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.

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