Experimental comparison between a counter-rotating axial-flow fan and a conventional rotor-stator stage

Based on the requirement of energy consumption level and weight and dimension restriction, compact axial machines are highly demanded in many industrial fields. The counter-rotating axial-flow fans could be a promising way to achieve these requirements. Because of the reduction of rotational speed and a better homogenization of the flow downstream of the rear rotor, these machines may have very good aerodynamic performances. However, they are rarely used in subsonic applications, mainly due to poor knowledge of the aerodynamics in the mixing area between the two rotors, where very complex structures are produced by the interaction of highly unsteady flows. The purpose of the present work is to compare the global performances (static pressure rise and static efficiency) and the wall pressure fluctuations downstream of the first rotor for three different stages operating at the same point: a single subsonic axial-flow fan, a conventional rotor-stator stage and a counter-rotating system that have been designed with inhouse tools. The counter-rotating system allows large savings of energy with respect to the other two systems, for lower rotation rates and by adjusting the distance between the two rotors, a solution with comparable wall pressure fluctuations levels for the three systems is found. NOMENCLATURE Symbols Acronyms D Pipe diameter CRS counter-rotating system f blade passing frequency FR front rotor N rotational rate (rpm) RR rear rotor Patm atmospheric pressure R1 front rotor alone Ps static pressure RSS rotor-stator stage Q Volumetric flow rate s axial distance W Power z axial coordinate ∆Ps static pressure rise ηs static efficiency φ volumetric flow rate coefficient ω angular velocity Ψs static head coefficient ρ density of the air τ torque θ ratio of angular velocities INTRODUCTION Nowadays, a revival of industrial interest for counter-rotating axial machines can be observed for various applications in subsonic regimes, as for instance fans and pumps, operating in ducted or freeflow configurations (Cho et al., 2009; Shigemitsu et al., 2009, 2010; Xu et al., 2009; Yoshihiko, 2003). 1 ha l-0 07 95 00 6, v er si on 1 27 F eb 2 01 3 Counter-rotating axial-flow fans for electronic devices cooling application are for instance developed by SANYO DENKI (manufacturer of fans) with various diameters. According to Yoshihiko (2003), these products have the advantages of large air volume, high static pressure while lower noise and power consumption, compared to 2 conventional fans used in series. For the same type of industrial application, Shigemitsu et al. (2010) have shown with numerical studies that counter-rotating axial small-size fans provided higher pressure and efficiency than one single rotor. However, detailed experiments and analysis are still demanded to reveal the physical mechanisms that improve their efficiency compared to the conventional facilities. The general idea of a counter-rotating system is that two rotors (front and rear) are rotating in opposite directions. The energy in the tangential velocity component of the flow after the first rotor is usually wasted in the wake (Dron, 2008). At the inlet of the rear rotor of a counter-rotating fan stage, this tangential velocity contributes to higher relative velocity, then it diffuses in the second rotor and is moreover converted to static pressure rise. Compared to a conventional rotor-stator stage, the rear rotor not only recovers the static head but also supplies energy to the fluid. Given all the advantages indicated above, the counter-rotating system attracts attention of a large number of researchers. An original method to design such a system has been developped in the DynFluid Laboratory and has been validated on a first prototype: CRS (Nouri et al., 2013). In this experiment, the rotors operate in a duct of diameter D = 380 mm, the ratio θ = NRR NFR of the rotation rates of the two rotors can be varied, and the axial distance s between the front rotor and the second rotor can be varied in a wide range (see Fig. 1). The main results of this study are: • the maximum of the peak static efficiency of CRS is 67 ± 1% whilst the peak static efficiency of the front rotor alone is 45± 1%; • at the design angular velocity ratio θ = 0.9 the overall performances are not significantly