Handbook of Turbomachinery Second Edition Revised - Ventech!

to subsonic conditions without passing through another shock. Near the tip
(with its higher inlet relative Mach number), a normal shock was present
near the trailing edge. Because the rotor had a swept leading edge, the shock
surface was also inclined, thereby generating an increased radial velocity
component behind the shock, due to flow turning across the shock.
Dunker et al. [12, 13] also examined the shock structure within a
transonic axial fan rotor. In a typical shock-wave system, they observed that
a detached bow shock was followed by a subsonic bubble around the rotor
leading edge. On the suction side of the blade, the flow then reaccelerated to
supersonic conditions, before encountering the normal shock branch of the
bow shock, running across the passage. Downstream of the passage shock,
the flow exited with subsonic velocity. Within the blade passage, the normal
shock interacted with the suction surface boundary layer to produce a l
shock next to the blade surface. The oblique branch of the bow shock
extended upstream, interacting with expansion waves from the supersonic
acceleration around the leading edge. As the inlet Mach number was
increased, the bow shock attached to the leading edge and developed an
oblique, rather than a normal, branch within the blade passage. Boundarylayer
interaction with this passage shock triggered a separation bubble on
the suction surface. Depending on the back pressure, an additional normal
passage shock was present near the trailing edge.
Structure of the blade wake is another important aspect of axial
compressor flow behavior. Vortices can be shed periodically from the blade
trailing edge, and the velocity defect is mixed out as the flow proceeds
downstream. Paterson and Weingold [14] simulated the behavior of a
compressor airfoil trailing-edge flow field, by utilizing a flat-plate model
with a thick, rounded trailing edge. This configuration produced trailingedge
boundary-layer separation, resulting in flow recirculation and
unsteadiness. The axial extent of the reversed-flow region downstream of
the trailing edge was found to be approximately 0.8 plate thicknesses.
Vortex shedding from the trailing edge strongly enhanced wake mixing and
also introduced unsteadiness in trailing-edge surface pressures and wake
static pressures. The trailing-edge interaction region extended from
approximately 10 plate thicknesses upstream of the trailing edge to 3 plate
thicknesses downstream. Further downstream, the velocity defect took on
the character of a far wake flow.
Prato and Lakshminarayana [15] studied the structure of an axial
compressor rotor wake flow in the trailing-edge, near-wake, and far-wake
regions. Asymmetric velocity profiles were observed in the trailing-edge and
near-wake regions; however, the profiles tended to become symmetric in the
far wake, due to mixing. Large gradients of static pressure existed across the
wake in the trailing-edge and near-wake regions, with maximum static
Copyright © 2003 Marcel Dekker, Inc.