Simulation und Modellierung turbulenter Verbrennung von Brennstoffgemischen mit hohem Wasserstoffanteil
Zusammenfassung der Projektergebnisse
Global climate change necessitates a cleaner and more efficient use of energy. For a future carbon-free energy system, it is clear that hydrogen will play a prominent role among synthetic fuels. It is carbon-free, very versatile in energy on-demand applications, and it can be converted into methane or liquid fuels. However, the deployment of hydrogen or high hydrogen content (HHC) fuels involves several scientific and technological challenges, e.g., such flames are prone to thermodiffusive instabilities. These instabilities have a leading-order effect on the flame dynamics, yielding consumption speeds several times higher than the laminar unstretched burning velocity. They arise from the strong differential diffusion of the hydrogen molecule or, in other words, the low Lewis number of hydrogen, which is the ratio of the thermal and its molecular diffusivity. The proposed project aims at understanding the effects of these instabilities on the combustion process and providing the foundation for the development of computational tools and models to support the design of fuel flexible combustion systems. To unravel the underlying physics of intrinsic flame instabilities, a series of direct numerical simulations (DNS) has been performed in a hierarchical simulation approach. For this, thermodiffusive instabilities are assessed first in laminar and then in turbulent flames. In particular, theoretical models are shown to be not capable of accurately describing the evolution of such flames, so the propensity of laminar lean hydrogen flames to develop thermodiffusive instabilities is studied in a large parametric space at different equivalence ratios, unburned temperatures, and pressures. A numerical perturbation analysis of a planar flame (linear flame regime) linked the characteristic growth rates of the intrinsic instabilities to the fundamental flame parameters, showing that an increase of the flame expansion ratio or the Zeldovich number or a decrease of the Lewis number enhances instabilities. Once intrinsic instabilities are fully developed (nonlinear regime), an analysis of the characteristic flame front corrugation revealed the existence of a smallest and largest flame intrinsic length scale, whose formation mechanisms are comprehensively discussed. Finally, the reference data of the laminar flames are used to develop a data-driven model for the consumption speed enhancement as a function of equivalence ratio, temperature, and pressure. To investigate the interactions of thermodiffusive instabilities and turbulence, large-scale DNS of turbulent lean hydrogen/air flames have been performed in a slot burner configuration. Thermodiffusive instabilities are shown to significantly affect the turbulent flame speed, which is not only enhanced by flame wrinkling, but is also greatly increased due to significant variations of the local reaction rates. The effects of differential diffusion are found to be even enhanced in a turbulent flame compared to a laminar flame at the same conditions due to larger curvature and strain rate values in the turbulent flow. Thus, thermodiffusive instabilities are sustained in turbulent flows and even show synergistic interactions with turbulence. Finally, the insights gained by DNS can be exploited for the development of combustion models. For this, a model for the progress variable source term to be used in lower fidelity simulations has been derived using a manifold of unstretched premixed flamelets at different equivalence ratios. For further model development and validation, the DNS represent a unique data base that will foster several research projects in the next years.
Projektbezogene Publikationen (Auswahl)
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Non-linear evolution of thermodiffuisvely unstable lean premixed hydrogen flames, Joint Meeting of the German and Italian Sections of the Combustion Institute, Sorrento (Italy), 23 May 2018 - 26 May 2018
L. Berger, K. Kleinheinz, A. Attili, H. Pitsch
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Characteristic patterns of thermodiffusively unstable premixed lean hydrogen flames, Proceedings of the Combustion Institute, 37, 1879–1886, 2019
L. Berger, K. Kleinheinz, A. Attili, H. Pitsch
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Flame fingers and interactions of hydrodynamic and thermodiffusive instabilities in laminar lean hydrogen flames, Proceedings of the Combustion Institute, 39, 2022
L. Berger, M. Grinberg, B. Jürgens, P.E. Lapenna, F. Creta, A. Attili, H. Pitsch
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Intrinsic instabilities in premixed hydrogen flames: Parametric variation of pressure, equivalence ratio, and temperature. Part 1 - Dispersion relations in the linear regime, Combustion and Flame, 240, 11935, 2022
L. Berger, A. Attili, H. Pitsch
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Intrinsic instabilities in premixed hydrogen flames: Parametric variation of pressure, equivalence ratio, and temperature. Part 2 - Non-linear regime and flame speed enhancement, Combustion and Flame, 240, 11936, 2022
L. Berger, A. Attili, H. Pitsch
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Synergistic interactions of thermodiffusive instabilities and turbulence in lean hydrogen flames, Combustion and Flame, 244, 112254, 2022
L. Berger, A. Attili, H. Pitsch
