Formation of a cool diffusion flame and its characteristics

Abstract

Low-temperature (∼700 K) “cool” flames formed with n-heptane fuel were observed in the experiments conducted under microgravity. Recently, Won et al. demonstrated experimentally that such “cool” flames could be established under normal gravity. However, as ozone was added to air for supporting the flames in that experiment, the fundamental question on the formation of a self-supported n-heptane/air “cool” flame remained unanswered. A numerical investigation is conducted for (1) finding an answer to this question and (2) determining a procedure for establishing a “cool” flame. A standard opposing-jet burner is considered. Investigations are performed using a well-tested CFD code. A reduced mechanism that successfully captured the “cool” flames in microgravity is used. Calculations have demonstrated that a “cool” diffusion flame can be established with n-heptane under normal gravity without resorting to flame-speed promoters such as ozone, pressure or temperature. Using velocities of 4 and 7 cm/s and temperatures of 400 and 300 K for the fuel and air jets, respectively, a stable “cool” flame was obtained. Flame was ignited through increasing the temperature of the air jet to a value >724 K and then a self-sustained “cool” flame was obtained by decreasing the temperature back to 300 K. Parametric studies are performed on the applied gravitational force such that the heavy n-heptane flowed upward from the bottom nozzle or downward from the top nozzle. In general, flame remained flat with or without the gravity. However, when the fuel jet was at the bottom, it expanded faster and moved the flame closer to the fuel nozzle. “Cool” flame could not be stabilized for the same velocity and temperature conditions when fuel flowed from the top. “Cool” and normal diffusion flames obtained with identical flow conditions are compared. Limiting stretch rate for extinguishing the “cool” flame is determined.