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Taking a new view on fuel injectors

The laser scattering patternator project grew out of the need to monitor the fuel sprayed from the injectors in gas turbines. The injectors must be set correctly to maximise efficiency yet produce the minimum pollution.
An array of collection tubes is normally used to sample a spray, but this is time-consuming, intrusive and inaccurate.
The project exploits non-intrusive laser optics to produce phase-Doppler measurements of fuel particle size and velocity. Scattered light signals are at the same time computer processed to produce, in conjunction with the phase-Doppler, accurate flow-rate readings, relatively quickly.
The technique is being commercialised and patent applications are being lodged in the US, Europe, Japan and other countries.

It takes just a single fuel injector to go wrong in a gas turbine for the engine efficiency to drop and for emissions of pollutants such as carbon monoxide and nitrogen oxides to rocket. Regular engine inspections therefore involve checks on all the injectors to ensure that they are producing sprays with the optimum distribution and volume of fuel.
A less-than-optimum spray can mean more expensive operations, and aircraft operators in particular are being subjected to increasingly stringent rules on emissions of pollutants. The situation is complicated because an engine burning fuel very efficiently may be producing large amounts of pollution.
For about 30 years sprays from injectors have been measured using an array of collection tubes that are positioned radially within a spray, with the tubes moved between readings to provide a map of the flow. This 'iso-kinetic' technique can take a couple of hours and is very intrusive. The poor collection efficiency also leads to large errors.
The MAT1 project Laser Scattering Patternator (LSP) was therefore launched to develop a much faster and non-intrusive system for measuring the flow of fuel from injectors.

Shining a light

The MAT project, led by the Spanish engineering company SENER Ingenieria y Sistemas of Madrid, was based on the phase-Doppler interferometry technique that is well-known in the laboratory for measuring sprays. By firing a laser at a spray and measuring the change in phase and frequency of the reflected signal, the size and velocity of droplets can be determined.
Using the phase-Doppler technique to calculate fuel flow rates, however, produces unreliable results when the sprays are optically dense and turbulent, such as from gas turbine fuel injectors. The flow from such injectors is characterised as dense because less than half of a ray of light directed across the spray will get through. The spray from an injector typically contains 10,000 fuel droplets/cm3.
The droplets are usually 10-150 microns in diameter. The phase-Doppler technique can produce flow-rate readings that differ by 100% from the nominal one.
However, the project partners - SENER, France's national aerospace technology development agency CERT-ONERA, Cranfield University of the UK and the Universidad Politecnica and Universidad Carlos III, both in Madrid - believed that they could overcome the flow-rate inadequacies of the phase-Doppler technique by exploiting latest optics and the rapid processing power of today's computers.

Added measurement

The partners decided to base their approach on the collection of a restricted amount of phase-Doppler information, which is particularly insensitive to the adverse optical conditions found in dense sprays.
An off-axis laser light-scattering system is also used to obtain readings of the spray activity at different points within the spray. Specifically developed correction algorithms are applied both to the scattering and the phase-Doppler information. By combining the phase-Doppler and the light-scattering data, the LSP can calculate the volume flux of the spray. The partners are unwilling to reveal more detail before winning patent protection.
The production of flow rate readings is relatively fast because only a few of the phase-Doppler measurements are used in the calculations. The software routines reduce the number of errors in the technique. The partners are aiming for maximum errors of 10%.

Practical application

The partners have tested the LSP system in the conditions found in industry on injectors with flow rates ranging from a few to 150/litre/hour at Madrid's Universidad Carlos III's premises. This was an important part of the MAT project and took about four months.
To test an injector using the iso kinetic system, the injector has to be removed from its engine and mounted vertically in a patternation rig. A similar size rig is needed for the new system, and people who have used the iso kinetic technique would have little difficulty in applying the LSP system, say the partners.
The lasers are common argon-iron lasers with a power of few hundred milliwatts. Suitable computers to handle the collection and processing of the huge amount of data required by the technique have only become available over the past few years. The software will automatically set up the system for a particular injector.

Speed and accuracy

One of the big attractions of the LSP system is the ability to produce flow-rate readings of much greater accuracy than those produced using the phase-Doppler system.
The time needed may also be as little as a tenth of that required for the phase-Doppler technique. In one test on an airblast type injector, the LSP took 28 minutes to produce a result, compared with 276 minutes using the phase-Doppler technique. In the same test, the LSP recorded a 96.6 litre/hour flow rate from the nominal 101.8 litre/hour injector, whereas the phase-Doppler reading was a much less accurate 203.5 litre/hour.
The partners have been evaluating the LSP technique with three types of injector covering the flow rate range from 5 litre/hour to 150 litre/hour. The bottom of the range corresponds to small engines of aircraft such as turboprops, and the top of the range covers the engines in wide-body turbofans.
They are encouraged by the results of the project, and have taken the first steps towards commercialising the technique, with patents being applied for in Europe, the USA, Japan and other countries.
They also plan to allow the main European gas turbine manufacturers, such as MTU, Snecma and Rolls-Royce, to assess the system.

Wider applications

The project also has possible applications wider than just in the aerospace sector. The technique could, for example, be used to analyse fuel injection in diesel engines or power plants at utility companies.
SENER works in aerospace and also in marine and other industrial sectors that could benefit from the technique. Universidad Politécnica and Universidad Carlos III, both of Madrid, are involved in research and development of technology related to combustion and power systems. CERT-ONERA works in many potential areas of exploitation, as does Cranfield of the UK.



Project Title:  
Laser scattering patternator for liquid flow measurement of industrial sprays

Industrial and Materials Technologies (BRITE-EURAM/CRAFT/SMT)

Contract Reference: MAT1-CT93-0030

Cordis DatabaseFor more information on this project,
go to the CORDIS Database Record