Flame geometry of downward buoyant turbulent jet fires under cross flows: Experiments, dimensional analysis and an integral model

Abstract

This study investigates the flame geometry of vertically downward facing buoyant turbulent jet fires issuing from a nozzle subject to cross flows, which deflect the jet flow by interacting with downward momentum issuing from the source and the upward buoyant forces owing to the flames. The flames turn eventually upwards as the buoyant forces dominate. The objective of this work is to understand the physics and develop non-dimensional correlations and modeling for the flame geometry of this jet fire configuration never investigated before. For this purpose, experiments are conducted having 4 circular nozzles (diameter at 3, 4, 5 and 7 mm) with propane employed as fuel under various heat release rates. The cross flows are generated by a wind tunnel having a uniform cross flow air speed varying from 0.31 m/s to 2.08 m/s. The horizontal and downward distance from the nozzle to the lowest point of the jet flame, the vertical thickness of the flame at the lowest point, the flame vertical height from lowest point to the flame tip, the flame tip horizontal projection distance are measured. Three characteristic length scales developed by dimensional analysis considering initial downward momentum M0, flame buoyancy g′, inertial cross flow force ua2 together with the normalized air required for complete combustion Sm˙f/ρaua×(ua2/g′)2, are found to represent well the aforementioned properties of the flame geometry. An integral model, considering the mass and momentum conservations, is developed to predict the trajectory and geometric properties of the downward jet flame under cross flow showing good agreement with the experimental results.