| OCR Text |
Show particle size relation shown above is valid only for droplets that pass through or near the center of the laser beams. The overall effect is for droplets to be sized as larger than actual and this also contributes to the low end roll-off in Figure 5. Recently, it has been shown that this problem can be overcome by using the average intensity of the scattered light signal in addition to visibility to obtain particle size. The average intensity, defined as (IM A X + L T J / 2 (see Figure 2) is used to restrict the measurement volume to the center of the beam crossing, where the particle visibility is an accurate measure of size. The visibility-intensity technique also aids in the rejection of signals from multiple particles. This technique was not available at the time this data was recorded but is currently being evaluated for coal-water slurry sprays. RESULTS After the apparatus was assembled, the operation of the particle sizing equipment was verified using glass beads and screened pulverized coal as reference particles. The aforementioned roll-off in the number of counts at the low end of the size range was noticeable, and there was also a slight broadening of the distribution toward larger sizes. Overall, the measured distributions of the reference particles were acceptable. Droplet size distributions for slurry sprays were measured for fuel flows ranging from about 100 - 500 lbm/hr and air-to-fuel mass flow ratios of about 0.2 to 2.0. Figures 5 and 6 show size distributions for test conditions typical of the combustion studies. At the low end of the distributions, the roll-off in the number of counts is very noticeable. The fact that small particles were actually present was verified by changing the transmitting optics to measure particles with an upper size limit of ~ 7 0 microns. This data indicated a continual increase in the number of particles with decreasing size. -9- |
