5.3 Effect of Axial inter-distance
IGV wake requires a greater mixing-out for a large axial inter-distance
which facilitates convection; on the contrary the wakes introduce a
considerable discontinuity into the velocity field for the small
inter-distance. As there is tangential fluctuation in the structure of
the blockage, and that means the tangential uniformity of the flow is
broken by IGV wakes discontinuities, a clear interaction between IGV
wakes and periodic blockage of the rotor blades occurs. The reduction in
inter-distance produces a clear spanwise redistribution of flow
velocity, with blockage pattern from mid-span to blade tip; so the flow
is concentrated near the hub. Moreover, the defect related to IGV wakes
is greater in the case of reduced inter-distance, because of the
inherent non-uniform flow pattern. Near the tip there is an important
reduction in axial velocity caused by the rotor blockage which is more
extreme when IGV wakes occur in particular angular positions as seen
before. To assess the effect of inter-distance six gaps were considered
for the unsteady flow simulations while recording the static pressure at
IGV/rotor interface. Table 7 shows that the amplitude of fundamental
harmonic of 1100 Hz has maximum amplitude at the minimum distance but
amplitude is less for larger inter-distance. It seems that the
amplitudes of pressure fluctuations recorded at points (P1, P2 and P3)
tend to decrease almost proportionally to the inverse of distance and
this trend is maintained until the optimal distance; afterwards the
amplitude of points (P2 and P3) rises again, but slightly for the point
(P1) which afterwards varies moderately for large inter-distance but
depending on the monitor point location. The amplitude of second
harmonic of 2200 Hz varies significantly with inter-distance, while the
minimum is still occurring around the optimum position. The monitor
point at the mid-span has smaller fluctuations in pressure. However, for
the third harmonic of frequency 3300 Hz, the amplitude seems varying
significantly to reach a minimum and then increases slightly, afterwards
drops for the large inter-distance. It appears that almost all the
harmonics produce the minimum fluctuations of pressure around the
optimum distance as depicted in Fig 17. Finally, it seems that the
potential interaction appearing in the form of periodic pressure
fluctuations propagating both upstream and downstream of blade-row
becomes dominant for small axial inter-distance for this large pitch by
chord ratio axial fan stage compared to an axial compressor.
Table 7. Pressure amplitude for main frequencies at IGV/rotor
interface with axial inter-distance