Rain Ingestion in Axial Flow Compressors at Part Speed

Abstract

New experimental work is reported on the effects of water ingestion on the performance of an axial flow compressor. The background to the work is the effect which heavy rain has on an aero-engine compressor when operating in ‘descent idle’ mode, i.e. when the compressor is operating at part-speed and when the aero-mechanical effects of water ingestion are likely to be more important than the thermodynamic effects. Most of our existing knowledge in this field comes from whole engine tests. The current work provides the first known results from direct measurements on a stand-alone compressor. The influence of droplet size on path trajectory is considered both computationally and experimentally to show that most rain droplets will collide with the first row of rotor blades. The water on the blades is then centrifuged towards the casing where the normal air flow patterns in the vicinity of the rotor tips are disrupted. The result of this disruption is a reduction in the delivery pressure and an increase the torque required to drive the compressor. Both effects reduce the efficiency of the machine. The behaviour of the water in the blade rows is examined in detail, and simple models are proposed to explain the loss of pressure rise and the increase in torque. The measurement were obtained in a low speed compressor making it possible to study the mechanical (increase in torque) and aerodynamic (reduction in pressure rise) effects of water ingestion without the added complication of thermodynamic effects.