The reactive force-field (ReaxFF) interatomic potential is a powerful computational tool for exploring, developing and optimizing material properties. Methods based on the principles of quantum mechanics (QM), while offering valuable theoretical guidance at the electronic level, are often too computationally intense for simulations that consider the full dynamic evolution of a system. Alternatively, empirical interatomic potentials that are based on classical principles require significantly fewer computational resources, which enables simulations to better describe dynamic processes over longer timeframes and on larger scales. Such methods, however, typically require a predefined connectivity between atoms, precluding simulations that involve reactive events. The ReaxFF method was developed to help bridge this gap. Approaching the gap from the classical side, ReaxFF casts the empirical interatomic potential within a bond-order formalism, thus implicitly describing chemical bonding without expensive QM calculations. This article provides an overview of the development, application, and future directions of the ReaxFF method.

https://biblio.ugent.be/publication/8514189/file/8514192.pdf

The ReaxFF reactive force-field: development, applications and future directions

Dynamics and stability of lean-premixed swirl-stabilized combustion

Ammonia-borane and related compounds as dihydrogen sources.

A review of oscillation mechanisms and the role of the precessing vortex core (PVC) in swirl combustion systems

Combustion dynamics constitutes one of the most challenging areas in combustion research. Many facets of this subject have been investigated over the past few decades for their fundamental and practical implications. Substantial progress has been accomplished in understanding analysis, modeling, and simulation. Detailed laboratory experiments and numerical computations have provided a wealth of information on elementary dynamical processes such as the response of flames to variable strain, vortex rollup, coupling between flames and acoustic modulations, and perturbed flame collisions with boundaries. Much recent work has concerned the mechanisms driving instabilities in premixed combustion and the coupling between pressure waves and combustion with application to the problem of instability in modern low NO x heavyduty gas turbine combustors. Progress in numerical modeling has allowed simulations of dynamical flames interacting with pressure waves. On this basis, it has been possible to devise predictive methods for instabilities. Important efforts have also been directed at the development of the related subject of combustion control. Research has focused on methods, sensors, actuators, control algorithms, and systems integration. In recent years, scaling from laboratory experiments to practical devices has been achieved with some successebut limitations have also been revealed. Active control of combustion has also evolved in various directions. A number of experiments on laboratory-scale combustors have shown that the amplitude of combustion instabilities could be reduced by applying control principles. Full-scale terrestrial application to gas turbine systems have allowed an increase of the stability margin of these machines. Feedback principles are also being explored to control the point of operation of combustors and engines. Operating point control has special importance in the gas turbine field since it can be used to avoid operation in unstable regions near the lean blowoff limits. More generally, closed loop feedback concepts are useful if one wishes to improve the combustion process as demonstrated by applications to automotive engines. Many future developments of combustion will use such concepts for tuning, optimization, and emissions reduction. This article proposes a broad survey of these fast-moving areas of research.

Combustion dynamics and control: Progress and challenges

An improved understanding of the mechanisms of unstable combustion in lean premixed combustors is essential to the development of stable gas turbine combustion systems. To obtain such understanding, detailed experimental studies of the phenomenology of unstable combustion are required. A number of experimental diagnostic techniques for characterizing unstable combustion and the underlying instability mechanisms are discussed. This includes techniques based on pressure, chemiluminescence emission, infrared absorption, and laser-induced fluorescence measurements. The techniques themselves are discussed briefly; however, the primary objective is to present and discuss results illustrating how these techniques can be used to characterize the mechanisms of unstable combustion, to gain an improved understanding of unstable combustion, and to develop strategies for suppressing unstable combustion in lean premixed gas turbine combustors.

