DOI: 10.5281/zenodo.15607526
title: Direct Numerical Simulations of NH₃/H₂ Flames Stabilized by a Splitter Plate: Effects of Inlet Temperature and Hydrogen Fraction Under Rich and Lean Conditions

description: ENCODING

The aim of this MSCA Doctoral Network, funded within the Horizon Europe Framework Programme is to build a generalized hybrid machine learning-based digital infrastructure in order to predict the impact of Renewable Synthetic Fuels (RSF) inexploitation on hard-to- abate sectors. The team is composed by 10 Doctoral Candidates, working in different fields: combustion science, fluid dynamics, and machine learning (ML) and the goal of the project is to work on their specific formation, promoting the exchange of ideas and tools.
description: INTRODUCTION

The growing interest in the decarbonization of hard-to-abate sectors has brought ammonia (NH₃) to the forefront of scientific research. However, its limited flammability properties present a significant challenge for combustion. One potential solution is the blending of ammonia with hydrogen, although this approach may lead to an increase in nitrogen oxide (NOₓ) emissions. This study focuses on the development of an ensemble of different simulations to investigate the impact of hydrogen blending on NOₓ emissions. In addition to offering further insights into the kinetics of NH3, these simulations will contribute to the creation of a dataset for Reduced Order Modeling (ROM) and facilitate the development of new models for both the kinetic and physical description of the combustion process.

DNS Dataset Configuration

The configuration consists of 2D Direct Numerical Simulations (DNS), primarily aimed at providing data on flame stability and quenching, in order to model flame-wall interactions. The configuration is based on a spatial mixing layer with a splitter plate (0.5x3.0 mm) inserted between the oxidizer and fuel streams to enhance flame stability. The lower boundary (on the fuel side) is defined by a symmetry condition, while the upper boundary represents a wall with a no-slip condition and a fixed temperature of 600 K. This temperature choice is consistent with typical operating conditions in a gas turbine. In all the simulations, homogeneous isotropic turbulence was injected in both the streams with an intensity of fluctuations fixed at 10 %.  The software utilized for the simulations is SiTCom-B [1], which solves the fully compressible form of the conservation equations for species mass fractions, momentum, and total sensible energy over a structured mesh using a finite volume formulation.

Included Datasets

The parameter variations in the simulations were built with the purpose to provide a comprehensive analysis of ammonia-hydrogen blend combustion in different operating conditions, with particular emphasis on the effect that their change can have on NOx emissions. The simulation data is stored as HDF5, with an accompanying XMF file provided for visualization in Paraview. An additional script is supplied to read the dataset in Python and generate visualizations for each iteration.

The stored variables include: the concentrations of all species, the source terms for species and temperature (omega_rhoY and omega_rhoE), the temperature, velocity (U, in components U_X, U_Y and U_Z) and vorticity (VORTICITY, in components VORTICITY_X, VORTICITY_Y and VORTICITY_Z) fields. An additional variable named “in_fluid” allows to make distinction between the area inside the fluid and the splitter plate. The variables are stored in groups of the same name, each one including one or more datasets (example: group omega_rhoY with datasets omega_rhoY_H2, omega_rhoY_NH3…).

Two folders are shared, named after the conditions in which the DNS was performed (rich: high global equivalence ratio and lean: low global equivalence ratio). In each one a Read_me file provides all the information relative to the presented simulations.

Conclusion

The parameter variations in the 2D simulations provides a thorough and comprehensive overview of ammonia kinetics in blends with hydrogen, as well as insights into how NOₓ emissions can be mitigated through dilution with burnt gases. The process is characterized by complex kinetics, involving the competition of three distinct kinetic processes occurring on vastly different timescales. The cracking of ammonia into hydrogen has been identified as a key factor in enhancing the flammability of the blend. Future perspectives are primarily focused on combustion modeling, particularly the extraction of fluxes along the stoichiometric iso-surface, which will contribute to a deeper understanding of differential diffusion and its effects. Additional simulations will be conducted for various hydrogen fractions and dilution levels in both rich and lean conditions. Selected cases will also be simulated in 3D to further investigate turbulence-flame interactions. This preliminary dataset, along with all future simulations, will serve as the foundation for the application of Proper Orthogonal Decomposition (POD) and other Reduced Order Modeling (ROM) techniques to extract the primary dynamic and kinetic features in collaboration with UPM.