Engine room

The accurate metering of fuel is important to get the best fuel economy and the lowest emissions from aircraft engines. Engine developers use complicated systems with a number of high-performance valves and pumps and other components to pump the fuel to the engines and then bleed off the excess fuel using control valves. However, they can suffer from instability, which produces self-excited oscillations in the valves and causes problems.

A project run by Bath University with Aero Engine Controls (a joint venture between Rolls-Royce and Goodrich Engine Control Systems) aims to develop accurate simulation models to better predict the behaviour of the systems and to inform the future design of valves, pumps and control systems.

Project leader Dr Nigel Johnston said: 'Although the ultimate aim is to reduce emissions and noise from engines, it will speed up the design process, too. Instead of having to build and test lots of prototypes with different details to see what effect they have, you should be able to do that at a computer by building up a computation fluid dynamics [CFD] model and using it to predict the flow and the pressure without having to cut any metal.' The researchers will start by modelling current parts to identify what features of their geometry are important. Seemingly small features such as tiny chamfers and curvatures can have a big effect on flow and pressure through the valves. As the project progresses, they will use the simulations to create improved designs that could then be built and tested at Aero Engine Controls.

Alongside standard sensors to measure pressure, flow and temperature, the researchers will use a novel technique for measuring dynamic flow. Flow rates are often measured using a turbine flow meter, which is like a miniature wind turbine inside a pipe. The flow of the fuel makes the blades turn and the speed of the blades is assessed, giving a measure of the flow rate. The mass and inertia of the turbine, however, mean it does not respond immediately to a short, sharp change in flow rates; it either continues turning at the same speed or speeds up or slows down gradually.

As the project is investigating rapid transience and instability, the researchers need to measure high-frequency flow fluctuations, which cannot be done with a traditional turbine flow meter. Instead, they will use pressure sensors at more than one point along a pipeline and then analyse the measurements on a computer using wave theory. By breaking down the pressure measurements into the waves travelling in the pipeline, the flow and pressure can be measured to indicate dynamic flow rates over very short timescales and at high frequencies.

By the end of the project, the team hopes to have a good idea of the way in which these fuel flow components operate and what the important parameters are that affect their performance. This will allow manufacturers to design valves and other components that are stable, fast acting and accurate without having to go through so many prototype stages.

Aero Engine Controls will get first sight of the result, but, when published, the project's findings could be used in other aero fuel systems, hydraulic fluid power systems and other fluid systems.

Berenice Baker