A general formalism for determining the unburnt composition in multi-stream species transport-based CFD simulations

The imperative to decarbonise combustion necessitates technical solutions that increasingly rely on the concurrent utilisation of different fuels and/or oxidisers. The complexity of the reactive mixture compositions in such scenarios poses additional challenges from a CFD modelling perspective. While species transport models can generally describe multi-stream combustion problems directly, the definition of turbulence-chemistry interaction closures or the proper comprehension of combustion regimes often requires the reconstruction of the non-reactive mixing field. This work proposes a general comprehensive formalism for determining the unburnt composition in multi-stream combustion environments. The method relies on the elemental mass fraction conservation for the definition of a linear system that can be solved at runtime to retrieve the local unburnt mixture composition. The introduced formalism allows to assess the number of auxiliary stream-tracking scalars a−priori, thereby minimising computational efforts and effectively enabling the use of the inherent information within the set of transported species. The study presents an application example where a dual-fuel turbulent combustion scenario is numerically investigated. In this context, the consistency of the method with respect to the use of passive scalars has been discussed with and without the species equi-diffusivity assumption. A procedure for the a−priori estimation of the error introduced by the species preferential diffusion has been proposed, providing insights about the expected uncertainty on the predicted mixture composition and the respective flame properties. Novelty and Significance Statement The determination of the non-reactive mixing field is crucial for understanding reactive CFD simulations based on species transport. Additionally, in turbulent combustion models, knowing the unburnt composition is often a pivotal requirement for the model closure. While recalculated mixture fractions can determine the unburnt composition in dual-stream problems, this approach is inappropriate for multi-stream problems. This research introduces a novel generalised method for determining unburnt mixture composition in multi-stream combustion scenarios using CFD calculations based on species transport. The proposed method minimises the need for additional passive scalars by efficiently utilising existing information from the solved equations and boundary conditions, leveraging elemental mass fraction conservation.