Analysis of blade tip timing data from fan blades with synchronous and non-synchronous vibration

Abstract

Blade tip timing (BTT) as an alternative method is capable of directly measuring the blade tip displacement and inferring the vibration characteristics of rotating blades. Its use is growing in popularity due to its ease of application and the ability to deliver information concerning all blades in a rotor. Although the interpretation of the collected data remains an issue, this paper puts forward a process of using such data to analyse the vibrational characteristics of fan blades under different operating conditions and find the blade endurance margin combined with finite element analysis (FEA).A test system with eight casing-mounted optical probes and one OPR (once per revolution) sensor is built to collect the blades’ arrival time of an axial fan. Results indicate that flutter occurs at engine order (EO) 1.5 with a combination of three nodal diameters (2, 3, 4), however, nodal diameter 2 is dominant. A noticeable response (EO≈0.5) is considered as stall. Because every blade vibrates with a frequency lower than its natural frequency (eigenfrequency), and all of them undergo a similar vibration form, continuously rising and falling, which caused by the rotating stall cells. The 1st flex mode – EO2 crossing is found at 48% rotor speed and it’s consistent with the results of FEA. Finally, the blade endurance in Goodman diagram demonstrates that flutter poses a higher risk in comparison with the synchronous vibration.