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.
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%.
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.
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.