We report the application of the Shadow Doppler Velocimeter (SDV) for spatial precise, simultaneous measurement of the size and velocity to assess the particle retention performance of a laboratory, 1/6 scale, 10 kW vertically‐fired atmospheric model of the pressurised pulverised‐coal furnace of Reichert et al. [1]. The SDV is based on the imaging of a conventional LDV probe volume onto a linear photodiode array and has the advantage over other sizing methods for irregular particles that it is tolerant of the optical misalignment and fouling which are inevitable when passing laser beams through windows in such furnaces. The size and two components of velocity of burning coal particles were measured in the present geometry which has 172 mm furnace diameter and 40 mm lateral exit duct diameter and a calculated exit bulk velocity of 4 m/s, evaluated at 300 K. The Sauter mean diameter of the particles is, within the experimental error, uniform at about 40 μm in the vertical profile normal to the axis of the exhaust pipe, 34.5 mm upstream of the exit. Coal particle velocities in the near‐exit region are directed towards the exit, closely following the gas‐phase velocities. Both these observations imply that particle retention efficiency due to streamline curvature is low and extrapolation suggests that there will be even less at large scales.
The velocity, size and volume flux of individual burning and non-burning particles were measured simultaneously in the near-burner region of a natural-gas supported 8 kW swirl burner for swirl number 0.68, using in the secondary stream unheated air as well as preheated vitiated air as oxidant, with oxygen mole fraction of 0.165 and 0.177, at 400 and 350 °C preheat temperatures respectively. An optical instrument based on a combined shadow Doppler velocimeter instrument (for sizing particles of arbitrary shape) and spatially-precise two-colour pyrometry was developed for the measurements. The results at axial distance z/D = 2.6 which lay within the recirculation zone showed that the fraction of burning particles (defined as the ratio of detected burning particles and the total measured) decreased with increasing radial distance in all flow conditions. The maximum fraction of burning particles decreased as a function of decreasing oxygen mole fraction from about 94% at a 0.21 mole fraction to 28% and 12% at oxygen mole fractions of 0.177 and 0.165 respectively and, as a consequence, the volume flux of burning particles was an order-of-magnitude smaller than the volume flux of the ensemble.
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.
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