Direct numerical simulations of head-on quenching of statistically planar turbulent premixed flames on rough walls

Rough walls are common in engineering applications. However, existing understanding of combustion near rough walls is lacking. In the present work, direct numerical simulations (DNS) of head-on quenching of statistically planar turbulent premixed flames on rough walls are reported for the first time. Hydrogen is considered as the fuel because of its importance in a zero-carbon economy. The temporal evolution of premixed flames propagating head-on towards walls with various wall roughnesses are compared. It is observed that rough walls result in incomplete consumption of hydrogen, with a more pronounced effect as the roughness amplitude increases. The impacts of wall roughness on wall heat transfer and local flame quenching are examined. The maximum wall heat flux in the rough-wall cases occurs at the roughness crests, and is significantly higher than that in the smooth-wall cases. The total wall heat loss increases with increasing wall roughness (i.e. increasing amplitude-to-wavelength ratio of roughness). The same trend is also observed in the corresponding laminar cases. A negative correlation between the quenching distance and the quenching wall heat flux exists in both the smooth and rough-wall cases. Moreover, it is found that rough walls lead to reduced quenching distances. The heat release rate on the wall is scrutinized. Remarkably high heat release rates are observed on the wall of the DNS cases, which is not observed in the head-on quenching process of the corresponding laminar flame. The heat release on the wall is dominated by radical recombination reactions. The heat release rate on rough walls is higher than that on smooth wall, which increases with increasing wall roughness. In the rough-wall cases, the heat release rate is the highest in the regions around the roughness crests, which can be explained by the distributions of species concentrations.

Novelty and significance

The work presented in this paper is new, original and of interest as it enhances our understanding of combustion near rough walls. For the first time, the interactions between flame and rough walls in turbulent environments are quantitatively examined using DNS. The temporal evolutions of the flow and flame structures during head-on quenching with various wall roughnesses are compared. The effects of wall roughness on wall heat transfer and local flame quenching are analyzed, and the heat release rate at the wall is closely scrutinized. The novel finding that rough walls may lead to incomplete fuel consumption is significant for the safe and efficient operation of industrial burners, and the analysis of wall heat transfer and flame quenching is essential for the design and optimization of advanced combustion devices.