MHE™Minimize Harmonic Excitation (MHE) is a Windows NT program for determining modifications to stator throat openings in axial flow turbines to minimize levels of harmonic excitation. By lowering excitation, blade high-cycle fatigue damage can be reduced. It is the non-uniform spacing of the stator row that produces an uneven flow field, resulting in different levels of excitation for the various harmonics.
The strongest level of harmonic forcing will always correspond to the number of openings in the stationary row, and rotating stages are designed to avoid damage at this 'nozzle passing frequency' (NPF). The other harmonics that are typically of interest are lower order harmonics, thus MHE considers the first 20 harmonics for analysis. There are two reasons for this: 1) higher order harmonics generally have lower levels of excitation associated with them, and 2) the rotating blade excitation frequencies of higher order harmonics are harder to control.
- Minimizes harmonic excitation of critical rotor frequencies
- Reduce blade vibrations at resonant conditions
- Increase the high-cycle fatigue life of blades
- Viable alternative to blade detuning, which risks moving other modes into resonance.
- Identifies adjustments to the nozzle throat openings to suppress harmonics of steam bending forcing functions.
- Stimulus from one or two critical harmonics of the NPF can be suppressed
This program is normally used in conjunction with finite element analysis (BLADE™) or a telemetry test of a rotor to identify the critical harmonics to be suppressed.
Harmonics of Spectra Before and After Modification
to Suppress 6th and 13th Harmonics