Large Eddy Simulations of a Low-Swirl Gaseous Partially Premixed Lifted Flame in Presence of Wall Heat Losses

Abstract

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.

Assessment of Thickened Flame Model Coupled With Flamelet Generated Manifold on a Low-Swirl Partially Premixed Gaseous Lifted Flame

“…In the previous works by the authors, the effects of heat losses were included within the FGM model through a correction factor applied by scaling the reaction rate [17,18]: outcomes have shown a discrepancy with the experimental measurements, resulting in a short flame stabilized low in the flame tube. A possible explanation for such behaviour could be incorrect modelling of the heat loss effects on the reaction, due to this simplistic approach, as described in [35].…”

“…This setup is similar to the one present in a first numerical study conducted by the authors [17] on the same low-swirl injector concept with a smaller effective area: here, the isothermal conditions were studied and reached a good agreement in terms of flow-field description. Indeed the present work focused only on the reactive point, similarly to the study conducted in [18] for evaluating the impact of the turbulent combustion modelling. Thus, it is assumed that the numerical setup can correctly represent the flow structures in absence of reaction.…”

“…No-slip condition is applied to the combustion chamber walls and a uniform temperature of 700 K is imposed as thermal boundary condition on the lateral and bottom walls. This point is crucial and it has been largely discussed in [18]. In the previous works, this value of wall temperature seems to better describe the temperature field in the proximity of the combustion chamber bottom walls.…”

“…The goal of the present work is therefore to investigate how the flame prediction concerning the KIT-EBI's burner is affected when turbulence-chemistry interaction is described through the use of Thickened Flame model with a chemistry tabulation from a non-adiabatic FGM manifold. To some extent, this work represents a continuation of the study in [18], where all the employed models have been found not fully satisfactory with respect to the experimental data. The expected outcomes are to extend the knowledge concerning this type of low-swirl lifted flame, other than highlighting the possible advantages of this approach for industrial applications.…”

“…The prediction can be improved when the FGM model is modified to take into account the effects of local stretch and heat loss on the global reactivity, referred as FGM-EXT in [17]. Further works from the authors [18] carried out a comparison among the previously mentioned models and the Thickened Flame (TF) model [19]: indeed, the use of the TF approach resulted in a better description of the flame lift-off and shape of the reaction zone. However, this first attempt employed the BFER 2-step mechanism by Franzelli et al [20] for the chemistry description, which is not adequate to quantitatively describe intermediate species such as carbon monoxide (CO) [21].…”