Large eddy simulation of premixed combustion using flamelets

Large Eddy Simulation (LES) has potential to address unsteady phenomena in turbulent premixed flames and to capture turbulence scales and their influence on combustion. Thus, this approach is gaining interest in industry to analyse turbulent reacting flows. In LES, the dynamics of large-scale turbulent eddies down to a cut-off scale are solved, with models to mimic the influences of sub-grid scales. Since the flame front is thinner than the smallest scale resolved in a typical LES, the premixed combustion is a sub-grid scale (SGS) phenomenon and involves strong interplay among small-scale turbulence, chemical reactions and molecular diffusion. Sub-grid scale combustion models must accurately represent these processes. When the flame front is thinner than the smallest turbulent scale, the flame is corrugated by the turbulence and can be seen as an ensemble of thin, one-dimensional laminar flames (flamelets). This allows one to decouple turbulence from chemistry, with a significant reduction in computational effort. However, potentials and limitations of flamelets are not fully explored and understood. This work contributes to this understanding. Two models are identified, one based on an algebraic expression for the reaction rate of a progress variable and the assumption of fast chemistry, the other based on a database of unstrained flamelets in which reaction rates are stored and parametrised using a progress variable and its SGS variance, and their potentials are shown for a wide range of premixed combustion conditions of practical interest. The sensitivity to a number of model parameters and boundary conditions is explored to assess the robustness of these models. This work shows that the SGS variance of progress variable plays a crucial role in the SGS reaction rate modelling and cannot be obtained using a simple algebraic closure like that commonly used for a passive scalar. The use of strained flamelets to include the flame stretching effects is not required when the variance is obtained from its transport equation and the resolved turbulence contains predominant part of the turbulent kinetic energy. Thus, it seems that SGS closure using unstrained flamelets model is robust and adequate for wide range of turbulent premixed combustion conditions.This work was supported by EPSRC [grant number EP/I0277556/1]

Large Eddy Simulation of Bluff Body Stabilised Premixed Flames Using Flamelets

Large Eddy Simulations of an unconfined turbulent lean practical flame stabilised behind a bluff body burner are computed using structured and unstructured numerical solvers. Unstrained flamelets are used as the sub-grid scale combustion closure using constant and dynamic formulations to model the flame curvature parameter βc. The model uses a presumed probability density function to calculate the filtered reaction rate. The aim of this study is to determine the numerical set-up that provides the most reliable results for the flame that is furthest from blow-off conditions (A1). This work will lead to modelling the flame closest to blow-off (A4) and the flame at blow-off conditions, since these flames are highly unstable. Comparisons will be drawn with experimental data obtained using PIV, OH chemiluminescence and OH-PLIF techniques.EPSRC DTP studentship (RG80792

The Role of CFD in Modern Jet Engine Combustor Design

Recent advances in the application of computational fluid dynamics (CFD) for turbulent combustion with the relevance for gas turbine jet engines are discussed. Large eddy simulation (LES) has emerged as a powerful approach to handle the highly turbulent, unsteady and thermochemically non-linear flows in the practical combustors, and it is a matter of time for the industry to replace the conventional Reynolds averaged Navier-Stokes (RANS) approach by LES as the main CFD tool for combustor research and development. Since combustion is a subgrid scale phenomenon in LES, appropriate modelling is required to describe the SGS combustion effects on the resolved scales. Among the various available models, the flamelet approach is seen to be a promising candidate for practical application because of its computational efficiency, robustness and accuracy. A revised flamelet formulation, FlaRe, is introduced to outline the general LES methodology for combustion modelling and then used for a range of test cases to demonstrate its capabilities for both laboratory burners and practical engine combustors. The LES results generally compare well with the experimental measurements showing that the important physical processes are captured in the simulations

LES of Reacting Flows in an Industrial Gas Turbine Combustor

The turbulent reacting flow in an industrial gas turbine combustor operating at 3 bar is computed using LES paradigm. The subgrid scale (SGS) combustion is modelled using a collection of unstrained premixed flamelets including mixture stratification. The non-premixed combustion mode is also included using a simple closure involving the scalar dissipation rate of the mixture fraction. A close attention is paid to maintain physical consistencies among sub-closure models for combustion and these consisten- cies are discussed on a physical basis. The importance of non-premixed mode and SGS mixture fraction fluctuations are investigated systematically. The results show that the SGS mixture fraction variance plays an important role and comparisons to measure- ments improve when contributions from the premixed and non-premixed modes are included. These numerical results and observations are discussed on a physical basis along with potential avenues for further improvements

Numerical investigation of a coupled blow-off/flashback process in a high-pressure lean-burn combustor

Large eddy simulation is used to investigate the flashback mechanism caused by the combustion-induced vortex breakdown (CIVB) in a high-pressure lean-burn annular combustor with lean direct injection of kerosene. A single sector of the geometry, including a central pilot flame surrounded by a main flame, is simulated at take-off conditions. A previously-developed flamelet-based approach is used to model turbulence-combustion interactions due to its relatively low cost, allowing to simulate a sufficiently long time window. In stable operations, the flame stabilises in an M-shape configuration and a periodic movement of the pilot jet, with the corresponding formation of a small recirculation bubble, is observed. Flashback is then observed, with the flame accelerating upstream towards the injector as already described in other studies. This LES, however, reveals a precursor partial blow-out of the main flame induced by a cluster of vortices appearing in the outer recirculation region. The combined effect of vortices and sudden quenching alters the mixing level close to the injector, causing first the main, then the pilot flame, to accelerate upstream and initiate the CIVB cycle before the quenched region can re-ignite. Main and pilot flames partly extinguish as they cross their respective fuel injection point, and re-ignition follows due to the remnants of the reaction in the pilot stream. The process is investigated in detail, discussing the causes of CIVB-driven flashback in realistic lean-burn systems.</p

Numerical investigation of a coupled blow-off/flashback process in a high-pressure lean-burn combustor

Large eddy simulation is used to investigate the flashback mechanism caused by the combustion-induced vortex breakdown (CIVB) in a high-pressure lean-burn annular combustor with lean direct injection of kerosene. A single sector of the geometry, including a central pilot flame surrounded by a main flame, is simulated at takeoff conditions. A previously developed flamelet-based approach is used to model turbulence–combustion interactions due to its relatively low cost, allowing to simulate a sufficiently long time window. In stable operations, the flame stabilizes in an M-shape configuration and a periodic movement of the pilot jet, with the corresponding formation of a small recirculation bubble, is observed. Flashback is then observed, with the flame accelerating upstream toward the injector as already described in other studies. This large eddy simulation (LES), however, reveals a precursor partial blow-out of the main flame induced by a cluster of vortices appearing in the outer recirculation region. The combined effect of vortices and sudden quenching alters the mixing level close to the injector, causing first the main, then the pilot flame, to accelerate upstream, and initiate the CIVB cycle before the quenched region can re-ignite. Main and pilot flames partly extinguish as they cross their respective fuel injection point, and re-ignition follows due to the remnants of the reaction in the pilot stream. The process is investigated in detail, discussing the causes of CIVB-driven flashback in realistic lean-burn systems.</p

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.</p