Propagation of ultra-lean hydrogen/air flames in a Hele-Shaw cell

Ultra-lean hydrogen flame is closely related to hydrogen safety. Recently, different types of hydrogen flame have been observed in experiments under ultra-lean conditions. However, the evolution and propagation of ultra-lean hydrogen flames are still not well understood. In this study, 3D simulations considering detailed chemistry and transport models are conducted for ultra-lean premixed hydrogen/air flames propagating in an open Hele-Shaw cell with isothermal walls. It is found that ultra-lean hydrogen flames are very sensitive to equivalence ratio, ϕ. As ϕ decreases from 0.225 to 0.21, different cellular flame regimes, including two-headed branching, two-headed finger and one-headed finger (ball-like flame) are sequentially observed. The cell size shows a decreasing tendency. Isolated ball-like flames and two-headed finger are stable in the ultra-lean mixture. During the flame cell propagation, both heat loss and heat release exhibit oscillatory characteristics since they are correlated with each other. The oscillation frequency is found to increase with ϕ. In order to balance the conductive heat loss to walls, two-headed flames split while isolated ball-like flames shrink, resulting in periodic changes in flame surface area and heat release rate. Moreover, ultra-lean flames are found to be characterized by high local equivalence ratio caused by strong differential diffusion of hydrogen over other species, highlighting the effect of diffusional-thermal instability (DTI) on sustaining the ultra-lean flame. Furthermore, stable ball-like flames and two-headed finger can exist simultaneously. Interestingly, flame instabilities play a stabilizing role in the ultra-lean flames. Darrieus-Landau instability (DLI) contributes to the stabilization of two-headed finger flames with strong mutual interaction between adjacent cells, whereas ball-like flames dominated by DTI tend to move away from each other to gain deficient fuel and drift in a zigzag manner. The present 3D simulations help to understand flame cell propagation and stabilization in ultra-lean hydrogen/air mixture within an open Hele-Shaw cell.