Experiments and modelling of biomass pulverisation in swirling and non-swirling bluff body-stabilised turbulent annular flows

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

Split-screen single-camera stereoscopic PIV application to a turbulent confined swirling layer with free surface

An annular liquid wall jet, or vortex tube, generated by helical injection inside a tube is studied experimentally as a possible means of fusion reactor shielding. The hollow confined vortex/swirling layer exhibits simultaneously all the complexities of swirling turbulence, free surface, droplet formation, bubble entrapment; all posing challenging diagnostic issues. The construction of flow apparatus and the choice of working liquid and seeding particles facilitate unimpeded optical access to the flow field. A split-screen, single-camera stereoscopic particle image velocimetry (SPIV) scheme is employed for flow field characterization. Image calibration and free surface identification issues are discussed. The interference in measurements of laser beam reflection at the interface are identified and discussed. Selected velocity measurements and turbulence statistics are presented at Re_λ = 70 (Re = 3500 based on mean layer thickness)

Thermo-fluid-dynamics of impinging swirling jets

The superimposition of a tangential motion on a conventional round jet has been demonstrated to significantly affect the large-scale topology of the flow. Swirling flows are widely employed, in the impinging configuration, in several industrial processes which involve both non-reacting and reacting applications. In the present dissertation, the simultaneously acquired thermal and three-dimensional velocity fields of an impinging hot jet emerging from a custom swirl generator in a cold ambient are presented. The velocity and temperature fields are experimentally measured using time-resolved Tomographic PIV and high-speed Infrared thermography in a combined system. A detailed description of a custom swirl generator is provided, and the time-averaged velocity profiles of a free swirling flow are discussed in order to estimate the swirl number. The instantaneous three-dimensional dynamics in proximity of the nozzle is discussed and the main features of a free swirling jet are investigated through the application of Proper Orthogonal Decomposition technique. The time-dependent features of velocity and temperature fields of an impinging swirling jet are investigated through the description of the time sequences of the temperature fluctuations and the synchronised instantaneous vortical structures. Taking advantage of the simultaneous acquisition and of the knowledge of the relative positioning of thermal and velocity frames, two different correlation techniques are applied, and their outcomes discussed

Experimental assessment of presumed filtered density function models

Measured filtered density functions (FDFs) as well as assumed beta distribution model of mixture fraction and “subgrid” scale (SGS) scalar variance, used typically in large eddy simulations, were studied by analysing experimental data, obtained from two-dimensional planar, laser induced fluorescence measurements in isothermal swirling turbulent flows at a constant Reynolds number of 29 000 for different swirl numbers (0.3, 0.58, and 1.07)

Experimental Assessment of ‘subgrid’ scale Probability Density Function Models for Large Eddy Simulation

Filtered density functions (FDFs) of mixture fraction are quantified by analyzing experimental data obtained from two-dimensional planar laser-induced fluorescence scalar measurements in the isothermal swirling flow of a combustor operating at a Reynolds number of 28,662 for three different swirl numbers (0.3, 0.58 and 1.07). Two-dimensional filtering using a box filter was performed on the measured scalar to obtain the filtered variables used for presumed FDF for Large Eddy Simulations (LES). A dependant variable from the measured scalar, which was a pre-computed temperature, was integrated over the experimentally obtained FDF as well as over the presumed beta or top-hat FDFs and a relative error in temperature prediction was calculated. The experimentally measured FDFs depended on swirl numbers and axial and radial positions in the flow. The FDFs were unimodal in the regions of low variance and bimodal in the regions of high variance. The influence of the filter spatial dimension on the measured FDF was evaluated and consequences for subgrid modeling for LES discussed