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In order to clarify the flow fields with the unique region, which is characterized by low meridional velocity, high tangential velocity and high velocity fluctuation, in a low specific-speed diagonal flow fan at higher flowrate condition than the design flowrate, the experimental and numerical investigations were carried out. The test fan is radial-outflow type diagonal flow fan similar to centrifugal fan and its specific-speed is 670 (min^-1, m³/min, m). The velocity fields at rotor outlet were measured using a single slanted hotwire probe and the wall pressures on the outer casing were measured using a highly responded pressure transducer. In order to obtain more detailed information inside the blade passage, the numerical simulations were carried out at the same flow condition. As results, the fan performance was very good near the design point. However, it decreased rapidly when the flowrate increased. The unique region was observed at all flowrate conditions in velocity field. This unique region is formed due to tip leakage vortex near design flowrate. At excessive flowrate, it is generated due to separated flow on blade pressure surface. And, when the flowrate increases from design point, the process to change from tip leakage vortex to separation on blade pressure surface was shown in this paper.

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