Numerical modelling of partially premixed low-swirl flames for aero-engine applications

The development of innovative aero-engine combustors has been devoted to drastically reducing pollutants emissions and improving engine performances in recent years. These aspects are not only crucial to meet the severe regulations imposed by ICAO-CAEP, but also to enable potential new engine architectures. Especially considering Nitrogen Oxidizes (NOx) emissions, the most promising concept carried out so far is represented by Lean burn combustors, which however introduce several challenges in terms of ame stability. A possible solution to this problem is the novel burner concept proposed in the EU project CHAiRLIFT (Compact Helical Arranged combustoRs with lean LIFTed ames). The proposal of this project sees the employment of low-swirl ultra-lean spray lifted flames with an inclined disposition of the burners. Both these concepts have been investigated separately at the Karlsruhe Institute of Technology (KIT). In particular, this type of flame has shown superior performances in terms of NOx emissions a good resistance to Lean Blow-Off (LBO) occurrences, while avoiding flashback risks. The inclined arrangement of the burners, instead, establishes a macro-recirculation in the combustion chamber responsible for the transport of vitiated gas among the burners, promoting flame stability. Moreover, it contributes to reduce the need of cooling air and the overall weight. This ambitious project indeed requires proper tools to study flame interaction between adjacent burners in deep. The present research effort is therefore devoted to numerically investigating the CHAiRLIFT concept through Computational Fluid Dynamics (CFD) simulations. With this goal, both Scale Resolved (SR) simulations of the low-swirl burner in single sector con guration and Reynolds Averaged Navier-Stokes (RANS) simulations of the multiburner rig, currently under investigation at KIT, have been employed. The numerical study of the multiburner con figuration had a twofold objective: assess the numerical approach with the available data and support the experimental investigations, especially concerning the sensitivity to the tilt angle. The outcomes have shown that, in non-preheated conditions, the numerical simulation can fairly reproduce the spray lifted flames with a reasonable computational effort. Also, it points out that the best setup in terms of tilt angle for maximizing the exhausts recirculation lays between 20 and 30 degrees, which is a lower value concerning the original experimental investigation with high-swrirl flames. Another point to be addressed is the turbulent combustion modelling in preheated conditions, which has shown to be more challenging from the modelling point of view. To this aim, the investigation is focused on the same burner in single sector con guration, operated with gaseous fuel. State-of-the-art numerical models for Large Eddy Simulations context are employed to understand how the ame is reproduced. The results highlighted that both the Flamelet Generated Manifold (FGM) approach and a modifi ed version taking into account the stretch and heat loss effects are mispredicting the flame lift-off height. Instead, a good reproduction is achieved with the Thickened Flame (TF) model. Despite the good agreement of the latter approach, this suffers of some disadvantages in terms of chemistry description: aiming to overcome these issues, a hybrid TF-FGM approach is introduced and validated in the conclusive part of this work, with very interesting results in terms of lift-off and flame shape. In the nal part, the hybrid TF-FGM model is applied to the same con guration operated with spray together with dedicated spray boundary conditions carried out from the research activity of the COmplexe de Recherche Interprofessionnel en Aerothermochimie (CORIA) research team. The results pointed out that although the ame is still not perfectly predicted, a large improvement is reached concerning the combustion and spray modeling approaches commonly present in the literature.