Turbulence–flame interaction and fractal characteristics of H2–air premixed flame under pressure rising condition

Abstract

Direct numerical simulation (DNS) of turbulent premixed flames in a constant volume vessel is conducted to investigate turbulence–flame interaction and fractal characteristics under pressure rising condition. DNS is conducted for hydrogen–air mixture with decaying homogeneous isotropic turbulence of which the Reynolds number based on Taylor micro scale is 97.1. The detailed kinetic mechanism including 12 reactive species and 27 elementary reactions is used to represent the hydrogen–air flames. Even for the pressure rising process, the local heat release rate, the flame curvature and the tangential strain rate can be scaled by the maximum laminar heat release rate corresponding to instantaneous mean pressure, the Kolmogorov length scale and the ratio of the turbulent intensity to the Taylor micro scale in the unburned mixture. These facts show a rapid response of flame characteristics to the pressure change. In the propagation process, flame displacement speed is very high compared to open flames due to the expansion of the burned gas. The compression of the unburned mixture due to the high displacement speed increases pressure in the vessel and induces slow decaying or increasing in the turbulent Reynolds number even for no mean flow. It is clarified that the fractal dimension of the flame surface does not show any dependence on pressure increase. The inner cutoff obtained in the pressure rising condition is in good agreement with the expression proposed in our previous study (Shim et al., 2011) [6] where the ratio of the diameter of coherent fine scale eddy, which is a universal fine scale structure of turbulence, to the laminar flame thickness is used as an important parameter.