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