Design analysis and development of a high temperature actuator for gas turbine blade tip clearance control

Official URL: http://192.168.1.20/record=b1378193 (Table of Contents)

During a typical startup cycle industrial gas turbine blades experience rapid radial thermal expansion while bulky shroud structure with larger thermal inertia requires much longer period to reach its operating temperature. Turbine designers have to leave a safe radial distance in order to prevent contact of blades to the surrounding annular casing. However, when thermal steady state in the turbine stage is achieved, shroud and casing grow and excessive amount of blade-shroud clearance remains. Engine efficiency is very sensitive to blade-shroud clearance. Just one millimeter of radial blade tip gap in fist stage turbine section of a 150 MW class engine leads to 4% efficiency drop due to blade tip leakage. To achieve better efficiency or higher power, turbine blade tip clearance has to be controlled. Attempts to address blade tip clearance problem were not applicable as designs were bulky and complex which required excessive modification on the turbine hardware and design. The goal of this study is to design, analyze and develop a low-cost and compact actuator system which is capable of controlling the tip clearance up to 0.25mm at elevated temperatures. Actuator will be positioned between inner and outer shrouds of the casing to force the inner shroud radially away from the blades during transients, and allow it to come back towards the blades when casing reaches operating temperature to decrease the tip leakage during steady state. Different actuator designs have been studied and finite element analysis solutions have been obtained for deflection and stress. Low cycle fatigue life of the actuator has been estimated via Coffin-Manson criterion. An experimental setup has been designed and fabricated to validate the simulation results. Furthermore, since actuator will be subjected to wear at elevated temperatures due to mechanical loading and vibrations in the gas turbine, friction and wear behavior of candidate actuator materials has to be investigated. High temperature scuffing combined with rapid oxidation can lead to failures and dramatic reductions service life. Therefore, another experimental setup has been developed to conduct friction and wear tests of the candidate actuator materials, i.e. Nickel and Cobalt based superalloys Haynes 25, 188 and 214. The tests have been conducted at 20, 200,400 and 540 °C. Overall, the results indicated that the compact actuator can achieve 0.25 mm tip clearance reduction leading to 1% efficiency increase for 880 startup cycles.