Figure 17. Amplitude of pressure fluctuations for main
harmonics vs. inter-distance
CONCLUSION
The aerodynamic characteristics of a high-speed high-reaction pre-whirl
axial fan stage are determined. The variation of axial inter-distance
has revealed an optimum spacing allowing for the majority of wake
diffusing and mixing and subsequently reaching a maximum aerodynamic
efficiency. The steady flow simulations have shown that the model of
frozen IGV/rotor interface has a weak influence on the average flows and
the pseudo interactions. Better results, including RSI, are obtained
from the unsteady flow simulation based on the frozen-rotor interface.
Time mode analysis of pressure fluctuations has permitted the
determination of different frequencies and the prevailing modes of a
lobed structure of pressure waves. The amplitudes of pressure
fluctuations and harmonics are shown to vary significantly depending on
the monitor locations and also on the axial inter-distance. It is clear
that IGV/rotor interactions are influenced by this distance, and the
potential interaction propagating upstream and downstream of blade-row
prevails more for the small inter-distances. Accordingly, an optimum
distance has been identified which to maximizes the efficiency and
minimize static pressure fluctuations which in turn may alleviate the
levels of vibration and noise. More details about the vortical
structures involvement in RSI studies requires adopting LES simulation
but this is still requiring powerful computing resources.