ANALYSIS OF UNSTEADY CASING PRESSURE MEASUREMENTS DURING SURGE AND ROTATING STALL

S. J. Anderson (CEng)

QinetiQ

SJANDERSON@qinetiq. com

Dr. N. H. S. Smith (CEng)

Rolls Royce PLC nigel. h.s. smith@rolls-royce. com

Abstract This paper presents a unique set of unsteady pressure measurements during a surge and rotating stall event in a modern transonic high speed single stage axial ft>w fan rig. The unsteady pressure data were acquired using an array of 22 high frequency response Kulite transducers positioned over the rotor tip to show detail of the in-passage fbw behaviour during stall and surge. In addition, Kulites were also positioned upstream, downstream and in the bypass duct of the rig to give a more general overview of the fan operation during the rotating stall and surge events.

1. Introduction

The modern large turbofan engine will experience unsteady ft>w phenom­ena when there is an excursion of the fan from its nominal operating line. An example of such behaviour is given in Ref. 1 (ASME 99-GT-344 Freeman & Rowe). To further understand typical unsteady ft>w phenomena – such as rotating stall and surge – detailed in-passage casing measurements of the un­steady pressures have been acquired from an experimental high speed single stage axial ft>w transonic fan rig using an array of 22 high frequency response pressure transducers. The transducer array was positioned in the rotor tip cas­ing to allow the in-passage pressure distribution and shock structure for each blade passing to be visualised with coverage extending upstream and down-

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K. C. Hall et al. (eds.),

Unsteady Aerodynamics, Aeroacoustics and Aeroelasticity of Turbomachines, 293-312. © 2006 Springer. Printed in the Netherlands.

stream of the rotor leading and trailing edges. The array data were recorded on a digital data acquisition system which incorporated a signal conditioning unit to give approximately 100 samples per blade passage on each transducer.

In addition to the in-passage array, additional rings of pressure transducers were positioned upstream and downstream of the rotor and in the bypass duct to give a more general overview of the behaviour of the fan during the rotat­ing stall and surge events. This set of data was sampled at a much reduced sample rate, and as a result only the rotating stall cells and the harmonics were detected.

The on-set of the rotating stall or surge event was achieved by throttling the bypass duct at constant speed, resulting in the fan operating further up its char­acteristic, the throttle was continually closed until the fan dropped into rotating stall or a surge cycle, once data had been captured the fan was recovered by rapidly opening the bypass valve.

This paper presents an example of rotating stall at part speed and a high speed surge event. The data for each event are firstly shown on the upstream and downstream rings of transducers to present an overview of each event and then the detailed in-passage data are shown.