In this work, numerical investigations were performed using large eddy simulations and validated against detailed measurements in the CeCOST swirl stabilised burner. Both cold and reactive flow have been studied and the model has shown a good agreement with experiments. The verification of the model was done using the LES index of quality and a single grid estimator. The cold flow simulations predicted results closely to experiments setting baseline for the reactive simulations. Coherent structures like the vortex rope above the swirler and a precessing vortex core in the combustion chamber were identified. The reactive conditions were modelled with the Flamelet generated manifold and artificially thickened flame models. Simulations were performed for an experimental syngas composition from black liquor gasification at three different CO2 dilution levels. Three different Reynolds numbers were investigated with the model matching closely to experimentally detected 2D flow field and OH for the most CO2 diluted mixture. It was found that the opening angles of the flames differ by a maximum of 13% between experiments and simulations. The most diluted fuel investigated experienced a liftoff distance of 23.5 mm at the Re 25 k. This was also the highest liftoff distance experienced in this cohort of fuels. The same fuel also proved to have the thickest flame annulus at 78.5 mm. Overall, in cases with no experimental data available the predictions made by the model follow the same trends which hints its applicability to higher Re cases.

The impact of CO2 dilution on combustion of syngas (a mixture of H2, CO, and CH4) was investigated in a lab-scale gas turbine model combustor at atmospheric pressure conditions. Two mild dilution levels of CO2, corresponding to 15% and 34% of CO2 mole fraction in the syngas/CO2 mixtures, were experimentally investigated to evaluate the effects of CO2 dilution on the flame structures and the emissions of CO and NOx. All experiments were performed at a constant Reynolds number (Re = 10000). High-speed flame luminescence, simultaneous planar laser-induced fluorescence (PLIF) measurements of the OH radicals and particle image velocimetry (PIV) were employed for qualitative and quantitative assessment of the resulting flame and flow structures. The main findings are: (a) the operability range of the syngas flames is significantly affected by the CO2 dilution, with both the lean blowoff (LBO) limit and the flashback limit shifting towards fuel-richer conditions as the CO2 dilution increases; (b) syngas flames exhibit flame-pocket structures with chemical reactions taking place in isolated pockets surrounded by non-reacting fuel/air mixture; (c) the inner recirculation zone tends to move closer to the burner axis at high CO2 dilution, and (d) the NOx emission becomes significantly lower with increasing CO2 dilution while the CO emission exhibits the opposite trend. The flame-pocket structure is more significant with increased CO2 dilution level. The low NOx emissions and high CO emissions are the results of the flame-pocket structures.

The aim of the work was to study the effects of neighboring particles with uniform Stefan flow in particle–fluid flows. Particle-resolved numerical simulations were carried out for particles emitting a uniform Stefan flow into the bulk fluid. The bulk fluid was uniform and isothermal. The Stefan flow volume emitted from the two particles is equal, such that it represents idealized conditions of reacting particles. Particles were located in tandem arrangement and particle distances were varied between 1.1 and 10 particle diameters (). Three particle Reynolds numbers were considered during the simulations ( and 14), which is similar to our previous studies. Three Stefan flow velocities were also considered during simulations to represent inward, outward, and no Stefan flow. The drag coefficient of the particles without Stefan flow showed that the results fit with previous studies on neighbor particle effects. When the particle distance is greater than 2.5 diameters (), the effects of Stefan flow and neighboring particles are independent of each other. I.e. an outward Stefan flow decreases the drag coefficient () while an inward Stefan flow increases it and the upstream particle experience a higher  than the downstream particle. When , the effect of Stefan flow is dominant, such that equal and opposite pressure forces act on the particles, resulting in a repelling force between the two neighboring particles. The pressure force showed a large increase compared to the viscous force at these distances. The effect of Stefan flow is weakened at higher Reynolds numbers. A model was developed for the calculation of the drag coefficient. The model, which reproduce the results from the numerical simulations presented above, is a product of independent models that describe the effects of both neighboring particles and two distinguished effects of the Stefan flow.

Inter-particle distance and particle dispersion during gasification of biomass have been found to significantly affect soot emission. Consequently, enhanced particle dispersion decreases energy losses and the risk for blockages of downstream equipment, increasing the efficiency and reliability of entrained flow reactors (EFRs). In this work, we investigated the interactions between imposed acoustic oscillations and particle dispersion under non-reacting conditions in a co-axial burner for a lab-scale EFR. A flow of air, laden with pulverized stem wood particles (Norwegian Spruce) of three different sizes (63–112 μm, 200–250 μm, and 500–600 μm), was forced axially through the burner center tube at Reynolds numbers ranged from 800 to 1700, and loading ratio of 0.7–4.2. The influences on particle dispersion from variations of the Strouhal number (0.12–0.6), the pressure amplitude at synthetic jet cavity (0.5–4.0 kPa_p-p), the swirl number (0–2.3), and the center jet velocity (1.9–3.9 m s⁻¹) were investigated. Post-processed shadowgraph images revealed the influence of acoustic perturbations, which generate large structures with high particle concentration for both swirling and non-swirling conditions. Time-averaged contour maps showed a significantly higher particle dispersion, quantified as dispersion angle, for higher values of forcing amplitude and swirl numbers, with a stronger influence from the forcing amplitude, especially at lower Stokes number.

