Simulating flame propagation in channels and tubes

The design and development of efficient and safe combustion devices depends on understanding the mechanisms of acceleration of premixed flame fronts in channels or tubes. Unexpected combustion in an industrial setting, such as a coal mine, can occur when combustible gases are present, and flame acceleration can lead to the possibility of detonation.

Many previous computational studies of flame propagation in these settings have employed several simplifications, including the assumption that mass and thermal diffusion are equal. In modeling terms, this would mean the Lewis number, Le, is equal to 1, a situation that is not always realistic, although it is often used in theoretical studies. In a recent paper, investigators have relaxed this assumption and considered Le values ranging from 0.2 to greater than 1. As a result, they found new behaviors and compared them to known trends.

The investigation considered a phenomenon known as finger flame acceleration where an initially hemispherical kernel of flame propagates into a tube or channel, acquiring an elongated finger-like shape. The investigators found flames with Le less than 1 had a higher scaled acceleration rate than did flames with Le equal to 1.

The flame Reynolds number (Re), which can be regarded as a scaled channel width, was found to have different effects on the burning rate for Le greater than 1 and Le less than 1 flames. In particular, for Le less than 1, the flame acceleration weakens as Re increases, but the effect is opposite for Le greater than 1 flames.

The authors considered two-dimensional geometries, both planar channels and cylindrical ones. If full three-dimensional geometries are considered, the simulation results could potentially change. Future studies will consider both 3D geometries and more realistic chemical reaction kinetics.

Source: “Impact of the Lewis number on finger flame acceleration at the early stage of burning in channels and tubes,” by Mohammed Alkhabbaz, Olatunde Abidakun, Damir Valiev, and V’yacheslav Akkerman, Physics of Fluids (2019). The article can be accessed at https://doi.org/10.1063/1.5108805 .