Characteristics of Rotating stall in High Specific Speed diagonal Flow Fan and Suppression of Rotating Stall

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In order to clarify the characteristics of rotating stall in a high specific speed diagonal flow fan with specific speed of 1620, and to propose an improved design for suppressing the rotating stall, experimental investigation has been conducted from the viewpoint of unsteady internal flow. The main results obtained are as follows.The rotating stall of the fan is observed in the range of low flow coefficient where total pressure curve indicates unstable character with positive gradient. Hysteresis appears in the pressure characteristics for small tip clearance only. The rotating stall disappears with increased tip clearance.The number of rotating stall cell is one, its speed is about 77% of the rotor speed. The cell occupies about 3/4 circumference and about 1/3 span at tip region.Velocity fluctuation in the cell is very strong at tip region and it becomes weak with decreasing radius.The inlet flow field with rotating stall is as follows. The flow outside of cell is similar to that of high flow region with regular variation by blade passing frequency. In the cell region flow field is very irregular, flow direction changes positive and negative.Total pressure field at rotor exit is measured with dual-phase-lock method. Total pressure is high in the cell region and its distribution is such that higher pressure is indicated at pressure side of the blade. Decay of the stall cell is very slow compared with decay of high pressure region with tip vortex.Two types of casing treatments with separator ring are developed and tested. With one of them the rotating stall is completely suppressed and fan characteristics curve is stabilized with negative gradient at low flow region. This is realized with little decrease in fan efficiency of 2% at design condition.The effectiveness of the casing treatment is discussed with the results of internal flow measurement from the viewpoint of unsteady flow.