Gas turbines for power generation are optimised to run with fossil fuels but as a response to tighter pollutant regulations and to enable the use of renewable fuels there is a great interest in improving fuel flexibility. One interesting renewable fuel is syngas from biomass gasification but its properties vary depending on the feedstock and gasification principle, and are significantly different from conventional fuels. This paper aims to give an overview of the differences in combustion behaviour by comparing numerical solutions with methane and several different synthesis gas compositions. The TECFLAM swirl burner geometry, which is designed to be representative of common gas turbine burners, was selected for comparison. The advantage with this geometry is that detailed experimental measurements with methane are publicly available. A two-stage approach was employed with development and validation of an advanced CFD model against experimental data for methane combustion followed by simulations with four syngas mixtures. The validated model was used to compare the flame shape and other characteristics of the flow between methane, 40% hydrogen enriched methane and four typical syngas compositions. It was found that the syngas cases experience lower swirl intensity due to high axial velocities that weakens the inner recirculation zone. Moreover, the higher laminar flame speed of the syngas cases has a strong effect on the flame front shape by bending it away from the axial direction, by making it shorter and by increasing the curvature of the flame front. A hypothesis that the flame shape and position is primarily governed by the laminar flame speed is supported by the almost identical flame shapes for bark powder syngas and 40% hydrogen enriched methane. These gas mixtures have almost identical laminar flame speeds for the relevant equivalence ratios but the heating value of the syngas is more than a factor of 3 smaller than that of the hydrogen enriched methane. The syngas compositions used are representative of practical gasification processes and biomass feedstocks. The demonstrated strong correlation between laminar flame speed and flame shape could be used as a rule of thumb to quickly judge whether the flame might come in contact with the structure or in other ways be detrimental to the function of the combustion system.

Forced intermittent combustion with periodical variations of pressure, velocity, and air-fuel ratios is a promising method to increase efficiency and reduce emissions from combustion and gasification applications. In this work, flame characteristics and emissions from a pulverized biomass burner are investigated under oscillations induced by an acoustically-driven synthetic jet. Instantaneous images of incandescent light emitted from flame were captured using high-speed cameras. The images were analyzed to identify the liftoff distance, flame length, and shape. The flame liftoff distance decreased under excited conditions, notably at high forcing amplitude applied to small particle size distribution (63-112 μm). In such conditions, acoustic forcing increases particle dispersion as presented in the previous work, providing conditions for earlier ignition due to enhanced fuel-air mixing besides reducing CO emissions. Flue gas emissions were influenced mainly by the particle size distribution, from which the 63-112 μm particle size presented the lowest values of CO and highest levels of NO emissions. The results presented stable flame edge positions for the particle size of 63-112 μm, while wide range particle distributions (0–600, 0-400 μm) had strong fluctuations, indicating high flame instability. The experimental work adds new insights regarding acoustic excitation in swirl burners, which could be used to optimize pulverized fuel combustion.

In Nordic countries, biomass gasification in a cyclone gasifier combined with a gas engine has been employed to generate small scale heat and power. Numerical simulations were carried out to analyze the effect of different operating conditions on the functioning of the gasifier. Reynolds-Averaged Navier-Stokes equations are solved together with the eddy-break up combustion model in conjunction with a modified k − ϵ model to predict the temperature and the flow field inside the gasifier. Results were compared with the experimental measurements in a 4.4 MW cyclone gasifier constructed by Meva Energy AB at Hortlax, Piteå, Sweden. The predicted results were in good agreement with the experimental data and the model provides detailed information about the gas compositions, cold gas efficiency and temperature field. Furthermore, the model allows different operating scenarios to be examined in an efficient manner such as the number of inlets, fuel to air velocity difference (slip-velocity) and moisture content in the fuel feedstock. The cold gas efficiency, composition of product gases and outlet temperature were monitored for each test case. These findings help to understand the importance of geometry modification, feedstock contents and make it possible to scale-up the gasifier for future applications.

A Stefan flow can be generated during a phase change or reactions of a particle immersed in a fluid. This study investigates the effect of Stefan flow on the exchange of momentum (drag coefficient (C_D)) and heat transfer (Nusselt number (Nu)) between the particle and bulk-fluid. Fully resolved simulations were carried out for a flow near a spherical particle immersed in a uniform bulk flow. The immersed boundary method is used for implementing fluid-solid interactions and the particle is considered as a static boundary with fixed boundary conditions. In a non-isothermal flow, the changes in thermophysical properties at the boundary layer played a role in the variation of C_D and Nu by a Stefan flow further. The previously developed model for the drag coefficient of a spherical particle in a uniform isothermal flow was modified for a uniform non-isothermal flow. The model is developed based on physical interpretation. A new model is developed for the Nusselt number for a spherical particle with a uniform Stefan flow combining available models in literature. The models are validated for Stefan Reynolds number −8⩽Re_sf,p⩽25 and particle Reynolds number of 2⩽Re_f⩽30 in gas flow (i.e. Pr≈0.7).

Particle laden flows with reactive particles are common in industrial applications. Chemical reactions inside the particle can generate a Stefan flow that affects heat, mass and momentum transfer between the particle and the bulk flow. This study aims at investigating the effect of Stefan flow on the drag coefficient of a spherical particle immersed in a uniform flow under isothermal conditions. Fully resolved simulations were carried out for particle Reynolds numbers ranging from 0.2 to 14 and Stefan flow Reynolds numbers from (-1) to 3, using the immersed boundary method for treating fluid-solid interactions. Results showed that the drag coefficient decreased with an increase of the outward Stefan flow. The main reason was the change in viscous force by the expansion of the boundary layer surrounding the particle. A simple model was developed based on this physical interpretation. With only one fitting parameter, the performance of the model to describe the simulation data were comparable to previous empirical models.