The use of lifted flames presents some very promising advantages in terms of pollutant emissions and flame stability. The focus here is on a specific low-swirl injection system operated with methane and derived from an air-blast atomizer for aero-engine applications, which is responsible for flame lift-off. The key feature of this concept is the interaction between the swirling jet and the confinement walls, leading to a strong outer recirculation zone and thus to an upstream transport of combustion products from the main reaction region to the flame base. Here, the representation of the physics involved is challenging, since finite-rate effects govern the lift-off occurrence, and only a few numerical studies have been carried out on this test case so far. The aim of the present work is therefore to understand the limits of some state-of-the-art combustion models within the context of LES. Considering this context, two different strategies are adopted: the Flamelet-Generated Manifold (FGM) approach and the Thickened Flame (TF) model. A modified version of the FGM model including stretch and heat loss effects is also applied as an improvement of the standard model. Numerical results are compared with the available experimental data in terms of temperature and chemical species concentration maps, showing that the TF model can better reproduce the lift-off than the FGM approach.

The present work focuses on the numerical modeling through Large Eddy Simulations (LES) of a low-swirl partially premixed lean flame operated with gaseous fuel using a hybrid Thickened Flame (TF)-Flamelet Generated Manifolds (FGM) combustion model. This approach aims to overcome the challenges of modeling the flame lift-off in this burner and the stabilization of the reaction zone at a remarkable distance from the nozzle outlet section, for which the reproduction of finite rate effects on combustion physics is crucial. The underlying strategy consists of applying the artificial thickening to the scalar equations required for the query of the look-up table computed a priori. The mentioned combustion model has been implemented in a general-purpose commercial CFD solver and Non-Adiabatic Flamelets have been employed for the look-up table computation. The goal is to include a detailed chemistry description while maintaining a cost-effective approach and improving the reproduction of the turbulence-chemistry interaction. Results are validated with experimental data in terms of temperature and chemical species concentration maps, showing the potential of the coupled TF-FGM approach for describing this type of flame.

Modern combustors operate with lean mixtures to prevent Nitrogen oxides (NOx) formation by limiting the peaks of the temperature inside the combustion chamber. One of the main drawbacks of these technologies is the higher risk of Lean Blow-Off (LBO) compared to the state-of-art Rich Quench Lean combustors. To limit this possibility, combustor designers introduced pioneering concepts for this component. In this fashion, the CHAiRLIFT (Compact Helical Arranged combustoRs with lean LIFTed flames) concept founds its advantages in the structure of the combustion chamber. It combines two concepts: the tilting of the burner’s axis relative to the engine axis with a low-swirl lifted spray flame. Here, the combustion can be stabilized at very low equivalence ratios thanks to the interaction between consecutive burners. A numerical analysis was carried out to support the experimental campaign aiming to investigate the performance of the burner under different tilting angles for the burners. Two-phase simulations of the CHAiRLIFT full rig burner were performed in the commercial CFD suite ANSYS Fluent and the results were compared with the available experimental data. Furthermore, a deeper sensitivity to the tilting angle was conducted through the introduction of specific performance parameters to assess the performance and to seek the best promising setup. The outcomes have shown that tilt angles between 20° and 30° could lead to an improvement of the exhaust recirculation, regarding the considered operating conditions.

During last decades several technologies were investigated to reduce the pollutant emissions and increase the overall gas-turbine efficiency. Unluckily, some of them hinder the ignition performances of combustors. Moreover, several tests under challenging operating conditions must be carried out to obtain the required certifications and assess the combustor with respect to the ignition process. Therefore, a deeper knowledge of the phenomena involved during ignition is mandatory to shorten the design process and achieve the required performances from the very beginning. In the last years, CFD simulations established as valid alternative to the experiments to investigate the ignition process. In particular, LES proved to be a reliable tool to uncover new mechanisms of ignition and flame stabilization in gas-turbines. In this work, LES of the ignition process was attempted using ANSYS-Fluent 2019R1, with the aim of testing the Thickened-Flame-Model. In fact, compared to the previous versions, a new formulation for the efficiency function based on the pioneering work of Colin was made available. Such promising tool was validated against detailed experimental results of a lean swirled flame, known as KIAI-CORIA spray flame. At first, non-reactive and reactive LES were carried out to validate the flowfield and the stabilized flame structure. Finally, two ignition simulations were performed, from initial spark deposition up to flame stabilization or kernel quenching. All the results have been extensively compared against the experimental data showing that the employed simulation setup is capable of describing the phenomena involved in the ignition.