Impact of “missing” third-body efficiencies on kinetic model predictions of combustion properties

Abstract A kinetic sensitivity analysis of almost any combustion observable will highlight pressure-dependent reactions important to predictions. Third-body efficiencies play a critical role in pressure-dependent reactions, serving as a species-specific multiplier on the rate constant of the reference collider (M). When third-body efficiencies are specified, the typical convention is to specify third-body efficiencies for a handful of common species relative to nitrogen, argon, or helium as the reference (in large part due to insufficient data for most other species). Species without explicit specification of a third-body efficiency are implicitly assumed to have a value of 1.0 – i.e. the same as the reference collider. While the impact of values used for third-body efficiencies for commonly specified species like water or carbon dioxide is well recognized, the impact of unspecified or “missing” third-body colliders is considerably less known. Naturally, the lack of specification of third-body efficiencies for some species could conceivably yield significant prediction errors. Their impact could be especially pronounced given that many unspecified third bodies, which are likely to have more rovibrational degrees of freedom or have permanent dipole moments, may be more effective in inducing energy transfer than the monatomic or diatomic gases used as a reference. Here, we present a screening procedure to estimate the potential impact of unspecified third-body efficiencies; outline key results across an array of models, fuels, and initial conditions; and explore implications for model development, optimization, and validation. In general, we find that the impact of unspecified third-body efficiencies can be substantial – large enough to potentially contaminate parameter inference in optimization studies in some cases and resolve factor-of-three differences between models and experimental data in other cases. These results therefore have further implications for the list of relevant colliders to be considered in both future modeling and energy transfer studies.