Intermittency oscillation behavior and physical model of anti-buoyancy turbulent downward flame in a jet reactor

Jet flames are commonly occurred in the fuel leakage and combustion chamber systems, which are closely related to safe production, especially in industries involving flammable gases or high-temperature processes. Flame oscillation due to combustion instability can generate severe risks to facilities and surroundings. This paper investigated the flame intermittency oscillation behavior of anti-buoyancy downward jet diffusion flames. Circular nozzles with four different diameters were used as the fire source openings with propane as fuel. The results show that the flame intermittent length of downward jets increases with increasing heat release rate, and it is smaller than the corresponding upward jet flame, which can be attributed to the axial velocity variation along the flame centerline. The ratio of flame intermittent length to flame downward distance decreases with heat release rate, showing an asymptotic behavior that approaches to a constant about 0.17. Moreover, the flame intermittent length normalized by nozzle diameter can converge well for different nozzles and increases with Froude number. A momentum-buoyancy length scale was proposed to represent the competition between jet momentum and flame buoyancy, and was employed to characterize the flame intermittent length of downward jets. Finally, a virtual source model was derived to predict the flame intermittent length showing a good agreement. This work provided knowledge on the oscillation physical mechanism of anti-buoyancy downward jet diffusion flame, which also provides important references for fire assessment and industrial process.