抄録
The work characteristics and loss-generation mechanism of a
single-stage axial flow compressor in windmilling operation were
investigated via experiments and computational fluid dynamics
analyses. The windmilling state occurs when air flowing through an
unlit engine drives the compressor rotor blades, similar to a turbine.
This phenomenon applies mostly to aircraft engines, where it is
caused by ram pressure. When the inlet flow coefficient is gradually
increased in the design, the rotor blades gradually enters the windmilling
operation from the tip toward the hub. This research has
focused on two windmilling operations: free windmilling (FW) and
highly loaded (HL) windmilling. In the case of FW, the net work
performed by the rotor blades to the fluid is canceled out (zero), and
the rotor is in an idle state. In the HL windmilling condition, the
work performed to the rotor blades by the fluid increases, the compressor
acts as a turbine, and power is generated.
According to the detailed numerical results, the total-pressure
loss under the free and HL windmilling conditions was mainly
caused by three flow structures: (1) tip leakage flow from the suction
surface (SS) to the pressure surface (PS) near the leading edge
and that from the PS to the SS near the trailing edge; (2) the interaction
of leading-edge separation vortices due to the highly negative
incidence and the rotor leading-edge vortex; and (3) the boundary-
layer separation near the hub wall. Surface-pressure measurement
on a rotating rotor blade revealed that the distribution of the
rotor operating mode existed not only in the spanwise direction but
also in the chordwise direction under the windmilling operations.
The turbine mode region was observed near the leading edge, while
the compressor mode region was observed near the trailing edge,
even in the HL windmilling condition. Therefore, the driving force
of the windmilling was dominated not by the area of the turbine
mode on the rotor surface but by the strength of the operating mode,
i.e., the static-pressure difference between the SS and PS on the rotor.
Finally, the unsteady flow field within blade-to-blades passages
was investigated via an unsteady detached eddy simulation,
and the differences in the loss-generation mechanism between the
FW and HL windmilling conditions were examined.
