Abstract
We report on the application of a combined shadow Doppler velocimeter and two-colour pyrometer instrument to make simultaneous measurements of the velocity, size and temperature of single coal particles in the near-burner region of 10 kW swirl-stabilised, natural-gas supported pulverised-coal flames with gas equivalence ratio of 0.69. Shadow Doppler velocimetry is an imaging technique for the simultaneous measurement of the velocity and the area of the projected image of an irregular particle illuminated by the laser beams of a conventional laser Doppler velocimeter (LDV), whilst the two-colour pyrometer was operating at wavelengths in the visible range, namely at 514.5 and 632.8 nm. Profiles of the fraction of burning particles and distributions of the size and velocity of the burning particles and the ensemble, which consisted of both burning and non-burning particles, were constructed from the raw data. In addition, an amplitude-based discrimination criterion was applied to the pyrometer measurements to produce temperature distributions of those measurements which corresponded to burning char as opposed to those corresponding to volatile flames surrounding particles. Increase of the swirl number from S = 0.41 to S = 0.57 resulted in penetration by the primary air jet of the recirculation zone on the centreline, 50% lower gas temperatures compared with S = 0.41 and accordingly, less than 5% burning particles for S = 0.57 in contrast to the 85% for S = 0.41 at the same location. For S = 0.41, except from locations near the centreline, where only particles with reverse velocities and size smaller than 60 μm were burning, only minor differences were found in the size-, velocity - distributions between burning particles and the ensemble. Despite the higher particle temperatures on the centreline, the relatively high particle burning fraction close to the oxygen-rich shear layer implies that those particles are more likely to yield NOx emissions from fuel-bound nitrogen.
