Unsteady Blade and Disk Resonant Stress Analysis Due to Supersonic Inlet Guide Vane Wakes

A high specific-power radial inflow turbine is currently being designed for installation in the fall of 2008. This turbine will generate 7,000–18,500 HP (5–14 MW) of electrical power. Operating with a relatively high pressure ratio and high molecular weight (58), which is common in geothermal power generation applications, the process has the ability to produce a very strong wake structure. Such wakes have caused catastrophic impeller fatigue failures in similar applications. To prevent a failure, the current design is based on a study of the interaction between the wake structure and the impeller. The SAFE and Campbell diagram are used for screening purposes, but the final design is analyzed using an unsteady CFD analysis coupled with time-dependent Finite Element Stress analysis. This pairing of CFD and FEA analysis allows the alternating torque to be determined both under resonant and non-resonant conditions. With this analysis, it is possible to approximate the stress resulting from higher order resonant frequencies which cannot be avoided in blade tuning. The analysis has shown that a high order harmonic of the vane passing frequency will not lead to unacceptable alternating stress levels. To facilitate future analysis, the 360° unsteady solution was compared to a harmonic analysis with two harmonic frequencies. The harmonic solution has shown good agreement with the full 360° solution.Copyright © 2008 by ASME