The application of laser measurement techniques to the pneumatic transport of fine particles

• Main Author: Birchenough, A.
• Published: University of Greenwich 1975
• Subjects: 620.106; QC Physics
• Online Access: https://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.449866

The application of laser anemometry techniques to the measurement of local velocities and turbulence intensities is now well established for gas and liquid flows. In these situations, light from a low powered laser is scattered by seeding particles in the fluid which are small enough to follow the flow. The resulting Doppler shift of the scattered light is proportionate to the velocity of the particles and hence, to the velocity of the fluid. When solid particles are conveyed pneumatically, for example in transport chemical reactors, in drag reducing flows, and in certain heat transfer processes, the particles are generally much larger than the usual seeding particles, and thus do not travel at the same velocity as the carrier fluid. During this investigation, local particle velocity and axial turbulence intensity measurements using a laser anemometer have been made in an upward flowing gas-solid suspension and some of the limitations of the method have been found. The measured velocity profiles were used to evaluate the mean solids velocity and these mean values were compared with the results obtained using a technique developed by the author, which measures the mean solids velocity directly. Solids loading ratios up to 2.0, and conveying velocities up to 56 m/s, have been analysed and it has been confirmed that all the alumina particles (size range 5 to 45 microns) contributed to the analysed signal. Measurements have also been made of the particle velocity in the wall region of the two-phase flow, using a back-scattered light collection method. The pneumatic conveying imposed an upper limit on the loading ratio of 4 for these experiments. However the measuring system itself suffers no such restriction. The results of the "back-scatter investigation" are discussed by reference to particle wall velocity curves which can be used to indicate that 'choking ' is imminent. Further work examined the 'clean' air velocity and axial turbulence intensity profiles obtained by seeding the flowing air with sub-micron particles of titanium dioxide. Finally, measurements of axial pressure gradient in the fully developed region of the gas-solid flow were made simultaneously with the particle velocity measurements.