Casing Treatment and Blade-Tip Configuration Effects on Controlled Gas Turbine Blade Tip/Shroud Rubs at Engine Conditions

Abstract

Experimental results obtained for an Inconel® compressor blade rubbing bare-steel and treated casings at engine speed are described. Since 2002 a number of experiments were conducted to generate a broad database for tip rubs, the Rotor-Blade Rub database obtained using the unique experimental facility at the The Ohio State University Gas Turbine Laboratory. As of 2007, there are seven completed groups of measurements in the database. Among them a number of blade-tip geometries and casing surface treatments have been investigated. The purpose of this paper is to provide a detailed interpretation of this database. Load cell, strain, temperature, and accelerometer measurements are discussed and then applied to analyze the interactions resulting from progressive and sudden incursions of varying severity, defined by incursion depths ranging from 13 μm to 762 μm (from 0.0005 in. to 0.030 in.). The influence of blade-tip speed on these measurements is described. The results presented describe the dynamics of rotor and casing vibro-impact response at representative operational speeds similar to those experienced in flight. Force components at the blade tip in the axial and circumferential directions are presented for rub incursions ranging in depth from very light (13 μm) to severe (406 μm). Trends of variation are observed during metal-to-metal and metal-to-abradable contacts for two airfoil tip shapes and tip speeds 390 m/s (1280 ft/s) and 180 m/s (590 ft/s). The nonlinear nature of the rub phenomena reported in earlier work is confirmed. In progressing from light rubs to higher incursion, the maximum incurred circumferential load increases significantly while the maximum incurred axial load increases much less. The manner in which casing surface treatment affects the loads is presented. Concurrently, the stress magnification on the rubbing blade at root midchord, at tip leading edge, and at tip trailing edge is discussed. Computational models to analyze the nonlinear dynamic response of a rotating beam with periodic pulse loading at the free-end are currently under development and are noted.