Figure 1: Temperature field, NOx mass fraction, and ammonia mass fraction in an intrinsically unstable laminar ammonia/hydrogen/air flame. Ammonia is a widely discussed carbon-free fuel candidate due to its high atomic hydrogen
content. While the combustion properties of pure ammonia are characterized by
low burning velocities and high ignition energies, they significantly improve when mixed
with molecular hydrogen. However, these fuel mixtures exhibit intrinsic flame instabilities
(IFIs) driven by thermodiffusive processes. For pure hydrogen, it is known that
these instabilities can cause a tremendous increase in the global fuel consumption rate.
Similar results are observed for fuel mixtures of ammonia and hydrogen. Furthermore, a
non-linear effect of these instabilities with the blending ratio on IFIs with the strongest
instabilities arising for hydrogen fractions of XH2 ≈ 40 % is reported experimentally.
Hence, a fundamental investigation of IFIs is crucial for efficiently utilizing ammonia/hydrogen
blends in various combustion applications. This study analyzes fully-developed
IFIs in laminar premixed ammonia/hydrogen flames over a wide parameter range of
hydrogen fraction, equivalence ratio, pressure, and unburned temperature using direct
numerical simulations (DNS). DNS resolve all relevant chemical processes and flow structures
directly on the computational grid. In reactive flows, this also includes the direct
solution of the chemistry. This approach allows for a nearly exact simulation of the flow
and is therefore often referred to as a numerical experiment. However, performing direct
numerical simulations requires significant computational resources.
Project Details
Project term
April 1, 2023–July 1, 2024
Affiliations
RWTH Aachen University
Institute
Institute for Combustion Technology (ITV)
Project Manager
Principal Investigator
Methods
For the simulations within the scope of this project, the C++ code PeleLMeX. It solves
the multi-species reactive Navier-Stokes equations in the low-Mach formulation. The
equations are advanced in time through a spectral-deferred corrections approach that
conserves species, mass, and energy. The advection term is discretized with a secondorder
Godunov scheme. The energy and species equations are treated implicitly utilizing
the ODE solver CVODE from the SUNDIALS package. PeleLMeX features adaptive
mesh refinement inherited from the AMReX package.
Results
During the project, a database of 30 two dimensional simulations of laminar premixed ammonia/hydrogen/air spanning a wide range of conditions was generated. The data confirm the non-monotonic behavior of intrinsic flame instabilities also for fully developed instabilities. Additionally, the data were analyzed with respect to the influence of intrinsic flame instabilities on NOx in ammonia/hydrogen/air flames. NO was shown to
be the dominant NOx pollutant, with negligible flux towards NO2 and N2O. High production rates of NO in the reaction zone resulted in significant diffusion of NO towards the unburned area, where a characteristic consumption zone formed. Formation of NO2 was seen to take place largely within this NO consumption zone by conversion of NO to NO2. However it should be noted that this consumption of NO plays a minor role in overall NO consumption. Characteristic behavior of the flame regarding NOx formation in relation to the local curvature was observed, where regions of the flame front concave towards the unburned mixture showed higher production rates for NO, resulting in trails of higher NO mass fractions. The opposite trend was observed for NO2, stemming from lower NO2 consumption rates of NO2 previously formed in negatively curved sections of
the flame. Strong wrinkling due to intrinsic flame instabilities was shown to lead to areas of local
flame extinction in the negatively curved regions. Here, reaction rates for NOx diminished
compared to the rest of the flame. These extinguished areas are responsible for
the slight decrease in NOx amounts observed in the burned region of the unstable flame
when compared to the one-dimensional unstretched flamelet and generally significantly
affect mean NOx formation in the unstable flame.
Discussion
To gain a more detailed understanding of NO formation in the investigated flame, different
pathways of NO formation and consumption were identified. NO formation was
found to be almost exclusively ascribable to fuel NO pathways, while thermal deNOx
as well as NO conversion to N2 via N2 arose as the most significant pathways for NO
consumption. The Zeldovich mechanism was found to be a net consumer of NO. In
the future, the constructed database will be used for further analysis of intrinsic flame
instabilities in ammonia/hydrogen/air flames with respect to NOx.
Additional Project Information
DFG classification: 404 Fluid Mechanics, Technical Thermodynamics and Thermal Energy Engineering
Software: PeleLMeX, FlameMaster
Cluster: CLAIX
Publications
Nikita Dimidziev, Effects of Intrinsic Flame Instabilities on NOx Formation in Laminar Premixed Ammonia/Hydrogen/Air Flames, Bachelor Thesis, Aachen 2024.