Session: 04-08 Combustion Modeling I

Paper Number: 151303

The Influence of Turbulence Intensity on NOx Emission and Flame Topology in Ammonia/Air Premixed Combustion 

Ammonia is recently drawing significant attention as a potential carbon-free fuel due to capability to be stored and transported easily. However, its slow flame speed and high nitrogen oxide (NOx) emissions present considerable challenges in industrial gas turbine applications. Hence, understanding the flame behavior and NOx formation characteristics in ammonia/air premixed flames is in urgent need for developing efficient and environmentally-friendly combustion systems which run on turbulent flow environments. This study investigates effects of turbulence intensities on flame propagation and NOx formation, particularly at an equivalence ratio of 1.2 which has low NOx emissions due to lower oxygen concentration.

The DNS was conducted using the PeleLM code from Lawrence Berkeley National Laboratory, an adaptive mesh hydrodynamics simulation code for low Mach number reacting flows. For chemical reactions, the Okafor mechanism was used without carbon-related reactions, which consists of 21 chemical species and 101 reactions. The simulations were performed on 7 turbulence intensities of u’/S_L = 5, 7.5, 10, 15, 20, 30 which are Karlovitz numbers of 14, 27, 41, 75, 116, and 212, respectively.

As the turbulence intensity increases, larger vortex structures were formed and the flame wrinkling increased, resulting in flame front distortion. At high turbulence intensities, the distorted flame front causes larger regions of high flame front curvature. The high flame curvature changes the typical laminar flame structure, which were distinguished across different turbulence intensities and captured through detailed 3D DNS visualizations.

Regarding NOx formation, as the turbulence intensity increases, more regions of high curvature appear, leading to higher NOx production in these areas. The increased NOx formation is driven by enhanced local heat release rates (HRR) in these curved regions, where vigorous chemical reactions occurred due to the flame’s increased surface area. Specifically, Due to reduced flame stretch, local chemical reactivity and heat release rates (HRR) are enhanced in regions with negative curvature, resulting in higher NOx concentration. Despite these localized increases in NOx production, the overall NO concentration measured downstream remains similar to laminar ammonia/air premixed flame, regardless of turbulence intensity.

This study provides insights for designing low-NOx ammonia combustion systems. By understanding the interactions between turbulence, flame structure, and NOx formation, the DNS data can assist in optimizing burner designs and operational strategies, while also contributing to the development of predictive NOx models for ammonia-fueled gas turbines.

Presenting Author: Inyeong Gu Korea Advanced Institute of Science & Technology (KAIST)

Presenting Author Biography: Inyeong Gu is currently a Ph.D. candidate at the Korea Advanced Institute of Science and Technology (KAIST) in the Department of Aerospace Engineering, having completed both a bachelor's and master's degree. Inyeong conducts research in Professor Donghyuk Shin's Combustion Modeling Lab, focusing on turbulent combustion, carbon-neutral fuel combustion, and NOx emissions.

Authors:

Inyeong Gu Korea Advanced Institute of Science & Technology (KAIST)
Dong-Hyuk Shin Korea Advanced Institute of Science & Technology (KAIST)