Handbook of Turbomachinery Second Edition Revised - Ventech!

The above considerations are all for pure 2D and highly simplified
situations. It should be noted that even in a 2D case with realistic blading
geometry, wakes may not be stretched all the time in a compressor passage
or compressed all the time in aturbine passage as depicted in Fig. 4. The 2D
computational result in Fig. 1 shows just how the wakes can be both
compressed in the frontal passage part and stretched in the rear in addition
to being distorted, giving a much more complex picture.
Furthermore, attention is drawn to the very 3D nature of wake–blade
row interaction in the near end-wall regions. An unsteady wake shed from
an upstream row is characterized by the cross-passage transportation by the
‘‘negative jet’’ within the wake, which convects low-momentum fluid from
the pressure surface to the suction surface for turbines and conversely for
compressors. On the other hand, end-wall flows in the blade row under
consideration are characterized by 3D secondary flow structures with
dominant streamiwse vorticity, e.g., passage vortex, tip-leakage vortex. A
basic feature of all these 3D flow structures is that the associated crosspassage
fluid movement near an end wall has an opposite sense to that close
to midspan. Here one can use a simple ‘‘phasing’’ argument to examine the
interaction between an incoming largely 2D unsteady wake with a 3D
‘‘steady’’ passage vortex. Just imagine two separate activities that could
behave linearly and thus be superimposed. One is the 2D incoming wake,
and the other is a steady passage vortex. Note that the sense of the crosspassage
fluid movement due to the incoming wake is the same regardless of
spanwise positions, while the cross-passage movement due to the passage
vortex changes the directions depending on the spanwise position. In the
very near-wall regions, the low-energy fluid is transported by the passage
vortex from the pressure to the suction surface and thus is in the same
direction as the wake transportation (for a turbine). On the other hand, in a
region away from the end wall, the cross-passage movement due to the
passage vortex will have an opposite sense to that due to the wake.
Consequently, the resultant flow structure when a 2D wake is interacting
with a passage vortex will have to be strongly 3D. The suppression or
enhancement of cross-passage movements at different spanwise sections has
been shown to generate a radial redistribution time-averaged entropy loss in
comparison with a steady flow without incoming wakes [6]. Given that a
major portion of aerodynamic losses is generated in highly 3D end-wall
regions for typical blading designs, one does need to be cautious in applying
the 2D flow arguments. More work is needed to identify 3D blade-row
interaction effects and understand the corresponding mechanisms.
Copyright © 2003 Marcel Dekker, Inc.