Reverse rotation operation of a multi-stage axial flow compressor

Abstract

Paper presented at the 6th International Conference on Heat Transfer, Fluid Mechanics and Thermodynamics, South Africa, 30 June - 2 July, 2008.

An experimental investigation of the third quadrant (reverse rotation) performance, operation and the internal flow structures of a three stage subsonic axial flow compressor is described. The compressor used for this investigation has three repeating stages, and a design speed of 3000 rpm, a design mass flow rate of 2.7 kg/s and a blade tip relative Mach number of 0.2 at design point. The non-dimensional third quadrant pressure rise and torque characteristics for this machine have previously been determined. The steady-state interblade row flow-field was experimentally obtained by means of a five-hole cobra-type pneumatic probe and time-dependent phenomena were resolved by means of a 50 μ diameter cylindrical constanttemperature hot-film sensor. The observed flow structures are compared with those observed in a single stage axial fan rotating in the reverse direction as described in literature. Flow within rotor blade passages is dominated by large areas of separated flow on the pressure surface of the blades. A large axial velocity gradient is observed in the radial direction, with velocities highest near the shroud. Third quadrant operation appears to be similar to stalled first quadrant operation. Separation was found to be less severe on the final stage rotor row, as this blade row functions as the inlet rotor row in this mode of operation, thus flow angles are less unsuited to blade metal angles. These results yield insight into the causes of the low maximum pressure rise and efficiency obtained for this operational mode for this machine, as well as the apparent lack of a stall point on the pressure characteristic obtained.