Experimental Investigation of Ducted Counter-Rotating Axial Flow Fans

D2 - Aerodynamic Design

An experimental study on counter-rotating axial-flow fans was carried out. The fans of diameter D= 375 mm were designed to match the following specification point : Q=3600 m3/h, total pressure rise ΔPt= 420 Pa rotating at 2000 RPM (specific speed Ω=2.46). The two rotors have been designed using the MFT software developed by the DynFluid Laboratory and based on the inverse method. The counter-rotating fans operate in a ducted-flow configuration and the overall performances are measured in a normalized test bench. The rotation rate of each fan is independently controlled. The axial spacing between the fans can vary from 10 to 150 mm.
The results show that the efficiency is strongly increased compared to a conventional rotor or to a rotor-stator stage. The effects of varying the rotation rates ratio on the overall performances are studied and show that the system is highly efficient on a wide range of flow-rates and pressure rises. However, the change of the axial distance between rotors from 10 to 50 mm does not seem to change the overall performances. This system has thus a very flexible use, with a large patch of high efficient operating points in the parameter space. On the other hand, wall pressure fluctuation measurements on the casing wall, 5 mm downstream the front rotor were performed. These measurements reveal tendencies about the overall machine noise and the rotors interaction. Results show that there is a strong interaction between rotors at S=10 mm with a richer power spectrum including interaction frequencies indeed the blade passing frequencies of each rotor. However, this interaction is highly decreased when the inter-rotor space is S=150 mm with still relatively high performances. The cross-correlation and auto-correlation of the wall-pressure fluctuations highlight the presence of coherent structures in between the fans, highly correlated and rotating at the same frequency and direction than the blade passing frequency of the rear fan when S=10mm. At S=150 mm however, the flow is less correlated but a rotating structure is still observed, rotating at the blade passing frequency of the front rotor. From these local measurements one could imagine that noise reduction could be expected when the axial spacing S is increased with relatively high performances.
Local measurements of the velocity field in-between the fans are scheduled soon, in order to compare the predicted and measured flow velocity and angle along the blades at the fans exit.