Project objectives

The objectives of the proposal are fourfold:

1. to understand in detail the direct effect of a pressure wave on flow aerodynamics,
2. to measure the behaviour of a combustor operating near lean extinction, under both steady and unsteady condtions, to establish how oscillations may affect the degree of extinction;
3. to study droplet and spray behaviour under unsteady conditions, so that the response of a spray to pressure waves may be determined;
4. to synthesise the individual process models into a single description of the unsteady combustion process and hence to provide a method for estimating pressure levels and frequencies in real gas turbine combustors.

Description of the work

All low-emission combustor designs are inherently prone to suffer from unstable combustion. In practice it is often this issue which places limits to attainable NOx levels in practical gas-turbine designs. Hence the understanding and prevention of acoustic oscillation in gas turbine combustors is fundamentally linked to the ability to deliver low NOx, and hence a cleaner environment.

The project consists of a series of coupled experiments at leading research centres in Europe, where local experts will be used to devise the experiments in detail and to develop existing measurement techniques to study the time-variable properties of the flow under investigation. Studies to date have concentrated on either eliminating problems with specific hardware or in describing a pre-supposed mechanism for coupling the flow pressure variations to the fluctuating heat release rate. This project is different in that no presumption will be made of the modes of oscillation, and so any methods developed should be equally applicable to understand oscillation issues at all frequencies of interest.

At present most models of combustion instability are based on linearised theory and also rely on the provision of a 'flame transfer function'. While these are undoubtedly useful, and can be run rapidly, they do suffer from two major flaws - all the physics is wrapped up in the flame-transfer function, and thus it is difficult to extrapolate to new geometries and conditions. As for the linearised theory, while it also predicts unstable frequencies well, it does not give information about the pressure levels which may be reached. Both these deficiencies are addressed in the proposed project, and a new model of the process will be delivered which can then be applied to all gas turbine combustors using the partners' own CFD codes.

Expected results

• Stationary lean combustion completed. Kinetic scheme devised. Droplet evaporation measurements made for one case. LPP rig set-up completed. Non-linear model devised, aerodynamic response measured.
• Aerodynamic studies completed, combusting system response measured. Spray response completed, LPP basic configuration done. CFD simulations of steady combustion complete, module delivered.
• Reports, model and data delivered to partners. M42-Model applied to real cases.