Experimental Study of the Instationary Flow Between Two Ducted Counter-Rotating Rotors

The purpose of this work is to study experimentally the aerodynamic characteristics of a subsonic counter-rotating axial-flow fans system operating in a ducted configuration. The fans of diameter $D=375$ mm were designed to match the specification point using an original iterative method: the front rotor blade cascade is designed with a conventional inverse method, setting the radial distribution of the Euler work. The through-flow is then computed using an axisymmetric and radial equilibrium assumption, with empirical models of losses. The rear rotor is not conventional but is designed to straighten the radial profile of the tangential velocity. The design of the front rotor is then modified until the stage meets the requirements. The experimental setup is arranged such that the rotation rate of each fan is independently controlled and that the axial distance between the rotors can be varied from $17%$ to $310%$ of the midspan chord length. Systematic measurements of the global performances and local measurements of the velocity field and of the wall pressure fluctuations are performed, in order to first validate the design method, and to explore the effects of the two specific free parameters of the system, the axial spacing and the ratio of rotation rates. The results show that the efficiency is strongly increased compared to a conventional rotor or to a rotor-stator stage. The developed design method slightly overpredicts the pressure rise and slightly underpredicts the best ratio of rotation rates. Flow angle measurements downstream of the stage show that the outflow is not completely straightened at the design point. Finally, the system is highly efficient on a wide range of flow rates and pressure rises: this system has thus a very flexible use, with a large patch of high efficient operating points in the parameter space.