Estimation of turbine blade natural frequencies from casing pressure and vibration measurements

Abstract

Non-contact measurement of gas turbine rotor blade vibration is a non-trivial task, with no method available which achieves this aim without some significant draw-backs. This paper presents a truly non-contact method to estimate rotor blade natural frequencies from casing vibration measurements at a single engine operating speed. An analytical model is derived to simulate the internal casing pressure in a turbine engine including the effects of blade vibration on this pressure signal. It is shown that the internal pressure inside a turbine contains measureable information about the rotor blade natural frequencies and in-turn the casing vibration response also contains this information. The results presented herein show the residual, pressure and casing vibration, spectrum can be used to determine the rotor blade natural frequencies with validation provided for the analytical model by experimental measurements on a simplified test rig. A simulated blade fault in one of the rotor blades is introduced with successful estimation of the simulated faulty blade natural frequency.

Modelling and experimental validation of active and passive eddy current sensors for blade tip timing

“…In order to demonstrate the reliability of the models described in this paper, a test rig was designed and manufactured in order to replicate a BTT application and generate BTT measurements using commercial A-ECS and P-ECS and therefore validate the models. Based on a brief review of the literature of the existing test rigs built for monitoring the structural health of a machine [34][35][36][37][38], a test rig was designed that was composed of a bladed disk with 12 blades surrounded by a cylindrical casing. The bladed disk is clamped to the end of a rotating shaft supported by two bearings.…”

Rotating blade vibration analysis using photogrammetry and tracking laser Doppler vibrometry

“…Blade tip timing [2][3][4] and casing pressure measurements [5] have been employed as non-contact techniques for blade vibration analysis. These techniques are more reliable than contact techniques in terms of life expectancy under the very hostile conditions of high temperatures and pressures under which turbine blades operate [2].…”

Turbine engine rotor blade fault diagnostics through casing pressure and vibration sensors

“…Forbes and Randall determined that the turbine engine casing acts as a filter to the internal dynamic pressure, and so the frequencies contained in both signals are the same [1]. In a turbine engine fan, the casing internal pressure peak travels with the rotor blade tips.…”

“…Forbes and Randall showed that each harmonic of the rotor speed carrier frequency produces its own set of sidebands, as shown in Figure 1 [1]. To determine rotor blade vibratory frequency from the casing internal pressure response in the frequency domain, Forbes and Randall developed the following equation ,…”

“…where ω is the blade vibratory frequency, Δf is the sideband spacing, Ω is the rotor rotational frequency, and q is an integer based on the possible sources of the sideband frequencies [1]. Equation 1 is linear in two variables (ω and q), and thus indeterminate.…”