Numerical Investigations on the Structure of Counterflow Nonpremixed Flames Interacting with a Single Vortex

초록

Turbulent flame is considered as a collection of flames interacting with vortices, which are featured by various scales and strengths, and can be modeled by flame-vortex interaction. It has, therefore, a valuable meaning to obtain details of physical phenomena for turbulent flames by investigating the flame-vortex interaction. A numerical simulation is a useful method to acquire reliable results for flame-vortex interaction. It is believed that the typical features of turbulent flame such as unsteady and curvature effects, which could not be observed in many previous studies for steady 1D flames, can be investigated by using a numerical simulation for unsteady 2D flames. Under the above background, we have performed unsteady 2D numerical studies for CH4/N2-Air counterflow nonpremixed flame interacting with single vortex. The flame configuration used in this study is the same as that of Sung et al. [1]. A 16-step Augmented Reduced Mechanism (ARM) [2], which consists of 20 species including NOx, is adopted in this calculation by considering a computational cost of 2D simulation. This 16-step ARM is modified to predict reasonably extinction point obtained by experiment. The steady flame structure of 2D simulation using above modified 16-step ARM is in very good agreement with those of experiment and calculation by 1D OPPDIF code. In case of flame-vortex interaction, to generate a vortex ring, fuel or air jet is issued from the small nozzle (d=2mm) in each side. The vortex formed from high velocity (v=5m/s) jet propagates downstream and interacts with flame. To quantify the strain on flame induced by a vortex, a scalar dissipation rate (SDR) is introduced. Results show that the fuel- and air-side vortex causes an unsteady extinction. In this case, the flame interacting with a vortex is extinguished in much larger SDR than steady flame. It was also seen that air-side vortex extinguishes a flame more rapidly than fuel-side vortex. In comparison between unsteady 2D an