat transfer coefficients and stresses on blade surfaces keys a role in thermal design of a gas turbine blade. The present study investigates heat transfer and stress in a gas turbine blade with 10 circular internal cooling passages. 3D-numerical conjugated simulations using a FVM and FEM commercial codes, CFX and ANSYS are performed to calculate distributions of the heat transfer coefficients and the stresses, respectively. The heat transfer coefficient is the highest on the stagnation point of leading edge due to impingement of incoming gas flow. It is the lowest at the trailing edge on both pressure and suction sides due to development of thermal boundary layer. However, the maximum material temperature and the maximum thermal stress occur at the trailing edge near the mid-span. Therefore, the failure of turbine blade should be predicted by total stress resulted from the combination of thermal load and cooling. © 2011 Elsevier Ltd. Source

Koo J.-M.,Sungkyunkwan University | Seok C.-S.,Sungkyunkwan University | Kang M.-S.,Sungkyunkwan University | Kim D.-J.,Sungkyunkwan University | And 2 more authors.
Transactions of the Korean Society of Mechanical Engineers, A

The thermal barrier coating of a gas turbine blade was degraded by isothermal heating in a furnace and by varying the exposure time and temperature. Then, a micro-Vickers hardness test was conducted on the cross section of the bond coat and Ni-based superalloy substrate. Further, the thickness of TGO(Thermally Grown Oxide) was measured by using an image analyzer, and the changes in the microstructure and element contents in the coating were analyzed by using an optical microscope and by performing SEM-EDX analysis. No significant change was observed in the Vickers hardness of the bond coat when the coated specimen was degraded at a high temperature; delamination was observed between the top coat and the bond coat when the coating was degraded for 50 h at a temperature 1, 151°C. Source

Kim K.M.,Yonsei University | Yun N.,Yonsei University | Jeon Y.H.,Yonsei University | Lee D.H.,Yonsei University | And 2 more authors.
Journal of Mechanical Science and Technology

Prediction of temperature distributions on hot components is important in development of a gas turbine combustion liner. The present study investigated conjugated heat transfer to obtain temperature distributions in a combustion liner with six combustion nozzles. 3D-numerical simulations using FVM commercial codes, Fluent and CFX were performed to calculate combustion and heat transfer distributions. The temperature distributions in the combustor liner were calculated by conjugation of conduction and convection (heat transfer coefficients) obtained by combustion and cooling flow analysis. The wall temperature was the highest on the attachment points of the combustion gas from combustion nozzles, but the temperature gradient was high at the after shell section with low wall temperature. © 2010 The Korean Society of Mechanical Engineers and Springer-Verlag Berlin Heidelberg. Source

Kim H.-I.,University of Delaware | Park H.-S.,KEPCO E&C | Koo J.-M.,Sungkyunkwan University | Seok C.-S.,Sungkyunkwan University | And 2 more authors.
Journal of Mechanical Science and Technology

A Ni-based super-alloy is widely used in manufacturing the first stage blade of high power land-based gas turbines as these first stage blades operate under high temperature and high pressure. The blade of a gas turbine must withstand the most severe combination of temperature, stress, and environment. After continued operation, the blade may be damaged by the turbine operation mode. To recover its initial mechanical properties, the blade of the Ni-based super-alloy undergoes a replacement repair process. Typical repair processes include blending, welding, re-machining and precision grinding. In this paper, the effects of manual overlay and laser cladding were investigated as part of the welding characteristic evaluation. Results are compared with those for post-heat treatment known as hot isostatic processing (HIP). © 2012 The Korean Society of Mechanical Engineers and Springer-Verlag Berlin Heidelberg. Source

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