Changing the flow direction in fans is frequently required in emergency situations in traffic tunnels, chemical plants and mines ventilation. Reverse flow in axial flow fan is often achieved using two methods: a) Changing direction of fan rotation and turning the stator vanes (Method I). b) Turning / resetting rotor blades during fan rotation (Method II). The required volume flow rate at flow reversal is usually at least 60% valid for normal fan working point. The motivation of the present paper is to compare the aerodynamic performance and 3D flow mechanism in fan stage at flow reversal carried out by the two methods above. In our paper conditions of the flow reversal are discussed. Theoretical relations are derived for both methods using fundamental equations valid for internal aerodynamics of axial flow compressors and fans. Parameters of three fan axial stages were measured on a 600 diameter test rig at standard and reverse conditions. The investigated fan ventilation stages had a design flow coefficient of 0.35 to 0.40 and pressure coefficient of 0.30. Flow field measurements were carried out with the use of 5-hole pressure probes in the stage planes. The blade rows flow mechanism at the standard and reverse conditions is described using test data obtained for both flow reversal methods. The flow simulation results were also used. It has been found in our investigations that moderate aerodynamic loading of the ventilation fans has better aerodynamic performance during flow reversal if Method II is used. Fan designers and users making the final decision relating to the selection of the flow reversal method should also include the reliability and cost of the reverse fan design with blade turning mechanism.

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