Investigating the effect of intrinsic instabilities on engine performance for Ammonia-Hydrogen blends
Creators: name: Nawaz, Behlol
name: Correa, Luis Fernando Alvarez
name: Das, Shubhra Kanti
name: Nasim, Md Nayer
name: Dumitrescu, Cosmin
name: Mack, John Hunter
Publisher: name: Zenodo
Year: 2024
Subjects: laminar flame
combustion instabilities
hydrogen
combustion engine
ammonia

description: Blends of ammonia (NH3) and hydrogen (H2) have great potential to be used as zero-carbon fuels notwithstanding challenges associated with combustion behavior, material compatibility, and distribution infrastructure. In most combustion applications, the effects of wrinkling caused by intrinsic combustioninstabilities are overshadowed by flow turbulence. However, this is not the case for H2-containing blends. This study investigates the link between the propensity for wrinkling in spherically-expanding laminar flames of NH3-H2 blends and their performance in spark-ignition engines in terms of pressure rise, cyclicvariability, and burn duration. A range of equivalence ratios (0.7 to 1.1) and blends (0 to 50% H2 in the fuel by volume) were first studied in an optically accessible constant volume combustion chamber (CVCC) to identify which blends exhibit instabilities. Classification was based on an analysis of the flame frontcaptured by schlieren imaging. Two specific blends by volume (50%H2-50%NH3 and 30%H2-70%NH3) were tested in a spark ignition (SI) engine under the same compression ratio for equivalence ratios between 0.5 and 1.0 in order to investigate performance differences between intrinsic instability regimes of NH3 – H2 blends.

For the spherical flames in the CVCC, higher H2 blend concentrations resulted in higher maximum crack lengths (Lcrack) and a faster increase, as well an earlier start (i.e., at smaller flame radii) to accelerated crack length growth. As the mixture becomes richer, the maximum number of cracks become increasingly smaller even at higher blend fractions of H2. The tendency of more instabilities at higher H2 fractions and lower Φ is consistent with how pure H2-air spherical flames behave. In the SI engine experiments, blends with higher H2 fraction show an increase in the peak in-cylinder pressure, pressure rise rate, and heat release, while the burn duration exhibits reduction. Similarly, as Φ was increased from 0.70 to 1.1 for a given fuel blend, the peak in-cylinder pressures became higher. There was no anomalous or counter-intuitive increase in in-cylinder pressure or decrease in burn duration.

As the increase in Φ towards the stoichiometric mixture (and even slightly higher) enhanced the pressure in the engine cylinder, while the crack lengths decreased for the corresponding spherical flame experiments in the CVCC, the overall effect is most likely due to the higher reactivity and heat release due to the increase of H2 fraction in the mixture. The added wrinkling due to intrinsic instabilities do not have a significant effect of their own on the engine performance. This is most likely because the flames in an internal combustion engine are turbulent enough to mask the effects of intrinsic instabilities. This methodology could be extended to devices with less turbulence, leaner mixtures, and/or higher H2 contents.