Evaluation of an Amplitude Sizing Anemometer and Application to a Pulverised Coal Burner

Abstract

The accuracy and precision of a sizing anemometer, based on diffraction of light from a laser beam of 240 μm diameter and 514.5 nm wavelength at a collection angle of 3.82° with a central “pointer volume” formed by a laser Doppler test volume of 41 μm diameter and 488 nm wavelength and using non-spherical coal particles, have been evaluated experimentally. The required independent measurement of the coal particle size was based on an aerodynamically determined diameter which was related to the measured exit velocity of the coal particles at the exit of a rapidly converging nozzle. The accuracy and precision were found to be +2, +7, −20 μm and ±2, ±7, ±10 μm, respectively, for mean values of 15,30, 60 μm at a coal feeding of 0.0133(g/min), corresponding to a number density of 7.1-10⁻¹⁰(m⁻³. The poor accuracy for the largest si2e is due to saturation of the sizing photodetector. The accuracy and precision became worse by 10% and 50%, respectively, when the feeding rate increased by a factor of 35 and thus when the number density also increased from 7.1-10⁻¹⁰ to 2.5 · 10⁻⁸. The application of the instrument is illustrated by simultaneous measurements of size and velocity in a small scale, 11.3 kW, burner operated at a swirl number of 0.5 and piloted by a gas pilot flame supporting a 0.24 g/min coal Same. The mean velocities showed that particle size classes characterised by mean diameters of 12 and 36 μm recirculated while those of 64 μm penetrated the recirculation zone. The arithmetic and Sauter mean diameters decreased with axial and radial distance, primarily due to the centrifuging effects of the swirl air.

Key Words:

• Particle sizing
• laser doppler velorimetry
• pulverised coal burners