An extended G-equation formulation for simulating thermodiffusively unstable hydrogen flames

Proceedings of the Combustion Institute, vol. 41, pp. 105945

Abstract

Hydrogen flames featuring thermodiffusive instabilities are challenging to model in large eddy simulations (LES) and Reynolds-averaged Navier–Stokes (RANS) simulations, where the relevant scales of the instability are typically not resolved. Approaches for modeling thermodiffusively unstable flames have mostly been formulated and validated for laminar flames, where interactions with turbulence are not considered. This study aims to extend the G -equation model for turbulent thermodiffusively unstable hydrogen flames in the context of RANS simulations. Despite the rise of the available computational resources and the development in high-fidelity LES and direct numerical simulations (DNS), RANS simulations are still the most employed approach in industrial applications, especially for design and operation optimization. In this study, the formulation of the turbulent flame speed as part of the G -equation combustion model was modified by coupling the empirical scaling relations for flame speed and thickness, which were derived from DNS of turbulent hydrogen flames and consider the effects of thermodiffusive instabilities and their interactions with turbulence. Here, a new relation is proposed to account for cases with high Karlovitz numbers, where increasing turbulence intensity leads to suppression of thermodiffusive instabilities. The derived model was applied to RANS simulations of two hydrogen flame configurations. The first configuration corresponds to a DNS database of two premixed turbulent jet flames. One accounts for thermodiffusive instabilities, while the other disables them by setting the Lewis numbers of all species to unity. The second configuration is performed for a research hydrogen engine, where two operating conditions were considered: one with a stoichiometric mixture and the other with a lean mixture representing conditions without and with strong thermodiffusive instabilities, respectively. Significant improvement in the prediction of the flame length of the jet flame and the in-cylinder pressure of the research engine was achieved with the extended model. The super-adiabatic temperatures due to thermodiffusive instabilities cannot be captured by the extended model, indicating a potential for further improvement for cases where the temperature distribution is of interest.

Authors 15

1. Hongchao Chu corresponding Aachen Institute for Combustion Technology

   RWTH Aachen University

   Affiliation as printed

   Institute for Combustion Technology, RWTH Aachen University, 52056 Aachen, Germany

2. Vignesh Varatharajan Aachen Institute for Combustion Technology

   RWTH Aachen University

   Affiliation as printed

   Institute for Combustion Technology, RWTH Aachen University, 52056 Aachen, Germany

3. Benjamin Pehlivanlar Aachen Chair of Thermodynamics of Mobile Energy Conversion Systems (TME)

   RWTH Aachen University

   Affiliation as printed

   Chair of Thermodynamics of Mobile Energy Conversion Systems, RWTH Aachen University, 52074 Aachen, Germany

4. Terence Lehmann Aachen Institute for Combustion Technology

   RWTH Aachen University

   Affiliation as printed

   Institute for Combustion Technology, RWTH Aachen University, 52056 Aachen, Germany

5. Lukas Berger Aachen Institute for Combustion Technology

   RWTH Aachen University

   Affiliation as printed

   Institute for Combustion Technology, RWTH Aachen University, 52056 Aachen, Germany

6. Dominik Golc Aachen Institute for Combustion Technology

   RWTH Aachen University

   Affiliation as printed

   Institute for Combustion Technology, RWTH Aachen University, 52056 Aachen, Germany

7. Stefania Esposito

   University of Bath

   Affiliation as printed

   IAAPS, Department of Mechanical Engineering, University of Bath, BA2 7AY Bath, United Kingdom

8. T.L. Howarth Aachen Institute for Combustion Technology

   RWTH Aachen University

   Affiliation as printed

   Institute for Combustion Technology, RWTH Aachen University, 52056 Aachen, Germany

9. Emanuele B. Porcelli

   Polytechnic University of Bari

   Affiliation as printed

   Polytechnic University of Bari, Department of Mechanics, Mathematics & Management, 70125 Bari, Italy

10. Davide Laera

    Polytechnic University of Bari

    Affiliation as printed

    Polytechnic University of Bari, Department of Mechanics, Mathematics & Management, 70125 Bari, Italy

11. Marco Günther Aachen Chair of Thermodynamics of Mobile Energy Conversion Systems (TME)

    RWTH Aachen University

    Affiliation as printed

    Chair of Thermodynamics of Mobile Energy Conversion Systems, RWTH Aachen University, 52074 Aachen, Germany

12. Michael Gauding Aachen Institute for Combustion Technology

    RWTH Aachen University

    Affiliation as printed

    Institute for Combustion Technology, RWTH Aachen University, 52056 Aachen, Germany

13. Joachim Beeckmann Aachen Institute for Combustion Technology

    RWTH Aachen University

    Affiliation as printed

    Institute for Combustion Technology, RWTH Aachen University, 52056 Aachen, Germany

14. Stefan Pischinger Aachen Chair of Thermodynamics of Mobile Energy Conversion Systems (TME)

    RWTH Aachen University

    Affiliation as printed

    Chair of Thermodynamics of Mobile Energy Conversion Systems, RWTH Aachen University, 52074 Aachen, Germany

15. Heinz G. Pitsch Aachen Institute for Combustion Technology

    RWTH Aachen University

    Affiliation as printed

    Institute for Combustion Technology, RWTH Aachen University, 52056 Aachen, Germany