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).