Handbook of Turbomachinery Second Edition Revised - Ventech!

ecommendation was made to minimize the amount of operating time in the
1–2 mode. It should be noted that this was a precautionary measure, as it
was also indicated that the stress increase was probably not sufficient by
itself to cause crack initiation by high cycle fatigue [51].
Sanders [67] has suggested that the stress effects of partial-arc
admission are applicable for any active arc less than 96% of full arc.
Flow bending at radial inlet and exhaust of steam. Radial inlet and
exhaust of steam at a turbine element require turning of the flow and
thus can cause circumferential pressure or velocity nonuniformities.
Steam extraction paths.
3. Unsteadiness in the flow passages. Flow perturbations can be
introduced by unsteadiness in the stationary flow passages, including
Acoustical resonances in inlet passages, extraction lines, or other
cavities excited by flow past them
Vortex shedding from stay-bars, etc. [52]
Unsteady flow separation from stationary blades
Unsteady shocks in choked stationary blade passages
Surface pressure fluctuations caused by impingement of turbulent flow
onto rotating blade shrouds, disks, etc.
‘‘Edge tones’’ or other acoustical noise generation phenomena
4. Flow over rotating blades can also induce unsteadiness such as by
Boundary-layer pressure fluctuations
Figure 41 Transient forced load response on rotating blades as a function of
turbine arc of admission.
Copyright © 2003 Marcel Dekker, Inc.