Title page for ETD etd-09232003-153947


Type of Document Master's Thesis
Author Wallace, Robert Malcolm
Author's Email Address rowalla3@vt.edu
URN etd-09232003-153947
Title Modal Response of a Transonic Fan Blade to Periodic Inlet Pressure Distortion
Degree Master of Science
Department Mechanical Engineering
Advisory Committee
Advisor Name Title
O'Brien, Walter F. Jr. Committee Chair
King, Peter S. Committee Co-Chair
Kenyon, James A. Committee Member
West, Robert L. Jr. Committee Member
Keywords
  • periodic flow
  • transonic fan blade
  • modal response
  • HCF
  • distortion
Date of Defense 2003-09-18
Availability unrestricted
Abstract
A new method for predicting forced vibratory blade response to total pressure distortion has been developed using modal and harmonic analysis. Total pressure distortions occur in gas turbine engines when the incoming airflow is partially blocked or disturbed. Distorted inlet conditions can have varying effects on engine performance and engine life. Short-term effects are often in the form of performance degradation where the distorted airflow causes a loss in pressure rise, and a reduction in mass flow and stall margin. Long-term effects are a result of vibratory blade response that can ultimately lead to high cycle fatigue (HCF), which in turn can quickly cause partial damage to a single blade or complete destruction of an entire compressor blade row, leading to catastrophic failure of the gas turbine engine. A better understanding and prediction of vibratory blade response is critical to extending engine life and reducing HCF-induced engine failures.

This work covers the use of finite element modeling coupled with computational fluid dynamics-generated pressure fields to create a generalized forcing function. The first three modes of a low-aspect-ratio, transonic, first stage blade of a two-stage fan were examined. The generalized forcing function was decomposed to the frequency domain to identify the dominant harmonic magnitude present, as well as other contributing harmonics. An attempt to define the relationship between modal force with varying total pressure distortion levels produced a sensitivity factor that describes the relationship in the form of a simple multiplier. A generalized force was applied to the blade and varied harmonically across a frequency range known to contain the first natural frequency. The mean rotor stress variation was recorded and compared to experimental results to validate the accuracy of the model and verify its ability to predict vibratory blade response accurately.

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