Flamelet modeling for combustion processes and NOx formation in the turbulent nonpremixed CO/H2/N2 jet flames

Two turbulent nonpremixed jet flames having the same jet Reynolds number but different nozzle diameters have been numerically investigated by using the steady and unsteady flamelet models. These validation cases have been chosen to critically evaluate the predicative capability of the present turbulent combustion models whether the model is able to predict correctly both the similarity of temperature and major species by scaling with nozzle diameter, sensitivity of minor species to fluid-dynamic scaling and distribution of radicals and NO. In order to accurately resolve the physically and geometrically complex reacting flow fields. the present study adopts the unstructured grid finite-volume method with the cell-centered scheme and the edge-based storage. In terms of unconditional mean scalar structures, the steady flamelet model with the additional treatment for the slow processes such as radiation and NOx formation is able to yield the reasonable results, On the other hand, without additional ad-hoc procedure, the unsteady flamelet model based on a Lagrangian approach can correctly predict the experimentally observed streamwise evolutions of conditional mean scalar structure and unconditional means as well as the full NOx chemistry. Furthermore, in the context of unsteady flamelet model, the detailed discussions have been made for the potential error of the optically thin radiation model, the full NO chemistry and the effect of differential diffusion, which the steady flamelet approach hardly addresses.