The purpose of the study is to assess the extinguishing concentration of inert gases in engine nacelle fire. Also, in staged combustion for reducing pollution such as NOx, if exhaust gas which has high concentrated fuel was re-ignited at wake, we would use bluff body for improve flame stability. The experiment was performed with a two dimensional rectangular bluff body stabilized flames, where the fuel was ejected to counter flow and co-flow against an oxidizer stream. Two inert gases, CO2 and N2, were used for extinguishing agent in the oxidizer and methane was used for fuel. The main experimental parameters were the direction of injecting fuel, the kinds of agent and the velocity ratio between air and fuel streams, which controlled the mixing characteristic near bluff body and the strength of recirculation zone in the down stream. The result shows the flame structure and the mode were strongly dependent with fuel/air ratio and the fuel jet direction. For both flow configurations, the extinguishing concentration of CO2 was smaller than the N2 because of the large heat capacity of CO2. However, the concentration of inert gases at blow out was much smaller than those in the cup-burner and co-flow jet diffusion flames, which implies that the extinction mechanism of bluff body stabilized flames was mainly due to the aerodynamic aspect. Compared to co-flow fuel injection, the extinguishing concentration of inert gases under counter flow configuration was lower. The effect of direction might result from the mixing characteristic and strength of recirculation zone around a bluff body.
For the more detail, in the complementary numerical simulation, FDS (fire dynamic simulator) based on the LES (Large Eddy Simulation) was employed to clarify the isothermal mixing characteristic and wake flow pattern near bluff body. The result shows the flame structure and blowout limit strongly depends on fuel/air velocity and the fuel ejection direction for both flow configurations. The extinguishing concentration of CO2 is smaller than the N2 because of the large heat capacity of CO2. However, the concentration of inert gases at blow out is much smaller than other diffusion flames, which implies that the extinction mechanism of bluff body stabilized flames is mainly due to the aerodynamic effect. The dilution effect on extinction is large for counter flow injection, which results from the mixing characteristic. The simulated result of isothermal flow field supports the facts that the difference of flame stabilization mechanism for counter flow configuration is mainly due to possibility of being of reacting wake and the controlling parameter on the flame mode is the vortex strength near bluff body by fuel injection for the co-flow configuration.