School of Engineering
Deeper insights into particle loading have practical significance in applications particularly when this affects fuel-oxidiser mixing and flame stabilisation, as occurs in pulverised fuel (biomass) combustion systems. The underlying particle flow and dispersion characteristics not only play a crucial role in controlling overall combustion performance but can also impact emissions. A fundamental understanding of particle flow dynamics and dispersion behaviour for raw pulverised biomass, under non-swirling and swirling conditions need further systematic investigation to better understand the interplay between swirling and non-swirling bluff-body stabilised recirculation zones and particles emitted from a centralised jet. This work uses Particle Image Velocimetry (PIV) with three-dimensional multi-phase simulations based on the Discrete Phase Model (DPM) and Reynolds Stress Model (RSM) to investigate the flow and dispersion characteristics in confined flows typical of many practical combustors. Raw pulverised biomass-laden is introduced through a central turbulent jet (Rej = 4500 and 7800) and also subjected to turbulent annular flows (Res = 35,500), both non-swirling (S = 0) and swirling conditions (S = 0.3). Simulations are first validated against Constant Temperature Anemometry (CTA) resolved inlet boundary conditions and flow field PIV data under similar conditions. Results show that when a pulverised biomass-laden central jet interacts with a surrounding turbulent annular flow (non-swirling, swirling) the presence of a bluff-body based recirculating zone (BB-RZ) leads to biomass particle entrainment (pick up) and their recirculation over the bluff-body before being dispersed further downstream. Under non-swirling conditions, a significant 35% decrease in the mean particle axial velocity is measured coupled with an even more substantive 177% increase in turbulent fluctuation along the centreline. These findings are indicative of intense upstream (x/D ≈ 0.64) turbulent mixing and more intense particle dispersion into a BB-RZ. For swirling annular flow conditions near the end of the BB-RZ, the interaction between a biomass-laden central jet and the annular flow is comparatively weaker in the upstream region relative to non-swirling conditions. However, swirl significantly enhances downstream particle dispersion and lateral spread as reflected by a 254% hike in the mean particle radial velocity. Numerical predictions show that for the same particle loading ratio, but different (higher and lower) Reynolds number of a central particle-laden jet (i.e., the carrier gas), the conditions at relatively lower central jet Reynolds number allow better particle recirculation in BB-RZ as well as enhancing downstream lateral particle dispersion and entrainment, compared to a higher Reynolds number in the central jet. Outcomes from this investigation may have implications on the design and operation of pulverised (solid) fuel combustors if operated on renewably sourced biomass rather than traditional fossil fuels.
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