Downstream air dilution and biomass pulverisation into bluff-body and swirl stabilised confined turbulent jets
Syed Ehtisham Gillani
Date of Award
Edith Cowan University
Doctor of Philosophy
School of Engineering
Zhao Feng Tian
Bluff-body and swirl-stabilised turbulent annular flows (single phase and multi-phase) find broad practical utility, especially in high-power non-premixed combustion applications. Their significance stems from features like their characteristic recirculation zones and intense upstream turbulence, both of which promote air-fuel mixing and aid in flame stabilisation. Moreover, in response to the push for low-emission and carbon-neutral fuel sources, there's a growing emphasis on integrating biomass, especially in its pulverised form, into major power generation systems. In practical gas turbine combustors featuring bluff-body and swirl stabilised flame burners, the incorporation of (air) side dilution jets plays a crucial role. These jets influence downstream reactant mixing by introducing cooler dilution air, effectively controlling combustion temperature and mitigating pollutant emissions, particularly oxides of Nitrogen.
Whilst literature reports investigations exploring bluff-body and swirl-stabilised annular turbulent annular flows, as well as others which separately look into the influence of dilution air in a crossflow, this thesis addresses the clear lack of fundamental insight that exists regarding (i) the interplay between turbulent side dilution jets and underlying flow behaviour of both swirling and non-swirling bluff-body stabilised annular jets in geometries typical of industrial combustors. This project also studies (ii) particle dispersion and the overall flow field associated with using pulverised raw biomass over a wide range of conditions spanning differing (particle phase) loadings, Reynolds numbers of both the (air) side dilution jets and annular jets as well as swirl numbers. The outcomes therefore not only progress our fundamental understanding of such complex two-phase flows but can ultimately assist in assessing and applying pulverised biomass to replace or reduce reliance on solid fossil fuels (coal) in power generation.
To achieve the above, the approach taken in this study follows a systematic progression of complexity from single phase to multi-phase flow investigations, both with and without (air) side dilution jets. In the initial phase (non-particle laden flows), the impact of turbulent side dilution jets on the flow field, turbulence, and mixing characteristics of non-particle-laden confined annular jets (both swirling and non-swirling) is investigated. This is achieved through a combination of Particle Image Velocimetry (PIV) and three-dimensional single phase numerical modelling. The subsequent phase (particle-laden flows) centres on investigating the incorporation of fine pulverised biomass particles (bark and walnut flour) within turbulent confined annular flows under both swirling and non-swirling conditions. Lastly, the study delves into the effect of turbulent side dilution jets on the flow and dispersion behaviour of pulverised biomass particles using two-dimensional PIV and three-dimensional multiphase CFD simulations.
Results for non-particle-laden (gaseous) flows reveal that irrespective of flow configuration (swirling or non-swirling), side dilution jets notably modify the overall flow field and prompt distinct recirculation zones at the annular jet's periphery, known as Peripheral Recirculation Zones (PRZ). However, the PRZ becomes significantly weaker when side dilution jets are introduced at more downstream locations (H=6.6D). The PRZ notably enhances upstream turbulence in the outer shear layer of the bluff-body stabilised annular jet, leading to a massive 176% and 218% increase in the peak turbulent kinetic energy at x/D=1.4 and 1.8, respectively. Downstream of the bluff-body stabilised annular jet (beyond x/D > 2), noticeable increases in centreline velocity decay (13%) and jet spreading (32%) are observed in the presence of a PRZ, thereby leading to improved mixing and flow entrainment.
Despite having well-defined (uniform) boundary conditions and an axisymmetric geometry at the nozzle exit plane, results show bluff-body stabilised annular jets are susceptible to asymmetry, particularly at larger blockage ratios. Whilst this is also reported elsewhere in the literature, this work goes further and uses two-dimensional time-averaged planar particle image velocimetry measurements to resolve the effects of swirl on such intrinsic asymmetry over transitional (Res=2,700) and turbulent (Res=17,800 and 35,500) bluff-body stabilised confined jets. Results show that swirl significantly mitigates asymmetric vortex shedding and restores annular jet mean flow symmetry. Quantification (through the introduced Asymmetry Index) reveals that swirl reduces flow asymmetry by 42% (for Res=17,800) and 78.6% (for Res=35,500) compared to non-swirling annular jets.
Measurements in multi-phase biomass-laden turbulent experiments and their modelling show that when a pulverised biomass-laden central jet interacts with a surrounding non-swirling turbulent annular flow, a bluff-body based recirculating zone (BB-RZ) leads to biomass particle entrainment and their recirculation over the bluff-body before being dispersed further downstream. Along the central axis, a 35% decrease and 177% increase in mean particle axial velocity and turbulent fluctuation, respectively, suggest intense upstream (x/D 0.64) turbulent mixing and effective particle dispersion into the BB-RZ. Furthermore, addition of swirl to annular flow significantly enhances the downstream dispersion and lateral spread of biomass particles as evident from the 20% and 254% (at x/D=2.2) massive increase in mean particle axial and radial velocities, respectively. Numerical predictions demonstrate that at a constant particle loading ratio, lower Reynolds numbers of the carrier gas (Rej = 4,500) result in better particle recirculation within the BB-RZ, fostering improved mixing which consequently enhances downstream lateral dispersion and entrainment of the biomass-laden jet, in contrast to higher Reynolds numbers of the carrier gas (Rej=7,800).
Interaction of turbulent side dilution jets with pulverised biomass turbulent annular flows reveals significant dispersion of biomass particles in the PRZ of annular flow. At a plane intersecting through the BB-RZ, a respective 22.4% reduction and a 52.6% increase in the mean particle axial velocity and turbulent fluctuation is recorded under non-swirling conditions. Swirl further intensifies this effect, yielding a 32.5% mean particle axial velocity reduction and a 100% increase in mean particle turbulent fluctuation. This corresponds to excessive turbulent mixing between solid particles and the surrounding gas phase. Side dilution jets notably enhance biomass particle lateral dispersion in the BB-RZ and PRZ, especially under swirling flow conditions. With swirl and side jets, downstream dispersion rises up to 145% compared to baseline flows (no side jets). In the PRZ, particle count increases by 108% (non-swirling) and 279% (swirling), indicating that swirl effectively doubles particle dispersion within the PRZ.
In summary, outcomes from this study advance the fundamental understanding of gaseous and particle-laden turbulent bluff-body stabilised annular jets in geometries representative of practical combustors. Insights gained into pulverised biomass flow dynamics provide assist in the optimisation of pulverised biomass combustor design and operation. Finally, the highly-resolved and well-defined boundary condition data acquired in the isothermal jets investigated, over a wide range of conditions, is likely to offer a valuable resource for validating numerical models used to predict the operation of industrial combustors.
Access to this thesis is embargoed until 17th May 2025.
Gillani, S. E. (2023). Downstream air dilution and biomass pulverisation into bluff-body and swirl stabilised confined turbulent jets. Edith Cowan University. https://doi.org/10.25958/0axn-bz37