Large eddy simulations of lean blow-off in a turbulent premixed flame

Abstract

In the context of reducing aviation emissions, understanding flame dynamics and improving combustion efficiency are key factors in the development of low-emission engines. Computational Fluid Dynamics (CFD), and in particular Large Eddy Simulation (LES), are powerful tools to investigate unsteady phenomena such as blow-off of turbulent flames.
Understanding flame stabilization and extinction phenomena in premixed turbulent flames is critical
for improving the design and safety of combustion systems. In particular, the use of alternative fuels, such as Sustainable Aviation Fuels (SAF), raises important questions about how these fuels behave under near-blow-off conditions. Experimental studies of stable combustion and blow-off with several fuels have been done in a turbulent bluff body flame configuration. The stabilized case has been reproduced using LES. This study focuses on the simulation of the lean blow-off case using two different fuels: Jet-A (A2), a petroleum-based fuel, and C1, an Alcohol-to-Jet (AtJ) bio-derived SAF.
The objective is to provide new knowledge on how LES captures blow-off and flame dynamics under
fuel-lean conditions. LES were performed in OpenFOAM, modeling combustion using a Finite Rate
Chemistry (FRC) approach with a Partially Stirred Reactor (PaSR) model. The gas phase chemistry
was described using the HyChem reaction mechanism.
Simulation results were compared with experimental data. A range of analyses were performed, including mean field distributions (OH, CH2O), blow-off event snapshots, and counting of unburnt pockets in the recirculation zone. Additional parameters were also computed to support further analysis of the blow-off dynamics and the influence of fuel chemistry. A2 gives good results with blow-off appearing at an equivalence ratio close to the experimental one. C1 does not blow-off in simulations, but the mean results are close to the experimental data. The C1 simulation shows that the C1 flame is less susceptible to instabilities and seems to dampen them.

Popular Abstract

Jet Fuels vs. Biofuels: How Flames Behave close to extinction?
Imagine an aircraft engine losing its flame in flight, a dangerous scenario called "blow-off". My thesis used advanced computer simulations to study how a traditional jet fuel and a biofuel behave close to blow-off, revealing surprising differences between the models.
Air travel is fast, efficient, and responsible for about 5% of global climate emissions. To tackle this, the industry is turning to Sustainable Aviation Fuels (SAFs), which are made from renewable sources
and aim to reduce emissions without changing current aircraft engines. But there’s still a lot we don’t know about how these new fuels burn, especially in extreme conditions, like just before the flame goes out. Currently, they are examined a lot experimentally and in simulation. I used advanced computer simulations (Large Eddy Simulations (LES)) combined with a special chemistry model called HyChem, to study two fuels: Jet-A (the standard fuel) and C1 (a bio-derived SAF). I looked at how their flames behave in turbulent conditions just before extinction: the so-called “blow-off”.
First, I validated the model by comparing the stable simulation results with the experimental results.
Both fuels showed good agreement overall. Then I gradually reduced the fuel input in the simulation to trigger a blow-off. Jet-A flames went out – just like in the experiments – confirming that the model works well for Jet-A in extreme conditions. But here’s the twist: the C1 flame never blew out, even when the fuel level was very low. It stayed stable and resisted turbulent disruptions that normally kill a flame. At first glance, this looks great for SAFs – but in reality, this unexpected stability is likely due to the simulation model, not the actual fuel. This means the HyChem model for C1 still needs improvement for this type of analysis. In short, my thesis confirmed that JET-A HyChem mechanism tool works well in blow-off scenarios, but not yet for C1. The thesis provides a solid foundation for further investigation on blow-off events in simulation.

• author: Legrand, Martin
• supervisor: Christer Fureby; Arvid Åkerblom
• organization: Department of Energy Sciences
• course: MVKM01 20251
• year: 2025
• type: H2 - Master's Degree (Two Years)
• subject: Technology and Engineering
• keywords: Combustion Large eddy simulation SAF
• report number: ISRN LUTMDN/TMPH-25/5621-SE
• ISSN: 0282-1990
• language: English
• id: 9195092
• date added to LUP: 2025-06-09 13:43:00
• date last changed: 2025-06-09 13:43:00