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Laser Doppler Anemometry
Laser Doppler Anemometry is used for in-cylinder velocity measurements, with a specific application to radial and swirl velocity components
Laser Doppler Anemometry/Velocimetry is a non-intrusive optical technique for measuring in-cylinder fluid velocities. LDA measures the velocity of particles seeded within the flow. The particles are typically 0.4-2 microns in diameter and may be made from various materials such as salt, red clay, silica coated alumina or titanium dioxide depending on whether the measurements are motored or fired; Liquid droplets may also be used though these have a tendency to evaporate or burn up in combustion. LDA is carried out in two different modes:
Forward scatter LDA:
For forward scatter LDA a line of sight path is required through the cylinder. This may be in the form of a quartz spacer or windows through the side of the combustion chamber, equally this may also mean using a transparent piston and cylinder head or quartz windows in both.
Back scatter LDA:
More common is back scatter LDA using a single window; scattered light is reflected back through the same window. The use of a single window facilitates optical access to the engine but makes measurements slightly more difficult. A significant problem with LDA in backscatter operation is the high level of background noise. Background noise may be generated by reflections from surfaces such as the cylinder window back into the receiving optics; Or by the piston as it approaches TDC when taking LDA measurements from above. Another reason is contamination build up on the quartz windows. Noise can be reduced by a good optical design and cleaning the quartz window every so often. A low noise level is desirable as it improves reliability and allows higher data rates in measurements hence a more detailed spectral analysis.
LDA is based on the Doppler effect, the motion of a particle will cause reflecting laser energy to change wavelength in proportion to its velocity as shown below.
The reflected beam may be mixed with a beam of a different
frequency, the beam will then interfere and beat. The beat frequency is used to
determine the velocity of the object. Two coherent beams of the same amplitude
and linear polarisation intersect at an angle 
.
In the region of intersection, bright and dark interference planes are produced
parallel to the plane bisecting the angle between the two beams.
A seeded particle passes through the interference area scattering light, blinking as it crosses light and dark planes. The scattered light passes back through a lens onto a receiving fibre that passes the light back onto a detector (avalanche photodiode or photomultiplier) The signal from the detector is periodic and has frequency;
Other velocity components are found by forming fringes orthogonal to the first set. Two orthogonal fringe sets can be formed by passing four beams through a single converging lens. Signal separation for each fringe is accomplished by using laser light of different colours. The diagram below shows fibre optic apparatus used for LDA measurements. The advantage of using fibre optics is greater flexibility and simplification of the set up procedure.
A laser beam is produced by a 4W Ar-Ion laser and passes through a Bragg cell where it is split into two beams of equal intensity. The first order beam is frequency shifted 40Mhz from the zero order beam. An amici prism then splits the light into separate wavelengths of 514.5 nm and 488 nm. The beams are then transmitted to the probe via graded index optical fibres. The scattered light from the combustion chamber is collected by a receiving fibre and split by a dichroic colour separator, then focused onto avalanche photodiodes. The signal is passed onto counter processors which time particles over a set number of fringes e.g., 32 fringes. The velocity is then calculated by dividing the spacing between 32 fringes by the time. The velocities are then sent to a computer where they are simultaneously recorded with signals from the crank angle encoder and pressure transducer.
LDA takes measurements at points along a traverse line to build up a time averaged picture of the mean flow velocity within the cylinder. Typical outputs are in the form of diagrams as follow;
Ref: [7],[F],[I],[N]
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