Counter-rotating fan stage provides significant benefits over the conventional fan in terms of overall performance and size. For electric propulsion application, counter-rotating fan provides compactness and reduction in weight to achieve higher pressure rise with less power consumption as compared to the unducted propeller. Past literature suggests counter-rotating fans, designed with higher loading in front rotor has a flat performance map and a wider range of stable operation. This, in particular, benefits the electrical vehicle to have higher maneuver capability during operation.
The paper discusses the design methodology of counter-rotating fans for application in roadable electric aircraft ‘Airavat’ and the effect of different loading (total pressure) distributions in front and rear rotor on its overall performance. The fan is required to provide 10 N thrust and hence is designed for total pressure rise of 1000 Pa. The dimension of the fan is decided according to the design constraints of the vehicle. Rotors are designed for the rotational speed of 7500 rpm (Counter-clockwise and clockwise respectively) and flow coefficient of 1.25 at the mid. There are 8 blades in the front and 7 blades in the rear rotor. Fans are designed for four different total pressure rise and hence loading distributions namely, 1) 50%-50%, 2) 55%-45%, 3) 60%-40% and 4) 65%-35% in front and rear rotor. Other design parameters are kept the same for all the four cases.
The performance of the fans with different loading distributions is evaluated through computational study in Ansys CFX using mixing plane approach. It is observed that, as the loading increases in the front rotor, blade camber increases. The blade becomes more prone towards flow separation near the trailing edge under an adverse pressure gradient. Wake coming out from the front rotor grows thicker with higher loading, leading to flow acceleration (thus total pressure loss) in the axial gap between these rotors. As a consequence, flow incidents on the rear rotor other than the design incidence and thus the rear rotor operates under off-design condition. For the equal loading distribution case, the rear rotor does not provide the designed total pressure rise even though the front rotor performs well. With 55%-45% loading distribution, both the rotors give desired total pressure rise and maximum stable operating range. The detailed flow field study is discussed to bring important outcomes for achieving the desired total pressure rise.