Christopher Freeman

Experiments in Active Control of Stall on an Aeroengine Gas Turbine

This paper describes work carried out between 1989 and 1994 to investigate the application of ‘Active Stall Control’ to a Rolls-Royce Viper turbojet. The results demonstrate that stall control is feasible and can increase the stable operating range by up to 25% of pressure rise. Stall disturbances were detected using rings of high response pressure transducers positioned at different axial planes along the compressor, and processed using a PC-based data acquisition and control system. Actuation was provided by six hydraulically operated sleeve valves positioned to recirculate air over all or part of the compressor.Stall was artificially induced using combinations of inbleed into the combustor outer casing, fuel spiking, hot gas ingestion and inlet pressure spoiling, thus replicating many of the transient conditions commonly observed to make a compressor prone to stall. Results are compared from a number of stall control strategies including those demonstrated at low speed by Paduano et al [1993] and Day [1993]. Best results were obtained with detection of non-axisymmetric disturbances coupled with axisymmetric control action. A control system of this type is demonstrated to be capable of extending the stable engine operating range at all speeds and with each method of inducing stall.Copyright

The Propagation of a Fuel-Air Ratio Disturbance in a Simple Premixer and its Influence on Pressure Wave Amplification

The propagation of a fuel concentration disturbance in a premixer is studied analytically with a 3-D spatio-temporal mixing model. A simple analytical method is introduced which permits the frequency response characteristics of multiple points of fuel injection in simple (yet meaningful) premixer passage geometries to be obtained. The results provide physical insight into some of the design parameters that may affect the amplitude of FAR fluctuations coming out of a premixer when it is subjected to finite amplitude pressure waves. The possibility of a given design to lead to amplification of pressure waves (Rayleigh’s criteria) can be influenced by the characteristics of the mixing processes in a premixer channel. Results obtained in this paper may affect the design implementation of some of the proposals made for the control of combustion instability such as introduction of multiple time delays through selective fuel placement in a premixer.Copyright

Combustion Instability Investigations on the BR710 Jet Engine

During the development of the BR710 jet engine, audible combustor instabilities (termed rumble) occurred. Amplitudes measured with test cell microphones were up to 130 dB at around 100 Hz. Disturbances of this amplitude are clearly undesirable, even if only present during start-up, and a research program was initiated to eliminate the problem. Presented here is the methodical and structured approach used to identify, understand, and remove the instability. Some reference is made to theory, which was used for guidance, but the focus of the work is on the research done to find the cause of the problem and to correct it. The investigation followed two separate, but parallel, paths--one looking in detail at individual components of the engine to identify possible involvement in the instability and the other looking at the pressure signals from various parts of a complete engine to help pinpoint the source of the disturbance. The main cause of the BR710 combustor rumble was found to be a self-excited aerodynamic instability arising from the design of the fuel injector head. In the end, minor modifications lead to spray pattern changes, which greatly reduced the combustor noise. As a result of this work, new recommendation are made formorexa0» reducing the risk of combustion instabilities in jet engines.«xa0less

Intake Engine Interactions of a Modern Large Turbofan Engine

An engine experiment has been carried out to investigate Fan Stability and response to ambient wind conditions during static high power running of a modern large turbofan engine. This paper describes the experiment and the conclusions.Intermittently the inlet would separate and drive the fan into stall from which it did not recover when the inlet cleared up: on high working lines one inlet separation could stall the fan; on lower working lines the inlet separation / fan flow / bypass duct pressure would develop a divergent 10Hz oscillation which could eventually stall the fan. The interaction of the stalled fan with the turbine and mixed nozzle would then raise the fan running line above the stall dropout level thus locking the fan into stall even when the inlet cleared up. A one dimensional dynamic model of the engine was created that would exhibit similar behaviour to the engine when a delay was introduced between the inlet loss and fan face loss. The engine never showed steady operation with the inlet separated.Copyright

Stall Inception and Development in an Axial Flow Aeroengine

This paper describes the phenomenon of stall and surge in an axial flow aeroengine using fast response static pressure measurements from the compressor of a Rolls-Royce VIPER engine. It details the growth of flow instability at various speeds, from a small zone of stalled fluid involving only a few blades into the violent surge motion of the entire machine.Various observations from earlier theoretical and compressor rig results are confirmed by these new engine measurements. The main findings are as follows:1. The point of stall inception moves rearwards as engine speed increases, and is shown to be simply related to the axial matching of the compressor.2. The final unstable operation of the machine can be divided into rotating stall at low speed and surge or multiple surge at high speed.3. The inception process is independent of whether the final unstable operation is rotating stall or multiple surge.4. Stall/surge always starts as a circumferentially small flow disturbance, rotating around the annulus at some fraction of rotor speed.Copyright