Differential diffusion effects in a lean-premixed bluff-body stabilized hydrogen flame

This paper describes the results of compressible large eddy simulations, with the Stochastic Fields method, of a lean-premixed bluff-body hydrogen flame. Both the LES isothermal and the reacting fields show good agreement with the measurements, revealing the capability of the Stochastic Fields method in bluff-body hydrogen flames. For the reacting field, two diffusive transport models, the constant Schmidt/Prandtl number approximation and mixture-averaged diffusion transport are implemented independently to isolate the effects of differential diffusion on species concentrations and temperature. The mixture-averaged diffusion transport model performs better than the constant Schmidt/Prandtl in terms of mean flame shape. Mixture-averaged diffusion results in local enrichment of the flame root, higher heat release rates, temperatures, fuel consumption rates and more diffused species profiles. High-temperature burnt gas in the recirculation zone elevates the significance of molecular diffusion, related to possible re-laminarization. The results reveal the importance of the inclusion of accurate molecular diffusive transport for hydrogen flames in lean-premixed bluff-body stabilized flames. DMD analysis suggests that both diffusion methods can recover the significant dynamic behaviour of the reacting flow system. Some physical-space differences between the two approaches are also shown in terms of their DMD modes. Novelty and Significance Statement The present work describes an LES study of the effect of differential diffusion on the hydrogen lean-premixed flames. The effect is isolated by comparing LES results obtained with two molecular diffusive transport models; the constant Schmidt/Prandtl number approximation and the mixture-averaged diffusion transport model applied to a bluff-body stabilized hydrogen flame. The solutions from the two diffusion transport methods are compared and analysed regarding flow field and flame characteristics. Dynamic mode decomposition is applied for further analysis. The results highlight the importance of including the differential diffusion effects in lean bluff-body stabilized hydrogen flames. The result is likely to helpful in the simulation of a wide range of hydrogen flame applications. The present work provides an example of hydrogen flame compressible LES with relatively manageable computational costs (3.5M mesh cells) and a multi-regime-robust flame model, i.e., the Stochastic Fields method. The method avoids any assumptions regarding flame regime burning and burning mode selection or parameter tuning. The method also allows the incorporation an ‘accurate’ diffusive transport model, e.g., mixture-averaged, in a relatively straightforward manner. This may significantly help the study and simulation of hydrogen flames. The present work systematically analyses the physical and chemical processes that occurred in the lean-premixed hydrogen flame arising in the NTNU bluff-body burner. Previous computational studies within this burner is limited in number and the present work will help the development hydrogen flame simulation methods. • This work describes the effect of differential diffusion on a hydrogen premixed flame. • This work presents Stochastic Fields LES with an ‘accurate’ diffusive transport model. • The physical–chemical processes of the flame are analyzed.