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Laser Doppler velocimetry (LDV) has become a common technique to measure the velocity field in combusting flows. In small-scale combusting chambers, the experiments are performed non-intrusively with the transmitting and receiving optics outside the hot flow. In large-scale flames (thermal input of 1 MW and higher) however, the unavoidable limitation of this approach is the decrease in signal quality associated with the beam steering and temperature gradients. Beam steering increases with the distance travelled by the laser beams in the hot flow until their crossing in the measurement volume. This eventually put a limit on the maximum furnace size in which non-intrusive laser velocimetry is practical.

At the IFRF, a solution has been developed by designing a water-cooled jacket which houses a small 25 mm diameter LDV probe equipped with fiber link. This 63 mm outer diameter jacket can been manufactured with a length up to 7 meters. The fiber-optics cable allows the maintenance of the laser, LDV optics and electronics in a clean room or compartment close to the furnace. The fiber-optics cable and the transmitting probe are the only components exposed to the furnace environment. The water-cooled jacket front end is sealed by two quartz windows through which flows a film of water. This arrangement provides a complete absorption of the flame radiation incident on the LDV probe optics. Window contamination by seeding or fuel particles is prevented by purging the probe tip with a small nitrogen flow. The average outlet velocity of the nitrogen purge flow is less than 5 m/s. The transmitting optics features a 20 mm diameter front lens with a 102 mm focal lens which places the measurement volume at 77 mm from the water- cooled probe tip.

Experiments in industrial flames of up to 12 megawatts have demonstrated fast and reliable operation of the laser velocimetry probe. Operation for long periods in flames with high particle loading and temperatures above 1800'C showed no fouling of the front window and no heating of the LDV optics. The effects of the probe intrusiveness and probe purge flow on the measurements accuracy were assessed and shown to be undetectable for measurements in semi- industrial scale swirling flames [11.

Since its development in 1991, the laser Doppler velocimetry probe has been routinely used by the IFRF in heavy fuel oil, coal and natural gas flames at thermal inputs up to 19 MW and in furnaces with diameters up to 4.2 m. Several companies in Europe and the United-States have chosen this probe for their laser velocimetry measurements in large scale industrial flames.

VENDOR: IFRF Research Station b.v.

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Please contact us:

IFRF Research Station B.V.
P.O. Box 10 000
1970 CA IJmuiden
The Netherlands

email: sales@ifrf.net