Effect of Unsteadiness and Scalar Dissipation Models on Flamelet Modeling of Differential Molecular Diffusion in Turbulent Non-Premixed DNS Flames.

Prediction of differential molecular diffusion remains a great challenge for flamelet modeling of turbulent non-premixed combustion. This work addresses this challenge through a priori and a posteriori testing of flamelet models by using DNS databases to enable a detailed examination of the model capability and limitation for the prediction of differential molecular diffusion under different combustion conditions characterized by different Reynolds numbers and Damköhler numbers. The emphasis is on the effect of unsteadiness and scalar dissipation models on the flamelet modeling. Two sets of Sandia DNS of temporally evolving turbulent non-premixed jet flames are used for the study, including six different cases. Three of the DNS cases are based on the CO/ H 2 fuel with different Reynolds numbers, and the other three cases are based on the C 2 H 4 fuel with different Damköhler numbers. The unsteady effect is examined by considering both steady and unsteady flamelet models in the context of Reynolds averaged Navier–Stokes simulations for the model examination. Different differential molecular diffusion models are incorporated in the flamelet models such as the linear differential diffusion model and the non-linear differential diffusion model (Wang, Phys Fluids 28:035102, 2019). It is found that the use of unsteady flamelet models can generally improve the model prediction for differential molecular diffusion when compared with the steady flamelet models. This suggests the importance of considering the unsteady effect in flamelet modeling of differential molecular diffusion. In the unsteady flamelet modeling, different models for the representative scalar dissipation rate are examined and compared. It is found that, in order to adequately capture differential molecular diffusion, an appropriate model for the scalar dissipation rate is needed in addition to the consideration of unsteadiness. [ABSTRACT FROM AUTHOR]