Investigation of tangential strain rate impact on NO emissions in turbulent premixed hydrogen flames using the Eulerian Stochastic Fields approach

Large Eddy Simulations (LES) coupled with the Eulerian Stochastic Fields (ESF) approach are used in this study to investigate the effects of tangential strain on NO emissions. The simulation framework is applied to a lean, hydrogen-air premixed flame stabilised by a conical bluff-body burner developed at the Norwegian University of Science and Technology (NTNU). Simulations are conducted at three different inlet conditions. The inlet mass flow rates of premixed fuel and oxidiser are increased to systematically vary the tangential strain rate and analyse its effect on the flame dynamics and NO formation. The results are validated against experimental measurements, showing good agreement for velocity statistics and flame structure. A detailed analysis reveals that, for the present test case, the tangential strain rate is the dominant contributor to flame stretch, while curvature effects are negligible. Increasing tangential strain enhances flame reactivity up to a critical threshold, beyond which the consumption speed decreases. Results show that increasing the mean tangential strain rate by 24% can lead to an almost 43% reduction in NO emissions per kW. These findings highlight the potential of strain-based control strategies for emission reduction in hydrogen combustion systems and demonstrate the suitability of the ESF method in modelling highly strained, turbulent premixed flames.Novelty and significance statementThis study provides the first demonstration of how tangential strain rate can be systematically exploited to reduce NO emissions in a practical turbulent premixed hydrogen flame configuration. While previous works have largely focused on laminar counterflow or simplified configurations, this research extends the analysis to a three-dimensional bluff-body stabilised flame, capturing realistic turbulence–chemistry interactions. By employing Large Eddy Simulation coupled with the Eulerian Stochastic Fields (LES–ESF) method, the work achieves a detailed representation of differential diffusion effects and flame–strain coupling without relying on empirical closure assumptions. The findings establish that tangential strain is the dominant contributor to flame stretch, with curvature playing a negligible role, and reveal a critical threshold beyond which increased strain reduces flame consumption speed and NO production. A moderate increase of roughly 24% in mean tangential strain rate was found to yield an almost 43% decrease in NO emissions per unit power. Considering the high tangential strain levels characterising the experimental flame, the results presented here not only demonstrate the robustness of the ESF framework in capturing the trends typical of highly strained hydrogen flames, but also open pathways for strain-based emission control strategies, offering practical relevance for the design of next-generation low-emission hydrogen combustion systems.