Comparison of NOx Predictions in Premixed & Non-premixed Laminar H2-air Flames- Detailed kinetics vs Inbuilt Fluent NOx Models

The present study develops a computation strategy for accurately predicting NOx emissions in laminar H2-air flames diluted with N2 using commercially available Ansys-Fluent software. Reactive flow 2D-CFD computations were performed on 2D-axisymmetric opposed-jet counterflow configurations. The hydrogen chemistry was solved using the latest detailed kinetic model by Konnov (2019). The two approaches adopted for NOx prediction are- (1) the use of fluent inbuilt NOx model and (2) the use of detailed kinetic steps for NOx containing N species from Gri-Mech 3.0 mechanism. The fluent post-processing NO model is coupled with the solved H and OH radical concentrations field from Konnov (2019) mechanism for NO production. Comparison of NO profiles results from details kinetics with fluent inbuilt NOx models was performed for both H2-air premixed and non-premixed flames. For non-premixed flames, hydrogen mole fraction in the fuel was varied from 100 % to 77.5 % to cover the effect of peak flame temperature on NO production. For premixed flames, the equivalence ratio was varied from 0.17 to 2.0 to quantify the effect of NO production from rich to lean regime. Comparing these NO profiles generated from the fluent NOx model with the detailed chemistry (Gri-Mech 3.0) highlights the inconsistency of this inbuilt fluent NOx model. Hence, it is concluded that detailed kinetic models containing N species should be preferred over the inbuilt fluent NOx model for accurate NO predictions.