Handbook of Turbomachinery Second Edition Revised - Ventech!

Figure 3 Classical distortion screen shapes.
magnitudes to stage performance characteristics are described in Ref. [2]. In
addition, Refs. [3–8] are several of many papers that describe inlet distortion
effects on gas turbine performance.
Unsteady inlet flow properties can also be a source of unwanted
mechanical excitation of stage blading and ducting hardware. Mechanical
design considerations must ensure that blade natural frequencies do not
coincide with those anticipated from unsteady inlet flows.
Reynolds Number
The impact of Reynolds number on performance is more often associated
with smaller turbomachines operating at extreme altitudes where the air
density is much less than at sea level. Under these high-altitude ambient
conditions, the stage Reynolds number (based on blade chord as the
characteristic length and mean stream fluid properties) will be much less
than at sea level. Fans, front ends of compressor stages, and the last turbine
stages are most susceptible to Reynolds number effects. For most larger
engines, Reynolds number effects are minimal. An empirical rule of thumb
suggests that for stage Reynolds numbers greater than 26105 , Reynolds
number impact on stage performance is relatively unimportant.
Blade boundary-layer behavior is a very strong function of Reynolds
number. In an axial compressor, for example, the smaller Reynolds number
due to extreme altitude operation can result in a transition delay from a
laminar to a turbulent boundary layer. A laminar boundary layer is more
likely to separate, which can result in blade stalling much earlier than at the
higher Reynolds numbers. Flow separation not only deteriorates local stage
performance but can also impact downstream stage performance.
For small gas turbine engines with high-altitude operating conditions,
design applications normally require larger blade chords (increased
characteristic length) to minimize Reynolds number effects.
Copyright © 2003 Marcel Dekker, Inc.