A numerical analysis of the structure of a turbulent hydrogen jet lifted flame

Abstract

This paper presents a direct numerical simulation (DNS) study of the flame structure of a turbulenthydrogen jet lifted flame. The diameter of the hydrogen injector is 2 mm, and the injection velocity is 680 m/s. The time-dependent three-dimensional simulation was made with full chemical kinetics and rigorous transport properties. More than 22 million grid points were used. The numerical analysis, in terms of the normalized flame index, has made clear that the lifted flame is not a single flame, but a complex flame consisting of three flame elements: (1) a stable laminar leading-edge flame, (2) a conical inner vigorous turbulent premixed flame, and (3) a number of floating diffusion flame islands, surrounding the inner premixed flame. The stable laminar leading-edge flame of ring shape is stabilized outside the turbulent jet and has a triple flamelike structure with the normalized flame index around unity, indicating that the incoming flow almost balances with the laminar burning velocity. The floating flame islands are produced by turbulent behavior and local extinction of the inner premixed flame. The detached gas volume flows downstream, continuing to burn by the molecular diffusion of oxidizers. The inner rich premixed flame is strongly turbulent by the instability of the hydrogen jet at the tip. The flame is strongly stabilized by the leading-edge flame, and the heat release layer of the flame is deviated from the hydrogen consumption layer, indicating that the turbulence modifies the inner flame structure. The respective flame elements have their own complicated three-dimensional structure, and further studies are required to understand in detail the structure and stability of the lifted flame. The present study has revealed that this kind of DNS study is very useful to investigate various very complicated flame structures, such as the lifted flame.