PREDICTION OF FLAME LIFTOFF HEIGHT OF DIFFUSION/PARTIALLY PREMIXED JET FLAMES AND MODELING OF MILD COMBUSTION BURNERS

Abstract

In this article, a new flame extinction model based on the k/ε turbulence time scale concept is proposed to predict the flame liftoff heights over a wide range of coflow temperature and O₂ mass fraction of the coflow. The flame is assumed to be quenched, when the fluid time scale is less than the chemical time scale (Da < 1). The chemical time scale is derived as a function of temperature, oxidizer mass fraction, fuel dilution, velocity of the jet and fuel type. The present extinction model has been tested for a variety of conditions: (a) ambient coflow conditions (1 atm and 300 K) for propane, methane and hydrogen jet flames, (b) highly preheated coflow, and (c) high temperature and low oxidizer concentration coflow. Predicted flame liftoff heights of jet diffusion and partially premixed flames are in excellent agreement with the experimental data for all the simulated conditions and fuels. It is observed that flame stabilization occurs at a point near the stoichiometric mixture fraction surface, where the local flow velocity is equal to the local flame propagation speed. The present method is used to determine the chemical time scale for the conditions existing in the mild/flameless combustion burners investigated by the authors earlier. This model has successfully predicted the initial premixing of the fuel with combustion products before the combustion reaction initiates. It has been inferred from these numerical simulations that fuel injection is followed by intense premixing with hot combustion products in the primary zone and combustion reaction follows further downstream. Reaction rate contours suggest that reaction takes place over a large volume and the magnitude of the combustion reaction is lower compared to the conventional combustion mode. The appearance of attached flames in the mild combustion burners at low thermal inputs is also predicted, which is due to lower average jet velocity and larger residence times in the near injection zone.

Keywords:

• Flame liftoff height
• Mild/
• flameless combustion
• NO_x emission

Notes

A – constant used for temperature correction in Eq. (7). T_ref – reference temperature for calculation of chemical time scale. E/R – activation temperature. τ_ref – reference chemical time scale.

The values of various constants used in Eq. (7) of Filatyev et al. (2005) are B₁ = 0.07, B₂ = 0.16, δ_f = 0.35 mm (laminar flame thickness), d = 6 mm (jet diameter).