Analysis of Flame Front Breaks Appearing in LES of Inhomogeneous Jet Flames Using Flamelets

The physical mechanism leading to flame local extinction remains a key issue to be further understood. An analysis of large eddy simulation (LES) data with presumed probability density function (PDF) based closure (Chen et al., 2020, Combust. Flame, vol. 212, pp. 415) indicated the presence of localised breaks of the flame front along the stoichiometric line. These observations and their relation to local quenching of burning fluid particles, together with the possible physical mechanisms and conditions allowing their appearance in LES with a simple flamelet model, are investigated in this work using a combined Lagrangian-Eulerian analysis. The Sidney/Sandia piloted jet flames with compositionally inhomogeneous inlet and increasing bulk speeds, amounting to respectively 70 and 90% of the experimental blow-off velocity, are used for this analysis. Passive flow tracers are first seeded in the inlet streams and tracked for their lifetime. The critical scenario observed in the Lagrangian analysis, i.e., burning particles crossing extinction holes on the stoichiometric iso-surface, is then investigated using the Eulerian control-volume approach. For the 70% blow-off case the observed flame front breaks/extinction holes are due to cold and inhomogeneous reactants that are cast onto the stoichiometric iso-surface by large vortices initiated in the jet/pilot shear layer. In this case an extinction hole forms only when the strain effect is accompanied by strong subgrid mixing. This mechanism is captured by the unstrained flamelets model due to the ability of the LES to resolve large-scale strain and considers the SGS mixture fraction variance weakening effect on the reaction rate through the flamelet manifold. Only at 90% blow-off speed the expected limitation of the underlying combustion model assumption become apparent, where the amount of local extinctions predicted by the LES is underestimated compared to the experiment. In this case flame front breaks are still observed in the LES and are caused by a stronger vortex/strain interaction yet without the aid of mixture fraction variance. The reasons for these different behaviours and their implications from a physical and modelling point of view are discussed in this study.

Grid Plate Flame Stabilizer for High Intensity Gas Turbine Combustion: The Influence of the Method of Fuel Injection on Mixing, Flame Development and NOx Emissions

Grid plate flame stabilizers for low NOx emissions have renewed interest in recent years due to their use in low NOx hydrogen gas turbine combustors. For non-premixed grid plate combustion, the difference in flame stabilizer design is in how the grid plate air flow is fueled. In the present work a simple four hole grid plate is investigated using CFD with three methods of fueling the air holes: radially inward fuel injection using 8 fuel nozzles per air hole (Grid Mix, GM 1 and Micromix); central fuel injection (FLOX method); and through a fuel annulus around each air hole (GM2). ANSYS FLUENT CFD predictions for GM2 are compared with axial gas composition traverses inside the combustor and with the mean combustor exit plane emissions. The three methods of fuel injection are also compared using isothermal CFD to determine which of the three methods offer the best mixing quality, which controls the relative NOx emissions. The predictions were for an equivalence ratio of 0.624 for the combustion stage and 0.5 for the isothermal study, using industrial propane. CFD modelling used RANS simulation with Realizable k-epsilon turbulence model, non-premixed combustion with the Steady Laminar Flamelet model. The temperature and mixing profiles obtained for GM2 were in reasonable agreement with the experiments and the other two fuel injection methods were then compared with GM2.

A Numerical Sensitivity Study of Modeling Parameters in the Combustion of a Swirler

In this study, a sensitivity analysis based on Reynolds Averaged Navier-Stokes (RANS) equations has been conducted to model the reacting turbulent flow in a swirler used in a (Disk-Oriented) gas-turbine using propane-air mixture. Several popular turbulence models and combustion models have been compared at different equivalence ratios. The effects of simulation parameters such as turbulence intensity, TKE Prandtl number, Schmidt number, and gravity direction have been studied. The contour plots of the species mass fraction (H2, OH) and temperature distributions from the CFD results are compared against the experimental visual results. The results showed that the realizable k-ε model and the steady diffusion flamelet model (SDF) are more suitable to model the turbulence combustion in the swirl domain. The computations further showed that the TKE Prandtl number and gravity are sensitive parameters to model the combustion from the swirler, while the Schmidt number and turbulence intensity showed less sensitivity.

Numerical and Experimental Analysis of the Effect of a Swirler with a High Swirl Number in a Biogas Combustor

Climate change as a worldwide phenomenon is the cause of multinational agreements such as the Kyoto Protocol and the Paris Agreement with the goal of reducing greenhouse gas emissions. Biogas is one of the most promising biofuels for the integration of clean energy sources; however, biogas has the disadvantage of a low calorific value. To overcome this problem, mechanical devices such as swirlers are implemented in combustion chambers (CCs) to increase their combustion efficiencies. A swirler induces rotation in the airstream that keeps a constant re-ignition of the air–fuel mixture in the combustion. We present the numerical modeling using computational fluid dynamics (CFD) and experimental testing of combustion with biogas in a CC, including an optimized swirler in the airstream with a swirl number (Sn) of 2.48. A turbulence model of the renormalization group (RNG) was used to analyze the turbulence. Chemistry was parameterized using the laminar flamelet model. The numerical model allows visualizing the recirculation zone generated at the primary zone, and partially at the intermediate zone of the CC caused by the strong swirl. Temperature distribution profiles show the highest temperatures located at the intermediate and dilution zones, with the last one being a characteristic feature of biogas combustion. A strong swirl in the airstream generates low-velocity zones at the center of the CC. This effect centers flame, avoiding hot spots near the flame tube and flashback at the structural components. Regarding pollutant emissions, the goal of a biogas that generates less pollutants than nonrenewable gases is accomplished. It is observed that the mole fraction of NO in the CC is close to zero, while the mole fraction of CO2 after combustion is lowered compared to the original mole fraction contained in the biogas (0.25). The mole fraction of CO2 obtained in experimental tests was 0.0127. Results obtained in the numerical model for temperatures and mole fractions of CO2 and NO show a behavior similar to that of the experimental model. Experimental results for mole fraction of CO emissions are also presented and have a mean value of 0.0009. This value lies within allowed pollutant emissions for CO according to national environmental regulations.