EVALUATION OF AN AMPLITUDE SIZING ANEMOMETER AND APPLICATION TO A PULVERIZED COAL BURNER

The accuracy and precision of a sizing anemometer, based on diffractio n of light from a laser beam of 240 mu m diameter and 514.5 nm wavelen gth at a collection angle of 3.82 degrees with a central ''pointer vol ume'' formed by a laser Doppler test volume of 41 mu m diameter and 48 8 nm wavelength and using non-spherical coal particles, have been eval uated experimentally. The required independent measurement of the coal particle size was based on an aerodynamically determined diameter whi ch was related to the measured exit velocity of the coal particles at the exit of a rapidly converging nozzle. The accuracy and precision we re found to be + 2, + 7, - 20 mu m and +/- 2, +/- 7, +/- 10 mu m, resp ectively, for mean values of 15, 30, 60 mu m at a coal feeding of 0.01 33(g/min), corresponding to a number density of 7.1 . 10(-10)(m(-3)). The poor accuracy for the largest size is due to saturation of the siz ing photodetector. The accuracy and precision became worse by 10% and 50%, respectively, when the feeding rate increased by a factor of 35 a nd 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 simul taneous measurements of size and velocity in a small scale, 11.3 kW, b urner operated at a swirl number of 0.5 and piloted by a gas pilot fla me supporting a 0.24 g/min coal flame. The mean velocities showed that particle size classes characterised by mean diameters of 12 and 36 mu m recirculated while those of 64 mu m penetrated the recirculation zo ne. The arithmetic and Sauter mean diameters decreased with axial and radial distance, primarily due to the centrifuging effects of the swir l air.