Structure and propagation of spherical turbulent iron-methane hybrid flame at elevated pressure

In this communication we demonstrate the role of turbulence intensity in the dual-front structure and self-similar propagation of spherical turbulent iron-methane hybrid flames. We first show that iron-methane hybrid mixture, whose iron concentration is below a critical threshold for the formation of a dust flame front in laminar or weak turbulent environment, can be burned strongly with both separated dual-front and merged single-front structures in intense turbulence. It is suggested that the formation of iron flame front would be attributed to local iron concentration accumulation by preferential sampling with near-unity Stocks number (St), heat transfer enhancement of iron particles to fluid and mixing promotion of iron particles with oxidants by strong turbulence. The propagation of iron front falls behind the methane front in the leading segments which is promoted by flame stretch for sub-unity Lewis number (Le), thus the separated dual-front structure occurs. Furthermore, the strong self-similar propagation of spherical turbulent iron-methane hybrid flame was observed under different turbulence intensities (u_rms). Mechanistically, such strong self-similar propagation of the hybrid flame is the consequence of the couple effects of flame mode transition at high u_rms with near-unity St and differential diffusion for sub-unity Le.