INTRODUCTION
The flow in a high performance axial fan stage is inherently unsteady
because of the interactions due to relative motion between the preceding
Inlet Guide Vanes (IGV) and rotating blades. This has been regarded as
the primary source of pressure fluctuations which in addition to noise
and vibrations introduce unfavourable changes in performance in terms of
aerodynamic efficiency, operating stability and range. Earliest
investigation on Rotor/Stator or Stator/Rotor interaction, known as RSI
phenomena, was due to Kemp et al. [1], and since many past
researchers have undertaken such studies [2-6]. The dominant causes
of such unsteadiness are attributed to the wake/blade interaction in
which wakes produced by upstream stator (IGV) are swept downstream into
the next rotor row, followed by the vortex shedding at trailing edges
added to the potential effect in which the pressure field associated
with the leading edge of a rotor sweeps past the trailing edge of an
upstream vane (stator) [2, 7]. Adamczyk et al. [8] have
described the wake mixing within a rotor passage and the tendency of
stator wake to drift toward the pressure side of a downstream rotor. As
the RSI phenomena do change the real flow; In case of an axial
compressor the wakes at rotor exit fluctuate strongly so that the
incidence angle to the next stator is higher and the flow turning is
higher too in the following blade [9]. Experimental studies of
wake/rotor interactions were performed by Kerrebrock and Mikolajczak
[10] for the compressors and by Binder et al. [11] and Hodson
[12] for the turbines. Hsu and Wo [13] carried out an
experimental investigation on unsteady interactions in a large-scale
low-speed 1. 5-stage axial compressor, followed a numerical study
by Lee and Feng [14]. In their two-part papers, Mailach et al.
[15, 16] presented detailed experimental investigations of unsteady
blade-row interactions in a low-speed research compressor and discussed
the effect of blade-row clocking on the unsteady profile pressures.
Later, Jia et al. [17] investigated numerically the same compressor
and compared with experiments. The study of the different aspects of RSI
phenomena may help in improving the design of high performance axial fan
stages and reduce noise emission. Tsuchiya et al. [18] attempted to
predict RSI in a high-speed fan stage acoustically based on Unsteady
Reynolds-Averaged Navier-Stokes equations (URANS) to acquire the
pressures pulsations induced by wakes. Also, Kodama et al. [19]
performed
URANS
analyses for the effects of axial spacing and vane geometry on the
unsteady pressure fluctuations. More recently, there are typically the
works of Oro et al. [20], who introduced an URANS modelling for the
flow in an axial fan stage of 13 vanes in IGV and 9 blades in rotor, and
their focus was on RSI and the influence of inter-distance. They showed
that the deterministic kinetic energy could be used effectively in
analyzing such unsteady phenomena. Later on, for the same axial fan
stage, Oro et al. [21] developed an experimental open-loop facility
to obtain a physical description of the flow, where exhaustive analyses
of measured wakes provided a comprehensive description of both
wake-transport phenomena and RSI mechanisms. The unsteadiness associated
to the wakes convected through the passages of the same low-speed axial
fan stage was further studied by Vega et al. [22] based on LES
simulations to resolve the largest scales of the vortical motion related
to vortex shedding. The numerical modelling was able to reproduce
accurately the unsteady phenomena that occur inside the axial fan;
specifically the chopping mechanisms and the periodic interactions of
the coherent turbulent structures were described and compared with
experiments.
The present contribution paper focuses on studying RSI phenomena in a
high-speed high-reaction pre-whirl axial fan stage, operating at a
variable speed, shown in Fig. 1. The main data are also provided in
Table 1. First, the aerodynamic performances were determined at
different speed and including the effect of axial inter-distance. Second
the unsteady flow simulations were carried out to predict the static
pressure fluctuations at different monitor points. As results, the
sequences of mode shapes of RSI are well illustrated for this high-speed
axial fan stage in terms of frequencies and amplitudes base on Fast
Fourier Transform (FFT).