An experimental study on the design of counter-rotating axial-flow fans wascarried out. The fans were designed using an inverse method. In particular, thesystem is designed to have a pure axial discharge flow. The counter-rotatingfans operate in a ducted-flow configuration and the overall performances aremeasured in a normalized test bench. The rotation rate of each fan isindependently controlled. The relative axial spacing between fans can vary from17% to 310%. The results show that the efficiency is strongly increasedcompared to a conventional rotor or to a rotor-stator stage. The effects ofvarying the rotation rates ratio on the overall performances are studied andshow that the system has a very flexible use, with a large patch of highefficient operating points in the parameter space. The increase of axialspacing causes only a small decrease of the efficiency
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