Experimental Identification of Blade Tip Rub Forces at Engine Relevant Temperatures and Speeds

Abradable casing liners are used to safely minimize the clearance between rotating turbomachinery blades and outer casings. Reduced clearance improves engine efficiency but can lead to more blade tip rub events. These blade tip rub interactions need to be well understood at engine relevant speeds and temperatures to safely design the engine. Advancements in experimental capabilities to meet these engine relevant conditions and the corresponding measurement and analysis tools will enable these developments. The Ohio State University Gas Turbine Laboratory (OSU GTL), in collaboration with Pratt & Whitney, has expanded its capability in carrying out blade tip rubs to vary the temperature of the casing and blades for temperatures seen in a fan before rubs occur. This expansion of experimental capabilities requires a matching expansion of the methodologies used to analyze and interpret experimental data. In analyzing rubs at varying temperatures, it becomes imperative to understand how the temperature in the rub shoe affects the force transfer functions between the blades contacting the abradable liner and the load cells recording vibrations behind the casing. These transfer functions are necessary for obtaining the actual forces exerted upon the blades during the experiment but are originally obtained at room temperature. This paper examines how the load measuring unit's frequency response function changes based on different temperature conditions and how force measurements are altered between similar tests at significantly different temperatures.