Comprehensive linear stability analysis for intrinsic instabilities in premixed ammonia/hydrogen/air flames

arXiv (Cornell University)

Abstract

Two-dimensional direct numerical simulations of planar laminar premixed ammonia/hydrogen/air flames are conducted for a wide range of equivalence ratios, hydrogen ($\rm H_2$) fractions in the fuel blend, pressures, and unburned temperatures to study intrinsic flame instabilities (IFIs) in the linear regime. For stoichiometric and lean mixtures at ambient conditions, a non-monotonic behavior of thermo-diffusive instabilities with increasing ($\rm H_2$) fraction is observed. Strongest instabilities occur for molar ($\rm H_2$) fractions of 40%. The analysis shows that this behavior is linked to the joint effect of variations of the effective Lewis number and Zeldovich number. IFIs in ammonia/hydrogen blends further show a non-monotonic trend with respect to pressure, which is found to be linked to the chemistry of the hydroperoxyl radical $\rm HO_2$. The addition of $\rm NH_3$ opens new reaction pathways for the consumption of $\rm HO_2$ resulting in a chain carrying behavior in contrast to its chain terminating nature in pure $\rm H_2$/air flames. Theoretically derived dispersion relations can predict the non-monotonic behavior for lean conditions. However, these are found to be sensitive to the different methods for evaluating the Zeldovich number available in the literature.

Authors 6

1. Lukas Berger Aachen Institute for Combustion Technology

   RWTH Aachen University

   Affiliation as printed

   Institute for Combustion Technology , RWTH Aachen University , Templergraben 64 , 52056 Aachen , Germany

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

   RWTH Aachen University

   Affiliation as printed

   Institute for Combustion Technology , RWTH Aachen University , Templergraben 64 , 52056 Aachen , Germany

3. Michael Gauding Aachen Institute for Combustion Technology

   RWTH Aachen University

   Affiliation as printed

   Institute for Combustion Technology , RWTH Aachen University , Templergraben 64 , 52056 Aachen , Germany

4. Sanket Girhe Aachen Institute for Combustion Technology

   RWTH Aachen University

   Affiliation as printed

   Institute for Combustion Technology , RWTH Aachen University , Templergraben 64 , 52056 Aachen , Germany

5. Bassam B. Dally

   King Abdullah University of Science and Technology

   Affiliation as printed

   Clean Energy Research Platform , King Abdullah University of Science and Technology , Thuwal , 23955-6900 , Saudi-Arabia

6. Heinz G. Pitsch Aachen Institute for Combustion Technology

   RWTH Aachen University

   Affiliation as printed

   Institute for Combustion Technology , RWTH Aachen University , Templergraben 64 , 52056 Aachen , Germany