Experimental Diagnostics for the Study of Combustion Instabilities in Lean Premixed Combustors

Results from an experimental study of the mechanism of unstable combustion in a coaxial, optically accessible, bluff-body-stabilizeddumpcombustorwithnaturalgasasthefuelarereported.Aparametricstudywasperformed to investigate the effects of equivalence ratio, inletvelocity, inlet fuel distribution, inlet swirl, and centerbody recess oncombustionstability. It wasfoundthatall of theseparametershadan effectonthestability characteristicsofthis combustor.Atselectedunstableoperatingconditions,phase-resolvedCHchemiluminescenceimageswerecaptured to study the heat-release structure during one period of pressure oscillation. The e ame‐ e owe eld interaction that is depicted in these images indicates that e ame‐ vortex interactions, and the resultant e ame area changes, play a signie cant role in the instabilities that occur when there is no swirl. A simple analysis of these images, however, showedthate uctuatinge ameareaandequivalenceratioe uctuationsbothcontributetotheheatreleasee uctuations that drive the instability. Unstable combustion with swirl appears to be fundamentally different from unstable combustion without swirl in that instabilities with swirl occur near lean blowout and appear to be associated with repeated detaching and reattaching of the e ame from the centerbody.

Mechanism of Combustion Instability in a Lean Premixed Dump Combustor

Ammonia borane (AB) has attracted significant attention due to its high hydrogen content (19.6% by mass). To investigate the reaction mechanism associated with the combustion of AB, a reactive force field (ReaxFF) has been developed for use in molecular dynamics (MD) simulations. The ReaxFF parameters have been derived directly from quantum mechanical data (QM). NVT-MD simulations of single- and polymolecular AB thermolysis were conducted in order to validate the force field. The release of the first equivalent H(2) is a unimolecular reaction, and MD simulations show an activation energy of 26.36 kcal mol(-1), which is in good agreement with experimental results. The release of the second H(2) is also a unimolecular reaction; however, the release of a third H(2) requires the formation of a B-N polymer. Similar simulations were conducted with a boron and oxygen system, since the oxidation of boron will be an integral step in AB combustion, and show good agreement with the established mechanism for this system. At low temperatures, boron atoms agglomerate into a cluster, which is oxidized at higher temperatures, eventually forming condensed and gas phase boron-oxide-species. These MD results provide confidence that ReaxFF can properly model the oxidation of AB and provide mechanistic insight into the AB dehydrogenation and combustion reactions.

ReaxFF Reactive Force Field Development and Applications for Molecular Dynamics Simulations of Ammonia Borane Dehydrogenation and Combustion

Spatially distributed flame transfer functions for predicting combustion dynamics in lean premixed gas turbine combustors

The effect of incomplete fuel-air mixing on NO x emission and combustion stability has been studied in an atmospheric pressure, optically accessible, laboratory-scale, premixed dump combustor. The degree of fuel-dimensional acetone fluorescence. Combustion stability was characterized both in terms of the level of combustion-generated noise during combustion oscillations and the lean blowouts limit. The structure of the flame zone was characterized using two-dimensional OH fluorescence measurements that were phase locked with the combustion-generated pressure oscillations. As has been observed previously, it was shown that incompletes fuel-air mixing results in increased NO x emissions at overall fuel-lean conditions. The results, however, indicate that “reaction zone” NO mechanisms, enhanced by superequilibrinm O-atom concentrations, are in part responsible for more than 50% of the increased NO production. It was also shown that incomplete fuel-air mixing in a premixed combustor can significantly reduce the stable operating range of the combustor both in terms of increasing the tendency for combustion oscillation and raising the lean blowont limit. Lastly, it was observed that during unstable combustion, the flame moves in a well-defined periodic manner: however, the details of the flame behavior and instability mechanism can vary significantly depending on the overall equivalence ratio and the degree of fuel-air mixing.

Jong Guen Lee

Papers

Experimental Diagnostics for the Study of Combustion Instabilities in Lean Premixed Combustors

Mechanism of Combustion Instability in a Lean Premixed Dump Combustor

ReaxFF Reactive Force Field Development and Applications for Molecular Dynamics Simulations of Ammonia Borane Dehydrogenation and Combustion

Spatially distributed flame transfer functions for predicting combustion dynamics in lean premixed gas turbine combustors

Stability and emissions characteristics of a lean premixed gas turbine combustor