Passive Control of the unstable characteristics of a high specific speed diagonal-flow fan has been proposed. It is possible to eliminate the unstable characteristics of the pressure-flow rate curve in a low-flow region without deterioration of performance at design point. The control action is done naturally (passively) without any energy input. The inlet nozzle of an ordinary diagonal-flow fan was replaced by an annular wing with a Go¨ttingen 625 airfoil section. The mechanism of the passive control and the optimum geometric parameter are discussed on the basis of performance tests and internal flow measurements.

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