Abstract
Loss of a blade from a running turbofan rotor introduces not only huge imbalance into the dynamical system rather it makes the entire rotor asymmetric as well. In a nonsymmetric rotor, the various terms of mass, gyroscopic and stiffness matrices also become time-dependent. In this paper, all the dynamical equations include the effect of the rotary inertia and gyroscopic moments as a result of both shaft bending as well as staggered blades flexing in-and-out of the plane of the disk. The governing equations also account for internal material damping in the shaft and the external damping in the support bearing system. In addition to the unbalance load at the disk location, the shaft may also be subjected to a torque and axial forces. Here, the fan blades are modeled as pre-twisted thin shallow shells. They have coupled flexural–torsional motion in the lateral out-of-plane direction as well as extensional degrees-of-freedom in the longitudinal spanwise direction of the blade airfoil. The effect of blade tip rub forces being transmitted to the shaft are analyzed in terms of the dynamic stability of the rotor, especially during windmilling.
Highlights
Losing a blade from a turbofan rotor of a jet engine during its normal operation either due to bird-strike or due to fatigue damage is a real possibility, and as such it is a major safety concern in commercial civil aviation [1,2,3,4,5,6]
A typical turbofan rotor starts behaving like a turbine rotor, when it is subjected to a head-wind, which feeds energy to the dynamical system
The analytical method is properly complemented by a detailed numerical simulation
Summary
Losing a blade from a turbofan rotor of a jet engine during its normal operation either due to bird-strike or due to fatigue damage is a real possibility, and as such it is a major safety concern in commercial civil aviation [1,2,3,4,5,6]. Any experiment simulating blade-loss or tip-rub under controlled conditions [11] is extremely expensive, rather sometimes they involve serious risk to the safety of the rig and the test personnel as well As such there is a scarcity of test data, to compare the results of any fan blade-out analytical model in the published literature. [14,15,16,17,18,19,20,21,22,23,24,25] In general, these studies invariably consider the bladed-disk assembly as rigid with entire flexibility of the dynamical system attributed to the shaft material with an artificial nonlinear spring to account for blade-tip rub forces. Due to large computation time involved, attempt is usually limited to obtain the solution only in frequency domain, where peak response are determined at certain pre-determined harmonic excitation frequencies, which happens to be the entire range of the running speed of the rotor during normal operation
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