Handbook of Turbomachinery Second Edition Revised - Ventech!

pressure at the wake centerline. The static pressure decayed rapidly in the
trailing-edge region, and much more gradually in the far wake. Wake width
was found to be largest in the tip region due to mixing caused by interaction
of the clearance flow, the wake, and the shroud boundary layer. Secondary
flow in the hub region also produced a local increase in wake width.
Other major influences on the performance of axial compressors are
unsteady flow and blade-row interaction. The effects of blade-row
interaction were examined by Okiishi et al. [16]. They observed that wakes
produced by an upstream rotor blade row were ‘‘chopped’’ into segments
and transported by the downstream stator blade row. The interaction of
these rotor wake segments with the stator blade boundary layers resulted in
higher losses through the stator than would have been generated by the
isolated wakes and boundary layers. In a follow-up study by Hathaway et
al. [17], it was observed that when the rotor and stator were not too closely
coupled, the transport of the rotor wakes through the stator blade row was
primarily controlled by steady-state inviscid flow effects. Spreading of the
rotor wake segments within the stator appeared to be minimal.
Blade-row interactions in multistage compressors become much more
complex. Cherrett and Bryce [18] examined unsteady viscous flow in the first
three stages of a high-speed multistage compressor. They observed that in
the embedded stage rotors, strong rotor–rotor interactions occurred, which
induced passage-to-passage variations in the phase-locked average (periodic)
total pressure field.
This discussion provides a sampling of the flow phemonena that occur
in axial fans and compressors and should give an indication of the
complexity of the flow structures that are present in such devices.
Flow in Centrifugal Compressors
Centrifugal compressors may be utilized in turbofan applications as the
high-pressure compressor downstream of multiple axial compressor stages.
In some turboprop or turboshaft applications, a one- or two-stage
centrifugal compressor will serve as the entire compression system.
Centrifugal compressors differ markedly from their axial counterparts.
Pressure rise per stage is significantly higher than for axial compressors. The
flow path undergoes a substantial increase in radius from inlet to exit, with
the flow entering the rotor, or impeller, axially and exiting radially. In most
applications, the flow then proceeds radially through a vaned diffuser.
As the flow-path radius increases, the circumferential distance between
blades increases. To compensate for this, and maintain the desired flow-path
area, impeller blade span decreases substantially from inlet to exit. In
addition, to allow blade loadings to be maintained at desired levels, without
Copyright © 2003 Marcel Dekker, Inc.