An improved seeding method for high temperature laser doppler velocimetry

Abstract

Measurement techniques such as laser Doppler velocimetry require the fluid under investigation to be uniformly seeded with micron-sized light scattering particles. Suitable methods to this end are available for aerosol generation in low-temperature flows [1]. However, high-temperature flows such as occur in combustion processes demand the use of refractory particle aerosols whose steady generation is difficult. Familiar techniques such as fluidised beds are available for this purpose, but as noted by Self and Whitelaw [2] their operation is usually far from satisfactory with large fluctuations in particle concentration. The device described in this note provides a significant improvement over the fluidised bed which suffers from erratic behaviour due to inherent factors such as channeling and slugging [31. The device is shown schematically in Fig. 1. Its operation is similar in principle to the reverse flow cyclone separator [4], but serves essentially the opposite purpose. The size graded refractory powder at the base of the column is entrained by the flow (see Fig. 1) and carried out by the central vortex. Experiments have been performed on a prototype with 0.1-1.0 /am A120 a to determine the steadiness of seeding and also the mean entrained particle size. All measurements were made at the nozzle exit plane of a 6.35-mm-diameter air jet. Laser light scattering in conjunction with a partiSEEDED NLITI ET