Paper presented at the 6th International Conference on Heat Transfer, Fluid Mechanics and Thermodynamics, South Africa, 30 June - 2 July, 2008.
An experimental investigation of the third quadrant
(reverse rotation) performance, operation and
the internal flow structures of a three stage subsonic
axial flow compressor is described. The compressor
used for this investigation has three repeating
stages, and a design speed of 3000 rpm, a design
mass flow rate of 2.7 kg/s and a blade tip relative
Mach number of 0.2 at design point. The
non-dimensional third quadrant pressure rise and
torque characteristics for this machine have previously
been determined. The steady-state interblade
row flow-field was experimentally obtained by
means of a five-hole cobra-type pneumatic probe
and time-dependent phenomena were resolved by
means of a 50 μ diameter cylindrical constanttemperature
hot-film sensor. The observed flow
structures are compared with those observed in a
single stage axial fan rotating in the reverse direction
as described in literature. Flow within rotor
blade passages is dominated by large areas of separated
flow on the pressure surface of the blades.
A large axial velocity gradient is observed in the
radial direction, with velocities highest near the
shroud. Third quadrant operation appears to be
similar to stalled first quadrant operation. Separation
was found to be less severe on the final stage
rotor row, as this blade row functions as the inlet
rotor row in this mode of operation, thus flow angles
are less unsuited to blade metal angles. These
results yield insight into the causes of the low maximum
pressure rise and efficiency obtained for this
operational mode for this machine, as well as the
apparent lack of a stall point on the pressure characteristic
obtained.
