Validation of Combustion Models Using Sustainable aviation Fuels

Abstract

The aviation industry is under increasing pressure to mitigate its environmental impact, particularly concerning greenhouse gas emissions. Traditionally, commercial aviation has relied on conventional kerosene-based fuels. Recent advancements and environmental concerns have prompted the exploration of Sustainable aviation fuels (SAF), with initiatives like the Commercial Aviation Alternative Fuel Initiative (CAAFI) at the forefront of this movement. SAFs derived from renewable resources such as biomass and waste, offer a promising solution to reduce the carbon footprint of aviation operations. To further the understanding and implementation of such fuels, several studies have been conducted. While experimental studies are important, they are time-consuming and expensive, and therefore Computational Fluid Dynamics, CFD, is utilized to simulate the combustion.

This master’s thesis, conducted in collaboration with GKN Aerospace Engine Systems, aims to validate combustion models for aviation fuels using commercial CFD software. The study focuses on comparing the results of different combustion models to experimental data, particularly investigating the combustion behavior and NOx emissions of both conventional Jet A (A2) and a specific sustainable Alcohol-To-Jet (C1) fuel from Gevo. The study is conducted in the simulation tool STAR-CCM+, initially modeling turbulence with Reynolds-Averaged Navier-Stokes (RANS) and then proceeding with Large Eddy Simulation (LES). Mesh sensitivity studies are performed in both cases, also considering turbulence sensitivity. The results are validated against the benchmark case which consists of the experiments conducted at Cambridge University by Pathania et al.

Key findings include the superior performance of the Eddy Dissipation Concept (EDC) and Thickened Flame Model (TFM) in combustion simulations, although the EDC model showed limited applicability in LES. Flamelet Generated Manifold (FGM) showed acceptable results in LES, for its relatively low computational cost. The small comprehensive Z79 reaction mechanism demonstrated better agreement with experimental data than the HyChem skeletal reaction mechanism. Notably, C1 fuel produced higher NOx emissions, attributed to a higher equivalence ratio and flame temperature. The study also highlights the significant impact of thermal radiation on simulation accuracy, advocating for its inclusion despite the somewhat increased computational cost.

Popular Abstract

The aviation industry is under increasing pressure to mitigate its environmental impact, particularly concerning greenhouse gas emissions. One solution for this is using sustainable aviation fuel.

The need for sustainable and environmentally friendly fuels in existing engines is significant. More research is needed, as it is crucial to first understand these fuels complete combustion behavior and parameters. Currently, certain sustainable aviation fuels are approved for use in a 50/50 mixture with conventional fuel, as this has been proven to work well. These fuels are expensive to produce, and to gain approval for mixtures exceeding 50/50, two steps are necessary: testing different mixtures to observe their behavior and also finding a way to produce more of these fuels. As experiments are both expensive and time-consuming, it can be complementary to use quick, inexpensive, and flexible computer simulations. We have conducted computer simulations based on Computational Fluid Dynamics to identify the best existing models for predicting combustion in aircraft engines, in the commercial software STAR-CCM+. In this comparative study, four different combustion models were evaluated across distinct turbulence modeling frameworks, notably including Reynolds-Averaged Navier-Stokes and the computationally intensive Large Eddy Simulation. One notable discovery pertains to the suitability of the combustion model Eddy Dissipation Concept. While it proves to be an appropriate choice for Reynolds-Averaged Navier-Stokes simulations, its performance worsen when applied in Large Eddy Simulation. This is caused by poor software implementation of the model. Furthermore, the investigation uncovers intriguing insights into the environmental impact of different aviation fuels. Surprisingly, the sustainable aviation fuel C1 exhibits a higher propensity for nitrogen oxide, NOx, emissions compared to the conventional Jet A fuel A2. Further studies are needed to validate this, as the equivalence ratios used for the fuels are different. Another significant revelation pertains to the computational trade-offs involved in incorporating thermal radiation into simulations. While enabling this feature enlarges the computational costs, the payoff is substantial, especially when analyzing temperature distributions or NOx emissions.
Moreover, these findings serve as a springboard for future explorations within the realm of STAR-CCM+. They provide a solid foundation for researchers to delve deeper into combustion models and reaction mechanisms.

• author: Larsen, Måns and Attoff, David
• supervisor: Christer Fureby
• organization: Department of Energy Sciences
• course: MVKM01 20241
• year: 2024
• type: H2 - Master's Degree (Two Years)
• subject: Technology and Engineering
• keywords: Sustainable Aviation Fuels, Simulation, Validation, CFD, Star-CCM+, Combustion, Fluid Mechanics, Aviation, Jet Fuels, Premixed Flame, RANS, LES, Reaction Mechanisms, Combustion Models.
• report number: LUTMDN/TMHP-24/5590-SE
• ISSN: 0282-1990
• language: English
• id: 9166169
• date added to LUP: 2024-06-20 12:51:19
• date last changed: 2024-06-20 12:51:19