Characteristics of laminar lifted flames in coflow jets with initial temperature variation

Characteristics of laminar lifted flames of propane highly diluted with nitrogen have been investigated by varying the initial temperature in coflow jets. The result showed that the lifted flame maintained the tribrachial structure up to the initial temperature of 900 K and the liftoff height decreased with initial temperature and dilution ratio. The overall behavior of liftoff heights correlated well with the jet velocity scaled by the stoichiometric laminar burning velocity, emphasizing the importance of the stoichiometric laminar burning velocity on the propagation speed of tribrachial flame. The exponent of the liftoff height with jet velocity in the relation of H_L ∼ U₀ⁿ increased with initial fuel mole fraction, which has been attributed to the differential diffusion between propane and diluent nitrogen. Consequently, nitrogen concentration varied along the stoichiometric contour, which affected the propagation speed. Also, the exponent increased with initial temperature due to the sensitiveness of the propagation speed variation with nitrogen dilution on initial temperature. The liftoff conditions have been observed for the jet velocity even smaller than the stoichiometric laminar burning velocity at relatively low initial temperatures. This can be attributed to the effect of the buoyancy. Liftoff velocities accounting for the relative buoyancy effect were found to have a satisfactory correlation with S_L^O|_st regardless of initial temperatures and nitrogen dilution.