Page 179 - Fluid Mechanics and Thermodynamics of Turbomachinery

160 Fluid Mechanics, Thermodynamics of Turbomachinery
High-pressure
air source
Adjustable
dashpot
Injection
plenum
Valve area detemined
by tip deflection of Reed Cantilevered
reed valve seal reed valve

Tip

Injection flow
Rotor
Air flow to compressor
Hub

FIG. 5.15. Schematic of the aeromechanical feedback system used to suppress the
onset of rotating stall (adapted from Gysling and Greitzer 1995).

region, the static pressure in the potential flow field upstream of the compressor
will increase. The increase in static pressure deflects the reed valves in that
region, increasing the amount of high momentum fluid injected and, hence, the
local mass flow and pressure rise across the compressor. The net result is an
increase in pressure rise across the compressor in the region of decreased axial
velocity. The feedback thus serves to add a negative component to the real part
of the compressor pressure rise versus mass flow transfer function.

Only a small amount (4%) of the overall mass flow through the compressor was used
for aeromechanical feedback, enabling the stall flow coefficient of the compression
system to be reduced by 10% compared to the stalling flow coefficient with the
same amount of steady-state injection.
It is claimed that the research appears to be the first demonstration of dynamic
control of rotating stall in an axial compressor using aeromechanical feedback.

Axial-flow ducted fans
In essence, an axial-flow fan is simply a single-stage compressor of low pressure
(and temperature) rise, so that much of the foregoing theory of this chapter is valid
for this class of machine. However, because of the high space chord ratio used in
many axial fans, a simplified theoretical approach based on isolated aerofoil theory
is often used. This method can be of use in the design of ventilating fans (usually of
high space chord) in which aerodynamic interference between adjacent blades can
be assumed negligible. Attempts have been made to extend the scope of isolated
aerofoil theory to less widely spaced blades by the introduction of an interference
factor; for instance, the ratio k of the lift force of a single blade in a cascade to the